Probing gas and dust in the tidal tail of NGC 5221 with the type Ia supernova iPTF16abc
R. Ferretti, R. Amanullah, A. Goobar, T. Petrushevska, S. Borthakur, M. Bulla, O. Fox, E. Freeland, C. Fremling, L. Hangard, M. Hayes
AAstronomy & Astrophysics manuscript no. abc c (cid:13)
ESO 2018October 8, 2018
Probing gas and dust in the tidal tail of NGC 5221 with the type Iasupernova iPTF16abc (cid:63)
R. Ferretti , R. Amanullah , A. Goobar , T. Petrushevska , S. Borthakur , M. Bulla , O. Fox , E. Freeland ,C. Fremling , L. Hangard , and M. Hayes Department of Physics, The Oskar Klein Centre, Stockholm University, Albanova, SE 106 92 Stockholm, Swedene-mail: [email protected] Department of Physics & Astronomy, john Hopkins University, 3701 San Martin Drive Baltimore, MD 21218 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Department of Astronomy, The Oskar Klein Center, Stockholm University, Albanova, SE 10691 Stockholm, SwedenReceived ... ; accepted ...
ABSTRACT
Context.
Type Ia supernovae (SNe Ia) can be used to address numerous questions in astrophysics and cosmology. Due to their wellknown spectral and photometric properties, SNe Ia are well suited to study gas and dust along the lines-of-sight to the explosions.For example, narrow Na I D and Ca II H&K absorption lines can be studied easily, because of the well-defined spectral continuum ofSNe Ia around these features.
Aims.
We study the gas and dust along the line-of-sight to iPTF16abc, which occurred in an unusual location, in a tidal arm, 80 kpcfrom centre of the galaxy NGC 5221.
Methods.
Using a time-series of high-resolution spectra, we examine narrow Na I D and Ca II H&K absorption features for variationsin time, which would be indicative for circumstellar (CS) matter. Furthermore, we take advantage of the well known photometricproperties of SNe Ia to determine reddening due to dust along the line-of-sight.
Results.
From the lack of variations in Na I D and Ca II H&K, we determine that none of the detected absorption features originatefrom the CS medium of iPTF16abc. While the Na I D and Ca II H&K absorption is found to be optically thick, a negligible amountof reddening points to a small column of interstellar dust.
Conclusions.
We find that the gas along the line-of-sight to iPTF16abc is typical of what might be found in the interstellar medium(ISM) within a galaxy. It suggests that we are observing gas that has been tidally stripped during an interaction of NGC 5221 withone of its neighbouring galaxies in the past ∼ years. In the future, the gas clouds could become the locations of star formation.On a longer time scale, the clouds might di ff use, enriching the circum-galactic medium (CGM) with metals. The gas profile along theline-of-sight should be useful for future studies of the dynamics of the galaxy group containing NGC 5221. Key words. supernovae: individual: iPTF16abc– Galaxies: individual: NGC 5221– Galaxies: ISM, interactions
1. Introduction
Type Ia supernovae (SNe Ia) have such uniform propertiesthat they are frequently described as "standard candles", which,among other applications, enables detailed measurements in cos-mology (see for instance Goobar & Leibundgut 2011). Thewell-known brightness and spectral energy distribution (SED)of SNe Ia can also be used to study gas and dust along the lines-of-sight.Due to the well defined continuum of SNe Ia, typical ab-sorption features of the interstellar medium (ISM) of their hostgalaxies, such as Na I D and Ca II H&K, can frequently be iden-tified. Furthermore, di ff use interstellar bands (DIBs), which arelargely of unknown chemical origin (Snow 2001; Snow & Mc-Call 2006), are sometimes detected in the spectra (e.g. Sollermanet al. 2005). One can also take advantage of the standard candleproperties of SNe Ia to study dust in the ISM. Comparing thelightcurves of a given SN Ia with an SED template, extinction (cid:63) Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere under ESO DDTprogramme 297.D-5005(A), P.I. Ferretti. and reddening can be determined to study the dust propertiesalong the line-of-sight.For example, the nearest SN Ia in modern times, SN 2014J,(Goobar et al. 2014), has been used to study details of the ISMcomposition and structure of M82 (Welty et al. 2014; Ritcheyet al. 2015; Yang et al. 2017). Interestingly, an unusual extinctioncurve was identified which possibly points to very small dustgrains in the ISM or dust in the circumstellar (CS) medium ofthe supernova (Amanullah et al. 2014; Foley et al. 2014).SNe Ia can be used to study a large variety of lines-of-sight.Due to the large delay times of ≈ –10 years between theformation of a SN Ia progenitor system and the explosion (Maozet al. 2012), they are known to occur almost anywhere in theirhost galaxies. An extreme example is PTF10ops, which was lo-cated at a projected distance of 148 kpc from the nearest possiblehost galaxy (Maguire et al. 2011).It is thus possible for SNe Ia to occur in regions with lowstellar density, such as tidally stripped spiral arms of galaxies.Tidal arms are thought to be the result of close interactions ofneighbouring galaxies (as shown in the simulations by Toomre& Toomre 1972). SNe Ia occurring in such locations can be usedto study the interstellar gas and dust properties of regions that Article number, page 1 of 8 a r X i v : . [ a s t r o - ph . GA ] A ug & A proofs: manuscript no. abc have been a ff ected by gravitational interactions. The ISM in tidalarms can be the location of future star formation (Schombertet al. 1990) and enrich the circumgalactic medium (CGM) withmetals.To date no SN Ia located in a tidal arm has been studied indetail, although at least one case is known (SN 2013de, Drakeet al. 2013a,b). Here we investigate iPTF16abc, a SN Ia whichoccurred in the tidal arm of its host galaxy, NGC 5221. Due tothe large displacement of iPTF16abc from its host, the super-nova was a good candidate to search for the presence of CS gasvia photoionisation using the method described in Ferretti et al.(2016). Furthermore, the obtained high-resolution spectra andphotometry could be used to study the spiral arm where the su-pernova is located.In the following, we present the discovery and observationsof iPTF16abc (Section 2). We then describe the narrow absorp-tion features detected in high-resolution spectra, as well as thephotometric reddening of iPTF16abc (Section 3). We furthershow that the deep absorption features cannot be due to CS gassurrounding the supernova, but are part of the tidally strippedISM of NGC 5221 (Section 4). Finally, we compare the line-of-sight with NGC 5221 and discuss the features in the contextof the galaxy group in which NGC 5221 is located (Sections 5and 6).
2. Discovery and Observations of iPTF16abc
The intermediate Palomar Transient Factory (iPTF) first re-ported the discovery of iPTF16abc (Miller et al. 2016), located170” from the galaxy NGC 5221 at R.A., Dec = + E ( B − V ) MW = .
028 mag (Schlafly & Finkbeiner2011). A colour composite image of NGC 5221 and the locationof iPTF16abc is shown in Figure 1. The supernova was discov-ered on 4.4 April 2016, while the first detection was on 3.4 Apriland the last non-detection occurred on 2.4 April (UT dates areused throughout the paper).A classification spectrum of iPTF16abc was obtained withthe DeVeny spectrograph on the Discovery Channel Telescope(DCT) (Cenko et al. 2016) on 5.3 April, which determinedit to be a SN Ia, possibly belonging to the subclasses resem-bling SNe 1991T or 1999aa. A spectrum obtained with GeminiN + GMOS on 5.4 April revealed the presence of deep Na I Dand Ca II H&K absorption features. In response, extensive pho-tometric and spectroscopic follow-up were triggered (Miller etal, 2017; submitted).As part of the follow-up of iPTF16abc, we received Direc-tor’s Discretionary Time at the European Organisation for As-tronomical Research in the Southern Hemisphere (ESO DDTprogramme 297.D-5005(A), P.I. Ferretti). The spectra takenwith Xshooter ( R (cid:39) R (cid:39) . g - and R -band photometry which wasobtained as part of the regular iPTF survey. Further we use gr -and i -band photometry of the Palomar Observatory 60-inch tele-scope (P60) and BVgr - and i -band from the Las Cumbres Ob- servatory (LCO). Ultraviolet (UV) photometry from the Ultra-violet / Optical Telescope (UVOT; Roming et al. 2005) on the
Swift spacecraft (Gehrels et al. 2004) uvm2 -filter and riZY J -and H -band with the Reionisation and Transient Infrared / OpticalProject (RATIR; Fox et al. 2012) was obtained and used for thisanalysis.
3. Absorption features and reddening
Deep well-defined narrow absorption lines corresponding toNa I D and Ca II H&K at the rest frame of NGC 5221 ( z = . ff erent radial velocities in the UVES spectra. Theabsorption line profiles can be seen in Figure 2, where the twodeep feature groups are centred at a radial velocity v r ≈ −
77 and −
51 km s − , with respect to the host galaxy rest frame.Both groups of absorption line features are visibly asymmet-ric, indicating that the absorption features are composed of sev-eral blended components. The Na I D lines are too saturated andblended to determine any further substructure with confidence.Therefore, only Na I column density upper limits can be de-termined. Using VPFIT , we fit Voigt profiles to Ca II H&K.In Table 2, we present the average Doppler width ( b ) and col-umn densities ( N ) of a 5-component fit. Since all the featuresare strongly blended, it cannot be excluded that there are moreunresolved components contributing to the profile.We further detect K I at λλ = ± ± ffi cult.We searched all spectra for traces of DIBs as well as CHand CH + . We found a feature in both UVES and Xshooter spec-tra which seemingly corresponds to the DIB at λ λλ λλ . . <
150 and <
50 mÅ, respectively given the signal-to-noise ratio.From the UV, optical and IR photometry of iPTF16abc wecan estimate the reddening of the SN by fitting the SED ofSN 2011fe (as in Amanullah et al. 2014, 2015) to the data. Asidefrom the unusually fast rise of iPTF16abc, the above SED fits thelightcurve and colours well. For this reason, we avoid includingthe early photometry and the light-curves using the photometrybetween phases p = −
10 – +
40 days.The measured reddening is estimated to E ( B − V ) = .
07 (0 .
05) mag for R V = . . R V = . . t B max = .
76 and s = . Article number, page 2 of 8. Ferretti et al.: Probing gas and dust in the tidal tail of NGC 5221 with the type Ia supernova iPTF16abc
Fig. 1.
The location of iPTF16abc, marked in the inset, with respect to its host galaxy NGC 5221. The tidal tail is visible spanning the spacebetween the galaxy and the supernova location. Coordinates at which SDSS spectra of NGC 5221 are available are marked and labeled S1 and S2.The main image is a colour composite of the publicly available PanSTARRS data (Flewelling et al. 2016). The inset is a deep stack of pre-explosioniPTF P48 R-band images. Instrument MJD UT Date Exp. time Phase Set-up(s) (days)Xshooter 57,492.2 Apr. 14.2 1755 -6.6 UVB 1.0" / VIS 0.9" / NIR 0.9"UVES 57,495.2 Apr. 17.2 2 × + + / VIS 0.9" / NIR 0.9"
Table 1.
Mid- and high-resolution spectra obtained with stretch-corrected phases in the rest frame with respect to B-band maximum. v b log { N Ca II } (km s − ) (km s − ) (cm − )-88. 3.1 ± ± ± ± ± ± ± ± ± ± Table 2.
Average Voigt profile parameters of Ca II H&K in the twoUVES spectra. value of R V . The fitted values of E ( B − V ) correspond to an ex-tinction of A V = . .
2) mag. The extinction can also be estimated by taking advantage ofthe standard candle property of SNe Ia and comparing the mea-sured peak brightness with the expected for the given redshift, z = . A V = − . ± .
04 whichis well within the expected intrinsic peak brightness dispersionof 0 . Article number, page 3 of 8 & A proofs: manuscript no. abc measurements of cosmological samples. Matter streaming froma galaxy is a suggested location where grey dust could be form-ing. In such a scenario the line-of-sight to iPTF16abc shouldprobe an overdensity of grey dust, and the standard candle prop-erties of this SN Ia can be used to constrain it. Since the peakbrightness of iPTF16abc is within the intrinsic scatter of SNe Iahowever, the grey dust column along the line-of-sight must benegligible.
4. Absence of circumstellar absorption
Although the majority of the absorption lines detected in SN Iaspectra are believed to be due to gas in the ISM of their hostgalaxies, there exist observations that indicate the presence ofCS gas. For example, cases of slight variations in absorption lineprofiles (Patat et al. 2007; Simon et al. 2009; Patat et al. 2013;Graham et al. 2015; Ferretti et al. 2016) hint at photoionisationor recombination of absorbers and predominantly blue-shiftedNa I D profiles (Sternberg et al. 2011; Maguire et al. 2013) sug-gest outflowing gas.Nevertheless, the existence of CS gas surrounding SNe Iaremains uncertain. Most of the known cases of varying absorp-tion lines all occurred in crowded fields, where geometric ef-fects (Patat et al. 2010) might be expected and a larger sample ofmulti-epoch high-resolution spectra has not turned up any moreexamples (Sternberg et al. 2014).SNe Ia peak in UV earlier than in optical. Because of this,most photoionisation should occur before the SNe reach max-imum brightness (Borkowski et al. 2009; Ferretti et al. 2016).Ideally, early spectra (around -14 days before maximum) needto be obtained to search for photoionisation of CS gases. The re-mote location and early discovery of iPTF16abc made it a goodcandidate SN Ia to search for CS matter by looking for changesin absorption line profiles.Knowing the SED of SNe Ia as well as the ionisation energyand cross-section of an absorption species, the rate of photoion-isation depends on the distance of the gas from the SN. Thusmeasuring the ionisation rate, is a method to determine the dis-tance of an absorber to the supernova. Thereby the di ff erent ioni-sation energies and cross-sections of di ff erent absorption speciesimplies that they are sensitive to photoionisation at di ff erent dis-tances.In the absence of any variations, one can determine the dis-tances at which there was no detectable gas, which would havebeen been ionised. The exclusion range is defined by two radii.The inner radius is determined by the distances at which all gasis ionised before the first spectrum is taken. The outer radius isdefined by the distance at which the amount of ionisation thatoccurs by the time the last spectrum is taken is negligible.In Table 4 we present the time series of the measured EWs ofNa I D and Ca II H&K. It can be seen that there is neither signif-icant time variability in the EW measurements, nor are there anyvisible changes in the line profiles. We investigate the profiles ofCa II H&K in the UVES spectrum in more detail than the satu-rated Na I D lines. A direct comparison of the Voigt profile pa-rameters, reveals no di ff erence between the two UVES epochs.Furthermore, we plot the di ff erence in the apparent optical depthof Ca II H&K between the two epochs in Figure 4. We find nosignificant changes in the apparent optical depth of any part ofthe profiles.Using the method of Ferretti et al. (2016), we can excludethe presence of Na I gas at a distance of R exclNa I ≈ × –2 × cm, and Ca II gas at R exclCa II ≈ × – 3 × cm from iPTF16abc at the 3 σ confidence level. To illustrate the exclusionrange, Figure 5 shows Na I ionisation curves of gas clouds at thelimiting and exclusion radii. The exclusion limits do not includepossible errors in the SNe Ia SED as described in Ferretti et al.(2016). In particular, the exclusion limits obtained from Ca IImust be taken with caution, due to the diversity of the SN IaSEDs.Thus all the detected absorption features must originate fromgas farther from iPTF16abc than the outer exclusion limit andare part of the ISM. It is not possible to further constrain the re-lation of iPTF16abc with the gas clouds other than stating thatthe supernova must have occurred behind them. Due to the broadnature of intrinsic supernova spectral features, the systemic ve-locity of the supernova can’t be determined with accuracy. It isnevertheless plausible that the progenitor system of iPTF16abcwas moving with the tidally stripped gas behind which it ex-ploded.
5. Discussion
The position of iPTF16abc suggests that we are probing the ISMin the tidal arm of NGC 5221. iPTF16abc is 170” from the centerof NGC 5221, which at a redshift of z = . −
100 – −
30 km s − , with respect to the rest-frame of the host galaxycore. These values are smaller than the rotation velocity ofNGC 5221. Courtois & Tully (2015) quote a line width (twicethe rotation velocity) of 510 ±
20 km s − . Furthermore, two SloanDigital Sky Survey (SDSS) spectra of spiral arms on oppositesides of the galaxy show a velocity di ff erence of 477 km s − . Thecoordinates at which the SDSS spectra have been taken prior toexplosion, are marked in Figure 1, whereby the eastern arm (S1)is approaching and the western arm (S2) is receding with − − , respectively. Interestingly, this implies that thegas in the tidal tail is receding from the galaxy in the oppositedirection to the spiral arm it is closest to. Although the tidal tailprojects in the opposite direction, this may indicate that the tidalstream connects to the eastern spiral arm.To further compare the radial velocity of the gas seen in theiPTF16abc spectra with NGC 5221, we consider an H I pro-file by the Arecibo L-band Feed Array (ALFA) as part of theArecibo Legacy Fast ALFA survey (ALFALFA, Giovanelli et al.2005; Haynes et al. 2011). The beam of the ALFA encompassesboth the position of iPTF16abc and NGC 5221. In Figure 6, theNa I D1 profile of the iPTF16abc line-of-sight is compared tothe ALFA H I profile of the entire galaxy.The position of iPTF16abc allows us to estimate a time-frame within which the gas must have been stripped fromNGC 5221. Assuming the gas is moving away from the galaxywith a velocity of v ≈
100 km s − , it would take at least8 × years to reach a distance of 80 kpc, depending on the exactprojection angle. Models show that this time frame is consistentwith close encounters and mergers of large galaxies (Toomre &Toomre 1972).A number of galaxies, listed in Table 5, are at a compara-ble redshift to NGC 5221 and within a few hundred kpc fromit. The two most prominent galaxies in the list, NGC 5222and NGC 5230, are likely candidates to have interacted withNGC 5221, producing the observed tidal stream. Hallenbecket al. (2016) have studied NGC 5230 as an example of a veryH I rich galaxy. In fact, the ALFALFA data they present in theirstudy show that H I gas encompasses the entire galaxy group,with column density maxima corresponding to NGC 5221, Article number, page 4 of 8. Ferretti et al.: Probing gas and dust in the tidal tail of NGC 5221 with the type Ia supernova iPTF16abc
Na I D20.00.20.40.60.81.01.2 Rest Frame Velocity (km s -1 ) N o r m a li s e d F l u x Na I D1120 100 80 60 40 20 00.20.40.60.81.01.2 Ca II H
Xshooter, Apr. 15UVES, Apr. 17UVES, Apr. 25Xshooter, May 12
120 100 80 60 40 20 0Ca II K
Fig. 2.
The Na I D doublet and Ca II H&K of iPTF16abc by UVES and Xshooter. In the UVES spectra, two distinct groups of absorption featureswith similar velocities are visible, while they are unresolved with Xshooter. The substructure of these groups is di ffi cult to further discern due tothe optical thickness of the absorbers.v Na I D1 Na I D2 ratio N(Na I) Ca II H Ca II K ratio N(Ca II)(km s − ) (mÅ) (mÅ) (D2 / D1) ( × cm − ) (mÅ) (mÅ) (K / H) ( × cm − )-77 241 ± ± > . ± ± ± ± ± > . ± ± ± Table 3.
Average Na I D and Ca II H&K equivalent widths of the features with centroids at −
77 and −
51 km s − measured from the UVESspectra. The equivalent width ratios are indicative of the optical depth of the absorbers. Column density lower limits are given for Na I, whichwere computed from Na I D1 assuming the optically thin relation to equivalent width. The Ca II column densities were determined from the 5component Voigt profile fit.Instrument Date Phase R ( λ/δλ ) S / N Na I D1 Na I D2 Ca II H Ca II K(days) (mÅ) (mÅ) (mÅ) (mÅ)XShooter Apr. 14 -6.1 7,450 †
113 512 ± ± ± ± ± ± ± ± ± ± ± ± †
73 497 ±
10 592 ±
10 138 ± ± ± ± ± ± † nominal XShooter resolution Table 4.
Equivalent widths of Na I D, Ca II H&K, of iPTF 16abc using XShooter and UVES. The resolution of the UVES spectra were estimatedfrom the full width at half-maximum intensity (FWHM) of several telluric features, and the S / N per pixel is measured around the wavelength ofNa I D.
NGC 5222 and NGC 5230. Thus the gas along the line-of-sightof iPTF16abc is representative of gas that can end up in the CGMof the group via interactions between the galaxies.Narrow absorption lines, such as those detected in the spec-tra of iPTF16abc, are not unusual features for SN Ia spectra(e.g. the sample of Sternberg et al. 2011). However, these fea-tures typically appear in lines-of-sight of supernovae clearly sit- uated within their host galaxies. The Na I and Ca II gas columnsalong the line-of-sight of iPTF16abc appear to be comparable totypical lines-of-sight within galaxies. To illustrate this, we con-sider the Na I D absorption in the two SDSS spectra available ofNGC 5221. We find Na I D EWs of 2 . ± . . ± . Article number, page 5 of 8 & A proofs: manuscript no. abc
400 300 200 100 0 100Velocity (km s − )0.900.951.001.051.101.15 N o r m a li s e d F l u x K I λ λ Fig. 3.
K I at λλ
120 100 80 60 40 20 0Velocity (km s − )-0.100.1-0.100.1 C a II K C a II H Fig. 4. Di ff erence in apparent optical depth between the UVES spectraof Ca II H&K. No pixels seem to be significant outliers.
15 10 5 0 5 10 15 20 25Phase (days)0.00.20.40.60.81.0 N ( t ) / N ( ) Fig. 5.
Fractional photoionisation curves of Na I gas at annotated radii incm. The vertical dashed lines indicate the phases of the obtained spectra.The red curve at 1 × cm, defines the inner exclusion radius at whicha gas cloud would have been ionised before the first spectrum was ob-tained. The blue curve defines the outer exclusion radius at 2 × cm,where photoionisation leads to negligible change. are unfortunately buried in noise in the SDSS spectra and cannotbe measured.Although the Na I columns in the tidal stream and theNGC 5221 di ff er by close to an order of magnitude, it is im-portant to note the di ff erences between the SDSS and supernovaNa I D EWs. In the case of the SDSS spectra, the absorptionlines represent the average absorption along the lines-of-sightto all stars covered by the spectral fibre. The investigated col-umn can thus be considered to be the average across an area of ≈ area of the galaxy. In comparison to this, the columnmeasured in the supernova spectrum, spans the projected area ofa photosphere which is of the order of ≈ − pc at maximumbrightness . We are thus comparing a very small area in the tidaltail to the average of a large area within the galaxy, where manymore gas clouds with di ff erent velocities can be situated alongthe line-of-sight.From ISM studies of the Milky Way, it is known thatNa I D and Ca II H&K correlate with reddening and also H Icolumn densities (Poznanski et al. 2012; Murga et al. 2015).These empirical relations can be used to estimate E ( B − V )and N (H I) for the line-of-sight of iPTF16abc. Using the sat-urated Na I D lines, the Poznanski et al. (2012) relation sug-gests that E ( B − V ) est > .
32. It is known that the empiricalrelations of Na I D to reddening work poorly for SNe Ia (Poz-nanski et al. 2011; Phillips et al. 2013). iPTF16abc is anotherexample with deep Na I D absorption lines, but comparativelylittle photometric reddening. Recently, Hoang (2017) suggestedthat the unusually large Na I columns could originate from gasreleased during dust grain collisions in clouds irradiated by theSNe. The ratio N (Na I) / N (Ca II) > . N (H I) est . > cm − .The H I column density estimates are more di ffi cult to com-pare to. From the ALFA spectrum, we can compute that the en-tire beam, spanning 3 . (cid:48) × . (cid:48) contains M H I = . × M (cid:12) ,corresponding to an average column density of N (H I) = . × cm − . Most of the H I in the ALFA spectrum must be situ-ated in NGC 5221, but could also have been stripped to locationssuch as the line-of-sight of iPTF16abc.
6. Summary and conclusions
We have presented the interstellar absorption lines observed inthe spectra of Type Ia supernova iPTF16abc. The gas corre-sponds to the tidally stripped ISM at least 80 kpc from the centreof the host galaxy NGC 5221. Compared with a line-of-sight in atypical galaxy, the ISM in the tidal tail appears to have a typicalgas content, but a surprisingly small column of dust.We detected Na I D and Ca II H&K absorption features intwo distinct clusters at velocities −
77 and −
51 km s − from thesystemic velocity of NGC 5221.From the lack of variations in the Na I D and Ca II H&K pro-files, we determined that the observed gas cannot be part of theCS environment of the supernova, since photoionisation wouldhave resulted in a significant change in the column density. Thusthe gas seen along the line-of-sight of iPTF16abc is the tidallystripped ISM of NGC 5221.At the same time the standard candle nature of iPTF16abchas allowed us to determine that the supernova is barely red-dened compared with the normal SNe Ia colours, implying thatthere is a negligible amount of dust along the line-of-sight. This Approximated from 10 km s − expansion after 21 days.Article number, page 6 of 8. Ferretti et al.: Probing gas and dust in the tidal tail of NGC 5221 with the type Ia supernova iPTF16abcGalaxy Redshift perp. Dist. † Direction Morphology / Notes(kpc)2MASX J13341283 + + + + + † At z = . Table 5.
Neighbouring galaxies of NGC 5221 at comparable redshift.
400 200 0 200 400Velocity (km s − ) A r b i t r a r y F l u x S1 S2 H INa I D1
Fig. 6.
Comparison of the radial velocity of the Na I gas along the line-of-sight of iPTF16abc and H I in and around NGC 5221. The ALFAH I spectrum encompasses the position of iPTF16abc and NGC 5221.The Na I D1 profile is shown from the Apr. 24 UVES spectrum. Verticaldashed lines indicate the line-of-sight velocity of the two SDSS spec-tra with respect to the rest-frame of NGC 5221. The labels S1 and S2corresponding to those in Figure 1. further excludes the presence of grey dust, which could be alongthe line-of-sight of iPTF16abc, if grey dust existed in the inter-galactic medium.NGC 5221 likely had a close encounter with one of its neigh-bouring galaxies in the past ≈ years, when large portions ofgas and the progenitor system of iPTF16abc were tidally strippedfrom it. While the velocity of the gas in the tidal arm appearscomparable to the velocity of the eastern spiral arm, the projec-tion of the tail points in the opposite direction. This suggests thatthe tidal tail might be connected to the eastern spiral arm.A map of H I content of the group of galaxies includingNGC 5221, presented by Hallenbeck et al. (2016), suggests thatthe group shares a common gas envelope. As the galaxies withinthe group are likely to continue to interact and merge in the fu-ture, the gas seen along the line-of-sight of iPTF16abc is repre-sentative of the gas that is transplanted into the CGM surround-ing the galaxy group. It has been shown that dense cold gasclouds can exist in the CGM of galaxy groups for > × years(Borthakur et al. 2015). In future interactions with neighbouringgalaxies, these clouds can be the sites of star formation. In thelong run however, the gas clouds could dissipate, enriching theCGM with the detected metals. The presented gas profiles along the line-of-sight ofiPTF16abc should be useful to future studies of NGC 5221, itstidal tail and the galaxy group it is located in. In particular, theprofiles could complement higher resolution observations of theH I gas in NGC 5221 and the tidal arm, resolving the dynamicsof the galaxy group. Acknowledgements.
We thank Gregory Hallenbeck for his help with extractingALFALFA data, David Martinez-Delgado, Darach Watson and Brice Menard fortheir helpful insights and Jesper Sollerman and Avishay Gal-Yam for their com-ments. R.A. and A.G. acknowledge support from the Swedish Research Counciland the Swedish Space Board. The Oskar Klein Centre is funded by the SwedishResearch Council. This work is based on observations made with the NordicOptical Telescope, operated by the Nordic Optical Telescope Scientific Associ-ation at the Observatorio del Roque de los Muchachos, La Palma, Spain, of theInstituto de Astrofisica de Canarias. This work makes use of observations fromthe LCOGT network. The Pan-STARRS1 Surveys (PS1) and the PS1 public sci-ence archive have been made possible through contributions by the Institute forAstronomy, the University of Hawaii, the Pan-STARRS Project O ffi ce, the Max-Planck Society and its participating institutes, the Max Planck Institute for As-tronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics,Garching, The Johns Hopkins University, Durham University, the University ofEdinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center forAstrophysics, the Las Cumbres Observatory Global Telescope Network Incor-porated, the National Central University of Taiwan, the Space Telescope Sci-ence Institute, the National Aeronautics and Space Administration under GrantNo. NNX08AR22G issued through the Planetary Science Division of the NASAScience Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the LosAlamos National Laboratory, and the Gordon and Betty Moore Foundation. References
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