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$1.
We investigate the evolution of the H$beta$+[OIII] and [OII] luminosity functions from $z sim 0.8$ to $sim5$ in four redshift slices per emission line using data from the High-{it z} Emission Line Survey (HiZELS). This is the first time that the H$beta$+[OIII] and [OII] luminosity functions have been studied at these redshifts in a self-consistent analysis. This is also the largest sample of [OII] and H$beta$+[OIII] emitters (3475 and 3298 emitters, respectively) in this redshift range, with large co-moving volumes $sim 1 times 10^6$ Mpc$^{-3}$ in two independent volumes (COSMOS and UDS), greatly reducing the effects of cosmic variance. The emitters were selected by a combination of photometric redshift and color-color selections, as well as spectroscopic follow-up, including recent spectroscopic observations using DEIMOS and MOSFIRE on the Keck Telescopes and FMOS on Subaru. We find a strong increase in $L_star$ and a decrease in $phi_star$ for both H$beta$+[OIII] and [OII] emitters. We derive the [OII] star-formation history of the Universe since $zsim5$ and find that the cosmic SFRD rises from $z sim 5$ to $sim 3$ and then drops towards $z sim 0$. We also find that our star-formation history is able to reproduce the evolution of the stellar mass density up to $zsim 5$ based only on a single tracer of star-formation. When comparing the H$beta$+[OIII] SFRDs to the [OII] and H$alpha$ SFRD measurements in the literature, we find that there is a remarkable agreement, suggesting that the H$beta$+[OIII] sample is dominated by star-forming galaxies at high-$z$ rather than AGNs.
We present results from the largest contiguous narrow-band survey in the near-infrared. We have used WIRCam/CFHT and the lowOH2 filter (1.187$pm$0.005 $mu$m) to survey ~10 deg$^2$ of contiguous extragalactic sky in the SA22 field. A total of ~6000 candidate emission-line galaxies are found. We use deep ugrizJK data to obtain robust photometric redshifts. We combine our data with the High-redshift Emission Line Survey (HiZELS), explore spectroscopic surveys (VVDS, VIPERS) and obtain our own spectroscopic follow-up with KMOS, FMOS and MOSFIRE to derive large samples of high-redshift emission-line selected galaxies: 3471 H$alpha$ emitters at z=0.8, 1343 [OIII]+H$beta$ emitters at z=1.4 and 572 [OII] emitters at z=2.2. We probe co-moving volumes of >10$^6$ Mpc$^3$ and find significant over-densities, including an 8.5$sigma$ (spectroscopically confirmed) over-density of H$alpha$ emitters at z=0.81. We derive H$alpha$, [OIII]+H$beta$ and [OII] luminosity functions at z=0.8,1.4,2.2, respectively, and present implications for future surveys such as Euclid. Our uniquely large volumes/areas allow us to sub-divide the samples in thousands of randomised combinations of areas and provide a robust empirical measurement of sample/cosmic variance. We show that surveys for star-forming/emission-line galaxies at a depth similar to ours can only overcome cosmic-variance (errors <10%) if they are based on volumes >5x10$^{5}$ Mpc$^{3}$; errors on $L^*$ and $phi^*$ due to sample (cosmic) variance on surveys probing ~10$^4$ Mpc$^{3}$ and ~10$^5$ Mpc$^{3}$ are typically very high: ~300% and ~40-60%, respectively.
We investigate the role of the delineated cosmic web/filaments on the star formation activity by exploring a sample of 425 narrow-band selected H{alpha} emitters, as well as 2846 color-color selected underlying star-forming galaxies for a large scale structure (LSS) at z=0.84 in the COSMOS field from the HiZELS survey. Using the scale-independent Multi-scale Morphology Filter (MMF) algorithm, we are able to quantitatively describe the density field and disentangle it into its major components: fields, filaments and clusters. We show that the observed median star formation rate (SFR), stellar mass, specific star formation rate (sSFR), the mean SFR-Mass relation and its scatter for both H{alpha} emitters and underlying star-forming galaxies do not strongly depend on different classes of environment, in agreement with previous studies. However, the fraction of H{alpha} emitters varies with environment and is enhanced in filamentary structures at z~1. We propose mild galaxy-galaxy interactions as the possible physical agent for the elevation of the fraction of H{alpha} star-forming galaxies in filaments. Our results show that filaments are the likely physical environments which are often classed as the intermediate densities, and that the cosmic web likely plays a major role in galaxy formation and evolution which has so far been poorly investigated.
We have observed a sample of typical z=1 star forming galaxies, selected from the HiZELS survey, with the new KMOS near-infrared, multi-IFU instrument on the VLT, in order to obtain their dynamics and metallicity gradients. The majority of our galaxies have a metallicity gradient consistent with being flat or negative (i.e. higher metallicity cores than outskirts). Intriguingly, we find a trend between metallicity gradient and specific star formation rate (sSFR), such that galaxies with a high sSFR tend to have relatively metal-poor centres, a result which is strengthened when combined with datasets from the literature. This result appears to explain the discrepancies reported between different high redshift studies and varying claims for evolution. From a galaxy evolution perspective, the trend we see would mean that a galaxys sSFR is governed by the amount of metal poor gas that can be funnelled into its core, triggered either by merging or through efficient accretion. In fact merging may play a significant role as it is the starburst galaxies at all epochs, which have the more positive metallicity gradients. Our results may help to explain the origin of the fundamental metallicity relation, in which galaxies at a fixed mass are observed to have lower metallicities at higher star formation rates, especially if the metallicity is measured in an aperture encompassing only the central regions of the galaxy. Finally, we note that this study demonstrates the power of KMOS as an efficient instrument for large scale resolved galaxy surveys.
We present the spatially resolved H-alpha (Ha) dynamics of sixteen star-forming galaxies at z~0.81 using the new KMOS multi-object integral field spectrograph on the ESO VLT. These galaxies were selected using 1.18 um narrow-band imaging from the 10 deg^2 CFHT-HiZELS survey of the SA22hr field, are found in a ~4Mpc over-density of Ha emitters and likely reside in a group/intermediate environment, but not a cluster. We confirm and identify a rich group of star-forming galaxies at z=0.813+-0.003, with thirteen galaxies within 1000 km/s of each other, and 7 within a diameter of 3Mpc. All our galaxies are typical star-forming galaxies at their redshift, 0.8+-0.4 SFR*(z=0.8), spanning a range of specific star formation rate of sSFR=0.2-1.1 Gyr^-1 and have a median metallicity very close to solar of 12+log(O/H)=8.62+-0.06. We measure the spatially resolved Ha dynamics of the galaxies in our sample and show that thirteen out of sixteen galaxies can be described by rotating disks and use the data to derive inclination corrected rotation speeds of 50-275 km/s. The fraction of disks within our sample is 75+-8, consistent with previous results based on HST morphologies of Ha selected galaxies at z~1 and confirming that disks dominate the star formation rate density at z~1. Our Ha galaxies are well fitted by the z~1-2 Tully-Fisher relation, confirming the evolution seen in the zero-point. Apart from having, on average, higher stellar masses and lower sSFRs, our group galaxies at z=0.813 present the same mass-metallicity and TF relation as z~1 field galaxies, and are all disk galaxies.
We obtained Subaru FMOS observations of Halpha emitting galaxies selected from the HiZELS narrow-band survey, to investigate the relationship between stellar mass, metallicity and star-formation rate 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 star formation rate (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 halos, 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.
We use the HiZELS narrow-band H-alpha survey in combination with CANDELS, UKIDSS and WIRDS near-infrared imaging, to investigate the morphologies, merger rates and sizes of a sample of H-alpha emitting galaxies in the redshift range z=0.40 - 2.23, an epoch encompassing the rise to the peak of the star formation rate density. Merger rates are estimated from space- and ground-based imaging using the M20 coefficient. To account for the increase in the specific star-formation rate (sSFR) of the star forming `main-sequence with redshift, we normalise the star-formation rate of galaxies at each epoch to the typical value derived from the H-alpha luminosity function. Once this trend in sSFR is removed we see no evidence for an increase in the number density of star-forming galaxies or the merger rate with redshift. We thus conclude that neither is the main driver of the enhanced star-formation rate density at z=1-2, with secular processes such as instabilities within efficiently fuelled, gas-rich discs or multiple minor mergers the most likely alternatives. However, we find that 40-50% of starburst galaxies, those with enhanced specific star formation at their epoch, are major mergers and this fraction is redshift independent. Finally, we find the surprising result that the typical size of a star-forming galaxy of a given mass does not evolve across the redshift range considered, suggesting a universal size-mass relation. Taken in combination, these results indicate a star-forming galaxy population that is statistically similar in physical size, merger rate and mass over the 6 Gyr covered in this study, despite the increase in typical sSFR.
We present adaptive optics assisted, spatially resolved spectroscopy of a sample of nine H-alpha-selected galaxies at z=0.84--2.23 drawn from the HiZELS narrow-band survey. These galaxies have star-formation rates of 1-27Mo/yr and are therefore representative of the typical high-redshift star-forming population. Our ~kpc-scale resolution observations show that approximately half of the sample have dynamics suggesting that the ionised gas is in large, rotating disks. We model their velocity fields to infer the inclination-corrected, asymptotic rotational velocities. We use the absolute B-band magnitudes and stellar masses to investigate the evolution of the B-band and stellar mass Tully-Fisher relationships. By combining our sample with a number of similar measurements from the literature, we show that, at fixed circular velocity, the stellar mass of star-forming galaxies has increased by a factor 2.5 between z=2 and z=0, whilst the rest-frame B-band luminosity has decreased by a factor ~6 over the same period. Together, these demonstrate a change in mass-to-light ratio in the B-band of Delta(M/L_B)/(M/L_B)_(z=0) sim 3.5 between z=1.5 and z=0, with most of the evolution occurring below z=1. We also use the spatial variation of [NII]/Halpha to show that the metallicity of the ionised gas in these galaxies declines monotonically with galacto-centric radius, with an average Delta(log O/H)/DeltaR=-0.027+/-0.005dex/kpc. This gradient is consistent with predictions for high-redshift disk galaxies from cosmologically based hydrodynamic simulations.
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.
