A large narrow band Hα\alpha survey at z∼0.2z\sim0.2: the bright end of the luminosity function, cosmic variance and clustering across cosmic time

We carried out the largest ($>3.5\times10^5$ Mpc$^3$, 26 deg$^2$) H$\alpha$ narrow band survey to date at $z\sim0.2$ in the SA22, W2 and XMMLSS extragalactic fields. Our survey covers a large enough volume to overcome cosmic variance and to sample bright and rare H$\alpha$ emitters up to an observed luminosity of $\sim10^{42.4}$ erg s$^{-1}$, equivalent to $\sim11 M_\odot$ yr$^{-1}$. Using our sample of $220$ sources brighter than $>10^{41.4}$ erg s$^{-1}$ ($>1 M_\odot$ yr$^{-1}$), we derive H$\alpha$ luminosity functions, which are well described by a Schechter function with $\phi^* = 10^{-2.85\pm0.03}$ Mpc$^{-3}$ and $L^*_{H\alpha} = 10^{41.71\pm0.02}$ erg s$^{-1}$ (with a fixed faint end slope $\alpha=-1.35$). We find that surveys probing smaller volumes ($\sim3\times10^4$ Mpc$^3$) are heavily affected by cosmic variance, which can lead to errors of over $100$ per cent in the characteristic density and luminosity of the H$\alpha$ luminosity function. We derive a star formation rate density of $\rho_\mathrm{SFRD} = 0.0094\pm0.0008$ $M_\odot$ yr$^{-1}$, in agreement with the redshift-dependent H$\alpha$ parametrisation from Sobral et al. (2013). The two-point correlation function is described by a single power law $\omega(\theta) = (0.159\pm0.012) \theta^{(-0.75\pm0.05)}$, corresponding to a clustering length of $r_0 = 3.3\pm0.8$ Mpc/h. We find that the most luminous H$\alpha$ emitters at $z\sim0.2$ are more strongly clustered than the relatively fainter ones. The $L^*_{H\alpha}$ H$\alpha$ emitters at $z\sim0.2$ in our sample reside in $\sim10^{12.5-13.5}$ $M_\odot$ dark matter haloes. This implies that the most star forming galaxies always reside in relatively massive haloes or group-like environments and that the typical host halo mass of star-forming galaxies is independent of redshift if scaled by $L_\mathrm{H\alpha}/L^*_{H\alpha}(z)$, as proposed by Sobral et al. (2010).Comment: Accepted for publication in MNRAS; 18 pages, 19 figures, 6 table

A Comparative Study of Density Field Estimation for Galaxies: New Insights into the Evolution of Galaxies with Environment in COSMOS out to z~3

It is well-known that galaxy environment has a fundamental effect in shaping its properties. We study the environmental effects on galaxy evolution, with an emphasis on the environment defined as the local number density of galaxies. The density field is estimated with different estimators (weighted adaptive kernel smoothing, 10$^{th}$ and 5$^{th}$ nearest neighbors, Voronoi and Delaunay tessellation) for a K$_{s}<$24 sample of $\sim$190,000 galaxies in the COSMOS field at 0.1$<$z$<$3.1. The performance of each estimator is evaluated with extensive simulations. We show that overall, there is a good agreement between the estimated density fields using different methods over $\sim$2 dex in overdensity values. However, our simulations show that adaptive kernel and Voronoi tessellation outperform other methods. Using the Voronoi tessellation method, we assign surface densities to a mass complete sample of quiescent and star-forming galaxies out to z$\sim$3. We show that at a fixed stellar mass, the median color of quiescent galaxies does not depend on their host environment out to z$\sim$3. We find that the number and stellar mass density of massive ($>$10$^{11}$M$_{\odot}$) star-forming galaxies have not significantly changed since z$\sim$3, regardless of their environment. However, for massive quiescent systems at lower redshifts (z$\lesssim$1.3), we find a significant evolution in the number and stellar mass densities in denser environments compared to lower density regions. Our results suggest that the relation between stellar mass and local density is more fundamental than the color-density relation and that environment plays a significant role in quenching star formation activity in galaxies at z$\lesssim$1.Comment: 20 pages, 11 figures, main figures 4,5,8 and 1

Identification of the brightest Ly\alpha\ emitters at z=6.6: implications for the evolution of the luminosity function in the re-ionisation era

Using wide field narrow-band surveys, we provide a new measurement of the $z=6.6$ Lyman-$\alpha$ Emitter (LAE) luminosity function (LF), which constraints the bright end for the first time. We use a combination of archival narrow-band NB921 data in UDS and new NB921 measurements in SA22 and COSMOS/UltraVISTA, all observed with the Subaru telescope, with a total area of $\sim 5$ deg$^2$. We exclude lower redshift interlopers by using broad-band optical and near-infrared photometry and also exclude three supernovae with data split over multiple epochs. Combining the UDS and COSMOS samples we find no evolution of the bright end of the Ly$\alpha$ LF between $z=5.7$ and $6.6$, which is supported by spectroscopic follow-up, and conclude that sources with \emph{Himiko}-like luminosity are not as rare as previously thought, with number densities of $\sim 1.5\times10^{-5}$ Mpc$^{-3}$. Combined with our wide-field SA22 measurements, our results indicate a non-Schechter-like bright end of the LF at $z=6.6$ and a different evolution of \emph{observed} faint and bright LAEs. This differential evolution is also seen in the spectroscopic follow-up of UV selected galaxies and is now also confirmed for Ly$\alpha$ emitters, and we argue that it may be an effect of re-ionisation. Using a toy-model, we show that such differential evolution of the LF is expected, since brighter sources are able to ionise their surroundings earlier, such that Ly$\alpha$ photons are able to escape. Our targets are excellent candidates for detailed follow-up studies and provide the possibility to give a unique view on the earliest stages in the formation of galaxies and re-ionisation process.Comment: 20 pages, main results shown in Fig. 6 and Fig. 7, accepted by MNRA

Neutral hydrogen gas, past and future star-formation in galaxies in and around the 'Sausage' merging galaxy cluster

CIZA J2242.8+5301 ($z = 0.188$, nicknamed 'Sausage') is an extremely massive ($M_{200}\sim 2.0 \times 10^{15}M_\odot$ ), merging cluster with shock waves towards its outskirts, which was found to host numerous emission-line galaxies. We performed extremely deep Westerbork Synthesis Radio Telescope HI observations of the 'Sausage' cluster to investigate the effect of the merger and the shocks on the gas reservoirs fuelling present and future star formation (SF) in cluster members. By using spectral stacking, we find that the emission-line galaxies in the 'Sausage' cluster have, on average, as much HI gas as field galaxies (when accounting for the fact cluster galaxies are more massive than the field galaxies), contrary to previous studies. Since the cluster galaxies are more massive than the field spirals, they may have been able to retain their gas during the cluster merger. The large HI reservoirs are expected to be consumed within $\sim0.75-1.0$ Gyr by the vigorous SF and AGN activity and/or driven out by the out-flows we observe. We find that the star-formation rate in a large fraction of H$\alpha$ emission-line cluster galaxies correlates well with the radio broad band emission, tracing supernova remnant emission. This suggests that the cluster galaxies, all located in post-shock regions, may have been undergoing sustained SFR for at least 100 Myr. This fully supports the interpretation proposed by Stroe et al. (2015) and Sobral et al. (2015) that gas-rich cluster galaxies have been triggered to form stars by the passage of the shock.Comment: Accepted to MNRAS, 14 pages, 9 figures, 7 table

VIS³COS. III. Environmental effects on the star formation histories of galaxies at z ∼ 0.8 seen in [O II], Hδ, and D_n4000

We present spectroscopic observations of 466 galaxies in and around a superstructure at z ∼ 0.84 targeted by the VIMOS Spectroscopic Survey of a Supercluster in the COSMOS field (VIS³COS). We use [OII]λ3727, Hδ, and D_n4000 to trace recent, medium-, and long-term star formation histories and investigate the effect of stellar mass and local environment on them. By studying trends in individual and composite galaxy spectra, we find that stellar mass and environment play a role in the observed galactic properties. Galaxies with low stellar mass (10  11) shows an increase in Hδ absorption strengths in intermediate-density environments (e.g. filaments). Galaxies with intermediate stellar mass (10.5 <  log₁₀ (M⋆/M⊙) < 11) have similar Hδ absorption profiles in all environments, but show an indication of enhanced [OII] emission in intermediate-density environments. This indicates that field galaxies with low stellar mass and filament galaxies with high stellar mass are more likely to have experienced a recent burst of star formation, while galaxies of the intermediate stellar-mass show an increase of star formation at filament-like densities. We also find that the median [OII] equivalent width (|EW_([OII])|) decreases from 27 ± 2 Å to 2.0^(+0.5)_(−0.4) Å and D_n4000 increases from 1.09 ± 0.01 to 1.56 ± 0.03 with increasing stellar mass (from ∼10^(9.25) to ∼10^(11.35) M⊙). For the dependence on the environment, we find that at fixed stellar mass, |EW_([OII])| is tentatively lower in environments with higher density. We find for D_n4000 that the increase with stellar mass is sharper in denser environments, which indicates that these environments may accelerate galaxy evolution. Moreover, we find higher D_n4000 values in denser environments at fixed stellar mass, suggesting that galaxies are on average older and/or more metal rich in these dense environments. This set of tracers depicts a scenario where the most massive galaxies have, on average, the lowest specific star formation rates and the oldest stellar populations (age ≳ 1 Gyr, showing a mass-downsizing effect). We also hypothesize that the observed increase in star formation (higher EW_([OII]|), higher specific star formation rate) at intermediate densities may lead to quenching because we find that the quenched fraction increases sharply from the filament to cluster-like regions at similar stellar masses

The rise and fall of star-formation in z∼0.2\bf z\sim0.2 merging galaxy clusters

CIZA J2242.8+5301 (`Sausage') and 1RXS J0603.3+4213 (`Toothbrush') are two low-redshift ($z\sim0.2$), massive ($\sim2\times10^{15}M_\odot$), post-core passage merging clusters, which host shock waves traced by diffuse radio emission. To study their star-formation properties, we uniformly survey the `Sausage' and `Toothbrush' clusters in broad and narrow band filters and select a sample of $201$ and $463$ line emitters, down to a rest-frame equivalent width ($13${\AA}). We robustly separate between H$\alpha$ and higher redshift emitters using a combination of optical multi-band (B, g, V, r, i, z) and spectroscopic data. We build H$\alpha$ luminosity functions for the entire cluster region, near the shock fronts, and away from the shock fronts and find striking differences between the two clusters. In the dynamically younger, $1$ Gyr old `Sausage' cluster we find numerous ($59$) H$\alpha$ emitters above a star-formation rate (SFR) of $0.17$ M_{\sun} yr$^{-1}$ surprisingly located in close proximity to the shock fronts, embedded in very hot intra-cluster medium plasma. The SFR density for the cluster population is at least at the level of typical galaxies at $z\sim2$. Down to the same star-formation rate, the possibly dynamically more evolved `Toothbrush' cluster has only $9$ H$\alpha$ galaxies. The cluster H$\alpha$ galaxies fall on the SFR-stellar mass relation $z\sim0.2$ for the field. However, the `Sausage' cluster has an H$\alpha$ emitter density $>20$ times that of blank fields. If the shock passes through gas-rich cluster galaxies, the compressed gas could collapse into dense clouds and excite star-formation for a few $100$ Myr. This process ultimately leads to a rapid consumption of the molecular gas, accelerating the transformation of gas-rich field spirals into cluster S0s or ellipticals.Comment: Accepted for publication in MNRAS after minor referee report. 21 pages, 15 figures, 5 table

The structural and size evolution of star-forming galaxies over the last 11 Gyrs

We present new results on the evolution of rest-frame blue/UV sizes and Sersic indices of H$\alpha$-selected star-forming galaxies over the last 11 Gyrs. We investigate how the perceived evolution can be affected by a range of biases and systematics such as cosmological dimming and resolution effects. We use GALFIT and an artificial redshifting technique, which includes the luminosity evolution of H$\alpha$-selected galaxies, to quantify the change on the measured structural parameters with redshift. We find typical sizes of 2 to 3 kpc and Sersic indices of n~1.2, close to pure exponential disks all the way from z=2.23 to z=0.4. At z=0 we find typical sizes of 4-5 kpc. Our results show that, when using GALFIT, cosmological dimming has a negligible impact on the derived effective radius for galaxies with

The HiZELS/UKIRT large area survey for bright Lyman-alpha emitters at z~9

We present the largest area survey to date (1.4 deg2) for Lyman-alpha emitters (LAEs) at z~9, as part of the Hi-z Emission Line Survey (HiZELS). The survey, which primarily targets H-alpha emitters at z < 3, uses the Wide Field CAMera on the United Kingdom Infrared Telescope and a custom narrow-band filter in the J band to reach a Lyman-alpha luminosity limit of ~10^43.8 erg/s over a co-moving volume of 1.12x10^6 Mpc^3 at z = 8.96+-0.06. Two candidates were found out of 1517 line emitters, but those were rejected as LAEs after follow-up observations. This improves the limit on the space density of bright Lyman-alpha emitters by 3 orders of magnitude and is consistent with suppression of the bright end of the Lyman-alpha luminosity function beyond z~6. Combined with upper limits from smaller but deeper surveys, this rules out some of the most extreme models for high-redshift Lyman-alpha emitters. The potential contamination of narrow-band Lyman-alpha surveys at z>7 by Galactic brown dwarf stars is also examined, leading to the conclusion that such contamination may well be significant for searches at 7.7 < z < 8.0, 9.1 < z < 9.5 and 11.7 < z < 12.2.Comment: To appear in proceedings of "UKIRT at 30: A British Success Story

HiZELS: the High Redshift Emission Line Survey with UKIRT

In these proceedings we report on HiZELS, the High-z Emission Line Survey, our successful panoramic narrow-band Campaign Survey using WFCAM on UKIRT to detect and study emission line galaxies at z~1-9. HiZELS employs the H2(S1) narrow-band filter together with custom-made narrow-band filters in the J and H-bands, with the primary aim of delivering large, identically-selected samples of H-alpha emitting galaxies at redshifts of 0.84, 1.47 and 2.23. Comparisons between the luminosity function, the host galaxy properties, the clustering, and the variation with environment of these H-alpha-selected samples are yielding unique constraints on the nature and evolution of star-forming galaxies, across the peak epoch of star-formation activity in the Universe. We provide a summary of the project status, and detail the main scientific results obtained so far: the measurement of the evolution of the cosmic star-formation rate density out to z > 2 using a single star-formation indicator, determination of the morphologies, environments and dust-content of the star-forming galaxies, and a detailed investigation of the evolution of their clustering properties. We also summarise the on-going work and future goals of the project.Comment: To appear in proceedings of "UKIRT at 30: A British Success Story"