Long Tidal Tails in Merging Galaxies and Their Implications
Jian Ren, Xian Zhong Zheng, David Valls-Gabaud, Pierre-Alain Duc, Eric F. Bell, Zhizheng Pan, Jianbo Qin, Dongdong Shi, Man Qiao, Yongqiang He, Run Wen
MMNRAS , 1–11 (2020) Preprint 28 September 2020 Compiled using MNRAS L A TEX style file v3.0
Long Tidal Tails in Merging Galaxies and Their Implications
Jian Ren, , (cid:63) X. Z. Zheng, , (cid:63) David Valls-Gabaud, Pierre-Alain Duc, Eric F. Bell, Zhizheng Pan, , Jianbo Qin, , D. D. Shi, , Man Qiao, , Yongqiang He, , Run Wen , Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, China LERMA, CNRS, PSL, Observatoire de Paris, 61 Avenue de l’Observatoire, 75014 Paris, France Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, 11 Rue de l’Université, F-67000 Strasbourg, France Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We investigate the properties of long tidal tails using the largest to date sample of 461 merginggalaxies with log ( M ∗ / M (cid:12) ) ≥ . within 0.2 ≤ z ≤ from the COSMOS survey in combi-nation with Hubble Space Telescope imaging data. Long tidal tails can be briefly divided intothree shape types: straight (41 per cent), curved (47 per cent) and plume (12 per cent). Theirhost galaxies are mostly at late stages of merging, although 31 per cent are galaxy pairs withprojected separations d > kpc. The high formation rate of straight tidal tails needs to beunderstood as the projection of curved tidal tails accounts for only a small fraction of thestraight tails. We identify 165 tidal dwarf galaxies (TDGs), yielding a TDG production rate of0.36 per merger. Combined with a galaxy merger fraction and a TDG survival rate from theliterature, we estimate that ∼ . ≤ log ( M ∗ / M (cid:12) ) ≤ . and appear compact with half-light radii following the M ∗ - R e relation of low-mass elliptical galaxies. However, their surface brightness profiles aregenerally flatter than those of local disc galaxies. Only 10 out of 165 TDGs have effectiveradii larger than 1.5 kpc and would qualify as unusually bright ultra-diffuse galaxies. Key words: galaxies: evolution – galaxies: interactions – galaxies: tidal tails – galaxies:dwarf
In the Λ Cold Dark Matter ( Λ CDM) cosmological model, cos-mic structure and dark matter haloes grow in a hierarchical man-ner, placing galaxy mergers as major drivers of galaxy formationand evolution (e.g. Hopkins et al. 2006). The mergers of galax-ies not only drive the ex situ mass growth to form more mas-sive galaxies (e.g. Bundy et al. 2009; Fakhouri, Ma & Boylan-Kolchin 2010; Oser et al. 2010; Moustakas et al. 2013; Rodriguez-Gomez et al. 2016; Martin et al. 2017), but also invoke gravita-tional torques/disruption to shape the kinematic and physical prop-erties of the merger remnants (e.g. Hopkins et al. 2007; Ellison etal. 2013; De Propris et al. 2014; Thorp et al. 2019). The past twodecades have witnessed significant progress in our understandingof galaxy merger rate across cosmic age both observationally (Pat-ton et al. 2002; Conselice et al. 2003b; Lin et al. 2004; Lotz et al.2008a; Jogee et al. 2009; Robaina et al. 2009; Lotz et al. 2011; Xuet al. 2012; Man, Zirm & Toft 2016; Wen & Zheng 2016; Mantha (cid:63)
E-mail: [email protected] (JR); [email protected] (XZZ) et al. 2018) and theoretically (Stewart et al. 2009; Hopkins et al.2010b; Fakhouri, Ma & Boylan-Kolchin 2010; Rodriguez-Gomezet al. 2015). Recently, more efforts have turned to characterizinggalaxy mergers of different types and their essential roles in regu-lating the evolution of galaxies over a wide range of properties.Galaxy mergers are often classified into different types interms of the properties of the merging galaxies, such as their massratio (major versus minor; Bournaud, Jog & Combes 2005; Lotz etal. 2010, 2011; Ownsworth et al. 2014) and gas fraction (wet versusdry; Bell et al. 2006; Lin et al. 2008). Major mergers, often definedby a mass ratio of m / M > . , have been widely studied in bothsimulations and observations. As the most violent process of galaxyinteractions, major mergers trigger intense star formation (Sanders& Mirabel 1996; Mihos & Hernquist 1996; Teyssier, Chapon &Bournaud 2010), in at least some cases ignite active galactic nu-clei (AGNs) (Sanders et al. 1988; Bahcall et al. 1997; Treister etal. 2012; Barrows et al. 2017; Ricci et al. 2017) and induce mor-phological transformation from discs into spheroids (e.g. Barnes1992; Genzel et al. 2001; Naab & Burkert 2003; Lotz et al. 2008b;Bournaud et al. 2011).In contrast, minor mergers occur more often than major merg- © 2020 The Authors a r X i v : . [ a s t r o - ph . GA ] S e p J. Ren et al. ers and supply stars and gas into the primary galaxies. Observa-tional evidence suggests that minor mergers are dominantly respon-sible for the size growth of massive galaxies (e.g. Newman et al.2012; Bluck et al. 2012; Hilz, Naab & Ostriker 2013) and for trig-gering star formation in at least the nearby universe (Kaviraj 2014).The minor mergers might even induce morphological transforma-tion through disc instabilities for z > galaxies (e.g. Welker et al.2014).Gas-rich mergers behave very differently from gas-poor merg-ers. At least some mergers drive gas inflows which fuel star forma-tion and can trigger AGN activity. These gas inflows are expectedto be important in influencing the growth of bulges (Hopkins et al.2009, 2010a), but the relationship between merger parameters andbulge growth is far from straightforward (Bell et al. 2017; Gargiuloet al. 2019). Disc survival and regrowth is expected to be impactedby gas content also, where even gas-rich major mergers are pre-dicted to lead to disc-dominated remnants (Robertson et al. 2006).On the contrary, dry mergers of gas-poor galaxies proceedwith dissipationless processes and sustain little star formation, be-ing a key driver of the mass growth and structural evolution of mas-sive elliptical galaxies (van Dokkum 2005; Bell et al. 2006) and theBrightest Cluster Galaxies (BCGs) at z < (e.g. Lin et al. 2013).Moreover, the internal structure and orbital parameters (progradeversus retrograde) of the merging galaxies also influence the prop-erties of the post-merger galaxies (Darg et al. 2010; Martin et al.2018). A comprehensive picture of shaping the detailed propertiesof present-day galaxies is still lacking.Indeed, tidal features generated in a galaxy merger stronglydepend on the mass ratio, gas fraction, internal structure and or-bital parameters of the merging galaxies, and thus can be usedto characterize the merger (Duc & Renaud 2013; Barnes 2016).For instance, bridges and long tidal tails can be generated in ma-jor mergers between equal-mass discs (Toomre & Toomre 1972;Barnes 1992), and tidal streams are often produced in the minormerger events such that satellite galaxies fall into a more massivegalaxy (Martínez-Delgado et al. 2010). Such tidal features are fre-quently detected around massive galaxies, tracing the merger eventshappened in the recent past (Duc et al. 2015; Morales et al. 2018;Hood et al. 2018; Kado-Fong et al. 2018). In particular, long tidaltails are exclusively created in the major mergers involving discgalaxies with preferentially prograde orbits (Struck & Smith 2012;Duc & Renaud 2013). The shape of tidal tails may be influencedby the shape of the gravitational potential wells and underlyingdark matter haloes. Dubinski, Mihos & Hernquist (1996) showedthrough simulations that galaxies with a low halo-to-disc mass ra-tio can produce long tidal tails in the merging process. Springel &White (1999) further pointed out that long tidal tails can be gener-ated when the halo spin in sufficiently large for merging galaxieswith a high halo-to-disc ratio. Moreover, using TDGs on tidal tails,Bournaud, Duc & Masset (2003) found that dark matter haloesaround spiral galaxies are at least ten times larger than the stellardiscs. Therefore, long tidal tails are useful probes for dark matterhalo parameters and galaxy structures. The characteristics of tidaltails are affected by the orbital parameters, disc orientations as wellas stellar and gas masses and the specific angular momentum of themerging galaxies (Struck & Smith 2012; Ploeckinger et al. 2018).Long tidal tails from mergers of disc galaxies thus provide hints onthe orientation between their spin and orbital parameters.Moreover, star clusters and tidal dwarf galaxies (TDGs) canbe formed in tidal tails (Barnes & Hernquist 1992; Bournaud &Duc 2006; Wetzstein, Naab & Burkert 2007). Previous observa-tional studies of TDGs focused on individual objects or small sam- ples (Duc et al. 2000; Duc & Mirabel 1998; Hancock et al. 2009;Mohamed, Reshetnikov & Sotnikova 2011; Kaviraj et al. 2012). Asystematic investigation of tidal tails using a sufficiently large sam-ple of merging galaxies is required to address how the tidal tails arecreated in reality, draw constraints on how spiral galaxies mergewith each other in the cosmic web, and examine the formation ofTDGs and star formation under these extreme conditions.Although there have been many simulations to study the mor-phology of merging galaxies and tidal tails. However, it is not clearhow many shapes real tidal tails have, how do they relate to themerger parameters and how they are formed. At the same time,solving the problem of the relationship between the TDG formedin the tidal tail and the shape of the tidal tail will help understandingmerging processes and star formation in tidal tails.In this work, we analyse a sample of 461 merging galaxieswith long tidal tails (defined as having sizes comparable to the ef-fective radii of the host galaxy, Wen & Zheng 2016) to character-ize tidal tails as well as their parent galaxies. We briefly introducethe sample and data in Section 2. Section 3 presents our analysisand results and we discuss the implications in Section 4. Throughout this paper, we adopt a concordance cosmology of H = km s − Mpc − , Ω m = . and Ω Λ = . . All photometric magni-tudes are given in the AB system. We adopt the sample of 461 long-tidal-tail merging galaxies(LTTGs) in the COSMOS field from Wen & Zheng (2016). Theyidentified these LTTGs using
Hubble Space Telescope ( HST )/ACSF814W ( I ) imaging data and multi-band deep survey data and cat-alogs in COSMOS (e.g. Leauthaud et al. 2007; Capak et al. 2007).The HST /ACS F814W science images have a scale of 0 . (cid:48)(cid:48)
03 perpixel and reach a limiting magnitude limit of 25.6 mag for extendedsources within a circular aperture radius of 0 . (cid:48)(cid:48)
3. The Full Width atHalf Maximum (FWHM) of the point-spread function (PSF) of theF814W images is 0 . (cid:48)(cid:48)
09, corresponding to a physical size of 297 pcat z = . and of 723 pc at z = (Koekemoer et al. 2007). The HST imaging data cover an area of 1.64 deg and provide high-resolution morphological information in the rest-frame optical forgalaxies at z < . The stellar mass and photometric redshift cata-logs come from Muzzin et al. (2013), based on the UltraVISTA sur-vey, providing a total of 154,803 K s -band-detected sources down to5 σ = . mag with a 90 per cent completeness for point sources.In Wen & Zheng (2016), a morphological classificationmethod, namely the A O - D O method, was used to select LTTGs.As a non-parametric method for galaxy morphology, the A O - D O method is dedicated to probing asymmetric structures in the out-skirts of a galaxy (Wen, Zheng & An 2014). In the method, the im-age of a galaxy is divided into two parts, the inner half-light region(IHR) and the outer half-light region (OHR). Each region containshalf the total flux of the galaxy. Two parameters are used: the outerasymmetry, A O , measures the asymmetry of OHR and the outercentroid deviation, D O , measures the deviation (or offset) betweenthe flux-weighted centroids of the IHR and the OHR. The parame-ters are sensitive to the asymmetric structures in the outskirts suchas long tidal tails. Galaxies with more disturbed morphologies tendto have higher A O and D O . In practice, A O is calculated using theformula: A O = (cid:205) | I − I | − δ (cid:205) | I | − δ , (1)where I refers to the light distribution of OHR of an image; I MNRAS , 1–11 (2020) haracterizing Tidal Tails in Merging Galaxies 6 ] K T M I Z ; M X I Z I \ Q W V , Q [ \ I V K M S X K 6 ] U J M Z ! ! , W ] J T M V ] K T M ] [ ; Q V O T M V ] K T M ] [ : M L [ P Q N \ : M L [ P Q N \ . Z I K \ Q W V Figure 1.
Distribution of nuclear separation distance in our sample merginggalaxies. The distance set as zero is for galaxies with a single nucleus, asshown in the red bar. The blue bars show the distribution for two-nucleusgalaxies. For the systems with more than two nuclei, the smallest separa-tion distance is adopted. The blank, half-shaded and filled regions representcontributions from . < z < . , . < z < . and . < z < . respectively, so that they correspond to the same time interval of 1.8 Gyr. represents the ◦ -rotation version of I ; I - I yields the resid-ual image; δ and δ are corrections for the noise contributions tothe flux image I and the residual image I - I , respectively. Theouter centroid deviation D O is measured as below: D O = (cid:112) ( x O − x I ) + ( y O − y I ) R e f f , (2)where ( x I , y I ) and ( x O , y O ) refer to the flux-weighted centroid po-sition of the IHR and of the OHR in pixels, respectively, and R e f f is the normalized effective radius in pixels estimated using R e f f = (cid:112) n c / π . Here n c is the area in pixels of the IHR. Moredetails about the method can be found in Wen, Zheng & An (2014).Two steps were adopted for identifying LTTGs in Wen &Zheng (2016). Firstly, they pre-selected 13,227 galaxies with mor-phologies showing a certain degree of disturbance using the A O - D O method from 35,076 galaxies with log ( M ∗ / M (cid:12) ) ≥ . within . ≤ z ≤ in the COSMOS field. Secondly, the 13,227 candi-dates were visually examined using the HST /ACS F814W scienceimages. A total of 461 merging galaxies were identified with longtidal tails. See Wen & Zheng (2016) for more details about the sam-ple selection. We use this sample to analyse their morphologies andthe substructures of their tidal tails.
Using
HST /ACS F814W imaging data, we characterize the mor-phological properties of our sample LTTGs, including the nuclearseparation distance, the shape and length of tidal tails and substruc-tures of tidal tails such as clumps, TDGs and gaps.
Simulations show that the growth of tidal tails is closely coupledwith the orbital parameters and the merging stage of a galaxymerger (Toomre & Toomre 1972; Barnes 1992). In a general merg-ing process, two galaxies approach each other, pass their pericentre,and undergo a damped oscillation with dynamical friction allowingthe galaxies to lose energy and angular momentum, while violentrelaxation happening in the final stages as they merge into a singleremnant. A close passage of one galaxy next to the other would
Figure 2.
Example
HST
F814W images of three types of tidal tails in oursample: straight (top), curved (middle) and plume (bottom). The image size,target ID and redshift are labelled on each image. Red boxes mark the tidaltails. lead to varying gravitational forces across the extended galaxies.The sides of the galaxies facing each other are more attracted thanthe opposite sides (Duc & Renaud 2013). This effect allows galax-ies to produce a long tidal tail and a short counter tail during themerging process. The nuclear separation distance is a crude tracerof the merging stage, although the separation oscillates a bit beforethe final collision. In particular, after the first pass the nuclear sepa-ration distance increases but the two galaxies appear different fromthe approaching pair before the first pass. The merging galaxies af-ter the first pass are strongly disturbed and thus can be distinguishedfrom the galaxy pair in the beginning of merging without apparenttidal features (e.g., tidal tails).If tidal tails are associated with the remnant of a previousmerger event, the nuclear separation distance in the ongoing mergeris irrelevant with the status of the tidal tails. We argue that suchcases should be rare and have marginal effects on our analysis, con-sidering that massive galaxies statistically underwent even less thanone major merger event over 8 Gyrs (e.g., Jogee et al. 2009).We measure the nuclear separation distance ( d ) from the HST images of our sample galaxies. For galaxy pairs, it is easy to find thecentre of merging galaxies. For galaxies in their late merger phase,we use the method from Cui et al. (2001) to find galactic nuclei. Wevisually examine the light distribution of a merging system to findthe core of the merging galaxies. If there is a dust lane, the singlenucleus of a galaxy could be mistaken as a double-nucleus galaxy.In this sample, dust lanes are found in only a few merging galaxiesin the final stage. If a galactic core is obscured by a dust lane, thenwe count it as a single core. Note that some sample galaxies containonly a single galactic nucleus, being most likely in the final stage ofthe merging process. We set d = for these single-nucleus mergers.In contrast, two nuclei are distinguishable even in a merger remnantwith disturbed morphology. We refer such systems to as mergerremnants with d < kpc. On the other hand, 8 per cent of our sam-ple merging systems contain more than two galaxies (or galactic MNRAS000
F814W images of three types of tidal tails in oursample: straight (top), curved (middle) and plume (bottom). The image size,target ID and redshift are labelled on each image. Red boxes mark the tidaltails. lead to varying gravitational forces across the extended galaxies.The sides of the galaxies facing each other are more attracted thanthe opposite sides (Duc & Renaud 2013). This effect allows galax-ies to produce a long tidal tail and a short counter tail during themerging process. The nuclear separation distance is a crude tracerof the merging stage, although the separation oscillates a bit beforethe final collision. In particular, after the first pass the nuclear sepa-ration distance increases but the two galaxies appear different fromthe approaching pair before the first pass. The merging galaxies af-ter the first pass are strongly disturbed and thus can be distinguishedfrom the galaxy pair in the beginning of merging without apparenttidal features (e.g., tidal tails).If tidal tails are associated with the remnant of a previousmerger event, the nuclear separation distance in the ongoing mergeris irrelevant with the status of the tidal tails. We argue that suchcases should be rare and have marginal effects on our analysis, con-sidering that massive galaxies statistically underwent even less thanone major merger event over 8 Gyrs (e.g., Jogee et al. 2009).We measure the nuclear separation distance ( d ) from the HST images of our sample galaxies. For galaxy pairs, it is easy to find thecentre of merging galaxies. For galaxies in their late merger phase,we use the method from Cui et al. (2001) to find galactic nuclei. Wevisually examine the light distribution of a merging system to findthe core of the merging galaxies. If there is a dust lane, the singlenucleus of a galaxy could be mistaken as a double-nucleus galaxy.In this sample, dust lanes are found in only a few merging galaxiesin the final stage. If a galactic core is obscured by a dust lane, thenwe count it as a single core. Note that some sample galaxies containonly a single galactic nucleus, being most likely in the final stage ofthe merging process. We set d = for these single-nucleus mergers.In contrast, two nuclei are distinguishable even in a merger remnantwith disturbed morphology. We refer such systems to as mergerremnants with d < kpc. On the other hand, 8 per cent of our sam-ple merging systems contain more than two galaxies (or galactic MNRAS000 , 1–11 (2020)
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