Small-scale systems of galaxies. IV. Searching for the faint galaxy population associated with X-ray detected isolated E+S pairs

In hierarchical evolutionary scenarios, isolated, physical pairs may represent an intermediate phase, or "way station", between collapsing groups and isolated elliptical (E) galaxies (or fossil groups). We started a comprehensive study of a sample of galaxy pairs composed of a giant E and a spiral (S) with the aim of investigating their formation/evolutionary history from observed optical and X-ray properties. Here we present VLT-VIMOS observations designed to identify faint galaxies associated with the E+S systems from candidate lists generated using photometric criteria on WFI images covering an area of ~ 0.2 h^{-1} Mpc radius around the pairs. The results are discussed in the context of the evolution of poor galaxy group associations. A comparison between the Optical Luminosity Functions (OLFs) of our E+S systems and a sample of X-ray bright poor groups suggest that the OLF of X-ray detected poor galaxy systems is not universal. The OLF of our X-ray bright systems suggests that they are more dynamically evolved than our X-ray faint sample and some X-ray bright groups in the literature. However, we suggest that the X-ray faint E+S pairs represent a phase in the dynamical evolution of some X-ray bright poor galaxy groups. The recent or ongoing interaction in which the E member of the X-ray faint pairs is involved could have decreased the luminosity of any surrounding X-ray emitting gas.Comment: accepted for publication in Astronomy and Astrophysic

Nature vs. nurture in the low-density environment: structure and evolution of early-type dwarf galaxies in poor groups

We present the stellar population properties of 13 dwarf galaxies residing in poor groups (low-density environment, LDE) observed with VIMOS@VLT. Ages, metallicities, and [alpha/Fe] ratios were derived from the Lick indices Hbeta, Mgb, Fe5270 and Fe5335 through comparison with our simple stellar population (SSP) models accounting for variable [alpha/Fe] ratios. For a fiducial subsample of 10 early-type dwarfs we derive median values and scatters around the medians of 5.7 \pm 4.4 Gyr, -0.26 \pm 0.28, and -0.04 \pm 0.33 for age, log Z/Zsun, and [alpha/Fe], respectively. For a selection of bright early-type galaxies (ETGs) from the Annibali et al.2007 sample residing in comparable environment we derive median values of 9.8 \pm 4.1 Gyr, 0.06 \pm 0.16, and 0.18 \pm 0.13 for the same stellar population parameters. It follows that dwarfs are on average younger, less metal rich, and less enhanced in the alpha-elements than giants, in agreement with the extrapolation to the low mass regime of the scaling relations derived for giant ETGs. From the total (dwarf + giant) sample we derive that age \propto sigma^{0.39 \pm 0.22}, Z \propto sigma^{0.80 \pm 0.16}, and alpha/Fe \propto sigma^{0.42 \pm 0.22}. We also find correlations with morphology, in the sense that the metallicity and the [alpha/Fe] ratio increase with the Sersic index n or with the bulge-to-total light fraction B/T. The presence of a strong morphology-[alpha/Fe] relation appears to be in contradiction to the possible evolution along the Hubble sequence from low B/T (low n) to high B/T (high n) galaxies. We also investigate the role played by environment comparing the properties of our LDE dwarfs with those of Coma red passive dwarfs from the literature. We find possible evidence that LDE dwarfs experienced more prolonged star formations than Coma dwarfs, however larger data samples are needed to draw more firm conclusions.Comment: Accepted for publication on A&

Hot gas in groups: NGC 5328 and the intriguing case of NGC 4756 with XMM-Newton

[Abridged] NGC 5238 and NGC 4756 are the brightest unperturbed elliptical galaxies in their respective loose groups. In the present study we aim at characterizing the properties of the hot gas in the halos of the brightest members and in the environment. In NGC 4756 we are also interested in the properties of a substructure identified to the SW and the region connecting the two structures, to search for a physical connection between the two. However, we have to take into account the fact that the group is projected against the bright, X-ray emitting cluster A1361, which heavily contaminates and confuses the emission from the foreground structure. We present a careful analysis of XMM-Newton data of the groups to separate different components. We also present a re-evaluation of the dynamical properties of the systems and . SPH simulations to interpret the results. We find that the X-ray source associated with NGC 4756 indeed sits on top of extended emission from the background cluster A1361, but can be relatively well distinguished from it as a significant excess over it out to r\sim150"\ (~40 kpc). NGC 4756 has an X-ray luminosity of ~10^41 erg/s due to hot gas, with an average temperature of kT\sim0.7 keV. We measure a faint diffuse emission also in the region of the subclump to the SW, but more interestingly, we detect gas between the two structures, indicating a possible physical connection. The X-ray emission from NGC 5328 is clearly peaked on the galaxy, also at 10^41 erg/s, and extends to r\sim110 kpc. Simulations provide an excellent reproduction of the SED and the global properties of both galaxies, which are caught at two different epochs of the same evolutionary process, with NGC 5328 ~2.5 Gyr younger than NGC 4756.Comment: Accepted for publication, Astronomy and Astrophysic

Small-scale systems of galaxies. II. Properties of the NGC 4756 group of galaxies

We present results from a study of the NGC 4756 group which is dominated by the elliptical galaxy NGC 5746. The characteristics of the group are investigated through (a) the detailed investigation of the morphological, photometric and spectroscopic properties of nine galaxies among the dominant members of the group (b) the determination of the photometric parameters of the faint galaxy population in an area of 34'x34' centered on NGC 4756 and (c) an analysis of the X-ray emission in the area based upon archival data. The 9 member galaxies are located in the core part of the NGC 4756 group (a strip diameter about 300 kpc in diameter which has a very loose configuration. The central part of the NGC 4756 group contains a significant fraction of early-type galaxies. Three new group members with previously unknown systemic velocities are identified, one of which is a dE. At about 7.5' SW of NGC 4756 a sub-structure of the group is detected, including IC 829, MCG -2-33-35, MCG -2-33-36 and MCG -2-33-38, which meets the Hickson criteria for being a compact group. Most of the galaxies in this sub-structure show interaction signatures. We do not detect apparent fine structure and signatures of recent interaction events in the early-type galaxy population, with the exception of a strong dust lane in the elliptical MCG -2-33-38. This galaxy displays however signatures of nuclear activity. Strong [O III], [N II] and [S II] line emission, combined with comparatively weak, but broad H alpha emission suggest an intermediate Seyfert type classification. Although the area is heavily contaminated by the background cluster Abell 1631, X-ray data suggest the presence of a hot intergalactic medium related to the group to the X-ray emission detected. The present results are discussed in the context of group evolution.Comment: 45 pages including 10 figures and 8 tables accepted for publication in A

The relationship between star formation rates, local density and stellar mass up to z ~ 3 in the GOODS NICMOS Survey

We investigate the relation between star formation rates and local galaxy environment for a stellar mass selected galaxy sample in the redshift range 1.5 < z < 3. We use near-infra-red imaging from an extremely deep Hubble Space Telescope survey, the GOODS-NICMOS Survey (GNS) to measure local galaxy densities based on the nearest neighbour approach, while star-formation rates are estimated from rest-frame UV-fluxes. Due to our imaging depth we can examine galaxies down to a colour-independent stellar mass completeness limit of log M\ast = 9.5 M\odot at z ~ 3. We find a strong dependence of star formation activity on galaxy stellar mass over the whole redshift range, which does not depend on local environment. The average star formation rates are largely independent of local environment apart from in the highest relative over-densities. Galaxies in over-densities of a factor of > 5 have on average lower star formation rates by a factor of 2 - 3, but only up to redshifts of z ~ 2. We do not see any evidence for AGN activity influencing these relations. We also investigate the influence of the very local environment on star-formation activity by counting neighbours within 30 kpc radius. This shows that galaxies with two or more close neighbours have on average significantly lower star formation rates as well as lower specific star formation rates up to z ~ 2.5. We suggest that this might be due to star formation quenching induced by galaxy merging processes.Comment: 12 pages, 6 figures, accepted for publication in MNRA

A WFC3 Grism Emission Line Redshift Catalog in the GOODS-South Field

We combine HST/WFC3 imaging and G141 grism observations from the CANDELS and 3D-HST surveys to produce a catalog of grism spectroscopic redshifts for galaxies in the CANDELS/GOODS-South field. The WFC3/G141 grism spectra cover a wavelength range of 1.1<lambda<1.7 microns with a resolving power of R~130 for point sources, thus providing rest-frame optical spectra for galaxies out to z~3.5. The catalog is selected in the H-band (F160W) and includes both galaxies with and without previously published spectroscopic redshifts. Grism spectra are extracted for all H-band detected galaxies with H<24 and a CANDELS photometric redshift z_phot > 0.6. The resulting spectra are visually inspected to identify emission lines and redshifts are determined using cross-correlation with empirical spectral templates. To establish the accuracy of our redshifts, we compare our results against high-quality spectroscopic redshifts from the literature. Using a sample of 411 control galaxies, this analysis yields a precision of sigma_NMAD=0.0028 for the grism-derived redshifts, which is consistent with the accuracy reported by the 3D-HST team. Our final catalog covers an area of 153 square arcmin and contains 1019 redshifts for galaxies in GOODS-S. Roughly 60% (608/1019) of these redshifts are for galaxies with no previously published spectroscopic redshift. These new redshifts span a range of 0.677 < z < 3.456 and have a median redshift of z=1.282. The catalog contains a total of 234 new redshifts for galaxies at z>1.5. In addition, we present 20 galaxy pair candidates identified for the first time using the grism redshifts in our catalog, including four new galaxy pairs at z~2, nearly doubling the number of such pairs previously identified.Comment: 25 Pages, 9 Figures, submitted to A

Small Scale Systems of Galaxies. III. X-ray detected E+S galaxy pairs in low density environments

We present a comprehensive study of the local environments of four E+S galaxy pairs with the main goal to investigate their formation/evolution histories. New XMM-Newton data were obtained for two pairs (RR 143 and RR 242) that complements existing ROSAT data for the other two (RR 210 and RR 216). The new observations reveal diffuse X-ray emission in both pairs. The emission is asymmetric in both cases and extends out to 120 kpc and 160 kpc in RR 143 and RR 242 respectively. The nucleus of RR 242 hosts a low luminosity mildly absorbed AGN. We find that the early-type components of pairs with diffuse hot gas appear to be relaxed objects while those in RR 210 and RR 216, where no diffuse emission has been found, display unambiguous signatures of ongoing interaction. Wide-field V and R-band data are used to study the photometric properties of the early-type components and to search for a candidate faint galaxy populations around each of the pairs. While no diffuse optical light is found for any of the pairs, all of the early-type members show very extended and concentric luminous envelopes. We identify a faint galaxy sample in each field and we consider whether they could be physically associated with the luminous pairs based upon (V-R) colors and photometric properties. We find that the distribution of r_e and M_R for the candidates are similar in three of the fields (RR 143, 216 and 242). The same selection criteria applied to the field of RR 210 suggest a fainter and more compact population possibly suggesting a larger background fraction than in the other fields.Comment: 47 pages, 18 figures, Astron. J. in pres

The GALEX UV emission in shell galaxies: tracing galaxy "rejuvenation" episodes

We present the GALEX far FUV and near NUV ultraviolet imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135. The system of shells and fine structures visible in the optical is detected in the NUV image of NGC 2865 and in both NUV and FUV images of NGC 7135. The NUV image of NGC 5018 does not present shell structures. We detect absorption features in the nuclear region of all three galaxies. NGC 2865 has a nearly flat colour profile with (FUV-NUV)~2 throughout the whole galaxy. NGC 7135 is blue in the center (FUV-NUV)~0 and as red as (FUV-NUV)~1.5 in the outskirts, including the faint shell-like feature. We investigate the ability of the nuclear GALEX (FUV-NUV) colour to provide information about rejuvenation phenomena in the stellar populations of the shell galaxies. To this aim, we derive from theory the relationship between the Mg2, Hbeta, HgammaA, HdeltaA Lick line-strength indices and the (FUV-NUV) colour. We extend the study to a sample of early-type galaxies with emission lines in their optical spectra (Annibali et al. 2007). In the index vs.(FUV-NUV) colour diagrams, most of the galaxies are well explained by passively evolving SSPs. On the average, ages and metallicities of the galaxies in our sample estimated from optical line-strength indices are consistent with those inferred from the (FUV-NUV) colour. In general, all the colours but for (FUV-NUV) and (FUV-V), become nearly age insensitive when 1-2 Gyr have elapsed from the last star forming event. Considering composite stellar population models with a recent burst of star formation, we suggest that the position of the NGC 7135 and NGC 2865 nuclei in the (FUV-NUV)-Hbeta plane could be explained in term of a recent rejuvenation episode. (Abridged)Comment: Accepted for publication in MNRAS Main Journal, 21 pages, 15 figure

The Tumultuous Formation of the Hubble Sequence at z > 1 Examined with HST/WFC3 Observations of the Hubble Ultra Deep Field

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 locally defined definition of disk and elliptical galaxies based on structure and spectral-type, we find no true ellipticals at z > 2, and a fraction of 3.2+/-2.3% at 1.5 < z < 2. Local counterparts of disk galaxies are at a similar level of 7-10%, much lower than the 75% fraction at lower redshifts. We further compare WFC3 images with the rest-frame UV view of galaxies from ACS imaging, showing that galaxies imaged with ACS that appear peculiar often contain an `elliptical' like morphology in WFC3. We show through several simulations that this larger fraction of elliptical-like galaxies is partially due to the courser PSF of WFC3, and that the `elliptical' class very likely includes early-type disks. We also measure the merger history for our sample using CAS parameters, finding a redshift evolution increasing with redshift, and a peak merger fraction of ~30% at z~2 for the most massive galaxies with M_*> 10^{10} M_sol, consistent with previous results from ACS and NICMOS. We compare our results to semi-analytical model results and find a relatively good agreement between our morphological break-down and the predictions. Finally, we argue that the peculiars, ellipticals and peculiar ellipticals have similar properties, suggesting similar formation modes, likely driven by major mergers.Comment: 21 pages, submitted to MNRA

Nearby early-type galaxies with ionized gas VI. The Spitzer-IRS view

We present low resolution Spitzer-IRS spectra of 40 ETGs, selected from a sample of 65 ETGs showing emission lines in their optical spectra. We homogeneously extract the mid-infrared (MIR) spectra, and after the proper subtraction of a "passive" ETG template, we derive the intensity of the ionic and molecular lines and of the polycyclic aromatic hydrocarbon emission features. We use MIR diagnostic diagrams to investigate the powering mechanisms of the ionized gas. The mid-infrared spectra of early-type galaxies show a variety of spectral characteristics. We empirically sub-divide the sample into five classes of spectra with common characteristics. Class-0, accounting for 20% of the sample, are purely passive ETGs with neither emission lines nor PAH features. Class-1 show emission lines but no PAH features, and account for 17.5% of the sample. Class-2, in which 50% of the ETGs are found, as well as having emission lines, show PAH features with unusual ratios, e.g. 7.7 {\mu}m/11.3 {\mu}m \leq 2.3. Class-3 objects have emission lines and PAH features with ratios typical of star-forming galaxies. 7.5% of objects fall in this class, likely to be objects in a starburst/post-starburst regime. Class-4, containing only 5% of the ETGs, is dominated by a hot dust continuum. The diagnostic diagram [Ne III]15.55{\mu}m/[Ne II]12.8{\mu}m vs. [S III]33.48{\mu}m/[Si II]34.82{\mu}m, is used to investigate the different mechanisms ionizing the gas. If we exclude NGC 3258 where a starburst seems present, most of our ETGs contain gas ionized via either AGN-like or shock phenomena, or both. Most of the spectra in the present sample are classified as LINERs in the optical window. The proposed MIR spectral classes show unambiguously the manifold of the physical processes and ionization mechanisms, from star formation, low level AGN activity, to shocks, present in LINER nuclei.Comment: Accepted for publication in Astronomy and Astrophysic