arXiv:1505.03407v1 [astro-ph.GA] 13 May 2015 rcro ayn nrc aaycutr,()t confirm ( to old (2) truly bright clusters, include a progenitors galaxy as Ia rich Ia SNe SNe in that of baryons light- utility is of the work “cosmic address tracer this of to as purpose (1) that The used threefold: structures directly. assess astrophysical also to diffuse difficult are are or Ia SNe faint they since for but bright, houses” – galaxies very dis- with large the are in of correlated most reside is and Ia SNe rate, Ia SNe covered formation after star of com- and rate occurs mass the galaxy giant explosion from, explosion The accretion red the mass a a panion. sufficient and in or or star, is with, WD merger WD sequence another the main either likely, or with Most under- system 2011). well binary yet Howell not (e.g., candles is stood scenario standard progenitor cosmological their as although WD) used (CO commonly dwarf white stars, carbon-oxygen of explosions clear e oeta.Drc esrmnso hslwsurface low clus- this the of in measurements reside Direct and galaxy potential. host ter their were that from stars (IC) must stripped intracluster this of and population mass, the baryonic include of accounting full a requires science these of each glob- motivate turn. and We galaxies in dwarf goals (GC). as such clusters hosts the faint ular on in constraints rate investigate Ia to SN (3) and systems, genitor OFRAINO OTESTP ASPROA SN UBESP HUBBLE USING SUPERNOVAE IA TYPE HOSTLESS OF CONFIRMATION i,P o 05 T S,Vcoi CVW36 Canada 3P6, V8W BC Victoria CSC, STN 3055, Box PO ria, 85721 79409 USA 94720-3411 CA ley, uenveo yeI SeI)aetethermonu- the are Ia) (SNe Ia Type of Supernovae nesadn h rwho tutr nteuniverse the in structure of growth the Understanding rpittpstuigL using typeset Preprint 4 3 2 1 eateto hsc n srnm,Uiest fVicto- of University AZ Astronomy, and Tucson Physics of Arizona, Department of University Observatory, TX Steward Lubbock, University, Tech Texas Department, Berke- Physics California, of University Astronomy, of Department e,pitsuc onietwt h orhS asepoinpositio explosion Ia’s SN fourth the with coincident source point red, ofis en h osrit nterrt ndaf n C ihd with GCs and dwarfs GC in or headings: Ia. re rate SNe dwarfs Subject higher their hostless at in on apparently light rate constraints intracluster low-redshift, of Ia of the SN tracers refine bright elliptic the first as an that to demonstra Ia of s SNe We implies proximity of local the constraints. use host observational the the to faint due previous estimate This likely, within We more remains is out. galaxy. 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M. . = )wihhsarltvl ihpoaiiyo en hneainet f A alignment. chance a being of probability high relatively a has which 2) − 1 . ) atro 0bte hnorgon-ae mgn.Too t of Two imaging. ground-based our than better 10 of factor a 0), > 1 .J Sand J. D. , y)pro- Gyr) 2 ABSTRACT 2 . .Zaritsky D. , 0 . %preto h tla asi lse galaxies cluster in mass stellar the of percent 2% ta.20) thge esit rgtrtae sre- is tracer brighter a redshifts and higher Virgo At and nearby quired, Neill 2005). 2004; the al. al. for et et and (Feldmeier done nebulae respectively been clusters planetary Fornax has as which such novae, population stellar lying aaycutr perdt ehsls,wihimplied low-redshift which rich, hostless, in Ia be SNe to 7 that their appeared of clusters (2003), 2 galaxy al. that et found Gal-Yam who by (WOOTS) Search Transients the stellar to IC the galaxies to cluster belong by and population. hostless hosted appear Ia that SNe number of number the rushv oa mass total a have groups es u ocsooia ufc rgtesdimming. light intracluster brightness uni- of ( local fraction De- surface the the of 2014; cosmological beyond measurements Trujillo Indirect difficult to & are due but Montes 2015), verse 2005; al. al. et al. et Maio et Gonzalez Zibetti (e.g., possible 2005; are component brightness aaygop perdt ehsls,wihimplied which hostless, be low-redshift to in Ia that appeared SNe 59 groups the of galaxy 19 Balogh that & found McGee by who (SDSS) (2010), Survey Sky Digital Sloan the esitmsieglx lses(ade l 01,which 2011), that al. implied et (Sand low- clusters in galaxy Ia massive Telescope SNe redshift hostless Hawaii apparently France four Survey identified Canada (CFHT) Cluster the Nearby from Multi-Epoch (MENeaCS) the recently, Most atsreslf 0 left surveys past being their on 0 eedn ntelgrtmcfitedsoeo h lu- the of slope faint-end function, logarithmic minosity are the values on These dependent thresholds. detection the below galaxies f . 05 ICL hswsfis oewt h ieOsraoyOptical Observatory Wise the with done first was This hsueuns fSeI staeso h C depends ICL the of tracers as Ia SNe of usefulness This − f a emd sn rgttaeso h under- the of tracers bright using made be can ) f ICL ICL 0 . 3 %(EeC)o h tla asi cluster in mass stellar the of (MENeaCS) 1% .J Pritchet J. C. , ≈ ≈ f M 0.Asmlrmaueetwsmd with made was measurement similar A 20%. lsesta eeietfidi the in identified were that clusters y sit,btta twl enecessary be will it that but dshifts, eta u eut ontpreclude not do results our that te ICL f efrteglx lse luminosity cluster galaxy the for pe ICL 0 o mle-cl tutrs(galaxy structures smaller-scale for 50% R truly N aapasna an disk faint a near appears Ia SNe e mgn o agrsample larger a for imaging eep > ( n a nta ecluae ycomparing by calculated be instead can . 03 0 = α − raedniiso C and GCs of densities urface M d otes tce mgn rmthe from imaging Stacked hostless. − 9 . lglx,btnihrcan neither but galaxy, al V ie h cehe ucin.The function). Schechter the (i.e. . u,ptnilyhostless, potentially our, 16 ) ofimn httheir that confirming 2), 0 a eehne,but enhanced, be may s . C EECP IMAGING TELESCOPE ACE = %(OT) %(DS,and (SDSS), 3% (WOOTS), 3% − +0 4 0 ∼ − . . 13 09 8 0 hto ihclusters). rich of that 10% . . )myb either be may 4) efour he aint, above results are based on αd = −1.0, which is true for 1.3. SN Ia Rates in Faint Hosts field galaxies (Blanton et al. 2003), but αd may be higher Over the past ten years, an increasing number of SN in rich galaxy clusters (e.g., Milne et al. 2007). Adopt- surveys have employed an unbiased, wide-field search ing αd = −1.5 as an upper limit, the MENeaCS survey strategy instead of targeting massive galaxies. This has estimated that . 2% of the stellar mass in dwarf cluster lead to the discovery that some types of luminous SNe galaxies below detection thresholds. If the SN Ia occur- have a higher explosion rate in dwarf hosts (e.g. Neill et rence rate per unit mass is equivalent in all cluster stel- al. 2011; Lunnan et al. 2014), attributed to high star lar populations – detected galaxies (∼ 82%), undetected formation rates providing more progenitor stars and/or dwarf galaxies (. 2%), and intracluster stars (∼ 16%) – lower metallicity leading to more luminous SNe, or per- then for MENeaCS we expect to find that . 2% of all the haps an elevated rate of binary star formation. As most discovered cluster SNe Ia (23) are hosted by faint dwarf SNe Ia occur in massive galaxies, it is difficult to assess galaxies (∼ 0.5 SNe Ia). In this work we use deep HST whether they might also have an enhanced rate in dwarf imaging at the locations of the 4 MENeaCS IC SNeIa galaxies because of the large statistical uncertainty on to further lower the fraction of undetected mass in faint the rate due to the relatively low number of SNe Ia in galaxies to 0.2% (0.003–0.007% with αd = −1.0), analyze low-mass hosts (e.g. Quimby et al. 2012). SN surveys of previously undetected objects in the vicinity of 2 SNe Ia, rich galaxy clusters are an efficient way to search many and discuss how our results affect the ability of IC SNe Ia low-mass galaxies at once, both due to a higher sky den- to measure fICL at higher redshifts. sity of galaxies and the (putative) upturn in the faint-end 1.2. SN Ia Delay Times slope of the cluster luminosity function (e.g., Milne et al. 2007; Yamanoi et al. 2007). In this work we find that One path towards understanding the progenitor sce- one IC SNIa may be hosted by a dwarf cluster galaxy, nario of SNe Ia is to constrain the range of possible ages and discuss the implication of this for SN Ia rates in faint via the SN Ia delay time distribution (DTD; the time be- hosts. tween star formation and explosion). While Type II and GCs are another potential very faint host for IC SNeIa. Ib/c supernovae – the core collapse of massive stars – are No SN has ever been confirmed to be hosted by a GC, associated with young stellar populations, SNe Ia occur although they are theoretically predicted to have a SN Ia with an explosion rate proportional to both galaxy stel- occurrence rate 1 − 10× that of elliptical galaxies due to lar mass and star formation rate (e.g., Mannucci et al. dynamical interactions that lead to more white dwarfs in 2005; Scannapieco & Bildsten 2005; Sullivan et al. 2006). binary systems in GCs (e.g., Ivanova et al. 2006; Shara & This indicates that SNe Ia are associated with both old Hurley 2006; Pfahl et al. 2009). Non-detections of GCs and young stellar populations. The current best mea- at the sites of ∼ 45 low-redshift SNe Ia in archival HST surements of the SN Ia DTD indicate that some SN Ia images have constrained the potential rate enhancement progenitors are quite old, >2 Gyr (e.g. Maoz, Mannucci to . 42× the rate in elliptical galaxies (Voss & Nelemans & Nelemans 2014), which implies long-lived progenitors 2012; Washabaugh & Bregman 2013). In this work we and/or that the timescales for mass transfer or merger find that one IC SN Ia may be hosted by a GC, and dis- are long. cuss the implication of this for SN Ia rates in GCs. Since The conclusion that at least some SNIa require pro- GCs are also comprised mainly of old (5–13) Gyr stellar genitors that are > 2 Gyr old relies on the SN Ia rate populations, confirming a GC-hosted SNe Ia would also in rich cluster galaxies, where the majority of the stellar meet our science goal of confirming SNeIa with long (> 2 population is old (e.g. Sand et al. 2012). However, there Gyr) delay times. is evidence that low levels of star formation are present in elliptical galaxies (e.g., Yi et al. 2005; Suh et al. 2011; 1.4. Paper Overview Graham et al. 2012). Could it be that all SNIa are ac- In this work, we use the Hubble Space Telescope (HST) tually from younger stellar populations? Probably not – Advanced Camera for Surveys (ACS) to obtain deep im- for example, Graham et al. (2012) show that the SNIa ages in filters F606W and F814W at the locations of 4 DTD result is robust to this small amount of young stars. apparently hostless SNe Ia from MENeaCS (Sand et al. Even so, direct confirmation of a SNIa progenitor in a 2011). These are the deepest images, and the largest stellar population of purely old stars would strengthen single survey sample, of IC SNe Ia locations in rich clus- and support the late-time DTD constraints on the pro- ters ever obtained and analyzed in this way. In Section genitor scenario. 2 we present our original CFHT and new HST observa- A suitable environment for this test is the population of tions and discuss our image processing and photometric intracluster (IC) stars in rich galaxy clusters: the colors calibrations. In Section 3 we analyze our deep stacks of and luminosities of IC red giant stars in Virgo show that HST ACS imaging: we characterize the faint sources in the IC stellar population is comprised of stars ≥ 2 Gyr the vicinity of the SNe Ia, derive our point-source lim- old (Durrell et al. 2002), and theoretical models also iting magnitudes, and determine the amount of cluster suggest the ICL is comprised of old stars (e.g. Sommer- stellar luminosity remaining below our detection thresh- Larsen et al. 2005; Purcell et al. 2007). All of the olds. In Section 4 we discuss the implications of these apparently hostless cluster SNe discovered to date are of results with respect to SNIa progenitor ages, the rates Type Ia; a lack of intracluster core collapse supernovae of SNe Ia in faint cluster objects, and the future use of lends credence to the idea that the ICL is composed of SNeIa as tracers of the ICL. We conclude in Section 5. an older stellar population. In this work, we use deep All dates are given in UT and a standard flat cosmology HST imaging to show that at least two of the apparently of ΩM =0.3, ΩΛ =0.7 is assumed throughout. hostless SNe Ia discovered by MENeaCS truly belong to 2. the IC stellar population of old stars. OBSERVATIONS Here we describe our past observations with the Mega- To make the deepest possible stacks of our HST +ACS Cam imager (Boulade et al. 2003) at the Canada-France- data, we use the Astrodrizzle software to median-combine Hawaii Telescope (CFHT), and our new deep imaging FLC images provided by that STScI pipeline (Gonzaga taken 4–5 years later with the Hubble Space Telescope et al. 2012). FLC files come fully reduced, drizzled, HST Advanced Camera for Surveys (ACS; Holland et al. and corrected for charge transfer efficiency by the STScI 1998). pipeline. We use the WCS astrometry during image com- 2.1. bination. We do this for the F606W and F814W images CFHT MENeaCS separately to make a deep stack for each filter that is free The Multi-Epoch Nearby Cluster Survey (MENeaCS) of cosmic rays, and then also create a single sum-combine monitored 57 low-redshift (0.05 MENeaCS SN Coordinates BCG Offset Redshift Spectral CFHT Detection Limit f( aFrom Table 1 in Sand et al. (2011). TABLE 2 HST+ACS Imaging Data for MENeaCS Intracluster Supernovae MENeaCS Orbits Observation Exposure Time (s) HST Detection Limits (mag) f( TABLE 3 TABLE 4 Photometric Transformations Source Extractor Parameters for the Detection Efficiency Natural Target C0 C1 C2 TCOL Filter Filter mV − mI Parameter Value F606W Johnson V 26.394 0.153 0.096 < 0.4 DETECT MINAREA 3 F606W Johnson V 26.331 0.340 −0.038 > 0.4 DETECT THRESH 2 F814W Cousins I 25.489 0.041 −0.093 < 0.1 SEEING FWHM 0.08 F814W Cousins I 25.496 −0.014 0.015 > 0.1 BACK SIZE 256 BACK FILTERSIZE 5 BACK TYPE AUTO the relevant detection parameters are given in Table 4. BACKPHOTO TYPE LOCAL We visually verify that these parameters are returning all and only real sources in the images. In Figure 1 we plot our detection efficiency (i.e. the fraction of objects recovered) as a function of apparent magnitude for the Point Source Limiting Magnitudes F606W and F814W images of each cluster. The “limit- 1.0 ing magnitude” is defined as the magnitude at which the detection efficiency drops to 50%, and is given for each 0.8 cluster and filter in the plot legend of Figure 1. In Section 3.3 below, we visually identify objects be- low the official limiting point source magnitude at the 0.6 Magnitude at which the locations of our SNe Ia in Abell 2495 and Abell 399. We recovery fraction < 50% find that these objects are only detected by Source Ex- F606W F814W 0.4 A1650 28.58 28.88 tractor if the threshold is lowered to 1σ, which also de- A2495 28.64 28.90 tects many peaks in the background noise and produces a Fraction Recovered A399 28.30 28.45 A85 28.40 28.49 source catalog with large uncertainties in their apparent 0.2 magnitude. This is why our official limiting magnitude – which needs to be robust because we use it to deter- mine the fraction of cluster stellar luminosity below our 0.0 detection thresholds in Section 3.2 – is slightly brighter 26.5 27.0 27.5 28.0 28.5 29.0 29.5 than some of the sources discussed in Section 3.3. The Simulated Apparent Magnitude caveat here is that brighter, but more extended, objects Fig. 1.— Detection efficiencies for simulated point sources in our may fall below our detection threshold also – but most of HST ACS images, as described in the text (§ 3.1). Filter and the faint cluster objects will be point-like (dwarf galaxies cluster are represented by color and symbol respectively, as shown and GCs). in the plot legend. The magnitude at which our detection efficiency falls to 50% is considered our limiting magnitude. 3.2. Fraction of Undetected Cluster Light To determine the fraction of cluster light remaining below our point-source limiting magnitudes, we follow a similar method to that presented by Sand et al. (2011) The absolute R-band luminosity function for the for our CFHT deep-stack images, the result of which is nearby Virgo cluster is modeled by Trentham & Tully reproduced in the last column of Table 1. (2002) with two components, a Gaussian for the bright end and a Schechter function (see their Equation 2): TABLE 5 Supernova Positional Uncertaintya 2 2 −(M−Mg ) /(2σg ) N(M)= Nge SN Ia PSF Centerb GEOMAPc Totald Error [−0.4(M−M )] e + N (10[−0.4(M−Md)])αd+1e−10 d , (2) Cluster ∆x ∆y ∆x ∆y ∆x ∆y Radius d Abell (pix) (pix) (pix) (pix) (pix) (pix) (′′) ′′ where N(M) is the number density of galaxies per square 1650 4.57 4.57 0.31 0.13 7.52 7.62 0.23 2495 6.55 6.31 1.81 2.00 6.42 6.46 0.34′′ Mpc per magnitude bin, M is the R-band absolute ′′ magnitude, and N = 17.6, M = −19.5, σ = 1.6, 399 6.15 6.00 0.63 1.56 7.76 7.82 0.31 g g g 85 7.39 7.21 2.57 3.52 10.41 10.15 0.40′′ Nd =3Ng, Md = −18.0, and αd = −1.03. However, the faint-end slope is known to steepen with redshift (e.g., a HST ′′ −1 Khochfar et al. 2007), and values down to α ≈ −1.5 All pixels are ACS pixels (0.05 pixel ). d bSource Extractor’s uncertainty on the SN’s coordinates in have been measured for the Coma cluster (Milne et al. the CFHT image post-transformation with GEOTRAN (i.e., the 2007) and most recently for Abell 85 (αd ≈−1.6; Agulli square root of the variance, the second moment of the barycenter). et al. 2014). In order to make a robust upper limit on the cGEOMAP’s uncertainty in mapping the CFHT image to the HST amount of cluster light below our detection efficiencies, image (i.e., a systematic, the output xrms and yrms values). dAdded in quadrature. − e ′′ −1 we use αd = 1.5 from here on. Average in x and y, ×0.05 pixel . In Sand et al. (2012) the total r′-band luminosity is calculated for all MENeaCS clusters. We convert this enclose nearby objects (sizes chosen to guide the eye). to R-band using the conversion factors from Blanton & Image information such as the cluster name, UT date of Roweis (2007), R = r − 0.0576 − 0.3718((r − i) − 0.2589). acquisition, filters, scale bar, and compass are shown in Because most of the cluster light is from old stellar popu- yellow along the bottom. These images all have a 30′′ × lations, we use the typical SDSS color of elliptical galax- 30′′ field of view. ies, r − i ∼ 0.4, from Eisenstein et al. (2001). We inte- As in Sand et al. (2011), we use the dimensionless pa- grate the galaxy luminosity function down to the CFHT rameter R to identify whether nearby objects could be point-source limiting magnitudes listed in Table 1 from considered as potential hosts of the SNeIa. This param- Sand et al. (2011), and then normalize to the total R- eter is defined in the Source Extractor manual as: band luminosity for each cluster. 2 2 2 We convert the absolute R-band luminosity function R = Cxxxr + Cyyyr + Cxyxryr (3) for each cluster into apparent V - and I-band using the cluster’s redshift, the elliptical galaxy template spec- where Cxx, Cyy, and Cxy are object ellipse parameters, trum from Kinney et al. (1996), and the line-of-sight xr = xSN −xgal and yr = ySN −ygal, and R ∼ 3 describes Galactic extinction for each cluster: AV,A1650 = 0.047, the isophotal limit of the galaxy (see also Sullivan et al. AV,A2495 = 0.211, AV,A399 = 0.467, AV,A85 = 0.103, 2006). A supernova is typically only classified as “host- AI,A1650 =0.026, AI,A2495 =0.116, AI,A399 =0.256, and less” if R > 5, but depending on the surface brightness AI,A85 = 0.057 (Schlafly & Finkbeiner 2011; Schlegel, profile of the galaxy, a significant amount of the stellar Finkbeiner & Davis 1998). We convert the V and I-band mass may reside beyond this radius (e.g., & 10% for the luminosity function into the HST ACS natural system fil- potential host of the SN in Abell 399, as determined in ters using the transformations of Sirianni et al. (2005), Sand et al. 2011). as described in Section 2.3. In Table 2, we report the 3.3.1. Abell 1650 point-source detection limit in absolute MR magnitudes, and the fraction of the cluster’s R-band stellar luminosity In Figure 2, we see that the location of the SNIa is remaining below this limit. For all clusters we find that truly devoid of objects. We ran Source Extractor with < 0.2% of the stellar mass in cluster galaxies remains very relaxed parameters and still recovered no sources in below our official point-source limiting magnitude. This this area. Of the three nearby objects enclosed by dashed result is discussed further in Section 4. circles in Figure 2, the SN location is R & 15 away. Our intracluster SNIa in Abell 1650 therefore appears to be 3.3. Potential Hosts in the Deep HST-ACS Images truly hostless. We use the IRAF tasks GEOMAP and GEOTRAN to 3.3.2. co-register our CFHT images to the new, deeper HST - Abell 2495 ACS images. In Figures 2–5 we show the results, side by In Figure 3 we see several sources near the location side, for comparison. The CFHT images are comprised of of the SN Ia that were not apparent in the CFHT deep two 120 second r′ exposures, and contain the SNIa – we stacks presented in Sand et al. (2011), but none are do not use our SN-free deep stacks here, because we need within the positional uncertainty of the SN Ia. We ran the SN’s coordinates in the co-registered frames. The Source Extractor with very relaxed parameters and still HST images are our deepest stacks, the sum-combined recovered no sources within the positional uncertainty. F606W and F814W filtered images. Green circles mark In Figure 6 we show this region in detail, and identify the position of the SN in each image, with a radius equal nearby sources A, B, C, and D. Sources A and B are to 3× the positional uncertainty of the SNIa. These posi- unlikely to be physically associated, as the SN Ia is R ∼ tional uncertainties, listed in Table 5, are a combination 17 away from them. We discuss objects C and D in turn. of Source Extractor’s uncertainty in the PSF centroid Object C is likely a part of object D, which is clumpy for the SN in the co-registered CFHT image (using win- and extended, as shown by the contour plot in the right- dowed output parameters), and the error in GEOMAP’s hand image of Figure 6. However, object C is identified transformation between images. The dashed cyan lines by Source Extractor as an independent source at the 1σ r=0.5" r=0.5" SN SN r=0.69" r=0.69" N N 2.0" Abell 1650 2.0" Abell 1650 CFHT r' HST 606W+814W 3.14 kpc 2009-12-14 E 3.14 kpc 2013-01-25 E Fig. 2.— Co-registered CFHT (left) and HST (right) images for the SN Ia in Abell 1650, as described in the text of Section 3.3. SN SN r=1.02" r=1.02" E E 2.0" Abell 2495 2.0" Abell 2495 CFHT r' HST 606W+814W 3.0 kpc 2009-05-23 N 3.0 kpc 2013-10-04 N Fig. 3.— Co-registered CFHT (left) and HST (right) images for the SN Ia in Abell 2495, as described in the text of Section 3.3. level in the F814W image, with mF814W = 29.0±0.2 mag. SN Ia or a chance alignment are discussed in Sections 3.4 In the F606W image it is not officially detected by Source and 3.5. Ultimately we find it unlikely that object C is Extractor, but it is just visible to the eye, and with aper- the host galaxy. ture photometry we estimate it to be mF606W = 29.8±0.2 Object D is an extended source with semi-major and mag. In both filters, object C falls below our 50% detec- semi-minor axes of A = 0.22′′ and B = 0.053′′, which tion efficiency for Abell 2495 (see Figure 1 in Section 3.1). leads to SN offsets of R = 4.9 and 4.1, in F606W and The SN’s location is R & 5 away from object C, which F814W respectively. As R < 5, this SNIa cannot be argues against a physical association. Object C is redder considered truly hostless unless we can show that object than the red sequence of Abell 2495, as shown in Figure D is unlikely to be a cluster member. In the natural 7, and so it is unlikely to be a cluster dwarf galaxy. The system of HST ACS, object D is mF606W = 25.56 and scenarios in which object C is lingering emission from the mF814W = 25.75 mag, and has a color = −0.20 mag. SN SN r=0.93" r=0.93" 2.0" Abell 399 E 2.0" Abell 399 E CFHT r' N HST 606W+814W N 2.36 kpc 2009-01-27 2.36 kpc 2013-12-04 Fig. 4.— Co-registered CFHT (left) and HST (right) images for the SN Ia in Abell 399, as described in the text of Section 3.3. r=1.0" r=1.0" SN SN r=1.20" r=1.20" G N N 2.0" Abell 85 2.0" Abell 85 CFHT r' HST 606W+814W 2.38 kpc 2.38 kpc 2009-06-18 E 2013-09-18 E Fig. 5.— Co-registered CFHT (left) and HST (right) images for the SN Ia in Abell 85, as described in the text of Section 3.3. As shown in Figure 7 it consistent with a red sequence Object D is clumpy and has an ellipticity of 0.7, which cluster galaxy. We use Sirianni et al. (2005) to convert is higher than the ellipticity of the bright red sequence this photometry into Landolt filter system and find that galaxies we identify in Abell 2495. Morphologically, ob- object D is mV ≈ 25.6 and mI ≈ 24.5 mag. We apply ject D resembles an inclined disk galaxy – can we use the distance modulus of Abell 2495 (µ ≈ 37.8 mag) and the disk scale length to assess whether it may be a blue find that intrinsically, object D is MV ≈−12.2 and MI ≈ spiral galaxy at higher redshift? Disk galaxies are gen- −13.3 mag. This is brighter than the limiting magnitudes erally well fit by an exponential function for the flux in- −R/Rd quoted for the CFHT deep stack of Abell 2495, but those tensity as a function of radius, I(R)= I0e , where limits are for point-like sources and object D is extended Rd is the characteristic disk scale length. We estimate ′′ – in fact, it is just barely visible as an extended source Rd . 0.4 , with no attempt to de-project or account for in Figure 3 of Sand et al. (2011). inclination. If object D is a cluster member, this corre- 0.5" SN 0.75 kpc 0.5" SN r=0.34" r=0.24" 0.75 kpc r=0.34" r=0.24" B A C B A D C D Abell 2495 Abell 2495 HST 606W+814W HST 606W+814W Fig. 6.— The HST +ACS image from the deepest stack of both filters F606W and F814W for the intracluster SN Ia in Abell 2495. The green circle marks the position of the SN Ia, with a radius equal to 1× the positional uncertainty from Table 5. In cyan we identify nearby objects A, B, C, and D (we cannot verify whether C is a part of D), discussed in the text. For objects A and B, the dashed cyan circle has r = 0.24′′, representing 3× the PSF FWHM of ACS (0.08′′). For the dashed ellipse around object D we use the parameters determined by Source Extractor for the F814W image: 3× the semi-major and -minor axes, A = 4.4 and B = 1.1 pixels (A = 0.22′′ and B = 0.053′′), and a position angle of θ = 74 degrees (in the x-y plane). In the image at right, we include flux contours in magenta to highlight the connection between objects C and D and the lack of low-significance sources within the SN’s positional error circle. ing they are not associated. We use these HST images to re-evaluate the parameter R from Equation 3, which de- scribes the SN’s offset in terms of the galaxy’s isophotes. We find that R ∼ 6 in both F606W and F814W, re- establishing the SN as independent of the stellar popu- lation of this large neighbor galaxy. In Figure 4 we see a small, faint source at the center of the green circle marking the SN Ia’s position, labeled object F in Figure 8. Object F is detected with low sig- nificance, but with coincidence in both the F606W and F814W images. It is a point-like source, with a FWHM of ∼ 3 pixels, and is offset by < 1 pixel from the SNIa location. Object F is therefore very likely to be physi- cally associated with the SN Ia in Abell 399 – the small probability of a chance alignment with a foreground star or background host is discussed in Section 3.4. In Fig- ure 8 we also label the next-nearest object E; it is a clumpy extended source, but we find it far less likely to Fig. 7.— Color-magnitude diagram in the natural HST ACS filter system for Abell 2495. Black points represent all sources extracted be physically associated with the SN and so we focus our with an apparent magnitude brighter than the 50% detection ef- attention on object F. ficiencies in both F606W and F814W, as discussed in § 3.1 and Object F is mF606W = 28.7 ± 0.3 and mF814W = shown in Figure 1. Black points surrounded by red indicate the 28.7 ± 0.2 mag, below our limiting magnitudes as shown galaxies used in the linear fit to the red sequence (red lines). The blue square and circle represent objects C and D respectively (see in Figure 1. Its photometry is consistent with a red se- Figure 6). quence galaxy in Abell 399, which is very similar to the red sequence shown for Abell 2495 in Figure 7. We use sponds to Rd ∼ 0.6 kpc, which is roughly appropriate for Sirianni et al. (2005) to convert this photometry into the a disk galaxy. In Section 3.4 we also discuss the relatively Landolt filter system and find that object F is mV ≈ 28.8 large probability that this is a chance alignment, but for- and mI ≈ 27.4 mag. We apply the distance modulus of mally we cannot exclude the possibility that object D is Abell 399 (µ ≈ 37.2 mag) and find that intrinsically, ob- a cluster member and the host of the SN in Abell 2495. ject F is MV ≈ −8.4 and MI ≈ −9.8 mag, and has a color of V − I ≈ 1.4. This is also well matched to the 3.3.3. Abell 399 expected magnitude and color of a bright GC: the lu- In Figure 4 we see a large elliptical galaxy near the lo- minosity function for GCs is a Gaussian that peaks at cation of the SNIa. Sand et al. (2011) describes how the MI ≈ −7.4 and has σ ≈ 1.2, and GC colors span from SN is > 7R from the large elliptical, and how the SN’s MV − MI ≈ 0.7–1.5 (e.g., West et al. 2011). Although and galaxy’s line-of-sight velocities differ by 3σ, indicat- GCs have been found distributed between galaxies in rich nearby elliptical galaxy (Figure 4), which has absolute 0.5" magnitudes of MV ≈−19.5 and MI ≈−20.7, and a stel- 9 0.59 kpc lar mass of ∼ 7.6×10 M⊙ (Bell et al. 2003). A galaxy of SN this stellar mass has between 10–100 GC within R< 50 kpc, and the surface density radial distribution for GC r=0.31" −2.4 −2 is N(r) = N0r GC kpc (Zaritsky et al. 2015); ∼ 75% of the GC are internal to 10 kpc (the distance E of the SN in Abell 399 from the nearby elliptical). Nor- malizing the radial distribution to a total of 10–100 GC gives N ≈ 0.3−3×10−2 GC kpc−2. Based on the GC lu- minosity function of West et al. (2011) ∼ 84% of all GC are below our limiting magnitudes, and so the observable surface density at 10 kpc is lowered to 0.5 − 5 × 10−3 − F GC kpc 2. This is higher than our estimated surface density for cluster dwarf galaxies, suggesting it is more likely to see a GC at the location of object F than a cluster dwarf. However, as our lower limit for the GC surface density is equal to our upper limit on the dwarf galaxy surface density the estimates are not significantly Abell 399 different enough to make a robust claim to the nature of HST 606W+814W object F. The caveat here is that we have used a radial distribution for all GC, but the population of red GCs has a significantly shorter radial extent than blue GCs – Fig. 8.— The HST +ACS image from the deepest stack of both in fact, for cluster ellipticals the radial distribution of red filters F606W and F814W for the candidate intracluster SN Ia in GCs appears to be truncated near the effective radius of Abell 399; a zoom-in of Figure 4. the parent galaxy (e.g., Brodie & Strader 2006). In summary, we find that the SN Ia in Abell 399 is clusters (e.g. Peng et al. 2011; West et al. 2011), their likely physically associated with object F. Without any density drops with clustercentric radius and we would constraints from expected SNIa rates, we find that object not expect a significant number of intracluster GCs at F is less likely to be a dwarf galaxy than a GC from the the location of this SNIa, 616 kpc from the BCG. How- nearby elliptical. We discuss the probability of chance ever, the presence of the nearby elliptical galaxy (Figure alignments in Section 3.4, and the possibility that object 4) makes the GC hypothesis more likely. F is lingering emission from the SN Ia in Section 3.5. We In Section 4.1, we show that the SN Ia rate enhance- discuss the implications of object F for SN Ia rates in ments in dwarf galaxies or GC implied by the nature of dwarf galaxies and GCs, and whether these implications object F are within all existing theoretical and observa- provide a prior on the nature of object F, in Section 4.1 tional limits. If we assume that the rate per unit mass and the impact of object F on using SNIa as tracers of of SNe Ia is not much larger in dwarf galaxies vs. GC the ICL at high-redshift in Section 4.2. (or vice versa) – which would put a strong prior on the nature of object F – we can estimate the expected sur- 3.3.4. Abell 85 face densities of dwarf red sequence galaxies and red GCs In Figure 5, we see that the location of the SNIa ap- at this location in order to assess which is more proba- pears devoid of objects. For the nearest source, labeled ble. Based on our extrapolation of the cluster luminosity object G in Figure 5, the SN is R ∼ 13 away. We function for Abell 399 (Section 3.2), we estimate there to ran Source Extractor with very relaxed parameters, but 9 be ∼ 1.5×10 L⊙ in dwarf cluster galaxies at MR > −10 found that the few sources detected are consistent with mag. That is about 100–500 dwarf galaxies of 5–10 ×106 noise peaks (i.e., they had very low significance, their de- L⊙ within a clustercentric radius of ∼ 1 Mpc, or 1–5 tections in F814W only and were not coincident with any ×10−4 dwarfs kpc−2. This would apply at any location F606W low-significance detections, and were not visually in the cluster, and so applies for the location of object F. confirmed in the F606W+F814W stack). We conclude To assess whether satellites of the nearby elliptical galaxy that the SN Ia in Abell 85 is truly hostless. might raise the predicted surface density at this location, we use the radial distribution of low-redshift, low-mass 3.4. Probabilities of Random Line-of-Sight Alignments satellite galaxies presented by Prescott et al. (2011; see In our discussion of the nature of object F in Abell their Figure 5). The location of the SN in Abell 399 is 399 we estimated the surface densities of faint cluster R ≈ 10 kpc from the center of the nearby elliptical; at objects such as dwarf red sequence galaxies and GCs to this radius Prescott et al. (2011) find that the surface be ∼ 5×10−4 objects per kpc2. The probability of a faint density of satellites is ∼ 7×10−6 kpc−2. The caveat here cluster object appearing randomly within the positional is that they consider only isolated primary galaxies, and uncertainty of our IC SNe Ia is negligible, . 0.0003%. so their results represent an upper limit on the radial dis- The presence of object F is not a chance alignment with tribution we could expect in rich clusters. However, as a cluster object. this is much lower than what we expect from the cluster We used TRILEGAL (Girardi et al. 2005) to simulate luminosity function we conclude that object F is unlikely a foreground star population in the directions of our four to be a satellite galaxy of the nearby elliptical. fields to a limiting magnitude of mV = 29. The prob- We can estimate the surface density of GC from the ability of a star appearing randomly within 0.2′′ of our IC SN Ia coordinates is ≤0.02%. We judge that it is ex- this subclass because 91T-like SNe Ia are associated with tremely unlikely that object F at the location of the SN younger stellar populations (Howell et al. 2009). Al- in Abell 399 is a foreground red star. though less is known about the late-time decline of sub- We used the Hubble Deep Field catalogs (Williams et luminous SNe Ia, SN 1991bg itself was already V ≈ −6 al. 1996) to simulate a population of faint objects be- by ∼ 600 days after peak brightness (Turatto et al. tween 25.0 < mF814W < 29.5. This is the magnitude 1996). Our intracluster SN Ia in Abell 85 was classified as range in which we detect sources in our HST images but SN 1991bg-like, but no object is detected at its location not our CFHT images. The new sources detected near in the HST ACS imaging. the SN Ia locations in Abell 2495 and Abell 399 fall in this Could we detect the emission from a shocked compan- magnitude range. We find the probability of a faint field ion star? For normal SNeIa, theoretical predictions for object randomly being within 0.31′′ and 0.34′′, the po- a non-degenerate companion shocked by the SN Ia ejecta sitional uncertainties on the locations of the SNe Ia, are include an increase in temperature and luminosity, up to 3 4 ∼ 2.0% and ∼ 2.5%, respectively. In our original work 10 to 10 L⊙ by 1–10 years after explosion (Pan et al. ∼ 2% of the cluster stellar mass in faint dwarf galaxies 2013; Shappee et al. 2013). Such a companion would be was below our detection limit (Sand et al. 2011). Assum- blue, and have V > −4 mag, which is well below our lim- ing the SN Ia occurrence rate per unit mass is equivalent iting magnitudes. A light echo is also likely to be blue, in all populations (high- and low-mass galaxies, and in- similar to the color of a SN Ia at peak light, and even tracluster stars), this means ∼ 2% of the all MENeaCS fainter. In order to formally rule out the possibility that SNe Ia should be hosted by undetected, faint dwarf galax- the objects identified near the SN locations in Abell 2495 ies (∼ 0.5 out of 23 SNe Ia). We now see that this is and Abell 399 are lingering emission from the SN Ia or approximately the same chance as finding a background its binary companion, we discuss each in turn. galaxy within the positional uncertainty. However, ob- Abell 2495 – In Figure 6 we identify object C as a ject F in Abell 399 is not just within the positional un- possible point source near the location of the SN Ia in certainty, but appears within 0.05′′ (1 ACS pixel) of the Abell 2495. The spectrum of this SNIa was obtained SN location. The probability of random alignment with on 2009-06-18.58 UT at Gemini Observatory as part of a background object within 0.10′′ is just 0.3%. On the the MENeaCS follow-up campaign, and we classified it other hand, if we increase the radius to 1.0′′ (i.e. the dis- as a normal SNIa at ∼ +3 months after peak bright- tance encompassing nearby objects for Abell 2495), the ness. At the time of our HST ACS images obtained probability of chance alignment increases to ∼ 18%. on 2013-10-04, the SNIa would be +1569 days old (4.3 As a final, alternative estimate we use our own source years). At this time, object C is mF606W = 29.8±0.2 and catalogs for all four HST -ACS fields and find that the mF814W = 29.0 ± 0.2 mag, or mV ≈ 30.0 and mI ≈ 27.8 fraction of our imaged area within R< 5 of an object is mag, in our HST ACS imaging. We apply the distance ∼ 3%. While it remains very unlikely that object F is a modulus of Abell 2495 (µ ≈ 37.8 mag) and find that chance alignment with the SN Ia in Abell 399 (Figure 8), intrinsically, object C is MV ≈ −7.8 and MI ≈ −10.0 we cannot say the same for the objects near the SN Ia magnitudes. This is both significantly brighter and red- location in Abell 2495 (Figure 6). der than a normal SN Ia is predicted to be at ∼ 4 years. 3.5. We conclude that object C is unlikely to be the SN Ia or Limits on the SNe Ia and/or its Companion an evolved companion star. A normal SN Ia has faded ∼ 4 magnitudes by ∼ 100 Abell 399 – In Figure 8 we identify object F as a days after peak brightness. After this time the decline point source at the location of the SN Ia in Abell 399. is set by the decay rate of Co56, and the SN Ia continues The classification spectrum of this SN Ia, obtained on to fade at ∼ 1 magnitude per 100 days in I-band, and 2008-11-28.49 UT at Gemini Observatory as part of the ∼ 1.3 − 1.5 mag per 100 days in BV R (e.g. as seen MENeaCS follow-up campaign, showed it to be a normal for normal twin SNeIa 2011fe and 2011by; Graham et SN Ia at ∼ +2 weeks after peak brightness. In our HST al. 2015a). For a normal SNIa such as SN2011fe, the ACS images obtained on 2013-12-04, the SNIa would be intrinsic brightness at ∼ 1000 days is V ∼−5 magnitudes +1832 days old (5 years). This object is MV ≈ −8.4 (e.g., Kerzendorf et al. 2014; Graham et al. 2015b). and MI ≈ −9.8 mag, has a color of V − I ≈ 1.4, both Most of the flux comes from blue emission features of significantly brighter and redder than a normal SN Ia is [Fe II] at < 6000 A˚ (e.g., Taubenberger et al. 2015; predicted to be at extremely late times. We conclude Graham et al. 2015b), and the late-time V − I color of a that object F is unlikely to be the SN Ia or an evolved normal SNIa is expected to be ∼ 0. After ∼ 1000 days, companion star. the predicted rate of decline for normal SNe Ia is even 4. slower (Seitenzahl et al. 2009), and so V ∼−5 mag is a DISCUSSION conservative upper limit on SN Ia brightness after 1000 Our analysis of the HST ACS images at the locations days. of our 4 intracluster MENeaCS SNe Ia has shown that Over-luminous SNe Ia that resemble SN 1991T have one is hosted by either a dwarf red sequence galaxy or been observed to decline more slowly. For example, red GC (Abell 399); one is potentially associated with a SN 1991T itself was V ∼ −10 at ∼ 600 days (Cappel- nearby spiral or irregular galaxy consistent with the red laro et al. 1997) and SN2000cx had a V -band slope sequence but also has a relatively high probability of be- of ∼ 0.65 mag per 100 days at ∼ 700 days after peak ing a chance alignment (Abell 2495); and two appear to brightness (Sollerman et al. 2004), so a SN 1991T-like be truly hostless (Abell 1650 and Abell 85). We discuss event could be V . −7 at 1000 days. However, none the implications of our results for the rates of SNe Ia in of the MENeaCS SNe Ia were spectroscopically similar faint cluster hosts in Section 4.1 and for the use of SNe Ia to SN 1991T, and are furthermore unlikely to belong to as tracers of the ICL in Section 4.2. 4.1. Implications for SN Ia Rates in Clusters the cluster luminosity function in later work, but con- In Section 3 we found that the SN Ia in Abell 399 was sider it beyond the scope of this paper. likely hosted by the faint object F, and that this source The potential SN Ia rate enhancement by a factor of is consistent with being either a cluster dwarf galaxy or ∼ 5 in the faintest dwarf galaxies is not too large to be a GC. Here we consider the implications of both scenar- unphysical, as the SN Ia rate is known to vary by factors ios on the rate of SNeIa in dwarf galaxies and GCs, and of that size (and more) with host galaxy properties such whether established SN Ia rates (or limits) in these pop- as the specific star formation rate (e.g. Mannucci et al. ulations can constrain the physical nature of object F. 2005; Scannapieco & Bildsten 2005; Sullivan et al. 2006; Smith et al. 2012). This potential rate enhancement is 4.1.1. Dwarf Galaxies also not so large that we expect to have already observed If object F in Abell 399 is a dwarf galaxy, does this it in wide-field sky surveys such as SDSS. For example, imply a significantly enhanced SN Ia rate in faint cluster Smith et al. (2012) find that the SN Ia rate per unit mass decreases as a function of host stellar mass, but galaxies? The rate per unit mass in a population, R, 8 is expressed by R = C × N/M, where C is a detection their lowest stellar mass bin is ∼ 5 × 10 M⊙ and the efficiency, N is the number of SNe Ia, and M is the mass uncertainty on its rate is a factor of ∼ 2–3. Furthermore, in the population. As described in Sand et al. (2011), they mention that ∼ 2% of the SNe Ia in their sample our original CFHT deep stacks left . 2% of the mass have undetected host galaxies. Current and future wide- in faint cluster galaxies undetected, but we now believe field surveys such as the Palomar Transient Factory and that population has hosted 1 SN Ia. We can estimate the the LSST should be able to improve the SN Ia rate in implied relative rate per unit mass in the faintest 2% of faint dwarf galaxies (e.g., Conroy & Bullock 2015). cluster galaxies with the following equation: 4.1.2. Globular Clusters R C × N /M If object F in Abell 399 is a GC, it would be the first 2% = 2% 2% 2% . (4) R C × N/M confirmed GC to host a SNIa. As GCs are purely old stellar populations (> 2 Gyr), such an association would Sand et al. (2011) describes how a small difference also be direct confirmation that SNe Ia progenitors in- in MENeaCS detection efficiencies between hosted and clude truly old star systems. The fraction of galaxy stel- hostless SNe Ia are introduced by two effects: (1) it is lar mass in GC systems is ∼ 2 × 10−3 for elliptical galax- more difficult to detect transients on top of a host galaxy ies (Harris et al. 2009; Peng et al. 2008; Zaritsky et (even with difference imaging techniques), and (2) the al. 2015). Based on this and the total stellar mass in spectroscopic follow-up coverage for the hostless popu- red sequence galaxies in MENeaCS clusters, we estimate 12 lation was slightly more extensive than for the hosted that & 10 M⊙ in GCs has been surveyed by MENeaCS. SNe (they were run under separate proposals). Together, If object F is a GC and has produced a SN Ia, it implies this difference works out to be C2% = 1.2C. Sand et a rate ∼25 times higher than the rate in our cluster red al. (2012) present that the number of SNe Ia hosted by sequence galaxies from Sand et al. (2012). This is on all cluster galaxies within 1 Mpc is N = 11, and so a similar scale to theoretically predicted enhancements with N2% = 1 we find that R2%/R ≈ 5.5. Repeating due to dynamical interactions in the dense stellar en- this calculation using only red sequence cluster galaxies vironments of GCs (Pfahl et al. 2009), and below the yields a similar rate enhancement because the number of current limits placed by non-detections of GC at the lo- SNe Ia in red sequence members within 1 Mpc is NRS =6 cations of SNe Ia in archival HST images (. 42×; Voss (i.e., 0.5N), the stellar mass in red sequence galaxies is & Nelemans 2012; Washabaugh & Bregman 2013). MRS ∼ 0.5M, and CRS = C. Would such an enhanced rate in GCs have already been Ultimately this potential rate enhancement by a fac- noticed? Potentially not. It would imply that . 5% of tor of ∼ 5 is quite uncertain, as it is based on just one the SNe Ia in ellipticals are hosted by their GCs, but SN Ia and an indirect estimate of the amount of mass in as we discussed in Section 3.3.3 the radial distribution faint cluster galaxies. As introduced in § 1.1 and 3.4, of GCs follows the galaxy light profile and drops off at by assuming the SN Ia rate per stellar mass is equal in ∼ 5R. It is entirely conceivable that GC-hosted SNe have all cluster populations we estimated that the expectation simply been assigned as belonging to the parent galaxy. value for the number of MENeaCS SNe Ia in dwarf hosts This might lead to a relative rate enhancement in the is ∼ 0.5. For a more restrictive estimate of the expecta- outer regions of ellipticals, which could be measured and tion value, we limit to red sequence galaxies within the attributed to GCs. However, such an effect would be clustercentric radius of 1 Mpc used above. Assuming the difficult to confidently measure for two reasons. First, rate in bright cluster red sequence galaxies (∼ 50% of there is a detection bias of SNe being easier to find when the stellar mass hosting NRS = 6 SNeIa) is the same as they are not embedded deep in the host galaxy. Second, that in dwarf red sequence galaxies (∼ 2% of the stellar a stellar population originating in GCs – able to produce mass), the expectation value is ∼ 0.24 SNe Ia in dwarf an enhanced rate of SNIa – may have previously released hosts. With Poisson statistics the probability of observ- into the halos (or bulges) of galaxies from GCs due to col- ing ≥ 1 SN Ia given this expectation value is 0.16, and lisions with clouds or other GCs, winds from supernovae the 1σ uncertainties our estimated rate enhancement are in the GC, tidal forces for GC on elliptical orbits about 12.7 5.5−4.5 (Gehrels 1986) – consistent with no rate enhance- their host, evaporation of stars during interal GC relax- ment. Furthermore, if the luminosity function is steeper ation, and/or dynamical friction (Fall & Rees, 1985). It than expected there could be more than 2% of the stellar is conceivable that these issues combined could conspire mass residing in such faint red galaxies. We plan to use to blur the signal of a SN Ia rate enhancement in GCs in our deep HST images to constrain the faint-end slope of the radial distribution of SNe Ia. 4.1.3. Rates Summary LSST itself will be able to provide this because its deep stack images have a projected detection limit of r ∼ 27.5 We find that the implied SN Ia rate enhancements in 5 either dwarf galaxies or GCs do not conflict with exist- mag . ing observations or theoretical predictions, but also do Until then, we can only caution that future surveys not provide a means to constrain the nature of object F. will have to consider that 25–30% of apparently hostless Additionally, if either of the 2 SNe Ia in Abell 1650 and SNe Ia might not belong to the IC stellar population. Is Abell 85 – which appear hostless in our deep HST images that too large for hostless SNe Ia to be scientifically use- – were in fact hosted by the ∼ 0.2% of the stellar mass ful tracers of fICL at higher redshifts? Some numerical in small galaxies that remains below our limiting magni- simulations indicate that fICL grows with cosmological tudes, then by Equation 4 this indicates a rate enhance- time, and by z ∼ 0 is ∼ 2× larger than at z ∼ 0.4 (e.g. ment by a factor of ∼ 55 in the faintest dwarf galaxies. Murante et al. 2007); others find that most IC stars This is conspicuously high and would have been noticed are stripped at z > 1, in which case fICL would remain in wide-field surveys. We therefore conclude that these 2 constant since then (Puchwein et al. 2010). Direct mea- SNe Ia were truly hosted by the population of intraclus- surements of this low surface brightness component have ter stars stripped from their host galaxy and residing in shown that fICL . 25% at z . 0.1 and ∼ 10% at z ∼ 0.2 the cluster’s gravitational potential. (e.g. Gonzalez et al. 2007; Zibetti et al. 2005), but HST imaging of 0.4 REFERENCES Agulli, I., Aguerri, J. A. 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