John P. Stott

A large Hα survey at z = 2.23, 1.47, 0.84 and 0.40: the 11 Gyr evolution of star-forming galaxies from HiZELS

This paper presents new deep and wide narrow-band surveys undertaken with United Kingdom Infrared Telescope (UKIRT), Subaru and the Very Large Telescope (VLT), a unique combined effort to select large, robust samples of Hα star-forming galaxies at z = 0.40, 0.84, 1.47 and 2.23 (corresponding to look-back times of 4.2, 7.0, 9.2 and 10.6 Gyr) in a uniform manner over ∼2 deg^2 in the Cosmological Evolution Survey and Ultra Deep Survey fields. The deep multi-epoch Hα surveys reach a matched 3σ flux limit of ≈3 M_⊙ yr^(−1) out to z = 2.2 for the first time, while the wide area and the coverage over two independent fields allow us to greatly overcome cosmic variance and assemble by far the largest samples of Hα emitters. Catalogues are presented for a total of 1742, 637, 515 and 807 Hα emitters, robustly selected at z = 0.40, 0.84, 1.47 and 2.23, respectively, and used to determine the Hα luminosity function and its evolution. The faint-end slope of the Hα luminosity function is found to be α = −1.60 ± 0.08 over z = 0–2.23, showing no significant evolution. The characteristic luminosity of star-forming galaxies, L*_Hα, evolves significantly as log  L*_Hα(z) = 0.45z + log  L*_z = 0. This is the first time Hα has been used to trace star formation activity with a single homogeneous survey at z = 0.4–2.23. Overall, the evolution seen with Hα is in good agreement with the evolution seen using inhomogeneous compilations of other tracers of star formation, such as far-infrared and ultraviolet, jointly pointing towards the bulk of the evolution in the last 11 Gyr being driven by a statistically similar star-forming population across cosmic time, but with a strong luminosity increase from z ∼ 0 to ∼2.2. Our uniform analysis allows us to derive the Hα star formation history (SFRH) of the Universe, showing a clear rise up to z ∼ 2.2, for which the simple parametrization log_10ρSFR = −2.1(1 + z)^(−1) is valid over 80 per cent of the age of the Universe. The results reveal that both the shape and normalization of the Hα SFRH are consistent with the measurements of the stellar mass density growth, confirming that our Hα SFRH is tracing the bulk of the formation of stars in the Universe for z < 2.23. The star formation activity over the last ∼11 Gyr is responsible for producing ∼95 per cent of the total stellar mass density observed locally, with half of that being assembled in 2 Gyr between z = 1.2 and 2.2, and the other half in 8 Gyr (since z < 1.2). If the star formation rate density continues to decline with time in the same way as seen in the past ∼11 Gyr, then the stellar mass density of the Universe will reach a maximum which is only 5 per cent higher than the present-day value.

The LABOCA Survey of the Extended Chandra Deep Field South : Clustering of submillimetre galaxies

We present a measurement of the spatial clustering of submillimetre galaxies (SMGs) at z= 1-3. Using data from the 870μm Large APEX Bolometer Camera (LABOCA) submillimetre survey of the Extended Chandra Deep Field-South, we employ a novel technique to measure the cross-correlation between SMGs and galaxies, accounting for the full probability distributions for photometric redshifts of the galaxies. From the observed projected two-point cross-correlation function we derive the linear bias and characteristic dark matter halo masses for the SMGs. We detect clustering in the cross-correlation between SMGs and galaxies at the >4σ level. Accounting for the clustering of galaxies from their autocorrelation function, we estimate an autocorrelation length for SMGs of r o = 7.7 -2.3 +1.8 h -1 Mpc assuming a power-law slope γ= 1.8, and derive a corresponding dark matter halo mass of log(M halo[h -1M ⊙]) = 12.8 -0.5 +0.3. Based on the evolution of dark matter haloes derived from simulations, we show that that the z= 0 descendants of SMGs are typically massive (~2-3L *) elliptical galaxies residing in moderate- to high-mass groups (log(M halo[h -1M ⊙]) = 13.3 -0.5 +0.3). From the observed clustering we estimate an SMG lifetime of ~100Myr, consistent with lifetimes derived from gas consumption times and star formation time-scales, although with considerable uncertainties. The clustering of SMGs at z~ 2 is consistent with measurements for optically selected quasi-stellar objects (QSOs), supporting evolutionary scenarios in which powerful starbursts and QSOs occur in the same systems. Given that SMGs reside in haloes of characteristic mass ~6 × 10 12h -1M ⊙, we demonstrate that the redshift distribution of SMGs can be described remarkably well by the combination of two effects: the cosmological growth of structure and the evolution of the molecular gas fraction in galaxies. We conclude that the powerful starbursts in SMGs likely represent a short-lived but universal phase in massive galaxy evolution, associated with the transition between cold gas-rich, star-forming galaxies and passively evolving systems.

The behaviour of dark matter associated with four bright cluster galaxies in the 10 kpc core of Abell 3827

Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10 kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and Very Large Telescope/Multi-Unit Spectroscopic Explorer integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62(-0.49)(+0.47) kpc, where the 68 per cent confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma(DM)/(m) similar to (1.7 +/- 0.7) x 10(-4) cm(2) g(-1) x (t(infall)/10(9) yr)(-2), where t(infall) is the infall duration.

The stellar mass function of star-forming galaxies and the mass-dependent SFR function since z=2.23 from HiZELS

We explore a large uniformly selected sample of Ha selected star-forming galaxies (SFGs) at z = 0.40, 0.84, 1.47, 2.23 to unveil the evolution of the star formation rate (SFR) function and the stellar mass function. We find strong evolution in the SFR function, with the typical SFR of SFGs declining exponentially in the last 11 Gyr as SFR*(T[Gyr]) = 104.23/T + 0.37 M⊙ yr-1, but with no evolution in the faint-end slope, α -1.6. The stellar mass function of SFGs, however, reveals little evolution: α -1.4, M* 1011.2 ± 0.2 M⊙ and just a slight increase of 2.3× in φ * from z = 2.23 to z = 0.4. The stellar mass density within SFGs has been roughly constant since z = 2.23 at 107.65 ± 0.08 M⊙ Mpc-3, comprising 100 per cent of the stellar mass density in all galaxies at z = 2.23, and declining to 20 per cent by z = 0.40, driven by the rise of the passive population.We find that SFGs with 1010.0 ± 0.2 M⊙ contribute most to the ρSFR density (ρSFR) per d log10M, and that there is no significant evolution in the fractional contribution from SFGs of different masses to ρSFR or ρSFR(d log10M)-1 since z = 2.23. Instead, we show that the decline of SFR* and of ρSFR is primarily driven by an exponential decline in SFRs at all masses. Our results have important implications not only on how SFGs need to be quenched across cosmic time, but also on the driver(s) of the exponential decline in SFR* from 66 M⊙ yr-1 to 5 ⊙; yr-1 since z 2.23.

An increase in the faint red galaxy population in massive clusters since z ∼ 0.5

We compare the luminosity functions for red galaxies lying on the rest-frame (U - V) color-magnitude sequence in a homogeneous sample of 10 X-ray-luminous clusters from the MACS survey at z ~ 0.5 to a similarly selected X-ray cluster sample at z ~ 0.1. We exploit deep Hubble Space Telescope ACS imaging in the F555W and F814W passbands of the central 1.2 Mpc diameter regions of the distant clusters to measure precise colors for the galaxies in these regions and statistically correct for contamination by field galaxies using observations of blank fields. We apply an identical analysis to ground-based photometry of the z ~ 0.1 sample. This comparison demonstrates that the number of faint, MV ~ -19, red galaxies relative to the bright population seen in the central regions of massive clusters has roughly doubled over the 4 Gyr between z ~ 0.5 and z ~ 0.1. We quantify this difference by measuring the dwarf-giant ratio on the red sequence, which increases by a factor of at least 2.2 ± 0.4 since z ~ 0.5. This is consistent with the idea that many faint, blue, star-forming galaxies in high-density environments are transforming onto the red sequence in the last half of the Hubble time.

LoCuSS: Connecting the Dominance and Shape of Brightest Cluster Galaxies with the Assembly History of Massive Clusters

We study the luminosity gap, ∆m_(12), between the first- and second-ranked galaxies in a sample of 59 massive (~10^(15)M_⊙) galaxy clusters, using data from the Hale Telescope, the Hubble Space Telescope, Chandra and Spitzer. We find that the ∆m_(12) distribution, p(∆m_(12)), is a declining function of ∆m_(12) to which we fitted a straight line: p(∆m_(12))∝−(0.13 ± 0.02)∆m_(12). The fraction of clusters with ‘large’ luminosity gaps is p(∆m_(12) ≥ 1) = 0.37 ± 0.08, which represents a 3σ excess over that obtained from MonteCarlo simulations of a Schechter function that matches the mean cluster galaxy luminosity function. We also identify four clusters with ‘extreme’ luminosity gaps, ∆m_(12) ≥ 2, giving a fraction of p(∆m_(12) ≥ 2) = 0.07^(+0.05)_(−0.03). More generally, large luminosity gap clusters are relatively homogeneous, with elliptical/discy brightest cluster galaxies (BCGs), cuspy gas density profiles (i.e. strong cool cores), high concentrations and low substructure fractions. In contrast, small luminosity gap clusters are heterogeneous, spanning the full range of boxy/elliptical/discy BCG morphologies, the full range of cool core strengths and dark matter concentrations, and have large substructure fractions. Taken together, these results imply that the amplitude of the luminosity gap is a function of both the formation epoch and the recent infall history of the cluster. ‘BCG dominance’ is therefore a phase that a cluster may evolve through and is not an evolutionary ‘cul-de-sac’. We also compare our results with semi-analytic model predictions based on the Millennium Simulation. None of the models is able to reproduce all of the observational results on ∆m_(12), underlining the inability of the current generation of models to match the empirical properties of BCGs. We identify the strength of active galactic nucleus feedback and the efficiency with which cluster galaxies are replenished after they merge with the BCG in each model as possible causes of these discrepancies.

A fundamental metallicity relation for galaxies at z = 0.84–1.47 from HiZELS

We obtained Subaru FMOS observations of Hα emitting galaxies selected from the HiZELS, to investigate the relationship between stellarmass, metallicity and star formation rate (SFR) at z = 0.84-1.47, for comparison with the fundamental metallicity relation seen at low redshift. Our findings demonstrate, for the first time with a homogeneously selected sample, that a relationship exists for typical star-forming galaxies at z ~ 1-1.5 and that it is surprisingly similar to that seen locally. Therefore, star-forming galaxies at z ~ 1-1.5 are no less metal abundant than galaxies of similar mass and SFR at z ~ 0.1, contrary to claims from some earlier studies.We conclude that the bulk of the metal enrichment for this star-forming galaxy population takes place in the 4 Gyr before z ~ 1.5. We fit a new mass-metallicity-SFR plane to our data which is consistent with other high-redshift studies. However, there is some evidence that the mass-metallicity component of this high-redshift plane is flattened, at all SFR, compared with z ~ 0.1, suggesting that processes such as star formation-driven winds, thought to remove enriched gas from low-mass haloes, are yet to have as large an impact at this early epoch. The negative slope of the SFR-metallicity relation from this new plane is consistent with the picture that the elevation in the SFR of typical galaxies at z≳ 1 is fuelled by the inflow of metal-poor gas and not major merging.

The LABOCA survey of the Extended Chandra Deep Field-South: clustering of submillimetre galaxies: Clustering of SMGs

We present a measurement of the spatial clustering of submillimetre galaxies (SMGs) at z= 1-3. Using data from the 870μm Large APEX Bolometer Camera (LABOCA) submillimetre survey of the Extended Chandra Deep Field-South, we employ a novel technique to measure the cross-correlation between SMGs and galaxies, accounting for the full probability distributions for photometric redshifts of the galaxies. From the observed projected two-point cross-correlation function we derive the linear bias and characteristic dark matter halo masses for the SMGs. We detect clustering in the cross-correlation between SMGs and galaxies at the >4σ level. Accounting for the clustering of galaxies from their autocorrelation function, we estimate an autocorrelation length for SMGs of r o = 7.7 -2.3 +1.8 h -1 Mpc assuming a power-law slope γ= 1.8, and derive a corresponding dark matter halo mass of log(M halo[h -1M ⊙]) = 12.8 -0.5 +0.3. Based on the evolution of dark matter haloes derived from simulations, we show that that the z= 0 descendants of SMGs are typically massive (~2-3L *) elliptical galaxies residing in moderate- to high-mass groups (log(M halo[h -1M ⊙]) = 13.3 -0.5 +0.3). From the observed clustering we estimate an SMG lifetime of ~100Myr, consistent with lifetimes derived from gas consumption times and star formation time-scales, although with considerable uncertainties. The clustering of SMGs at z~ 2 is consistent with measurements for optically selected quasi-stellar objects (QSOs), supporting evolutionary scenarios in which powerful starbursts and QSOs occur in the same systems. Given that SMGs reside in haloes of characteristic mass ~6 × 10 12h -1M ⊙, we demonstrate that the redshift distribution of SMGs can be described remarkably well by the combination of two effects: the cosmological growth of structure and the evolution of the molecular gas fraction in galaxies. We conclude that the powerful starbursts in SMGs likely represent a short-lived but universal phase in massive galaxy evolution, associated with the transition between cold gas-rich, star-forming galaxies and passively evolving systems.

Little change in the sizes of the most massive galaxies since z = 1

Recent reports suggest that elliptical galaxies have increased their size dramatically over the last ~8 Gyr. This result points to a major rethink of the processes dominating the late-time evolution of galaxies. In this paper we present the first estimates for the scale sizes of brightest cluster galaxies (BCGs) in the redshift range 0.8 <z <1.3 from an analysis of deep Hubble Space Telescope imaging, comparing to a well-matched local sample taken from the Local Cluster Substructure Survey at z~ 0.2. For a small sample of five high-redshift BCGs we measure half-light radii ranging from 14 to 53kpc using de Vaucuoleurs profile fits, with an average determined from stacking of 32.1 ± 2.5kpc compared to a value 43.2 ± 1.0kpc for the low-redshift comparison sample. This implies that the scale sizes of BCGs at z= 1 are ≃30 per cent smaller than at z= 0.25. Analyses comparing either Sersic or Petrosian radii also indicate little or no evolution between the two samples. The detection of only modest evolution at most out to z= 1 argues against BCGs having undergone the large increase in size reported for massive galaxies since z= 2 and in fact the scale-size evolution of BCGs appears closer to that reported for radio galaxies over a similar epoch. We conclude that this lack of size evolution, particularly when coupled with recent results on the lack of BCG stellar mass evolution, demonstrates that major merging is not an important process in the late-time evolution of these systems. The homogeneity and maturity of BCGs at z= 1 continues to challenge galaxy evolution models.

Near-infrared evolution of brightest cluster galaxies in the most X-ray luminous clusters since z = 1

We investigate the near-infrared evolution of brightest cluster galaxies (BCGs) from a sample of rich galaxy clusters since z ~ 1. By employing an X-ray selection of L_X > 10^(44) erg s^(−1), we limit environmental effects by selecting BCGs in comparably high-density regions. We find a positive relationship between X-ray and near-infrared luminosity for BCGs in clusters with L_X > 5 × 10^(44) erg s^(−1). Applying a correction for this relation, we reduce the scatter in the BCG absolute magnitude by a factor of 30 per cent. The near-infrared J − K colour evolution demonstrates that the stellar population in BCGs has been in place since at least z = 2, and that we expect a shorter period of star formation than that predicted by current hierarchical merger models. We also confirm that there is a relationship between ‘blue’ J − K colour and the presence of BCG emission lines associated with star formation in cooling flows.