It has been suggested that merging plays an important role in the formation and the evolution of elliptical galaxies(EGs). In order to understand the past merging history of EGs, we studied the optical-NIR color gradients of 204 EGs. The use of the optical and the NIR data provides large wavelength baselines, and breaks the age-metallicity(Z) degeneracy, allowing us to derive age and Z gradients. The use of the deep SDSS Stripe 82 images make it possible for us to examine how the color/age/Z gradients are related to merging features. We find that the optical-NIR color and the age/Z gradients of EGs with tidal features are consistent with those of relaxed EGs suggesting that the two populations underwent a similar merging history on average and that mixing of stars was more or less completed before the tidal features disappear. EGs with dust features have steeper color gradients than the other two types, even after masking out dust features during the analysis, which can be due to a process involving wet merging. More importantly, we find that the scatter in the color/age/Z gradients of the relaxed and merging feature types, decreases as their luminosities increase at M > 10^{11.4} M_{\odot} but stays to be large at lower luminosities. Mean Z gradients appear nearly constant over the explored mass range, but a possible flattening is observed at the massive end. According to our toy model that predicts how the distribution of Z gradients changes as a result of major dry merging, the mean Z gradient should flatten by 40% and its scatter become smaller by 80% per a mass doubling scale if EGs evolve only through major dry merger. Our result, although limited by a number statistics at the massive end, is consistent with the picture that major dry merging is an important mechanism for the evolution for EGs at M > 10^{11.4} M_{\odot}, but less important at the lower mass range.
We investigate the influence of major mergers on the radio emission of elliptical galaxies. We use a complete sample of close pairs, which contains 475 merging and 1828 non-merging paired elliptical galaxies of M_r<-21.5 selected from the Sloan Digital Sky Survey. In addition, a control sample of 2000 isolated field galaxies is used for comparison. We cross-identify the optical galaxies with the radio surveys of FIRST and NVSS. We find that the radio fraction of merging paired galaxies is about 6%, which is slightly higher than the 5% obtained for non-merging paired galaxies, although these va…
[Abridged] We study the properties, evolution and formation mechanisms of isolated field elliptical galaxies. We create a mock catalogue of isolated field elliptical galaxies from the Millennium Simulation Galaxy Catalogue, and trace their merging histories. The formation, identity and assembly redshifts of simulated isolated and non-isolated elliptical galaxies are studied and compared. Observational and numerical data are used to compare age, mass, and the colour-magnitude relation. Our results, based on simulation data, show that almost seven per cent of all elliptical galaxies brighter tha…
We present a combined X-ray and optical analysis of three bimodal galaxy clusters selected as merging candidates at z ~ 0.1. These targets are part of MUSIC (MUlti--Wavelength Sample of Interacting Clusters), which is a general project designed to study the physics of merging clusters by means of multi-wavelength observations. Observations include spectro-imaging with XMM-Newton EPIC camera, multi-object spectroscopy (260 new redshifts), and wide-field imaging at the ESO 3.6m and 2.2m telescopes. We build a global picture of these clusters using X-ray luminosity and temperature maps together w…
We study the correlation between the morphological mix of cluster galaxies and the assembly history of the parent cluster by taking advantage of two independently developed semi-analytic models for galaxy formation and evolution. In our models, both the number of cluster members and that of elliptical members increase as a function of cluster mass, in such a way that the resulting elliptical fractions are approximately independent of cluster mass. The population of cluster ellipticals exhibit a marked bimodal distribution as a function of galaxy stellar mass, with a dip at masses $\sim 10^{10}…
We have explored the buildup of the local mass-size relation of elliptical galaxies using two visually classified samples. At low redshift we compiled a subsample of 2,656 elliptical galaxies from SDSS, whereas at higher redshift (up to z~1) we extracted a sample of 228 object from the HST/ACS images of the GOODS. All the galaxies in our study have spectroscopic data, allowing us to determine the age and mass of the stellar component. Using the fossil record information contained in the stellar populations of our local sample, we do not find any evidence for an age segregation at a given stell…
How did the dwarf galaxy population of present-day galaxy clusters form and grow over time? We address this question by analysing the history of dark matter subhaloes in the Millennium-II cosmological simulation. A semi-analytic model serves as the link to observations. We argue that a reasonable analogue to early morphological types or red-sequence dwarf galaxies are those subhaloes that experienced strong mass loss, or alternatively those that have spent a long time in massive haloes. This approach reproduces well the observed morphology-distance relation of dwarf galaxies in the Virgo and C…
Galaxy interactions are known to trigger starbursts. Young massive star clusters formed in interacting galaxies and mergers may become young globular clusters. The ages of these clusters can provide clues about the timing of interaction-triggered events, and thus provide an important way to reconstruct the star formation history of merging galaxies. Numerical simulations of galaxy mergers can implement different star formation rules. For instance, star formation dependent on gas density or triggered by shocks, predicts significantly different star formation histories. To test the validity of t…
We measure star-formation rates (SFRs) and specific SFRs (SSFRs) of Ks-selected galaxies from the VIDEO survey by stacking 1.4-GHz Very Large Array data. We split the sample, which spans 0 < z < 3 and stellar masses 10**8.0 < Mstellar/Msol < 10**11.5, into elliptical, irregular or starburst galaxies based on their spectral-energy distributions. We find that SSFR falls with stellar mass, in agreement with the `downsizing' paradigm. We consider the dependence of the SSFR-mass slope on redshift: for our full and elliptical samples the slope flattens, but for the irregular and starburst samples th…
We combine UV/Optical/near-IR/mid-IR data on a sample of ~240 S0 galaxies to examine various star formation related processes in them. We split the sample into bright and faint S0 galaxies based on their K band luminosity. Comparing the FUV-NUV versus NUV-K color-color diagram with a Simple Stellar Population (SSP) model shows that ellipticals and bright S0 galaxies are dominated by a stellar population of age > 10^9 years while faint S0 galaxies may contain stars as young as 10^8 years, providing evidence for relatively recent star formation activity. The strength of the 4000A break is also s…
Fossil galaxy groups are speculated to be old and highly evolved systems of galaxies that formed early in the universe and had enough time to deplete their $L^{*}$ galaxies through successive mergers of member galaxies, building up one massive central elliptical, but retaining the group X-ray halo. Considering that fossils are the remnants of mergers in ordinary groups, the merger history of the progenitor group is expected to be imprinted in the fossil central galaxy (FCG). We present for the first time radial gradients of single-stellar population (SSP) ages and metallicites in a sample of F…
The formation and evolution of elliptical galaxies (EGs) is still an open question. In particular, recent observations suggest that elliptical galaxies are not only simple spheroidal systems of old stars. In this paper we analyze a sample of elliptical galaxies selected from the Sloan Digital Sky Survey in order to study the star-forming activity in local elliptical galaxies. Among these 487 ellipticals we find that 13 EGs show unambiguous evidence of recent star-formation activity betrayed by conspicuous nebular emission lines. Using the evolutionary stellar population synthesis models and Li…
We examine in this paper a stellar mass selected sample of galaxies at 1 < z < 3 within the Hubble Ultra Deep Field, utilising WFC3 imaging to study the rest-frame optical morphological distribution of galaxies at this epoch. We measure how apparent morphologies (disk, elliptical, peculiar) correlate with physical properties, such as quantitative structure and spectral-types. One primary result is that apparent morphology does not correlate strongly with stellar populations, nor with galaxy structure at this epoch, suggesting a chaotic formation history for Hubble types at z > 1. By using a lo…
In the present study, the dependences of the morphological types of the first and second ranked group galaxies on the magnitude gap were studied. It is shown that there is no increase in the relative number of elliptical galaxies among the first and second ranked group galaxies with a large magnitude gaps (in comparison with the expected, assuming that the morphological type of these galaxies does not depend on the magnitude gap). This result contradicts the merger hypothesis. The hypothesis proposed by Ambartsumian does not contradict this result.
The discovery of a population of massive, compact and quiescent early-type galaxies has changed the view on plausible formation scenarios for the present day population of elliptical galaxies. Traditionally assumed formation histories dominated by 'single events' like early collapse or major mergers appear to be incomplete and have to be embedded in the context of hierarchical cosmological models with continuous gas accretion and the merging of small stellar systems (minor mergers). Once these processes are consistently taken into account the hierarchical models favor a two-phase assembly proc…
We report on a new merging group of galaxies, Suzaku J1552+2739 at z ~ 0.08 revealed by a SUZAKU observation. The group was found by observing a junction of galaxy filaments optically identified in the Sloan Digital Sky Survey spectroscopic data. Suzaku J1552+2739 exhibits an irregular morphology and presents several peaks in its X-ray image. A bright elliptical galaxy, observable in the central peak, allows the localization of the group at z = 0.083. We found a significant hot spot visible in the X-ray hardness map, close to the second peak. The spectroscopic temperature is T = 1.6+0.4-0.1 ke…
We examine the stellar haloes of the Auriga simulations, a suite of thirty cosmological magneto-hydrodynamical high-resolution simulations of Milky Way-mass galaxies performed with the moving-mesh code AREPO. We study halo global properties and radial profiles out to $\sim 150$ kpc for each individual galaxy. The Auriga haloes are diverse in their masses and density profiles; mean metallicity and metallicity gradients; ages; and shapes, reflecting the stochasticity inherent in their accretion and merger histories. A comparison with observations of nearby late-type galaxies shows very good agre…
We present the first systematic 1.4 GHz Very Large Array radio continuum survey of fossil galaxy group candidates. These are virialized systems believed to have assembled over a gigayear in the past through the merging of galaxy group members into a single, isolated, massive elliptical galaxy and featuring an extended hot X-ray halo. We use new photometric and spectroscopic data from SDSS Data Release 7 to determine that three of the candidates are clearly not fossil groups. Of the remaining 30 candidates, 67% contain a radio-loud (L_1.4GHz > 10^23 W Hz^-1) active galactic nucleus (AGN) at the…
Building galaxy merger trees from a state-of-the-art cosmological hydrodynamics simulation, Horizon-AGN, we perform a statistical study of how mergers and smooth accretion drive galaxy morphologic properties above $z > 1$. More specifically, we investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that smooth accretion tends to flatten small galaxies over cosmic time, leading to the formation of disks. On the other hand, mergers, and not only the major ones, exhibit a propensit…
We use the Sloan Digital Sky Survey to investigate the properties of massive elliptical galaxies in the local Universe (z\leq0.08) that have unusually blue optical colors. Through careful inspection, we distinguish elliptical from non-elliptical morphologies among a large sample of similarly blue galaxies with high central light concentrations (c_r\geq2.6). These blue ellipticals comprise 3.7 per cent of all c_r\geq2.6 galaxies with stellar masses between 10^10 and 10^11 h^{-2} {\rm M}_{\sun}. Using published fiber spectra diagnostics, we identify a unique subset of 172 non-star-forming ellipt…
The Cheshire Cat is a relatively poor group of galaxies dominated by two luminous elliptical galaxies surrounded by at least four arcs from gravitationally lensed background galaxies that give the system a humorous appearance. Our combined optical/X-ray study of this system reveals that it is experiencing a line of sight merger between two groups with a roughly equal mass ratio with a relative velocity of ~1350 km/s. One group was most likely a low-mass fossil group, while the other group would have almost fit the classical definition of a fossil group. The collision manifests itself in a bimo…
