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Mergers of gas-poor galaxies, so-called dry mergers, may play a fundamental role in the assembly of the most massive galaxies, and therefore, in galaxy formation theories. Using the Sloan Digital Sky Survey, we have serendipitously discovered a rare system in the observational and theoretical context, possibly a quintuple dry merger at low redshift. As a follow-up, we have obtained Nordic Optical Telescope long-slit spectra of the group, in order to measure the individual redshifts and gain insight into its merger fate. Our results show an isolated, low-redshift galaxy group consisting of massive, quiescent, early-type galaxies, composed of two clumps (possibly themselves in the process of merging), which we estimate will hypothetically merge in roughly less than a Gyr. With the possible exception of the high line-of-sight velocity dispersion, the overall properties of the system may be comparable to a compact Shakhbazyan group. However, when the small projected separations and relative mass ratios of the galaxies are taken into account in cosmological simulations, we find that this system is rather unique. We hypothesize that this group is a dry merger, whose fate will result in the assembly of an isolated, massive elliptical galaxy at low redshift. [ABSTRACT FROM PUBLISHER]

We use the United Kingdom Infrared Telescope Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS), the deepest degree-scale near-infrared survey to date, to investigate the clustering of star-forming and passive galaxies to z ∼ 3.5. Our new measurements include the first determination of the clustering for passive galaxies at z > 2, which we achieve using a cross-correlation technique. We find that passive galaxies are the most strongly clustered, typically hosted by massive dark matter haloes with Mhalo > 5 × 1012 M⊙ irrespective of redshift or stellar mass. Our findings are consistent with models in which a critical halo mass determines the transition from star-forming to passive galaxies. Star-forming galaxies show no strong correlation between stellar mass and halo mass, but passive galaxies show evidence for an anticorrelation; low-mass passive galaxies appear, on average, to be located in the most massive haloes. These results can be understood if the termination of star formation is most efficient for galaxies of low stellar mass in very dense environments. [ABSTRACT FROM AUTHOR]

ABSTRACT 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-infrared 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 ultraviolet fluxes. Due to our imaging depth, we can examine galaxies down to a colour-independent stellar mass completeness limit of log M*= 9.5 M⊙ 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 those in the highest relative overdensities. Galaxies in overdensities 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 active galactic nucleus 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. [ABSTRACT FROM AUTHOR]

ABSTRACT We examine in this paper a stellar-mass-selected sample of galaxies at 1 < z < 3 within the Hubble Ultra Deep Field, utilizing Wide-Field Camera-3 (WFC3) imaging, to study the rest-frame optical morphological and structural distribution of galaxies at this epoch. We measure how apparent morphologies (disc, elliptical, peculiar) correlate with physical properties, such as quantitative structure and spectral type. 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 disc 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 per cent at 1.5 < z < 2. Local counterparts of disc galaxies are at a similar level of 7-10 per cent, much lower than the 75 per cent fraction at lower redshifts. We further compare WFC3 images with the rest-frame ultraviolet view of galaxies from ACS imaging, showing that galaxies imaged with the ACS, which appear peculiar, often contain an 'elliptical'-like morphology in the WFC3. We show through several simulations that this larger fraction of elliptical-like galaxies is partially due to the courser point spread function of the WFC3, and that the 'elliptical' morphological class very likely includes early-type discs. We also measure the merger history for our sample using concentration, asymmetry and clumpiness parameters, finding a redshift evolution increasing with redshift, and a peak merger fraction of ∼30 per cent at z∼ 2 for the most massive galaxies with M* > 1010 M⊙, consistent with previous results from the ACS and NICMOS. We also compare our results to semi-analytical model results and find a relatively good agreement between our morphological breakdown and the predictions. We finally argue that galaxies classified visually as peculiar, elliptical or peculiar ellipticals, all have similar structural and stellar population properties, suggesting that these galaxies are in a similar formation mode, likely driven by major mergers. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a study of the star-forming properties of a stellar mass-selected sample of galaxies in the GOODS (Great Observatories Origins Deep Survey) NICMOS Survey (GNS), based on deep Hubble Space Telescope ( HST) imaging of the GOODS North and South fields. Using a stellar mass-selected sample, combined with HST/ACS and Spitzer data to measure both ultraviolet (UV) and infrared-derived star formation rates (SFRs), we investigate the star forming properties of a complete sample of ∼1300 galaxies down to log M*= 9.5 at redshifts 1.5 < z < 3. Eight per cent of the sample is made up of massive galaxies with M*≥ 1011 M⊙. We derive optical colours, dust extinctions and UV and infrared SFR to determine how the SFR changes as a function of both stellar mass and time. Our results show that SFR increases at higher stellar mass such that massive galaxies nearly double their stellar mass from star formation alone over the redshift range studied, but the average value of SFR for a given stellar mass remains constant over this ∼2 Gyr period. Furthermore, we find no strong evolution in the SFR for our sample as a function of mass over our redshift range of interest; in particular we do not find a decline in the SFR among massive galaxies, as is seen at z < 1. The most massive galaxies in our sample (log M*≥ 11) have high average SFRs with values SFRUV, corr= 103 ± 75 M⊙ yr−1, and yet exhibit red rest-frame ( U− B) colours at all redshifts. We conclude that the majority of these red high-redshift massive galaxies are red due to dust extinction. We find that A2800 increases with stellar mass, and show that between 45 and 85 per cent of massive galaxies harbour dusty star formation. These results show that even just a few Gyr after the first galaxies appear, there are strong relations between the global physical properties of galaxies, driven by stellar mass or another underlying feature of galaxies strongly related to the stellar mass. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the details and early results from a deep near-infrared survey utilizing the NICMOS instrument on the Hubble Space Telescope centred around massive M* > 1011 M ⊙ galaxies at 1.7 < z < 2.9 found within the Great Observatories Origins Deep Survey (GOODS) fields North and South. The GOODS NICMOS Survey (GNS) was designed to obtain deep F160W ( H-band) imaging of 80 of these massive galaxies and other colour-selected objects such as Lyman-break dropouts, BzK objects, distant red galaxies (DRGs), extremely red objects (EROs), Spitzer-selected EROs, BX/BM galaxies, as well as flux-selected submillimetre galaxies. We present in this paper details of the observations, our sample selection, as well as a description of the properties of the massive galaxies found within our survey fields. This includes photometric redshifts, rest-frame colours and stellar masses. We furthermore provide an analysis of the selection methods for finding massive galaxies at high redshifts, including colour-selection methods and how galaxy populations selected through these colour methods overlap. We find that a single colour selection method cannot locate all of the massive galaxies, with no one method finding more than 70 per cent. We however find that the combination of these colour methods finds nearly all of the massive galaxies that would have been identified in a photometric redshift sample, with the exception of apparently rare blue massive galaxies. By investigating the rest-frame ( U− B) versus MB diagram for these galaxies, we furthermore show that there exists a bimodality in colour-magnitude space at z < 2, driven by stellar mass, such that the most massive galaxies are systematically red up to z∼ 2.5, while lower mass galaxies tend to be blue. We also discuss the number densities for galaxies with stellar masses M* > 1011 M ⊙, whereby we find an increase of a factor of 8 between z= 3 and 1.5, demonstrating that this is an epoch when massive galaxies establish most of their stellar mass. We also provide an overview of the evolutionary properties of these galaxies, such as their merger histories, and size evolution. [ABSTRACT FROM AUTHOR]

We present the Galaxy Evolution Explorer ( GALEX) far-ultraviolet (FUV) and near-ultraviolet (NUV) imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135 located in low-density environments. 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 centre (FUV−NUV) ≈ 0 and as red as (FUV−NUV) ≈ 1.5 in the outskirts, including the faint shell-like feature. The three shell galaxies are members of poor groups of galaxies. We compare GALEX NUV observations with available H i large-scale measurements, and determine the UV magnitudes of likely companions. Most of the known (and possible) companions are gas-rich late-type galaxies, suggesting that our shell galaxies inhabit the ideal environment for hosting rejuvenating episodes driven by accretion events. 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, Hβ, HγA, HδA Lick line-strength indices and the (FUV−NUV) colour. We extend the study to a sample of early-type galaxies in low-density environments which includes shell galaxies and/or galaxies with emission lines in their optical spectra. In the index versus (FUV−NUV) colour diagrams, most of the galaxies are well explained by passively evolving single stellar populations. 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. We note that the GALEX (FUV−NUV) and (UV−V) colours have different response to age and metallicity. 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. Finally, considering composite stellar population models with a recent burst of star formation, we suggest that the positions of the NGC 7135 and NGC 2865 nuclei in the (FUV−NUV)–Hβ plane could be explained in terms of a recent rejuvenation episode. [ABSTRACT FROM AUTHOR]

We present the results of a photometric and spectroscopic study of seven members of the NGC 5328 group of galaxies, a chain of galaxies spanning over 200 kpc . We analyse the galaxy structure and study the emission-line properties of the group members looking for signatures of star formation and active galactic nucleus activity. We finally attempt to infer, from the modelling of line-strength indices, the stellar population ages of the early-type members. We investigate also the presence of a dwarf galaxy population associated with the bright members. The group is composed of a large fraction of early-type galaxies including NGC 5328 and 5330, two ‘bona fide’ ellipticals at the centre of the group. In both galaxies no recent star formation episodes are detected by the Hβ versus MgFe line-strength indices of these galaxies. 2MASX J13524838−2829584 has extremely boxy isophotes which are believed to be connected to a merging event: line-strength indices suggest that this object probably had a recent star formation episode. A warped disc component emerges from the model subtracted image of 2MASX J13530016−2827061, which is interpreted as a signature of an ongoing interaction with the rest of the group. Ongoing star formation and nuclear activity are present in the projected outskirts of the group. The two early-type galaxies 2MASX J13523852−2830444 and 2MASX J13525393−2831421 show spectral signatures of star formation, while a Seyfert 2 type nuclear activity is detected in MCG–5-33-29. [ABSTRACT FROM AUTHOR]

It is now clear that the epoch when the Universe was half of its current age is a crucial period during which galaxies assembled their mass and evolved into the galaxies we observe in the local Universe. However, so far only very few direct studies of mass assembly in action, hence galaxy merging, were conducted over z=1 and usually relied on very small or biased samples. Based on very deep near infrared survey data, the latest UKIDSS-UDS DR8, combined with the optical data conducted by Subaru and CFHT and Spitzer IRAC observations, we explored the evolution of the merging rate up to z = 2, over the largest volume of the Universe at 0.4