ALBERT

Description: We describe a statistical approach for measuring the influence that a galaxy's closest companion has on the galaxy's properties out to arbitrarily wide separations. We begin by identifying the closest companion for every galaxy in a large spectroscopic sample of Sloan Digital Sky Survey galaxies. We then characterize the local environment of each galaxy by using the number of galaxies within 2 Mpc and by determining the isolation of the galaxy pair from other neighbouring galaxies. We introduce a sophisticated algorithm for creating a statistical control sample for each galaxy, matching on stellar mass, redshift, local density and isolation. Unlike traditional studies of close galaxy pairs, this approach is effective in a wide range of environments, regardless of how faraway the closest companion is (although a very distant closest companion is unlikely to have a measurable influence on the galaxy in question). We apply this methodology to measurements of galaxy asymmetry, and find that the presence of nearby companions drives a clear enhancement in galaxy asymmetries. The asymmetry excess peaks at the smallest projected separations (〈10 kpc), where the mean asymmetry is enhanced by a factor of 2.0 ± 0.2. Enhancements in mean asymmetry decline as pair separation increases, but remain statistically significant (1–2) out to projected separations of at least 50 kpc.

Description: We use the relations between aperture stellar velocity dispersion ( ap ), stellar mass ( M SPS ) and galaxy size ( R e ) for a sample of ~150 000 early-type galaxies from Sloan Digital Sky Survey/DR7 to place constraints on the stellar initial mass function (IMF) and dark halo response to galaxy formation. We build cold dark matter-based mass models that reproduce, by construction, the relations between galaxy size, light concentration and stellar mass, and use the spherical Jeans equations to predict ap . Given our model assumptions (including those in the stellar population synthesis models), we find that reproducing the median ap versus M SPS relation is not possible with both a universal IMF and a universal dark halo response. Significant departures from a universal IMF and/or dark halo response are required, but there is a degeneracy between these two solutions. We show that this degeneracy can be broken using the strength of the correlation between residuals of the velocity–mass (log ap ) and size–mass (log R e ) relations. The slope of this correlation, VR log ap /log R e , varies systematically with galaxy mass from VR ~= –0.45 at M SPS  ~ 10 10 M to VR ~= –0.15 at M SPS  ~ 10 11.6 M . The virial Fundamental Plane (FP) has VR = –1/2, and thus we find that the tilt of the observed FP is mass dependent. Reproducing this tilt requires both a non-universal IMF and a non-universal halo response. Our best model has mass-follows-light at low masses ( M SPS   10 11.2 M ) and unmodified Navarro, Frenk and White haloes at M SPS  ~ 10 11.5 M . The stellar masses imply a mass-dependent IMF which is ‘lighter’ than Salpeter at low masses and ‘heavier’ than Salpeter at high masses.

Description: New spectral line observations, obtained with the Jansky Very Large Array (VLA), of a sample of 34 galaxies in 17 close pairs are presented in this paper. The sample of galaxy pairs is selected to contain galaxies in close, major interactions (i.e. projected separations 〈30 $h_{70}^{-1}$ kpc, and mass ratios less extreme than 4:1), while still having a sufficiently large angular separation that the VLA can spatially resolve both galaxies in the pair. Of the 34 galaxies, 17 are detected at 〉3. We compare the H i gas fraction of the galaxies with the triggered star formation present in that galaxy. When compared to the star formation rates (SFRs) of non-pair galaxies matched in mass, redshift, and local environment, we find that the star formation enhancement is weakly positively correlated (~2.5) with H i gas fraction. In order to help understand the physical mechanisms driving this weak correlation, we also present results from a small suite of binary galaxy merger simulations with varying gas fractions. The simulated galaxies indicate that larger initial gas fractions are associated with lower levels of interaction-triggered star formation (relative to an identical galaxy in isolation), but also show that high gas fraction galaxies have higher absolute SFRs prior to an interaction. We show that when interaction-driven SFR enhancements are calculated relative to a galaxy with an average gas fraction for its stellar mass, the relationship between SFR and initial gas fraction dominates over the SFR enhancements driven by the interaction. Simulated galaxy interactions that are matched in stellar mass but not in gas fraction, like our VLA sample, yield the same general positive correlation between SFR enhancement and gas fraction that we observe.

Description: We analyse the sizes, colour gradients and resolved stellar mass distributions for 36 massive and passive galaxies in the cluster XMMUJ2235-2557 at z  = 1.39 using optical and near-infrared Hubble Space Telescope ( HST ) imaging. We derive light-weighted Sérsic fits in five HST bands ( i 775 , z 850 , Y 105 , J 125 , H 160 ), and find that the size decreases by ~20 per cent going from i 775 to H 160 band, consistent with recent studies. We then generate spatially resolved stellar mass maps using an empirical relationship between $M_{{\ast }}/L_{H_{160}}$ and ( z 850  –  H 160 ) and use these to derive mass-weighted Sérsic fits: the mass-weighted sizes are ~41 per cent smaller than their rest-frame r -band counterparts compared with an average of ~12 per cent at z  ~ 0. We attribute this evolution to the evolution in the $M_{{\ast }}/L_{H_{160}}$ and colour gradient. Indeed, as expected, the ratio of mass-weighted to light-weighted size is correlated with the M * / L gradient, but is also mildly correlated with the mass surface density and mass-weighted size. The colour gradients ( z  –  H ) are mostly negative, with a median value of ~0.45 mag dex –1 , twice the local value. The evolution is caused by an evolution in age gradients along the semimajor axis ( a ), with age  = dlog ( age )/dlog ( a ) ~– 0.33, while the survival of weaker colour gradients in old, local galaxies implies that metallicity gradients are also required, with Z  = dlog ( Z )/dlog ( a ) ~– 0.2. This is consistent with recent observational evidence for the inside–out growth of passive galaxies at high redshift, and favours a gradual mass growth mechanism, such as minor mergers.

Description: We quantify the impact that a variety of galactic and environmental properties have on the quenching of star formation. We collate a sample of ~400 000 central and ~100 000 satellite galaxies from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Specifically, we consider central velocity dispersion ( c ), stellar, halo, bulge and disc mass, local density, bulge-to-total ratio, groupcentric distance and galaxy–halo mass ratio. We develop and apply a new statistical technique to quantify the impact on the quenched fraction ( f Quench ) of varying one parameter, while keeping the remaining parameters fixed. For centrals, we find that the f Quench – c relationship is tighter and steeper than for any other variable considered. We compare to the Illustris hydrodynamical simulation and the Munich semi-analytic model (L-Galaxies), finding that our results for centrals are qualitatively consistent with their predictions for quenching via radio-mode AGN feedback, hinting at the viability of this process in explaining our observational trends. However, we also find evidence that quenching in L-Galaxies is too efficient and quenching in Illustris is not efficient enough, compared to observations. For satellites, we find strong evidence that environment affects their quenched fraction at fixed central velocity dispersion, particularly at lower masses. At higher masses, satellites behave identically to centrals in their quenching. Of the environmental parameters considered, local density affects the quenched fraction of satellites the most at fixed central velocity dispersion.

Description: Using artificial neural network predictions of total infrared luminosities (L IR ), we compare the host galaxy star formation rates (SFRs) of ~21 000 optically selected active galactic nuclei (AGN), 466 low-excitation radio galaxies (LERGs) and 721 mid-IR-selected AGN. SFR offsets (SFR) relative to a sample of star-forming ‘main-sequence’ galaxies (matched in M * , z and local environment) are computed for the AGN hosts. Optically selected AGN exhibit a wide range of SFR, with a distribution skewed to low SFRs and a median SFR = –0.06 dex. The LERGs have SFRs that are shifted to even lower values with a median SFR = –0.5 dex. In contrast, mid-IR-selected AGN have, on average, SFRs enhanced by a factor of ~1.5. We interpret the different distributions of SFR amongst the different AGN classes in the context of the relative contribution of triggering by galaxy mergers. Whereas the LERGs are predominantly fuelled through low accretion rate secular processes which are not accompanied by enhancements in SFR, mergers, which can simultaneously boost SFRs, most frequently lead to powerful, obscured AGN.

Description: In order to investigate the effects of galaxy mergers throughout the interaction sequence, we present a study of 10 800 galaxies in close pairs and a smaller sample of 97 post-mergers identified in the Sloan Digital Sky Survey. We find that the average central star formation rate (SFR) enhancement ( x 3.5) and the fraction of starbursts (20 per cent) peak in the post-merger sample. The post-mergers also show a stronger deficit in gas phase metallicity than the closest pairs, being more metal-poor than their control by –0.09 dex. Combined with the observed trends in SFR and the time-scales predicted in merger simulations, we estimate that the post-mergers in our sample have undergone coalescence within the last few hundred Myr. In contrast with the incidence of star-forming galaxies, the frequency of active galactic nuclei (AGN) peaks in the post-mergers, outnumbering AGN in the control sample by a factor of 3.75. Moreover, amongst the galaxies that host an AGN, the black hole accretion rates in the closest pairs and post-mergers are higher by a factor of ~3 than AGN in the control sample. These results are consistent with a picture in which star formation is initiated early on in the encounter, with AGN activity peaking post-coalescence.

Description: Interactions between galaxies are predicted to cause gas inflows that can potentially trigger nuclear activity. Since the inflowing material can obscure the central regions of interacting galaxies, a potential limitation of previous optical studies is that obscured active galactic nuclei (AGNs) can be missed at various stages along the merger sequence. We present the first large mid-infrared study of AGNs in mergers and galaxy pairs, in order to quantify the incidence of obscured AGNs triggered by interactions. The sample consists of galaxy pairs and post-mergers drawn from the Sloan Digital Sky Survey that are matched to detections by the Wide-Field Infrared Sky Explorer . We find that the fraction of AGNs in the pairs, relative to a mass-, redshift- and environment-matched control sample, increases as a function of decreasing projected separation. This enhancement is most dramatic in the post-merger sample, where we find a factor of 10–20 excess in the AGN fraction compared with the control. Although this trend is in qualitative agreement with results based on optical AGN selection, the mid-infrared-selected AGN excess increases much more dramatically in the post-mergers than is seen for an optical AGN. Our results suggest that energetically dominant optically obscured AGNs become more prevalent in the most advanced mergers, consistent with theoretical predictions.

Description: We investigate the origin of galaxy bimodality by quantifying the relative role of intrinsic and environmental drivers to the cessation (or ‘quenching’) of star formation in over half a million local Sloan Digital Sky Survey galaxies. Our sample contains a wide variety of galaxies at z  = 0.02–0.2, with stellar masses of 8 〈 log(M * /M ) 〈 12, spanning the entire morphological range from pure discs to spheroids, and over four orders of magnitude in local galaxy density and halo mass. We utilize published star formation rates and add to this recent GIM2D photometric and stellar mass bulge + disc decompositions from our group. We find that the passive fraction of galaxies increases steeply with stellar mass, halo mass, and bulge mass, with a less steep dependence on local galaxy density and bulge-to-total stellar mass ratio (B/T). At fixed internal properties, we find that central and satellite galaxies have different passive fraction relationships. For centrals, we conclude that there is less variation in the passive fraction at a fixed bulge mass, than for any other variable, including total stellar mass, halo mass, and B/T. This implies that the quenching mechanism must be most tightly coupled to the bulge. We argue that radio-mode active galactic nucleus feedback offers the most plausible explanation of the observed trends.

Description: A well-calibrated method to describe the environment of galaxies at all redshifts is essential for the study of structure formation. Such a calibration should include well-understood correlations with halo mass, and the possibility to identify galaxies which dominate their potential well (centrals), and their satellites. Focusing on z  ~ 1 and 2, we propose a method of environmental calibration which can be applied to the next generation of low- to medium-resolution spectroscopic surveys. Using an up-to-date semi-analytic model of galaxy formation, we measure the local density of galaxies in fixed apertures on different scales. There is a clear correlation of density with halo mass for satellite galaxies, while a significant population of low-mass centrals is found at high densities in the neighbourhood of massive haloes. In this case, the density simply traces the mass of the most massive halo within the aperture. To identify central and satellite galaxies, we apply an observationally motivated stellar mass rank method which is both highly pure and complete, especially in the more massive haloes where such a division is most meaningful. Finally, we examine a test case for the recovery of environmental trends: the passive fraction of galaxies and its dependence on stellar and halo mass for centrals and satellites. With careful calibration, observationally defined quantities do a good job of recovering known trends in the model. This result stands even with reduced redshift accuracy, provided the sample is deep enough to preserve a wide dynamic range of density.