Probing the mass assembly of massive nearby galaxies with deep imaging
aa r X i v : . [ a s t r o - ph . C O ] F e b The intriguing life of massive galaxiesProceedings IAU Symposium No. 295, 2013D. Thomas, A. Pasquali & I. Ferreras, eds. c (cid:13) Probing the mass assembly of massivenearby galaxies with deep imaging
P.–A. Duc, J.-C. Cuillandre, K. Alatalo, L. Blitz, M. Bois, F.Bournaud, M. Bureau, M. Cappellari, P. Cˆot´e, R. L. Davies, T.A. Davis, P. T. de Zeeuw, , E. Emsellem, , L. Ferrarese, E.Ferriere, S. Gwyn, S. Khochfar, D. Krajnovic, H. Kuntschner, P.-Y. Lablanche, R. M. McDermid, L. Michel-Dansac, R.Morganti, T. Naab, T. Oosterloo, M. Sarzi, N. Scott, P.Serra, A. Weijmans and L. M. Young AIM Paris-Saclay, France; CFHT, USA; University of California, Berkeley, USA; Observatoire de Lyon, France; University of Oxford, UK; Herzberg Institute ofAstrophysics, Victoria, Canada; ESO, Garching, Germany; Leiden University, TheNetherlands; MPE, Garching, Germany; Gemini Observatory, Hilo, USA; ASTRON,Dwingeloo, The Netherlands; MPI, Garching, Germany; University of Hertfordshire,Hatfield, UK; Dunlap Institute for Astronomy & Astrophysics, University of Toronto,Canada; New Mexico Tech, Socorro, USA
Abstract.
According to a popular scenario supported by numerical models, the mass assem-bly and growth of massive galaxies, in particular the Early-Type Galaxies (ETGs), is, belowa redshift of 1, mainly due to the accretion of multiple gas–poor satellites. In order to get ob-servational evidence of the role played by minor dry mergers, we are obtaining extremely deepoptical images of a complete volume limited sample of nearby ETGs. These observations, donewith the CFHT as part of the ATLAS , NGVS and MATLAS projects, reach a stunning 28.5– 29 mag.arcsec − surface brightness limit in the g’ band. They allow us to detect the relics ofpast collisions such as faint stellar tidal tails as well as the very extended stellar halos whichkeep the memory of the last episodes of galactic accretion. Images and preliminary results fromthis on-going survey are presented, in particular a possible correlation between the fine structureindex (which parametrizes the amount of tidal perturbation) of the ETGs, their stellar mass,effective radius and gas content. Keywords. galaxies: evolution, galaxies: interactions, galaxies: elliptical and lenticular, cD
1. Introduction
Early-Type Galaxies play a key role in modern cosmology: according to the standardhierarchical cosmological model, galaxies build up from successive mergers, associatedby a series of morphological transformations. The massive ETGs we see today are theend-product of this process.
At high redshifts , few ETGs are observed, but surprisingthey have not all disappeared, raising questions on how they formed so quickly in thetraditional merging scenario. They appear to be also very compact (e.g. Buitrago et al.,2008).
At low redshift , the early-type galaxies are observed to be larger but also to bemuch more complex and lively that once believed. As shown in the presentations of theATLAS results in this volume, a large fraction of them contain in particular gas, somewith non-regular kinematics, and thus may still be involved in active transformationprocesses.Simulations indicate that while the global light profile of ETGs can be rather easilyobtained with various processes, getting their total mass and large radius is much more1 Duc et al.challenging and may require multiple collisions in the recent past. Several recent papershave highlighted the role of minor mergers in the growth of galaxies (e.g., Johansson etal., 2012; Newman et al., 2012). These late mergers impact the properties of the stellarpopulations mostly at large galactocentric radii. In particular minor mergers bring lowmetallicity stars from the infalling dwarf satellites, and create radial color gradients. Onthe other hand, a major merger induces large radial mixing, leading to a washing up ofmetallicity/age ad thus color gradients.Furthermore, the mass assembly of galaxies leaves various imprints in their surround-ings, such as shells, streams and tidal tails. The frequency, shape and properties of thesefine structures depend on the mechanism driving the mass assembly (see the review byDuc & Renaud, 2013). Any analysis of the fine structures around galaxies, however,should take into account (1) that the resulting stellar debris have a very low surfacebrightness, (2) that such debris fade with age, or may be dispersed by the local environ-ment, and (3) that galaxies may have formed by multiple processes.Deep imaging surveys coupled with numerical simulations done in cosmological contextcan now address these issues. Several studies have quantified the importance of finestructures around massive galaxies and their variation with environment and redshift(Tal et al., 2009; Bridge et al., 2010; Adams et al., 2012) but were restricted to thecensus of luminous tidal features. So far, the surface brightness limit required to apply agenuine galactic archeology technique has been reached for only local galaxies for whichstellar counts may be done. As however shown, among others, by Martinez-Delgado etal. (2010) and Duc et al. (2011), deep optical images obtained under specific conditionscan also reveal the diffuse light associated with very low-surface brightness structures.
2. Observations and data reduction
Our targets are the 260 nearby ETGs located at distances below 42 Mpc from theATLAS survey (Cappellari et al., 2011). Very deep optical images in the g’,r’ andi’ bands are currently being obtained with the large field of view camera MegaCaminstalled on the CFHT. The observations are carried out as part of several projects:ATLAS , the Next Generation Virgo Cluster Survey (NGVS, covering the full VirgoCluster area; Ferrarese et al., 2012), and the recently accepted CFHT MATLAS LargeProgramme. The typical integration time is about one hour per band. The limitingsensitivity is outstanding with respect to previous generations of optical images: about28.5 – 29 mag.arcsec − in the g’–band. This could be achieved using dedicated observingstrategies and data reduction softwares. On traditional images obtained by MegaCam,the presence of scattered light patterns masks extended features below surface brightnessof 27 mag.arcsec − . Investigations motivated by the NGVS have shown that this problemcan be overcome carrying out a sequence of observations with large offsets between theimages, as it is usually done with infrared observations. A sky is then computed andsubtracted from the individual images, before they are stacked.The majority of the fine structures disclosed by the survey are very extended anddirectly show up on surface brightness or color maps. To disclose those located more to-wards the inner regions, several methods were used: unsharp masking, best at revealingsharp-edged structures such as shells or narrow filaments; subtraction of the ETGs mod-eled by an ellipse fitting or by a multi-component GALFIT model, which helps to detectextended asymmetric features. As a first step to quantify the amount of tidal perturba-tions, a fine structure index was determined, counting by eye the number of structures,and weighting them according to their nature. eep imaging of massive galaxies
3. Preliminary results
Figure 1 illustrates the variety of low surface brightness structures detected aroundthe ETGs: 200 kpc long tidal tails revealing past 2-3 Gyr old major merger (for instancearound NGC 5557; Duc et al., 2011), narrow filaments around a disrupted dwarf, withtheir typical S-shape and wrapping around the host, diffuse halos, some remaining regulareven up to large radii.At time of writing, more than half of the ATLAS ETGs benefit from deep Mega-Cam images. This initial sub-sample is however somehow biased towards the most mas-sive ETGs, those that are slow rotating and/or gas–rich. In such conditions, providingthe percentage of disturbed galaxies is premature. Nevertheless some initial trends werefound.First of all, statistically, the galaxies that contain atomic hydrogen, in particular intheir outskirts, have a higher fine structure index. This is a strong indication that theHI clouds are associated with collisional debris. A rather large fraction of ATLAS ETGs contain detectable molecular gas, as traced by CO. Those for which the CO iskinematically misaligned with respect to the stellar component and for which an externalorigin of the molecular gas had been proposed (Davis et al, 2011), exhibit collisionaldebris, just as expected. Finally one of the most promising results is the trend withmass and size, two fundamental parameters in the scaling laws of galaxies. As shownon Figure 2, the more massive and the more extended † the galaxy is, the higher its finestructure index. Conversely, many, usually fast rotating low–mass, ETGs do not showany sign of external perturbation. This tells that the most massive galaxies in the localUniverse have had a rich recent mass accretion history. At which level it accounts fortheir mass/size growth remains to be determined. Since the optical images only revealstructures brighter than 29 mag.arcsec − , simulations will be key to extrapolate from theobservations the quantity of accreted material since z = 1. Another crucial parameterwill be the survival time of tidal features, which likely depends on the environment. Thestudy of the fine structure index as a function of the local density – our volume limitedsample covers a large range of environments: field, groups and the Virgo Cluster – willgive interesting constrains on that matter. References
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Figure 1.
Deep optical images of a sub-sample of nearby Early-Type Galaxies obtained withMegaCam on the CFHT as part of the ATLAS and Next Generation Virgo Cluster Survey.The figure illustrates the variety of low surface brightness structures that show up around thesegalaxies: long tidal tails and shells, telling us about past major mergers (a,d); narrow stellarfilaments associated with disrupted dwarf satellites, revealing future minor mergers (b); regularlow surface brightness star–forming disks (c); extended featureless stellar halos (e). Figure 2.
Effective radius, R max e , vs M JAM , a proxy of the stellar mass, for the full sampleof ATLAS ETGs (open circles, Cappellari et al., 2012). The galaxies shown with the halffilled circles have already deep MegaCam images available from either the ATLAS or NGVSsurveys. The images for the other galaxies will be obtained as part of the MATLAS CFHT LargeProgramme. Those that have a high fine structure index, i.e. galaxies that are either stronglytidally perturbed, or have multiple stellar streams in their vicinity, are shown with the red filledcircles.or NGVSsurveys. The images for the other galaxies will be obtained as part of the MATLAS CFHT LargeProgramme. Those that have a high fine structure index, i.e. galaxies that are either stronglytidally perturbed, or have multiple stellar streams in their vicinity, are shown with the red filledcircles.