Old and New Major Mergers in the SOSIMPLE galaxy, NGC 7135
Thomas A. Davison, Harald Kuntschner, Bernd Husemann, Mark A. Norris, Julianne J. Dalcanton, Alessandra De Rosa, Pierre-Alain Duc, Stefano Bianchi, Pedro R. Capelo, Cristian Vignali
MMNRAS , 1–13 (2020) Preprint 19 January 2021 Compiled using MNRAS L A TEX style file v3.0
Old and New Major Mergers in the SOSIMPLE galaxy, NGC 7135
Thomas A. Davison, , ★ Harald Kuntschner, Bernd Husemann, Mark A. Norris, Julianne J. Dalcanton, Alessandra De Rosa, Pierre-Alain Duc, Stefano Bianchi, Pedro R. Capelo, Cristian Vignali , European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-87548 Garching bei Muenchen, Germany Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA INAF - Istituto di Astrofisica e Planetologie Spaziali, Via Fosso del Cavaliere, 00133 Rome, Italy Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg (ObAS), UMR 7550, 67000 Strasbourg, France Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, I-00146 Roma, Italy Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zurich,Winterthurerstrasse 190, CH-8057 Zürich, Switzerland Dipartimento di Fisica e Astronomia, Alma Mater Studiorum, Università degli Studi di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
The simultaneous advancement of high resolution integral field unit spectroscopy and robustfull-spectral fitting codes now make it possible to examine spatially-resolved kinematic, chem-ical composition, and star-formation history from nearby galaxies. We take new MUSE datafrom the Snapshot Optical Spectroscopic Imaging of Mergers and Pairs for Legacy Explo-ration (SOSIMPLE) survey to examine NGC 7135. With counter-rotation of gas, disruptedkinematics and asymmetric chemical distribution, NGC 7135 is consistent with an ongoingmerger. Though well hidden by the current merger, we are able to distinguish stars originatingfrom an older merger, occurring 6-10 Gyr ago. We further find a gradient in ex-situ materialwith galactocentric radius, with the accreted fraction rising from 0% in the galaxy centre, to ∼
7% within 0.6 effective radii.
Key words: galaxies: interactions – galaxies: evolution – galaxies: stellar content
Galaxy merger research has shown how fundamental merging isto galaxy evolution, with historical merger rates generally increas-ing with galaxy mass (Bundy et al. 2009; Schawinski et al. 2010;L’Huillier et al. 2012; Pillepich et al. 2018). Distant galaxies (z ≈ ≈ ★ E-mail: [email protected] ( (cid:46) © a r X i v : . [ a s t r o - ph . GA ] J a n T. Davison et al. is an important aspect to understand. For one thing, mergers areknown to drive gas towards the galaxy centre (Mihos & Hernquist1995), causing AGN activity and black hole growth, which in turncan shut down or suppress star formation in the galaxy at large (Cales& Brotherton 2015; Choi et al. 2015). On the other hand, mergerscan cause sudden and significant bursts of star formation due tothe disruption of previously unperturbed gas kinematics (Di Mat-teo et al. 2008; Ellison et al. 2013; Moreno et al. 2015; Capeloet al. 2015). Disruption in the gas kinematics of galaxies can leavekey fingerprints in identification of merger events. One of the mostreadily identifiable features of a recent or ongoing merger is counterrotating components, with up to 40% of S0 galaxies displaying sig-natures of counter-rotation (Rubin 1994; Davis et al. 2011; Coccatoet al. 2015; Bassett et al. 2017). Galaxy-galaxy mergers of the rightcombination can change the very morphological type of a galaxy.As such, mergers hold the power to define entire galaxy futures.The S01-pec galaxy NGC 7135 (AM 2146–350, IC 5136) in theconstellation of Piscis Austrinus is a merger remnant galaxy (Keel1985a) that is likely en route to forming an S0 galaxy. It currentlydisplays several immediately striking visual features including anextended tail, shell features, and curved structure (Figure 1) basedon photometry from the Carnegie-Irvine Galaxy Survey (Ho et al.2011).NGC 7135 was first described as having ‘a curious jet andshell’ in Malin & Carter (1983) with the ‘jet’ later shown to be atail in Rampazzo et al. (2003). The shell structures of the galaxywere found to be particularly clear in UV (Rampazzo et al. 2007;Marino et al. 2011), with FUV gas structure further linked to anaccretion event that also likely formed the shells. Ueda et al. (2014)found CO emitting gas that was unassociated with the nucleus,along with 3 mm continuum associated with the nucleus. Despitespeculation, NGC 7135 was determined to have no active nucleusas shown in Zaw et al. (2009) through optical spectra analysis.Analysis in Keel (1985b) identifies NGC 7135 as a merger galaxy,and in Rampazzo et al. (2003) NGC 7135 is shown to possess anelongated, asymmetric gas structure relative to the stellar material.The local environment of NGC 7135 is described by Samiret al. (2016) as being ‘low density’, with the classification of ‘lowdensity’ (Annibali et al. 2010) a result of the richness parameter 𝜌 𝑥𝑦𝑧 =0.32 gal Mpc − (Tully & Fisher 1988). Early type galaxies inlow density environments are known to possess on average youngerpopulations ( ∼ We observed NGC 7135 with the Multi Unit Spectroscopic Explorer(MUSE, Bacon et al. 2010, 2014) at the Very Large Telescope (VLT)as part of the Snapshot Optical Spectroscopic Imaging of Mergersand Pairs for Legacy Exploration (SOSIMPLE) survey (ProgramID: 0103.A-0637(A), PI: B. Husemann). The aim of the SOSIM-PLE survey is to provide complementary IFU observations for anongoing Hubble filler gap snapshot imaging program (Program ID:15446, PI: J. Dalcanton). HST imaging of NGC 7135 is not yettaken due to the filler nature of the HST program, thus these MUSEobservations act as a first look at the data, to which HST data canbe compared to at a later date. Combining IFU spectroscopy witha large set of high-quality ancillary data will hopefully provide ob-servational and theoretical insights into the evolution of mergingsystems.The MUSE observations were conducted on 6 July 2019 duringdark sky conditions and split into 3 ×
560 s dithered pointings alongwith a 300 s dedicated blank sky field exposure for backgroundsubtraction of this extended galaxy. Rotations of 90 ◦ were appliedbetween exposures covering approximately 3.4 arcmin as shown inFig 1. The seeing during the observations maintained at ∼ (cid:48)(cid:48) , andthe sky was covered with thin clouds during strong wind conditionsfrom North-West direction.The data were reduced with the standard ESO pipeline (Weil-bacher et al. 2020) which performs detector calibrations, flat-fielding, wavelength calibration, flux calibration as well as skysubtraction, exposure alignment, and cube reconstruction of thecombined exposures. We performed an additional correction forresidual sky lines using a simple PCA algorithm. The MUSE pixelscale is 0.2 arcsec pixel − , with a mean spectral resolution of ∼ Spaxels were Voronoi binned to a minimum SN of 50 per Å, therebypoor signal regions were made available for analysis, whilst higherSN spaxels remained unbinned. This optimally allowed for spatialinvestigation of spectral properties, without losing valuable highresolution data at high SN locations.The wavelength was restricted to 4759 - 6849 Å for all spaxelsto ensure the strongest Balmer lines were included, and to excludenoisier sky-dominated regions at redder wavelengths. All spectra ofspaxels within a bin were summed into a single spectra representingthe area covered by the bin. An area containing a foreground starwas masked from analysis in the West of the image (see Figure 1).To analyse the spectra from the binned NGC 7135 data weutilised the Penalized PiXel-Fitting (pPXF) method, described inCappellari & Emsellem (2004) and upgraded in Cappellari (2017).With this method, single-age single-metallicity stellar population(SSP) models are fit to spectra to build a map of stellar populationsacross age and metallicity space. By identifying the combinationof SSP models that approximate a given spectrum, the estimatedconstituent populations are extracted, as well as velocity and dis-persion. Stellar models are weighted as per the estimated fraction ofthe population present in the galaxy. As a result, output weights ofstellar models indicate the fractions of specific stellar populations
MNRAS , 1–13 (2020) ergers in NGC 7135 present in the spectrum. The output model of combined spectra ismade more physical by the use of template regularisation (see e.g.section 3.5 of Cappellari 2017), the methodology of which is ex-plained in detail below. Standard pPXF cleaning algorithms wereincluded to mask emission lines where necessary.A total of 552 MILES SSP models (Vazdekis et al. 2010) wereused to fit to galaxy spectra. These models were of Kroupa revisedinitial mass function (log slope of 1.3, M 𝑚𝑎𝑥 =100M (cid:12) ) using BaSTIisochrones, with a metallicity range of -2.27 to +0.4 [M/H] in 12non-linear steps, and an age range of 0.1 to 14.0 Gyr in 46 non-linear steps (Kroupa 2001; Cassisi et al. 2006; Pietrinferni et al.2006; Falcón-Barroso et al. 2011; Vazdekis et al. 2012).Application of regularisation allows smoothing over stellarmodel weights to reproduce a population map consistent with phys-ical results. The weighted templates that have been combined to pro-duce a target spectrum will often be unphysically localised to onlythe strongest of possible solutions, with many other valid solutionsbeing overlooked, despite their physicality. To produce more repre-sentative distributions, regularisation seeks to smooth the solutionsto a physical state. The challenge is to smooth the template weightsto a solution that most accurately represents observed conditions,whilst not overlooking genuine fluctuations and details present inthe model-fit. The regularisation parameter controls the strength ofthe smoothing and is deduced through a robust iterative approachfor each spectrum individually. The regularisation parameter is de-rived such that it corresponds to the maximum value consistentwith observations. Thus the derived star formation history will bethe smoothest that is consistent with the observations. This has beenshown in literature to be an accurate and useful method of galaxypopulation extraction (see e.g. Comerón et al. 2015; Norris et al.2015; Guérou et al. 2016; Faifer et al. 2017; Ge et al. 2019; Boeckeret al. 2020).In this work an iterative routine is applied to extract the op-timal regularisation parameter. For the best possible fit, the 𝜒 ofthe solution is expected to be approximately equal to the numberof available voxels in the spectrum, 𝑁 (i.e. the number of voxelsavailable after any masking). To obtain this optimal solution, the 𝜒 must be increased from the unregularised 𝜒 (referred to as 𝜒 ) by √ 𝑁 .After rescaling noise from the unregularised solution such that 𝜒 𝑁 = 1, we make a number of primary guesses at the regularisationparameter. We find the Δ 𝜒 of these initial guesses and fit a functionto the input regularisation guesses and output Δ 𝜒 values. By doingso we can precisely find the optimal regularisation parameter suchthat 𝜒 = 𝜒 +√ 𝑁 . This action is performed for every bin, resultingin optimal solutions across the entire image map. We separate the analysis of NGC 7135 into three components; thestellar component analysis, encompassing the stellar kinematics; thegaseous component analysis, encompassing gas kinematics, emis-sion lines and star formation aspects; and the population analysis,examining the various stellar populations and the resulting implica-tions for the assembly history of NGC 7135.To examine the stellar component we utilise Voronoi binning asdescribed in Section 3. From this we are able to examine the stellarrotation and bulk velocities, as well as mean age and metallicitiesspatially across the galaxy (Fig 2). To investigate details relatedto the gaseous component we use regular binning to view the gas
Figure 1.
A colour image of NGC 7135 showing the MUSE cube footprint.Photometry of NGC 7135 is from the Carnegie-Irvine Galaxy Survey (Hoet al. 2011). The blue border shows the boundaries of the reduced MUSEIFU data used in this study. A green circle traces an area containing a brightforeground star that was entirely excluded from the analysis. velocities and rotation, as well as the line strengths of H 𝛼 and H 𝛽 (Fig 3). Though we see reasonable amounts of H 𝛼 emission, thereis scant evidence for significant ongoing star formation. This isexplained in detail in Section 4.2. Finally, in Section 4.3 we furtheranalyse age and metallicity distributions for sampled regions acrossthe galaxy to diagnose assembly history and current merger status,then go on to examine underlying metal poor populations in Section4.4. Application of the pPXF method to the NGC 7135 data cube pro-vides mean kinematic properties which are extracted from each bin.Demonstrations of this for velocity and velocity dispersion of thegalaxy are found in the top panels of Figure 2. Application of regu-larisation and mass-to-light ratios produce maps of the constituentstellar populations within each bin of the galaxy. From these bins wecan derive mean mass-weighted stellar age and metallicity values,as demonstrated in the lower panels of Figure 2.The stellar kinematic, age, and metallicity maps of NGC 7135reveal much about the galaxy. Stellar rotation is immediately visible.This is of key interest when comparing to gas which rotates counterto the direction of stellar rotation. This is explored in detail inSection 4.2. One prominent kinematic feature, perhaps most clearlyseen in the velocity map (top left panel) of Figure 2, is an arcof incongruous material at higher than average velocity, stretchingfrom the South West of the Figure to the West. The Southern endof this arc is matched in the metallicity map (lower right panel,Figure 2) by a higher metallicity region, which is also distinct invelocity and velocity dispersion. Upon inspection, this is revealedto be an infalling galaxy currently merging onto NGC 7135. This
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MNRAS000 , 1–13 (2020)
T. Davison et al. -34°52'00"30"53'00"30" D e c V NE2 kpc21 h m s s s s s -34°52'00"30"53'00"30" Ra D e c Age h m s s s s s Ra [M/H] V e l o c i t y , m e a n ( k m s ) V e l o c i t y D i s p e r s i o n ( k m s ) A g e ( G y r ) M e t a lli c i t y [ M / H ] Figure 2.
Voronoi map of NGC 7135 showing 4 different stellar kinematic or mass-weighted population properties. The top left panel shows the mean velocityin km/s for each bin. The top right panel shows mean velocity dispersion within bins in km/s. The lower left panel shows the mean age of populations withinthe bin in Gyr. Finally the lower right panel shows mean metallicity within each bin. North is to the top of the image, and East is to the left. The stars show clearrotation in the centre. Velocity dispersion, age and metallicity all increase towards the galaxy centre. Distinct kinematics and metallicity south of the centrehighlight a distinct component. can be clearly seen in photometry shown in Figure 6, and even morecompelling evidence comes from population analysis below.
To explore gas kinematics and distribution in NGC 7135, regularbinning was employed to avoid biases caused by the stellar lightcontrolling Voronoi binning. Large square bins containing 64 pixelswere selected across the face of the data cube, and spectra withina given bin were summed and analysed with ppxf as describedin Section 3. Following this, those bins with signal-to-noise thatexceeded the minimum detection threshold were re-binned to ahigher resolution. This adaptive ‘zoom’ binning gave high resolutionin areas of strong H 𝛼 emission. The zoom resolution was limited tocentral regions of the galaxy, where the finest detail was required.NGC 7135 displays localised areas of strong Balmer emission,shown in Figure 3 with a cropped version showing the galaxy centre in Figure 4. As seen from all panels, the gas is asymmetric in dis-tribution as well as in kinematics. The rotation of the gas highlightsthe decoupled nature of the stellar material in the core.Gas is counter-rotating to the stellar component, strongly in-dicating a disrupted system. A slight deviation to the coherent gasmovement is seen in the galaxy centre, giving an ‘S’ shaped gas ro-tation profile. Counter rotation has long been associated with galaxymergers (see e.g. Bertola et al. 1988). Total decoupling of gas ro-tation from stellar components as a result of prograde-progrademerger shocks has been shown in simulation in Capelo & Dotti(2017), and a similar event appears to be in play here, wherein amajor merger has resulted in a counter rotation of the gas com-ponent. Plausibly this is the result of a previous merger providingcounter rotation from a prograde-prograde merger, this is expandedfurther in section 4.3. Alternatively, counter rotation could havearisen as a result of a first pass of the currently infalling galaxy.Velocity vectorisation of the gas and stars allows us to measure MNRAS , 1–13 (2020) ergers in NGC 7135 -34°52'00"30"53'00"30" D e c V gas NE2 kpc gas h m s s s s -34°52'00"30"53'00"30" Ra D e c H h m s s s s Ra H V e l o c i t y , m e a n ( k m s ) V e l o c i t y D i s p e r s i o n ( k m s ) H F l u x ( e r g / s / c m ) H F l u x e r g / s / c m ) Figure 3.
Regularly binned map of NGC 7135 showing 4 different gas kinematic and strength properties. The top left panel shows the mean velocity of gasin km/s for each bin. The top right panel shows mean velocity dispersion of gas within bins in km/s. The lower left panel shows the H 𝛼 flux throughoutNGC 7135. The scale has been limited from the true maximum to better display regions of intermediate strength. This limits the core from a true strengthof at most 36.2 × − erg/s/cm (limited to 2.5 × − erg/s/cm ). The lower right panel shows H 𝛽 flux throughout NGC 7135. The scale has been limitedfrom the true maximum to better display regions of intermediate strength. This limits the core from a true strength of at most 5 × − erg/s/cm (limited to2.1 × − erg/s/cm ). The gas velocity shows counter rotation compared to the stellar component, and on a slightly different axis, suggesting a merger origin. the gas and stellar rotation misalignment. The rotation consensus inthe gas is fairly standard, with the gas rotating around the centre.In the stellar component however, matters are complicated by thevelocity of the in-falling galaxy, which shifts the positive rotationvector compared to the core. If we consider only the core, themisalignment of gas and stars is 176 ◦ , whereas when the entire cubeis considered, the misalignment is 139 ◦ . This is entirely within therealm of expected values for an interacting galaxy (see e.g. Barrera-Ballesteros et al. 2015; Bryant et al. 2019). This is shown in Figure4 as solid and dashed arrows for the directions of mean positivestellar and gas rotation respectively, with associated errors shownas shaded regions.Regions of H 𝛼 emission can be seen in the southern areasof the lower left panel of Figure 3. This forms a large arc withpatches exhibiting particularly strong emission. These are seeminglymatched by arcs in the north in an asymmetrical manner.Considering the gas asymmetry and the increase in both gas ve- locity and velocity dispersion, a large amount of gas can be attributedto material stripped from the outskirts of the infalling galaxy andwhich is currently in the process of accreting onto the host galaxy.This is seen in the largest area of gas velocity dispersion occurringoutside the core, located in a tight region south of the galaxy core.This region indicates a quantity of gas that is not associated withthe cohort gas of NGC 7135, as it displays a region where infallinggas is interacting with the galaxy interstellar medium. This area ofhigher than expected dispersion is in the plane of the galaxy gasrotation, again evidence that gas is infalling, creating high velocitydispersion at the position where in-situ gas meets ex-situ gas.A strong presence of H 𝛼 in concentrated regions is consistentwith the picture of NGC 7135 as a galaxy that has perhaps recentlyundergone star formation as suggested in Rampazzo et al. (2007),though at low levels. Despite this, there is little to no evidence ofstrong ongoing star formation. This can be seen in the emissionline diagnostic diagram in Figure 5. Almost all the sources of emis- MNRAS000
Regularly binned map of NGC 7135 showing 4 different gas kinematic and strength properties. The top left panel shows the mean velocity of gasin km/s for each bin. The top right panel shows mean velocity dispersion of gas within bins in km/s. The lower left panel shows the H 𝛼 flux throughoutNGC 7135. The scale has been limited from the true maximum to better display regions of intermediate strength. This limits the core from a true strengthof at most 36.2 × − erg/s/cm (limited to 2.5 × − erg/s/cm ). The lower right panel shows H 𝛽 flux throughout NGC 7135. The scale has been limitedfrom the true maximum to better display regions of intermediate strength. This limits the core from a true strength of at most 5 × − erg/s/cm (limited to2.1 × − erg/s/cm ). The gas velocity shows counter rotation compared to the stellar component, and on a slightly different axis, suggesting a merger origin. the gas and stellar rotation misalignment. The rotation consensus inthe gas is fairly standard, with the gas rotating around the centre.In the stellar component however, matters are complicated by thevelocity of the in-falling galaxy, which shifts the positive rotationvector compared to the core. If we consider only the core, themisalignment of gas and stars is 176 ◦ , whereas when the entire cubeis considered, the misalignment is 139 ◦ . This is entirely within therealm of expected values for an interacting galaxy (see e.g. Barrera-Ballesteros et al. 2015; Bryant et al. 2019). This is shown in Figure4 as solid and dashed arrows for the directions of mean positivestellar and gas rotation respectively, with associated errors shownas shaded regions.Regions of H 𝛼 emission can be seen in the southern areasof the lower left panel of Figure 3. This forms a large arc withpatches exhibiting particularly strong emission. These are seeminglymatched by arcs in the north in an asymmetrical manner.Considering the gas asymmetry and the increase in both gas ve- locity and velocity dispersion, a large amount of gas can be attributedto material stripped from the outskirts of the infalling galaxy andwhich is currently in the process of accreting onto the host galaxy.This is seen in the largest area of gas velocity dispersion occurringoutside the core, located in a tight region south of the galaxy core.This region indicates a quantity of gas that is not associated withthe cohort gas of NGC 7135, as it displays a region where infallinggas is interacting with the galaxy interstellar medium. This area ofhigher than expected dispersion is in the plane of the galaxy gasrotation, again evidence that gas is infalling, creating high velocitydispersion at the position where in-situ gas meets ex-situ gas.A strong presence of H 𝛼 in concentrated regions is consistentwith the picture of NGC 7135 as a galaxy that has perhaps recentlyundergone star formation as suggested in Rampazzo et al. (2007),though at low levels. Despite this, there is little to no evidence ofstrong ongoing star formation. This can be seen in the emissionline diagnostic diagram in Figure 5. Almost all the sources of emis- MNRAS000 , 1–13 (2020)
T. Davison et al. -34°52'25"30"35"40"45" D e c V h m s s s s s -34°52'25"30"35"40"45" Ra D e c H h m s s s s s Ra H V e l o c i t y , m e a n ( k m s ) V e l o c i t y D i s p e r s i o n ( k m s ) H F l u x ( e r g / s / c m ) H F l u x e r g / s / c m ) Figure 4.
Regularly binned and zoomed in map of NGC 7135 showing 4 different gas kinematic and strength properties. The top left panel shows the meanvelocity of gas in km/s for each bin. The top right panel shows mean velocity dispersion of gas within bins in km/s. The lower left shows the H 𝛼 flux throughoutNGC 7135. The scale has been limited from the true maximum to better display regions of intermediate strength. This limits the strongest emission near thecore from a true strength of at most 36.2 × − erg/s/cm (limited to 2.5 × − erg/s/cm ). The lower right panel shows H 𝛽 flux throughout NGC 7135. Thescale here has also been limited. This limits the strongest emission from a true strength of at most 5 × − erg/s/cm (limited to 2.1 × − erg/s/cm ). In theupper left panel, arrows show the average positive rotation direction. The solid arrow indicates the average stellar component positive rotation whilst the dottedarrow shows the average gas positive rotation direction. Shaded regions show the standard deviation of vectors for both components for bins of 0.1 effectiveradii. In the lower left panel, contours show integrated CO(J=1–0) emission detected in ALMA observations (Ueda et al. 2014). Contours show the 0.8, 1.0and 1.2 Jy km s − levels. There is pervasive H 𝛼 emission with a high luminosity and high velocity dispersion component in the centre, though there is littleevidence of star formation. sion are associated with low-ionization nuclear emission-line re-gions (LINERs). Though a handful of active galactic nuclei (AGN)sources can be seen, they largely lie in the outer noisier regionsof the data-cube, which makes the presence of true AGN sourcesdoubtful, as shown in Zaw et al. (2009). This strong bias towardsLINER emission is typical of merging systems with shock drivenLINER emission (Monreal-Ibero et al. 2010; Rich et al. 2011).ALMA data (Ueda et al. 2014) showing the CO(J=1–0) emis-sion is overlaid in the lower left panel of Figure 4. The ALMAobservations reveal a significant peak in CO emission offset fromthe galaxy core with an integrated molecular gas mass of 𝑀 H2 = ( . ± . ) × 𝑀 (cid:12) adopting an 𝛼 CO = . 𝑀 (cid:12) pc − ( K km s − ) − (Solomon & Barrett 1991). This cold gas mass would correspondto an expected SFR of only ∼ . 𝑀 (cid:12) yr − if a normal depletion time of 2 Gyr for galaxies is assumed (Bigiel et al. 2011; Leroyet al. 2013). Although there is no similarly distinct ionised gasstructure observed with MUSE, there is plenty of ionized gas whichmay partially originate from star formation despite the LINER-likeclassification. The extinction-corrected H 𝛼 flux within the centralr=1 (cid:48)(cid:48) is ( ± . ) × − erg s − cm − which would correspond toSFR = . ± . 𝑀 (cid:12) yr − following Kennicutt Jr (1998). So only5% of the central H 𝛼 would need to be hidden among LINER-like classified ionised gas to be in agreement with ongoing starformation. Such a low fraction of star formation would not alterthe line diagnostics significantly and would remain hidden. Hence,we cannot rule out ongoing star formation based on the centralcold gas mass observed by Ueda et al. (2014). Given the highlydisturbed kinematics, the possibility that dynamical suppression of MNRAS , 1–13 (2020) ergers in NGC 7135 l o g ([ O III ] / H ) SF inter LINERAGN
Figure 5.
An emission line diagnostic diagram (Baldwin et al. 1981) dividedinto various sources. Each bin is shown as a point according to its emissionratios of [NII]/H 𝛼 and [OIII]/H 𝛽 allowing for the identification of regionsof star formation, AGN emission or Low-ionization nuclear emission-lineregion (LINER) emission. Detailed description of the line equations canbe found in Park et al. (2013). NGC 7135 shows no bins where currentstar formation is clear in the emission. Slight overlaps outside the LINERemission bin are unlikely to be genuine, but rather likely arise becauseof noise and intrinsic variations. The galaxy emission is overwhelminglyLINER type. star formation is preventing cold gas collapse cannot be tested byour observations. Populations of a galaxy evolve in metallicity over time, graduallyenriching with age. The exact quantities and rates of this enrichmentare well known (Carraro & Chiosi 1994; Layden & Sarajedini 2000;Pont & Eyer 2004), with the rate of enrichment directly tied to galaxymass resulting in the mass-metallicity relation. Thus, we can quicklyestablish whether a galaxy has followed the standard enrichment ofits population as would be expected from an isolated galaxy.In reality, galaxies are more often than not experiencing regulardisturbances in the form of mergers, fly-bys and intracluster mediuminteraction such as ram-pressure stripping (Lotz et al. 2011; Sinha &Holley-Bockelmann 2012; Ebeling et al. 2014; Ventou et al. 2017).One effect of this is the variation of the age-metallicity relationof a galaxy from the modelled form. This is most strikingly clearwhen a galaxy accretes material from a lower mass galaxy (Spolaoret al. 2009; Leaman et al. 2013). Due to the lower metal enrichmentrate of lower mass galaxies than that of larger mass galaxies, onefinds that in general a smaller mass galaxy will exhibit far lowervalues of metallicity at late ages. Because of the ability for fullspectral fitting methods to identify populations based on age andmetallicity models, one would see these two populations as distinctand separate areas on an age-metallicity diagram. This is dependenton the difference in mass of the mergers however, as if two galaxiesof similar mass were to merge, the separation of populations onthe age-metallicity diagram would be too little to distinguish atthe current resolutions of full-spectral fitting methods. Using theseprinciples we can estimate which of the populations present arethose which have accreted onto the host galaxy, and are thereforeex-situ in origin.We apply these principles to the population maps of NGC 7135 in order to derive the history of formation and evolution. In Figure6, nine regions are marked with sequential letters correspondingto population maps, which are similarly sequentially lettered, withmaps taken from the Voronoi bin below the labelled cross. Eachposition marks an area of interest or standard uniformity acrossthe maps of Figure 2 with which we can build a picture of theassembly and current status of NGC 7135. Region ‘A’ marks thecore of NGC 7135. Regions ‘B’ and ‘C’ sample the tidal tail clearlyseen in the unsharp mask image (lower right panel of Figure 6),with increasing galactocentric radius. Regions ‘D’, ‘E’, and ‘F’ alsosample with increasing galactocentric radius, however they do sooutside of any prominent tidal features. These are assumed to bea ‘control’ sample which are chosen to represent the underlyinggalaxy, though show signs of probing accreted material. Regions‘G’ and ‘H’ sample the tidal regions opposite the tail, with ‘H’particularly covering unstripped remnants of the infalling galaxy.Finally region ‘K’ covers the core of the infalling galaxy.Starting with region ‘A’, we see a very high metallicity, very oldpopulation associated with the galaxy core. This is to be expectedand is commonly seen in galaxy cores (see e.g. Guérou et al. 2016).There is little obvious evidence for accreted populations as expected,as shown by the old and high metallicity population, and lack of anyclear population bimodality.Moving along the main tidal tail in region ‘B’ we see a muchyounger population at high metallicity. When comparing to regionsnot associated with tidal features but at similar radius such as ‘E’and ‘F’, we see that the population of ‘B’ is not comparable to ‘E’or ‘F’. This is largely due to a lack of older material that wouldbe expected to be associated with the host galaxy. Plausibly thisis the result of the vast majority of the stellar material originatingin the infalling galaxy and comprising the tidal tail, and thus thepopulations visible are instead associated with this infalling object,rather than original populations of NGC 7135. A small amountof material is also visible as a young and metal poor population.This can be attributed to ex-situ material that merged onto eitherNGC 7135 or the infalling galaxy in the past prior to the currentmerger, and thus shows a separate population signature.As we move further out along the tidal tail to region ‘C’, manyof the features become more prominent. For one thing, the highmetallicity population associated with the stripped material fromthe infalling galaxy remains. Furthermore, low metallicity ex-situpopulations increase in the fraction of contributed mass (as seenas a distinctly separate low metallicity population). Care must betaken in comparison due to colour normalisation differences onthe plot, however the maximum low metallicity ex-situ fractionincreases from ∼ ∼ MNRAS000
An emission line diagnostic diagram (Baldwin et al. 1981) dividedinto various sources. Each bin is shown as a point according to its emissionratios of [NII]/H 𝛼 and [OIII]/H 𝛽 allowing for the identification of regionsof star formation, AGN emission or Low-ionization nuclear emission-lineregion (LINER) emission. Detailed description of the line equations canbe found in Park et al. (2013). NGC 7135 shows no bins where currentstar formation is clear in the emission. Slight overlaps outside the LINERemission bin are unlikely to be genuine, but rather likely arise becauseof noise and intrinsic variations. The galaxy emission is overwhelminglyLINER type. star formation is preventing cold gas collapse cannot be tested byour observations. Populations of a galaxy evolve in metallicity over time, graduallyenriching with age. The exact quantities and rates of this enrichmentare well known (Carraro & Chiosi 1994; Layden & Sarajedini 2000;Pont & Eyer 2004), with the rate of enrichment directly tied to galaxymass resulting in the mass-metallicity relation. Thus, we can quicklyestablish whether a galaxy has followed the standard enrichment ofits population as would be expected from an isolated galaxy.In reality, galaxies are more often than not experiencing regulardisturbances in the form of mergers, fly-bys and intracluster mediuminteraction such as ram-pressure stripping (Lotz et al. 2011; Sinha &Holley-Bockelmann 2012; Ebeling et al. 2014; Ventou et al. 2017).One effect of this is the variation of the age-metallicity relationof a galaxy from the modelled form. This is most strikingly clearwhen a galaxy accretes material from a lower mass galaxy (Spolaoret al. 2009; Leaman et al. 2013). Due to the lower metal enrichmentrate of lower mass galaxies than that of larger mass galaxies, onefinds that in general a smaller mass galaxy will exhibit far lowervalues of metallicity at late ages. Because of the ability for fullspectral fitting methods to identify populations based on age andmetallicity models, one would see these two populations as distinctand separate areas on an age-metallicity diagram. This is dependenton the difference in mass of the mergers however, as if two galaxiesof similar mass were to merge, the separation of populations onthe age-metallicity diagram would be too little to distinguish atthe current resolutions of full-spectral fitting methods. Using theseprinciples we can estimate which of the populations present arethose which have accreted onto the host galaxy, and are thereforeex-situ in origin.We apply these principles to the population maps of NGC 7135 in order to derive the history of formation and evolution. In Figure6, nine regions are marked with sequential letters correspondingto population maps, which are similarly sequentially lettered, withmaps taken from the Voronoi bin below the labelled cross. Eachposition marks an area of interest or standard uniformity acrossthe maps of Figure 2 with which we can build a picture of theassembly and current status of NGC 7135. Region ‘A’ marks thecore of NGC 7135. Regions ‘B’ and ‘C’ sample the tidal tail clearlyseen in the unsharp mask image (lower right panel of Figure 6),with increasing galactocentric radius. Regions ‘D’, ‘E’, and ‘F’ alsosample with increasing galactocentric radius, however they do sooutside of any prominent tidal features. These are assumed to bea ‘control’ sample which are chosen to represent the underlyinggalaxy, though show signs of probing accreted material. Regions‘G’ and ‘H’ sample the tidal regions opposite the tail, with ‘H’particularly covering unstripped remnants of the infalling galaxy.Finally region ‘K’ covers the core of the infalling galaxy.Starting with region ‘A’, we see a very high metallicity, very oldpopulation associated with the galaxy core. This is to be expectedand is commonly seen in galaxy cores (see e.g. Guérou et al. 2016).There is little obvious evidence for accreted populations as expected,as shown by the old and high metallicity population, and lack of anyclear population bimodality.Moving along the main tidal tail in region ‘B’ we see a muchyounger population at high metallicity. When comparing to regionsnot associated with tidal features but at similar radius such as ‘E’and ‘F’, we see that the population of ‘B’ is not comparable to ‘E’or ‘F’. This is largely due to a lack of older material that wouldbe expected to be associated with the host galaxy. Plausibly thisis the result of the vast majority of the stellar material originatingin the infalling galaxy and comprising the tidal tail, and thus thepopulations visible are instead associated with this infalling object,rather than original populations of NGC 7135. A small amountof material is also visible as a young and metal poor population.This can be attributed to ex-situ material that merged onto eitherNGC 7135 or the infalling galaxy in the past prior to the currentmerger, and thus shows a separate population signature.As we move further out along the tidal tail to region ‘C’, manyof the features become more prominent. For one thing, the highmetallicity population associated with the stripped material fromthe infalling galaxy remains. Furthermore, low metallicity ex-situpopulations increase in the fraction of contributed mass (as seenas a distinctly separate low metallicity population). Care must betaken in comparison due to colour normalisation differences onthe plot, however the maximum low metallicity ex-situ fractionincreases from ∼ ∼ MNRAS000 , 1–13 (2020)
T. Davison et al. as expected following galaxy population age gradients. Little to noex-situ material is clear. Moving further out in radius, we cometo region ‘E’. This also shows the expected populations previouslyseen in ‘A’ and ‘D’. This time however there is a more significantlow metallicity ex-situ population, which as mentioned previouslyis expected as one reaches regions further from the galaxy centreaccording to galaxy simulations. Also prominent in region ‘E’ is apopulation of intermediate age and high metallicity stars. As shownbelow in region ‘H’, this is almost certainly associated with theinfalling galaxy.Region ‘F’ samples at a slightly greater radius than ‘E’, againwith more prominent features, though in similar positions to ‘E’.We see an increase in the low metallicity ex-situ population radiallyalong the tidal tail (‘A’, ‘B’ and ‘C’) and well as radially in areasnot associated with tidal features (‘D’, ‘E’ and ‘F’).The final regions sample the galaxy shell and associated in-falling object. Region ‘G’ examines an area of tidal shell seeminglyalso originating from the infalling galaxy. The region almost iden-tically matches ‘H’ which is placed to examine the outskirts of theinfalling object, in regions that have yet to be stripped. The factthat these two populations are quite so similar suggests they are ofthe same origin, and that the tidal shells and tails are the result ofscattered accreted material from the infalling galaxy.Finally region ‘K’ examines the core of the infalling galaxy atapproximately 0.5 effective radii from the centre of NGC 7135. Itshows a highly metal rich and old population with the exact ten-dencies of a galaxy nucleus. It shows largely the same properties asthe nucleus of NGC 7135, though with marginally lower metallicityand a greater extent in age, suggesting a lower mass.The velocity dispersion of region ‘K’ (seen in Fig 2) is ata much lower average velocity dispersion than the host galaxy,again suggesting a lower mass of the merging galaxy comparedto NGC 7135. This is curious considering its high metallicity. Oneexplanation would be that the in-falling galaxy is the remnant of agalaxy core stripped of its halo, which would explain both its rela-tively high brightness and high metallicity. This is also supported bythe large amounts of seemingly ex-situ gas that are seen in Figure 3,where this gas would have formed the outer regions of the infallinggalaxy as explained further in section 4.2.The velocity dispersion (Fig 2) increases significantly midwaybetween the accreting galaxy core and the host galaxy core. Thisfurther lends weight to the idea that material is accreting onto thehost galaxy, as the high velocity dispersion area indicates a regionwhere accreted material begins encountering large amounts of in-situ material, and the difference in velocities becomes more evident,inflating the velocity dispersion, prior to mixing.In summary, the population maps are indicative of three dis-tinct galaxy populations, in which two significant merger eventsare present. The first is ongoing, with an intact core of a secondgalaxy currently in close proximity to NGC 7135, with material be-ing stripped off, accreted onto NGC 7135, and creating large tidalfeatures. These make up the high metallicity populations at interme-diate ages. Yet another population is consistently present, as a lowmetallicity, intermediate to old aged population. As discussed pre-viously, chemical enrichment and mass-metallicity relations meanthis population is not associated with either galaxy. Therefore weattribute these stars to older historical mergers, now mixed looselywith the main populations. It is unclear which of these two presentgalaxies these populations accreted to, however as mentioned previ-ously, the ex-situ population is likely present in both galaxies inde-pendently, and was captured by each prior to this ongoing merger.
As seen in Figure 6, many bins display a bimodality in populationdistribution (see e.g. panels ‘B’, ‘C’, ‘E’, ‘F’, ‘G’, and ‘H’). Sucha strong separation in populations suggests stellar material beingobtained from more than a single source. Galaxies not associatedwith the main galaxy will evolve with a different metallicity dueto the mass metallicity relation. As such, when the galaxies merge,there will be a distinct separation in the Age-Metallicity relation ofeach galaxy. The most obvious explanation for the bimodal popula-tions seen in Figure 6 would be the merger of a less massive, lowermetallicity galaxy to the host galaxy or onto the infalling galaxy,beginning ∼
10 Gyr ago. Furthermore, the fact that the bi-modalityof populations is seen at almost all positions across the galaxy out-side of the cores (panels ‘B’, ‘C’, ‘E’, ‘F’, ‘G’, and ‘H’) suggeststhat this material has been well mixed and is distributed throughoutthe galaxy, with the exception of the two galaxy cores (see panels‘A’, ‘D’, and ‘K’).To explore the population bi-modality, the fraction of starsnot associated with the main host population was determined fromeach bin. To identify two discontinuous populations, a dividing linewas sought across the population map, which would follow thelowest saddle points. This ‘path of least resistance’ then dividedthe populations into two distinct sources; one being material fromNGC 7135 and the in-situ material of the infalling galaxy; and theother source being low metallicity populations accreted onto bothgalaxies at earlier times. This can be imagined as the valley betweentwo hills, with the dividing line taking the natural path of a riverat the lowest potential. This is visualised in Figure 7 with a redline showing the calculated separation path for one random bin,separating the populations into two sources.Application of this to all bins provides a map such as in Figure8, where we can examine the fraction of stellar material associatedwith the lower metallicity source. Figure 8 shows a polar view ofNGC 7135 to better examine radial features. By examining fractionacross the galaxy we can infer regions of higher or lower concen-tration of the accreted material.At the centre of NGC 7135 we see no accreted material sug-gesting the core is dominated by in-situ stars. The density of accretedmaterial rises with radius which is indicative of galaxy mergers de-positing material on the outer regions of the galaxy. The materialseems to be unevenly radially mixed, with proportionally higherquantities of ex-situ material deposited between 0 and 1 radiansfrom North. This is likely a projection effect, as the area at the southof the galaxy (the left and right extents of Figure 8) aligns with thepreviously mentioned high metallicity galaxy, with the stream ofstellar material obscuring the host galaxy structure, and dominatingthe spectral light.We can further see evidence of the division of the variouspopulations by examining stellar mass estimates per population,determined with the division of the age-metallicity plane in combi-nation with mass-to-light ratios. We show this in Figure 9, with threeregions of different populations separated roughly. Using mass tolight ratios from Thomas et al. (2003), we estimate the stellar massper population division, per pixel. The panel labelled ‘1’ corre-sponds to intermediate age stars with high metallicity which wereassociated with the infalling galaxy. This is confirmed in the firstmap in the Figure (panel 2) in which there is a noticeably higherstellar mass associated with the infalling object for only this pop-ulation. This panel also encompasses much of the stellar materialof NGC 7135 near to the centre though at a slight distance, as isexpected from standard galaxy age gradients. Though effects from
MNRAS , 1–13 (2020) ergers in NGC 7135 Figure 6.
NGC 7135 population sampling diagram. The upper nine panels display mass weighted metallicity space of NGC 7135 for various regions.Corresponding regions are marked in the lower left panel with crosses marking the position extracted, and the corresponding letter. The lower right panel showsthe same region as an unsharp masked image to highlight tidal features. Data for the unsharp masked image are taken from the VST ATLAS survey (Shankset al. 2015). The diagrams build a narrative in which a recent and ongoing merger creates large tidal features in NGC 7135. There are also populations of farlower metallicity which are well mixed in the galaxy. These populations indicate historical mergers of high merger-mass ratio. the pointing overlaps are visible, it is notable that we see a smallamount of material tracing the tidal tail and other tidally derivedfeatures. This suggests that the intermediate age material and tidaltail is associated with the infalling galaxy exclusively, though fur-ther data analysis from a higher resolution stellar model grid wouldbe required for verification of this.In the second labelled map (panel 3) we see that the mostmetal rich and oldest material is associated heavily with the hostgalaxy, with a strong gradient from the galaxy centre. This in-situpopulation is generally undisturbed and centrally concentrated, incomparison to the largely ex-situ population represented in the 1stmap. Finally in the third labelled map (panel 4), we see again agradient of stellar mass associated with the host galaxy. This thirdmap shows only stars at far lower metallicities than the majorityof the stellar material. This material is assumed to be low massobjects which have historically accreted to NGC 7135, and are nowwell mixed into the galaxy. It should be noted that these are rigiddivisions, and that the true population distributions from each objectundoubtedly bleed over into the other divided regions (especially inregions ‘1’ and ‘2’).
Analysis of the galaxy kinematics and gas of NGC 7135 yieldedevidence for both historical galaxy mergers, as well as an ongoingdisruptive major merger. Despite the kinematics of past mergersbeing hidden (to the available resolution of data) due to mixing overtime, ex-situ populations were extracted from the galaxy using fullspectral fitting. This allowed for the identification of a well mixedlow-metallicity stellar population relative to the larger fraction ofhigher metallicity stellar population. Considering expected enrich-ment patterns, this can only have occurred if either gas or stars(or both) originating in an ex-situ galaxy rapidly accreted or fullymerged onto NGC 7135. The lower metal content of this populationmade it distinct from the original population.Potentially, all the stellar material in this population could havebeen created in-situ using gas that was accreted from another galaxy.This is highly unlikely however considering the specificity of theage and metallicity of the two distinct populations. Were these starsto be the product of new infalling gas, we would expect to see amixing of the gas, and for the metallicity of new stars born afterthe merger event to be at a more intermediate metallicity. Instead,we see the two populations continuing to form stars without a sharp
MNRAS000
MNRAS000 , 1–13 (2020) T. Davison et al.
Figure 7.
Population map of one bin projected on 3D axes. A line is soughtfor each map to bisect the lower metallicity population from the older usinglow saddle points. For this example, the path is marked by a red line. change in metallicity, thus the lower metallicity population stars areconsidered to be born ex-situ.The bimodality of these stars allowed for clean separation ofthe ex-situ and in-situ populations. Thus the relative fraction of ex-situ material could be ascertained. This allowed for the explorationof ex-situ fraction with galactocentric radius, as shown in Figure 8.The Figure shows a clear preference for ex-situ material to be locatedat the outer edges of the galaxy, with no detectable ex-situ materialin the centre of the galaxy. This is akin to simulated results showingthe same preference for ex-situ fraction increase with galactocentricradius (Schaye et al. 2014; Crain et al. 2015; Rodriguez-Gomezet al. 2016; Davison et al. 2020), as well as observational studiesshowing the same principles (Forbes et al. 2011; Pillepich et al.2015b; Oyarzún et al. 2019). The mean ex-situ fraction measured forNGC 7135 at approximately 0.6 effective radii (the greatest extentcaptured by the MUSE image) is 7%. This is only representativeof the low metallicity populations from low-mass systems. Highermetallicity populations from mergers of smaller mass-ratio mergerswould be disguised amongst in-situ populations.Limitations of this technique largely arise from the ability toseparate populations. At current resolutions of full spectral fittingtechniques, populations must be wholly distinct in metallicity to benoticeably separable from the host population. Accreted materialwith age and metallicity similar to that of the host galaxy wouldbe largely indistinguishable from the main population. Further lim-itations are the inability to directly distinguish between stars thatare born ex-situ, and those born in-situ but of ex-situ material. Asdiscussed above, these limitations are unlikely to be dominant inthis scenario.One interesting area to consider is the eventual fate ofNGC7135. Will it retain some semblance of a spiral structure,or evolve into an S0 or elliptical galaxy? Conversion into an S0galaxy seems to be a distinct possibility as S0 galaxies with coher-ent disk kinematics form through merger mechanisms, though theexact merger specifics continue to be debated within the community.Some evidence suggests that S0 galaxies are rarely expected to beformed through major mergers (<4:1 merger ratio) (Bournaud et al.2005; Lofthouse et al. 2016), with the conclusion given that major mergers are a plausible but non-dominant mechanism for early typeformation. Conversely, other arguments suggest that S0 galaxies canindeed be formed from major mergers (Querejeta et al. 2015a,b).Furthermore major mergers can be shown to give rise to much ofthe inner structure often found in early types (Eliche-Moral et al.2018). Perhaps the most consistent agreement for the formationrequirements of an S0 via mergers is the necessity for a misalign-ment of angular momentum between the in-situ and ex-situ accretedbaryonic components (see e.g. Sales et al. 2012). Considering theexisting baryonic misalignment present in NGC 7135 in the form ofa counter rotating disk, and considering the seemingly misalignedorbit of the ongoing merger, it is perhaps likely that the ongoingdisruption will lead to NGC 7135 tending towards S0 morphology.Plausibly the kinematics would increasingly reflect those of generalspheroid galaxies as newly formed stars with an opposing angularmomentum to the mean, and those recently accreted, would beginto reduce kinematic coherence. Though this is a distinct possibility,the true future of NGC 7135 will remain unknown until more deci-sive techniques and modelling are developed. Due to the complexnature of the recent history of NGC 7135, any predictions on futureevolution are speculation.
We have used a Voronoi binned map of NGC 7135 to explore kine-matic stellar features such as velocity and velocity dispersion, aswell as the distributions of stellar properties such as age and metal-licity. Gas properties were also explored in regular bins, with bothkinematic gas properties and gas distribution investigated. Gas wasshown to be counter rotating compared to stellar material, withsignificant evidence of disturbance in the galaxy core. This alongwith population evidence shows a galaxy currently merging ontoNGC 7135. Despite gas being present, little to no current star for-mation was identified. ALMA data of the galaxy core points to a starformation rate of only 0 . 𝑀 (cid:12) yr − assuming normal depletiontimes. Strong LINER emission likely obscures emission associatedwith star formation and as such a higher SFR cannot be ruled out.During population analysis of NGC 7135 from data providedby the SOSIMPLE project, we have identified both historic andongoing merger activity. This was achieved using a ‘full spectralfitting’ method to disentangle strong bi-modalities in stellar popu-lations. We show that in a snapshot of a ‘single’ galaxy, we are inreality witnessing the product of three distinct galaxy populations.An ongoing merger or large accretion event is clear from thestellar kinematic maps, showing a distinct area of stellar materialnot associated with the host galaxy, but interacting with the galaxystructure. Likewise in gas maps we see large velocity dispersion inareas where ex-situ infalling gas interacts with in-situ gas.At least one historical large merger event took place at 6-10 Gyr ago according to star-formation history derived by full spec-tral fitting. This potentially provided gas with lower enrichmentwith which NGC 7135 birthed stars of lower metallicity; howeverthe timeline of stellar ages, matched with the likely merger datemakes it highly likely that most, if not all of the stars belongingto this population are ex-situ stars originating in another galaxy.Considering there is no discernible change in the host populationmetallicity of new stars born after the merger, we assume that alllower metallicity population stars are ex-situ in origin. The timelineof star formation history suggests that this merger caused a generalshut-down of star formation in NGC 7135, not long after the mergerevent. MNRAS , 1–13 (2020) ergers in NGC 7135 Figure 8.
The left panel shows a polar oriented map of NGC 7135. Blue colour shows the mass fraction of derived material not associated with the host galaxypopulation, with contouring shown in red-orange-yellow. The angle is shown with 0 radians as the North of the image and positive angle increase showingclockwise movement around the galaxy. Gaussian smoothing has been applied to show more clearly larger structures of ex-situ material. The radius from centrehas been limited to include only radii in which a complete circle can be arranged within the image. The adjoining right-hand panel shows the same radialpositions as the left side, however it shows the mean discontinuous mass fraction for a complete circle for the radii. Mean fraction was calculated using circularannuli of radius 3 pixels with a moving average. The effective radius is taken from table 1 of Marino et al. (2011). The fraction of accreted material increaseswith radius, with a roughly 7% increase within 0.6 effective radii. [ M / H ] l o g M P i x e l l o g M P i x e l l o g M P i x e l Figure 9.
The first panel shows a general galaxy age-metallicity map. This is divided by the red boxes into 3 groups of populations to examine the massassociated with each area. Panel labels correspond to the numbers on the age-metallicity map. These show the divided nature of the populations, in which theintermediate age high metallicity population is more strongly associated with the infalling object and tidal features, whilst the older metal rich population isassociated with the host galaxy.
We calculate the fraction of the ex-situ material as a functionof galactocentric radius, finding a steep increase in ex-situ materialas we probe further to the outskirts of the galaxy. The centre of thegalaxy exhibits no signs of ex-situ material, whilst by 0.6 effectiveradii, this fraction is at 7%. This is in common with literature expec-tations of ‘two phase’ galaxy assembly seen both observationallyand in simulation, where ex-situ material is preferentially depositedon the outskirts of a galaxy.Many more SOSIMPLE galaxies are available from the survey,with much left to explore.
Many thanks to an anonymous referee for useful comments. Thiswork was completed with the support of the ESO studentship pro- gram and the Moses Holden Scholarship. BH acknowledges supportby the DFG grant GE625/17-1 and DLR grant 50OR1911. Basedon observations collected at the European Southern Observatoryunder ESO programme 0103.A-0637(A).
The data described in this article are accessible via the ESO archiveof MUSE data.
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