arXiv subjects

David Sobral

Publications and source records attributed to David Sobral.

At least 109 records · Page 6Linked to original sources

arXiv2016

The Nature of H$β+$[O{\sc iii}] and [O{\sc ii}] emitters to $z \sim 5$ with HiZELS: stellar mass functions and the evolution of EWs

We investigate the properties of $\sim7000$ narrow-band selected galaxies with strong H$β+$[OIII] and [OII] nebular emission lines from the High-$z$ Emission Line Survey (HiZELS) between $z \sim 0.8 - 5.0$. Our sample covers a wide range in stellar mass ($M\sim10^{7.5 - 12.0}$ M$_\odot$), rest-frame equivalent widths (EW$_\mathrm{rest}\sim 10 - 10^5$ Å), and line luminosities ($L\sim10^{40.5 - 43.2}$ erg s$^{-1}$). We measure the H$β+$[OIII] and [OII]-selected stellar mass functions out to $z \sim 3.5$ and find that both $M_\star$ and $ϕ_\star$ increases with cosmic time, which may be due to the [OIII] selection including an increasing fraction of AGN at lower redshifts. The [OII]-selected stellar mass functions show a constant $M_\star\sim10^{11.6}$ M$_\odot$ and a strong, increasing evolution with cosmic time in $ϕ_\star$ in line with H$α$ studies. We also investigate the EW$_\mathrm{rest}$ evolution as a function of redshift with a fixed mass range (10$^{9.5 - 10.0}$ M$_\odot$) and find an increasing trend best represented by $(1+z)^{3.81\pm0.14}$ and $(1+z)^{2.72\pm0.19}$ up to $z\sim 2$ and $\sim 3$ for H$β$+[OIII] and [OII] emitters, respectively. This is the first time that the EW evolution has been directly measured for H$β+$[OIII] and [OII] emitters up to these redshifts. There is evidence for a slower evolution for $z > 2$ in the H$β+$[OIII] EW and a decreasing trend for $z > 3$ in the [OII] EW evolution, which would imply low [OII] EW at the highest redshifts and higher [OIII]/[OII] line ratios. This suggests that the ionization parameter at higher redshift may be significantly higher than the local Universe. Our results set the stage for future near-IR space-based spectroscopic surveys to test our extrapolated predictions and also produce $z > 5$ measurements to constrain the high-$z$ end of the EW$_\mathrm{rest}$ and [OIII]/[OII] evolution.

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arXiv2016

The nature of H-alpha star-forming galaxies at z~0.4 in and around Cl 0939+4713: the environment matters

Cluster star-forming galaxies are found to have an excess of Far-Infrared emission relative to H-alpha (Ha), when compared to those in the field, which could be caused by intense AGN activity, dust and/or declining star formation histories. Here we present spectroscopic observations of Ha emitters in the Cl 0939+4713 (Abell 851) super-cluster at z=0.41, using AF2+WYFFOS on the WHT. We measure [OII], Hbeta (Hb), [OIII], Ha and [NII] for a sample of 119 Ha emitters in and around the cluster. We find that 17+-5% of the Ha emitters are AGN, irrespective of environment. For star-forming galaxies, we obtain Balmer decrements, metallicities and ionisation parameters with different methods, individually and by stacking. We find a strong mass-metallicity relation at all environments, with no significant dependence on environment. The ionisation parameter declines with increasing stellar mass for low-mass galaxies. Ha emitters residing in intermediate environments show the highest ionisation parameters (along with high [OIII]/Ha and high [OIII]/[OII] line ratios, typically twice as large as in the highest and lowest densities), which decline with increasing environmental density. Dust extinction (A$_{Hα}$) correlates strongly with stellar mass, but also with environmental density. Star-forming galaxies in the densest environments are found to be significantly dustier (A$_{Hα}$~1.5-1.6) than those residing in the lowest density environments (A$_{Hα}$~0.6), deviating significantly from what would be predicted given their stellar masses.

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arXiv2016

Lyman Alpha Signatures from Direct Collapse Black Holes

`Direct collapse black holes' (DCBHs) provide possible seeds for supermassive black holes that exist at redshifts as high as z~7. We study Lyman Alpha (Lya) radiative transfer through simplified representations of the DCBH-scenario. We find that gravitational heating of the collapsing cloud gives rise to a Lya cooling luminosity of up to ~ 1e38(M_gas/1e6 Msun)^2 erg/s. The Lya production rate can be significantly larger during the final stages of collapse, but collisional deexcitation efficiently suppresses the emerging Lya flux. Photoionization by a central source boosts the Lya luminosity to L~1e43(M_BH/1e6 M_sun) erg/s during specific evolutionary stages of the cloud, where M_BH denotes the mass of the black hole powering this source. We predict that the width and velocity off-set of the Lya spectral line range from a few tens to few thousands km/s, depending sensitively on the evolutionary state of the cloud. We also compare our predictions to observations of CR7 (Sobral et al. 2015), a luminous Lya emitter at z~7, which is potentially associated with a DCBH. If CR7 is powered by a black hole, then its Lya flux alone requires that M_BH> 1e7 M_sun, which exceeds the mass of DCBHs when they first form. The observed width of the Lya spectrum favors the presence of only a low column density of hydrogen, log [N_HI/cm^-2]~19-20. The shape of the Lya spectrum indicates that this gas is outflowing. These requirements imply that if CR7 harbors a DCBH, then the physical conditions that enabled its formation have been mostly erased, which is in agreement with theoretical expectations.

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arXiv2016

The CALYMHA survey: Ly$α$ escape fraction and its dependence on galaxy properties at $z=2.23$

We present the first results from our CAlibrating LYMan-$α$ with H$α$ (CALYMHA) pilot survey at the Isaac Newton Telescope. We measure Ly$α$ emission for 488 H$α$ selected galaxies at $z=2.23$ from HiZELS in the COSMOS and UDS fields with a specially designed narrow-band filter ($λ_c$ = 3918 Å, $Δλ$= 52 Å). We find 17 dual H$α$-Ly$α$ emitters ($f_{\rm Lyα} >5\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$, of which 5 are X-ray AGN). For star-forming galaxies, we find a range of Ly$α$ escape fractions (f$_{\rm esc}$, measured with 3$"$ apertures) from $2$\%$-30$\%. These galaxies have masses from $3\times10^8$ M$_{\odot}$ to 10$^{11}$ M$_{\odot}$ and dust attenuations E$(B-V)=0-0.5$. Using stacking, we measure a median escape fraction of $1.6\pm0.5$\% ($4.0\pm1.0$\% without correcting H$α$ for dust), but show that this depends on galaxy properties. The stacked f$_{\rm esc}$ tends to decrease with increasing SFR and dust attenuation. However, at the highest masses and dust attenuations, we detect individual galaxies with f$_{\rm esc}$ much higher than the typical values from stacking, indicating significant scatter in the values of f$_{\rm esc}$. Relations between f$_{\rm esc}$ and UV slope are bimodal, with high f$_{\rm esc}$ for either the bluest or reddest galaxies. We speculate that this bimodality and large scatter in the values of f$_{\rm esc}$ is due to additional physical mechanisms such as outflows facilitating f$_{\rm esc}$ for dusty/massive systems. Ly$α$ is significantly more extended than H$α$ and the UV. f$_{\rm esc}$ continues to increase up to at least 20 kpc (3$σ$, 40 kpc [2$σ$]) for typical SFGs and thus the aperture is the most important predictor of f$_{\rm esc}$.

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arXiv2016

The KMOS Redshift One Spectroscopic Survey (KROSS): Dynamical properties, gas and dark matter fractions of typical z~1 star-forming galaxies

The KMOS Redshift One Spectroscopic Survey (KROSS) is an ESO guaranteed time survey of 795 typical star-forming galaxies in the redshift range z=0.8-1.0 with the KMOS instrument on the VLT. In this paper we present resolved kinematics and star formation rates for 584 z~1 galaxies. This constitutes the largest near-infrared Integral Field Unit survey of galaxies at z~1 to date. We demonstrate the success of our selection criteria with 90% of our targets found to be Halpha emitters, of which 81% are spatially resolved. The fraction of the resolved KROSS sample with dynamics dominated by ordered rotation is found to be 83$\pm$5%. However, when compared with local samples these are turbulent discs with high gas to baryonic mass fractions, ~35%, and the majority are consistent with being marginally unstable (Toomre Q~1). There is no strong correlation between galaxy averaged velocity dispersion and the total star formation rate, suggesting that feedback from star formation is not the origin of the elevated turbulence. We postulate that it is the ubiquity of high (likely molecular) gas fractions and the associated gravitational instabilities that drive the elevated star-formation rates in these typical z~1 galaxies, leading to the ten-fold enhanced star-formation rate density. Finally, by comparing the gas masses obtained from inverting the star-formation law with the dynamical and stellar masses, we infer an average dark matter to total mass fraction within 2.2$r_e$ (9.5kpc) of 65$\pm$12%, in agreement with the results from hydrodynamic simulations of galaxy formation.

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arXiv2016

The most luminous H$α$ emitters at z~0.8-2.23 from HiZELS: evolution of AGN and star-forming galaxies

We use new near-infrared spectroscopic observations to investigate the nature and evolution of the most luminous Hα(Ha) emitters at z~0.8-2.23, which evolve strongly in number density over this period, and compare them to more typical Ha emitters. We study 59 luminous Ha emitters with $L_{Hα}>L_{Hα}^*$, roughly equally split per redshift slice at z~0.8, 1.47 and 2.23 from the HiZELS and CF-HiZELS surveys. We find that, overall, 30$\pm$8% are AGN (80$\pm$30% of these AGN are broad-line AGN, BL-AGN), and we find little to no evolution in the AGN fraction with redshift, within the errors. However, the AGN fraction increases strongly with Ha luminosity and correlates best with $L_{Hα}/L_{Hα}^*(z)$. While $L_{Hα} 80%), the most luminous Ha emitters ($L_{Hα}>10L_{Hα}^*(z)$) at any cosmic time are essentially all BL-AGN. Using our AGN-decontaminated sample of luminous star-forming galaxies, and integrating down to a fixed Ha luminosity, we find a factor of ~1300x evolution in the star formation rate density from z=0 to z=2.23. This is much stronger than the evolution from typical Ha star-forming galaxies and in line with the evolution seen for constant luminosity cuts used to select "Ultra-Luminous" Infrared Galaxies and/or sub-millimetre galaxies. By taking into account the evolution in the typical Ha luminosity, we show that the most strongly star-forming Ha-selected galaxies at any epoch ($L_{Hα}>L^*_{Hα}(z)$) contribute the same fractional amount of ~15% to the total star-formation rate density, at least up to z=2.23.

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arXiv2015

Spectroscopic Study of Star-forming Galaxies in Filaments and the Field at $z\sim$0.5: Evidence for Environmental Dependence of Electron Density

We study the physical properties of a spectroscopic sample of 28 star-forming galaxies in a large filamentary structure in the COSMOS field at $z\sim$0.53, with spectroscopic data taken with the Keck/DEIMOS spectrograph, and compare them with a control sample of 30 field galaxies. We spectroscopically confirm the presence of a large galaxy filament ($\sim$ 8 Mpc), along which five confirmed X-ray groups exist. We show that within the uncertainties, the ionization parameter, equivalent width (EW), EW versus specific star-formation rate (sSFR) relation, EW versus stellar mass relation, line-of-sight velocity dispersion, dynamical mass, and stellar-to-dynamical mass ratio are similar for filament and field star-forming galaxies. However, we show that on average, filament star-forming galaxies are more metal-enriched ($\sim$ 0.1$-$0.15 dex), possibly due to the inflow of the already enriched intrafilamentary gas into filament galaxies. Moreover, we show that electron densities are significantly lower (a factor of $\sim$17) in filament star-forming systems compared to those in the field, possibly because of a longer star-formation timescale for filament star-forming galaxies. Our results highlight the potential pre-processing role of galaxy filaments and intermediate-density environments on the evolution of galaxies, which has been highly underestimated.

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arXiv2015

Nebular and Stellar Dust Extinction Across the Disk of Emission-Line Galaxies on Small (KPC) Scales

We investigate resolved kpc-scale stellar and nebular dust distribution in eight star-forming galaxies at z~0.4 in the GOODS fields. Constructing the observed Spectral Energy Distributions (SEDs) per pixel, based on seven bands photometric data from HST/ACS and WFC3, we performed pixel-by-pixel SED fits to population synthesis models and estimated small-scale distribution of stellar dust extinction. We use Halpha / Hbeta nebular emission line ratios from Keck/DEIMOS high resolution spectra at each spatial resolution element to measure the amount of attenuation faced by ionized gas at different radii from the center of galaxies. We find a good agreement between the integrated and median of resolved color excess measurements in our galaxies. The ratio of integrated nebular to stellar dust extinction is always greater than unity, but does not show any trend with stellar mass or star formation rate. We find that inclination plays an important role in the variation of the nebular to stellar excess ratio. The stellar color excess profiles are found to have higher values at the center compared to outer parts of the disk. However, for lower mass galaxies, a similar trend is not found for the nebular color excess. We find that the nebular color excess increases with stellar mass surface density. This explains the absence of radial trend in the nebular color excess in lower mass galaxies which lack a large radial variation of stellar mass surface density. Using standard conversions of star formation rate surface density to gas mass surface density, and the relation between dust mass surface density and color excess, we find no significant variation in the dust to gas ratio in regions with high gas mass surface densities, over the scales probed in this study.

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arXiv2015

Evolution of the H$β$+[OIII] and [OII] luminosity functions and the [OII] star-formation history of the Universe up to $z$ ~ 5 from HiZELS

We investigate the evolution of the H$β$+[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$β$+[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$β$+[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 $ϕ_\star$ for both H$β$+[OIII] and [OII] emitters. We derive the [OII] star-formation history of the Universe since $z\sim5$ 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 $z\sim 5$ based only on a single tracer of star-formation. When comparing the H$β$+[OIII] SFRDs to the [OII] and H$α$ SFRD measurements in the literature, we find that there is a remarkable agreement, suggesting that the H$β$+[OIII] sample is dominated by star-forming galaxies at high-$z$ rather than AGNs.

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arXiv2015

A large narrow band H$α$ survey at $z\sim0.2$: 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$α$ 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$α$ 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$α$ luminosity functions, which are well described by a Schechter function with $ϕ^* = 10^{-2.85\pm0.03}$ Mpc$^{-3}$ and $L^*_{Hα} = 10^{41.71\pm0.02}$ erg s$^{-1}$ (with a fixed faint end slope $α=-1.35$). 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$α$ luminosity function. We derive a star formation rate density of $ρ_\mathrm{SFRD} = 0.0094\pm0.0008$ $M_\odot$ yr$^{-1}$, in agreement with the redshift-dependent H$α$ parametrisation from Sobral et al. (2013). The two-point correlation function is described by a single power law $ω(θ) = (0.159\pm0.012) θ^{(-0.75\pm0.05)}$, corresponding to a clustering length of $r_0 = 3.3\pm0.8$ Mpc/h. We find that the most luminous H$α$ emitters at $z\sim0.2$ are more strongly clustered than the relatively fainter ones. The $L^*_{Hα}$ H$α$ 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 $L_\mathrm{Hα}/L^*_{Hα}(z)$, as proposed by Sobral et al. (2010).

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arXiv2015

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$α$ 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.

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arXiv2015

Evidence for PopIII-like stellar populations in the most luminous Lyman-$α$ emitters at the epoch of re-ionisation: spectroscopic confirmation

Faint Lyman-$α$ (Ly$α$) emitters become increasingly rarer towards the re-ionisation epoch (z~6-7). However, observations from a very large (~5deg$^2$) Ly$α$ survey at z=6.6 (Matthee et al. 2015) show that this is not the case for the most luminous emitters. Here we present follow-up observations of the two most luminous z~6.6 Ly$α$ candidates in the COSMOS field: `MASOSA' and `CR7'. We used X-SHOOTER, SINFONI and FORS2 (VLT), and DEIMOS (Keck), to confirm both candidates beyond any doubt. We find redshifts of z=6.541 and z=6.604 for MASOSA and CR7, respectively. MASOSA has a strong detection in Ly$α$ with a line width of $386\pm30$ km/s (FWHM) and with high EW$_0$ (>200 Å), but it is undetected in the continuum. CR7, with an observed Ly$α$ luminosity of $10^{43.93\pm0.05}$erg/s is the most luminous Ly$α$ emitter ever found at z>6. CR7 reveals a narrow Ly$α$ line with $266\pm15$ km/s FWHM, being detected in the NIR (rest-frame UV, with $β=-2.3\pm0.1$) with an excess in $J$, and also strongly detected in IRAC/Spitzer. We detect a narrow HeII1640$Å$ emission line ($6σ$) which explains the excess seen in the $J$ band photometry (EW$_0$~80 Å). We find no other emission lines from the UV to the NIR in our X-SHOOTER spectra (HeII/OIII]1663A>3 and HeII/CIII]1908A>2.5). We find that CR7 is best explained by a combination of a PopIII-like population which dominates the rest-frame UV and the nebular emission, and a more normal stellar population which dominates the mass. HST/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Ly$α$ and HeII emission, and two red components (~5 kpc away), which dominate the mass. Our findings are consistent with theoretical predictions of a PopIII wave, with PopIII star formation migrating away from the original sites of star formation.

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arXiv2015

Identification of the brightest Lyα 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-$α$ 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$α$ 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$α$ 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$α$ 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.

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arXiv2015

CF-HiZELS, a 10 deg$^2$ emission-line survey with spectroscopic follow-up: Hα, [OIII]+Hβ and [OII] luminosity functions at z=0.8, 1.4 and 2.2

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 $μ$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$α$ emitters at z=0.8, 1343 [OIII]+H$β$ 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$σ$ (spectroscopically confirmed) over-density of H$α$ emitters at z=0.81. We derive H$α$, [OIII]+H$β$ 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 $ϕ^*$ 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.

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arXiv2015

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.

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arXiv2015

MC$^2$: Boosted AGN and star-formation activity in CIZA J2242.8+5301, a massive post-merger cluster at z=0.19

Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. (2014) revealed unexpected over-densities of candidate H$α$ emitters near the ~1 Mpc-wide shock fronts of the massive (~2x10$^{15}$M$_{\odot}$) "Sausage" merging cluster, CIZA J2242.8+5301. We used Keck/DEIMOS and WHT/AF2 to confirm 83 H$α$ emitters in and around the merging cluster. We find that cluster star-forming galaxies in the hottest X-ray gas and/or in the cluster sub-cores (away from the shock fronts) show high [SII]6716/[SII]6761 and high [SII]6716/H$α$, implying very low electron densities (<30x lower than all other star-forming galaxies outside the cluster) and significant contribution from supernovae, respectively. All cluster star-forming galaxies near the cluster centre show evidence of significant outflows (blueshifted Na D~200-300km/s), likely driven by supernovae. Strong outflows are also found for the cluster H$α$ AGN. H$α$ star-forming galaxies in the merging cluster follow the z~0 mass-metallicity relation, showing systematically higher metallicity (~0.15-0.2 dex) than H$α$ emitters outside the cluster (projected R>2.5 Mpc). This suggests that the shock front may have triggered remaining metal-rich gas which galaxies were able to retain into forming stars. Our observations show that the merger of impressively massive (~10$^{15}$M$_\odot$) clusters can provide the conditions for significant star-formation and AGN activity, but, as we witness strong feedback by star-forming galaxies and AGN (and given how massive the merging cluster is), such sources will likely quench in a few 100 Myrs.

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arXiv2014

The rise and fall of star-formation in $\bf z\sim0.2$ 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 ($13$Å). We robustly separate between H$α$ and higher redshift emitters using a combination of optical multi-band (B, g, V, r, i, z) and spectroscopic data. We build H$α$ 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$α$ 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$α$ galaxies. The cluster H$α$ galaxies fall on the SFR-stellar mass relation $z\sim0.2$ for the field. However, the `Sausage' cluster has an H$α$ 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.

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arXiv2014

MC^2: Constraining the Dark Matter Distribution of the Violent Merging Galaxy Cluster CIZA J2242.8+5301 by Piercing through the Milky Way

CIZA J2242.8+5301 is a merging system with a prominent (~2 Mpc long) radio relic, which together with the morphology of the X-ray emission provides strong evidence for a violent collision along the N-S axis. We present our constraints on the dark matter distribution of this unusual system using Subaru and CFHT imaging data. Measuring a high S/N lensing signal from this cluster is potentially a challenging task because of its proximity to the Milky Way plane (|b|~5 deg). We overcome this challenge with careful observation planning and systematics control, which enables us to successfully map the dark matter distribution of the cluster with high fidelity. The resulting mass map shows that the mass distribution is highly elongated along the N-S merger axis inferred from the orientation of the radio relics. Based on our mass reconstruction, we identify two sub-clusters, which coincide with the cluster galaxy distributions. We determine their masses using MCMC analysis by simultaneously fitting two NFW halos without fixing their centroids. The resulting masses of the northern and southern systems are $M_{200}=11.0_{-3.2}^{+3.7}\times10^{14} M_{\sun}$ and $9.8_{-2.5}^{+3.8}\times10^{14} M_{\sun}$, respectively, indicating that we are witnessing a post-collision of two giant systems of nearly equal mass. When the mass and galaxy centroids are compared in detail, we detect ~ 1' (~190 kpc) offsets in both northern and southern sub-clusters. We find that the galaxy luminosity-mass offset for the northern clump is statistically significant at the ~2 sigma level whereas the detection is only marginal for the southern sub-cluster in part because of a relatively large mass centroid error. We conclude that it is yet premature to uniquely attribute the galaxy-mass misalignment to SIDM and discuss caveats.

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