ALBERT

Description: Galaxies arrive on the red sequences of clusters at high redshift ( z 〉 1) once their star formation is quenched and evolve passively thereafter. However, we have previously found that cluster red sequence galaxies (CRSGs) undergo significant morphological evolution subsequent to the cessation of star formation, at some point in the past 9–10 Gyr. Through a detailed study of a large sample of cluster red sequence galaxies spanning 0.2 〈 z 〈 1.4 we elucidate the details of this evolution. Below z ~ 0.5–0.6 (in the last 5–6 Gyr) there is little or no morphological evolution in the population as a whole, unlike in the previous 4–5 Gyr. Over this earlier time (i) disc-like systems with Sérsic n 〈 2 progressively disappear, as (ii) the range of their axial ratios similarly decreases, removing the most elongated systems (those consistent with thin discs seen at an appreciable inclination angle) and (iii) radial colour gradients (bluer outwards) decrease in an absolute sense from significant age-related gradients to a residual level consistent with the metallicity-induced gradients seen in low-redshift cluster members. The distribution of their effective radii shows some evidence of evolution, consistent with growth of at most a factor 〈1.5 between z ~ 1.4 and ~0.5, significantly less than for comparable field galaxies, while the distribution of their central (〈1 kpc) bulge surface densities shows no evolution at least at z 〈 1. A simple model involving the fading and thickening of a disc component after comparatively recent quenching (after z ~ 1.5) around an otherwise passively evolving older spheroid component is consistent with all of these findings.

Description: Radio galaxies are among the most massive galaxies in the high-redshift Universe and are known to often lie in protocluster environments. We have studied the fields of seven z = 2.2 radio galaxies with High Acuity Wide field K-band Imager (HAWK-I) narrow-band and broad-band imaging in order to map out their environment using Hα emitters (HAEs). The results are compared to the blank field HAE survey HiZELS. All of the radio galaxy fields are overdense in HAEs relative to a typical HiZELS field of the same area and four of the seven are richer than all except one of 65 essentially random HiZELS subfields of the same size. The star formation rates of the massive HAEs are lower than those necessary to have formed their stellar population in the preceding Gyr – indicating that these galaxies are likely to have formed the bulk of their stars at higher redshifts, and are starting to quench.

Description: We present a meta-analysis of star formation rate (SFR) indicators in the Galaxy And Mass Assembly (GAMA) survey, producing 12 different SFR metrics and determining the SFR– M * relation for each. We compare and contrast published methods to extract the SFR from each indicator, using a well-defined local sample of morphologically selected spiral galaxies, which excludes sources which potentially have large recent changes to their SFR. The different methods are found to yield SFR– M * relations with inconsistent slopes and normalizations, suggesting differences between calibration methods. The recovered SFR– M * relations also have a large range in scatter which, as SFRs of the targets may be considered constant over the different time-scales, suggests differences in the accuracy by which methods correct for attenuation in individual targets. We then recalibrate all SFR indicators to provide new, robust and consistent luminosity-to-SFR calibrations, finding that the most consistent slopes and normalizations of the SFR– M * relations are obtained when recalibrated using the radiation transfer method of Popescu et al. These new calibrations can be used to directly compare SFRs across different observations, epochs and galaxy populations. We then apply our calibrations to the GAMA II equatorial data set and explore the evolution of star formation in the local Universe. We determine the evolution of the normalization to the SFR– M * relation from 0 〈  z  〈 0.35 – finding consistent trends with previous estimates at 0.3 〈  z  〈 1.2. We then provide the definitive z  〈 0.35 cosmic star formation history, SFR– M * relation and its evolution over the last 3 billion years.

Description: We present an Integral Field Unit survey of 73 galaxy clusters and groups with the VIsible Multi Object Spectrograph on the Very Large Telescope. We exploit the data to determine the H α gas dynamics on kpc scales to study the feedback processes occurring within the dense cluster cores. We determine the kinematic state of the ionized gas and show that the majority of systems (~2/3) have relatively ordered velocity fields on kpc scales that are similar to the kinematics of rotating discs and are decoupled from the stellar kinematics of the brightest cluster galaxy. The majority of the H α flux (〉50 per cent) is typically associated with these ordered kinematics and most systems show relatively simple morphologies suggesting they have not been disturbed by a recent merger or interaction. Approximately 20 per cent of the sample (13/73) have disturbed morphologies which can typically be attributed to active galactic nuclei activity disrupting the gas. Only one system shows any evidence of an interaction with another cluster member. A spectral analysis of the gas suggests that the ionization of the gas within cluster cores is dominated by non-stellar processes, possibly originating from the intracluster medium itself.

Description: We have carried out a joint photometric and structural analysis of red sequence galaxies in four clusters at a mean redshift of 〈 z 〉 ~ 1.25 using optical and near-infrared Hubble Space Telescope imaging reaching to at least three magnitudes fainter than M *. As expected, the photometry and overall galaxy sizes imply purely passive evolution of stellar populations in red sequence cluster galaxies. However, the morphologies of red sequence cluster galaxies at these redshifts show significant differences to those of local counterparts. Apart from the most massive galaxies, the high-redshift red sequence galaxies are significantly discier than their low-redshift analogues. These galaxies also show significant colour gradients, again not present in their low-redshift equivalents, most straightforwardly explained by radial age gradients. A clear implication of these findings is that red sequence cluster galaxies originally arrive on the sequence as disc-dominated galaxies whose discs subsequently fade or evolve secularly to end up as high Sérsic index early-type galaxies (classical S0s or possibly ellipticals) at lower redshift. The apparent lack of growth seen in a comparison of high- and low-redshift red sequence galaxies implies that any evolution is internal and is unlikely to involve significant mergers. While significant star formation may have ended at high redshift, the cluster red sequence population continues to evolve (morphologically) for several gigayears thereafter.

Description: We present a multiwavelength morphological analysis of star-forming clouds and filaments in the central (50 kpc) regions of 16 low-redshift ( z 〈 0.3) cool core brightest cluster galaxies. New Hubble Space Telescope imaging of far-ultraviolet continuum emission from young (10 Myr), massive (5 M ) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Lyα, narrow-band Hα, broad-band optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot (~10 7–8 K) and warm ionized (~10 4 K) gas phases, as well as the old stellar population and radio-bright active galactic nucleus (AGN) outflows. Nearly half of the sample possesses kpc-scale filaments that, in projection, extend towards and around radio lobes and/or X-ray cavities. These filaments may have been uplifted by the propagating jet or buoyant X-ray bubble, or may have formed in situ by cloud collapse at the interface of a radio lobe or rapid cooling in a cavity's compressed shell. The morphological diversity of nearly the entire FUV sample is reproduced by recent hydrodynamical simulations in which the AGN powers a self-regulating rain of thermally unstable star-forming clouds that precipitate from the hot atmosphere. In this model, precipitation triggers where the cooling-to-free-fall time ratio is t cool / t ff ~ 10. This condition is roughly met at the maximal projected FUV radius for more than half of our sample, and clustering about this ratio is stronger for sources with higher star formation rates.

Description: Measurement of the evolution of both active galactic nuclei (AGN) and star-formation in galaxies underpins our understanding of galaxy evolution over cosmic time. Radio continuum observations can provide key information on these two processes, in particular via the mechanical feedback produced by radio jets in AGN, and via an unbiased dust-independent measurement of star formation rates. In this paper, we determine radio luminosity functions at 325 MHz for a sample of AGN and star-forming galaxies by matching a 138 deg 2 radio survey conducted with the Giant Metrewave Radio Telescope, with optical imaging and redshifts from the Galaxy And Mass Assembly survey. We find that the radio luminosity function at 325 MHz for star-forming galaxies closely follows that measured at 1.4 GHz. By fitting the AGN radio luminosity function out to z = 0.5 as a double power law, and parametrizing the evolution as (1 + z ) k , we find evolution parameters of k = 0.92 ± 0.95 assuming pure density evolution and k = 2.13 ± 1.96 assuming pure luminosity evolution. We find that the Low Excitation Radio Galaxies are the dominant population in space density at lower luminosities. Comparing our 325 MHz observations with radio continuum imaging at 1.4 GHz, we determine separate radio luminosity functions for steep- and flat-spectrum AGN, and show that the beamed population of flat-spectrum sources in our sample can be shifted in number density and luminosity to coincide with the unbeamed population of steep-spectrum sources, as is expected in the orientation-based unification of AGN.

Description: We present multifrequency observations of the radio galaxy Hydra-A (3C218) located in the core of a massive, X-ray luminous galaxy cluster. Integral field unit spectroscopy is used to trace the kinematics of the ionized and warm molecular hydrogen which are consistent with an ~5 kpc rotating disc. Broad, double-peaked lines of CO(2–1), [C ii ] 157 μm and [O i ] 63 μm are detected. We estimate the mass of the cold gas within the disc to be M gas  = 2.3 ± 0.3 10 9 M . These observations demonstrate that the complex line profiles found in the cold atomic and molecular gas are related to the rotating disc or ring of gas. Finally, a Hubble Space Telescope image of the galaxy shows that this gas disc contains a substantial mass of dust. The large gas mass, star formation rate and kinematics are consistent with the levels of gas cooling from the intracluster medium (ICM). We conclude that the cold gas originates from the continual quiescent accumulation of cooled ICM gas. The rotation is in a plane perpendicular to the projected orientation of the radio jets and ICM cavities hinting at a possible connection between the kpc-scale cooling gas and the accretion of material on to the black hole. We discuss the implications of these observations for models of cold accretion, AGN feedback and cooling flows.

Description: Star-forming galaxies at high redshift show ubiquitously high-ionization parameters, as measured by the ratio of optical emission lines. We demonstrate that local ( z  〈 0.2) sources selected as Lyman break analogues also manifest high line ratios with a typical [O iii ]/H $\beta =3.36^{+0.14}_{-0.04}$ – comparable to all but the highest ratios seen in star-forming galaxies at z  ~ 2–4. We argue that the stellar population synthesis code bpass can explain the high-ionization parameters required through the ageing of rapidly formed star populations, without invoking any AGN contribution. Binary stellar evolution pathways prolong the age interval over which a starburst is likely to show elevated line ratios, relative to those predicted by single stellar evolution codes. As a result, model galaxies at near-solar metallicities and with ages of up to ~100 Myr after a starburst typically have a line ratio [O iii ]/Hβ ~ 3, consistent with those seen in Lyman break galaxies and local sources with similar star formation densities. This emphasises the importance of including binary evolution pathways when simulating the nebular line emission of young or bursty stellar populations.

Description: Identifying galaxy clustering at high redshift (i.e. z  〉 1) is essential to our understanding of the current cosmological model. However, at increasing redshift, clusters evolve considerably in star formation activity and so are less likely to be identified using the widely used red-sequence method. Here we assess the viability of instead identifying high-redshift clustering using actively star-forming galaxies (submillimetre galaxies, SMGs, associated with overdensities of BzKs /LBGs). We perform both a 2D and 3D clustering analysis to determine whether or not true (3D) clustering can be identified where only 2D data are available. As expected, we find that 2D clustering signals are weak at best and inferred results are method dependent. In our 3D analysis, we identify 12 SMGs associated with an overdensity of galaxies coincident both spatially and in redshift – just 8 per cent of SMGs with known redshifts in our sample. Where an SMG in our target fields lacks a known redshift, their sight line is no more likely to display clustering than blank sky fields; prior redshift information for the SMG is required to identify a true clustering signal. We find that the strength of clustering in the volume around typical SMGs, while identifiable, is not exceptional. However, we identify a small number of highly clustered regions, all associated with an SMG. The most notable of these, surrounding LESS J033336.8–274401, potentially contains an SMG, a quasi stellar object (QSO) and 36 star-forming galaxies (a 〉20 overdensity) all at z  ~ 1.8. This region is highly likely to represent an actively star-forming cluster and illustrates the success of using star-forming galaxies to select sites of early clustering. Given the increasing number of deep fields with large volumes of spectroscopy, or high quality and reliable photometric redshifts, this opens a new avenue for cluster identification in the young Universe.