arXiv subjects

Stephen M. Wilkins

Publications and source records attributed to Stephen M. Wilkins.

At least 145 records · Page 8Linked to original sources

arXiv2017

GAMA/G10-COSMOS/3D-HST: The 0<z<5 cosmic star-formation history, stellar- and dust-mass densities

We use the energy-balance code MAGPHYS to determine stellar and dust masses, and dust corrected star-formation rates for over 200,000 GAMA galaxies, 170,000 G10-COSMOS galaxies and 200,000 3D-HST galaxies. Our values agree well with previously reported measurements and constitute a representative and homogeneous dataset spanning a broad range in stellar mass (10^8---10^12 Msol), dust mass (10^6---10^9 Msol), and star-formation rates (0.01---100 Msol per yr), and over a broad redshift range (0.0 < z < 5.0). We combine these data to measure the cosmic star-formation history (CSFH), the stellar-mass density (SMD), and the dust-mass density (DMD) over a 12 Gyr timeline. The data mostly agree with previous estimates, where they exist, and provide a quasi-homogeneous dataset using consistent mass and star-formation estimators with consistent underlying assumptions over the full time range. As a consequence our formal errors are significantly reduced when compared to the historic literature. Integrating our cosmic star-formation history we precisely reproduce the stellar-mass density with an ISM replenishment factor of 0.50 +/- 0.07, consistent with our choice of Chabrier IMF plus some modest amount of stripped stellar mass. Exploring the cosmic dust density evolution, we find a gradual increase in dust density with lookback time. We build a simple phenomenological model from the CSFH to account for the dust mass evolution, and infer two key conclusions: (1) For every unit of stellar mass which is formed 0.0065---0.004 units of dust mass is also formed; (2) Over the history of the Universe approximately 90 to 95 per cent of all dust formed has been destroyed and/or ejected.

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arXiv2017

Dust Obscured Star Forming Galaxies in the Early Universe

Motivated by recent observational constraints on dust reprocessed emission in star forming galaxies at $z\sim 6$ and above we use the very-large cosmological hydrodynamical simulation \bluetides\ to explore predictions for the amount of dust obscured star formation in the early Universe ($z>8$). \bluetides\ matches current observational constraints on both the UV luminosity function and galaxy stellar mass function and predicts that approximately $90\%$ of the star formation in high-mass ($M_{*}>10^{10}\,{\rm M_{\odot}}$) galaxies at $z=8$ is already obscured by dust. The relationship between dust attenuation and stellar mass predicted by \bluetides\ is consistent with that observed at lower redshift. However, observations of several individual objects at $z>6$ are discrepant with the predictions, though it is possible their uncertainties may have been underestimated. We find that the predicted surface density of $z\ge 8$ sub-mm sources is below that accessible to current {\em Herschel}, SCUBA-2, and ALMA sub-mm surveys. However, as ALMA continues to accrue additional surface area the population of $z>8$ dust-obscured galaxies may become accessible in the near future.

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arXiv2017

The properties of the first galaxies in the BLUETIDES simulation

We employ the very large cosmological hydrodynamical simulation BLUETIDES to investigate the predicted properties of the galaxy population during the epoch of reionisation ($z>8$). BLUETIDES has a resolution and volume ($(400/h\approx 577)^{3}\,{\rm cMpc^3}$) providing a population of galaxies which is well matched to depth and area of current observational surveys targeting the high-redshift Universe. At $z=8$ BLUETIDES includes almost 160,000 galaxies with stellar masses $>10^{8}\,{\rm M_{\odot}}$. The population of galaxies predicted by BLUETIDES closely matches observational constraints on both the galaxy stellar mass function and far-UV ($150\,{\rm nm}$) luminosity function. Galaxies in BLUETIDES are characterised by rapidly increasing star formation histories. Specific star formation rates decrease with redshift though remain largely insensitive to stellar mass. As a result of the enhanced surface density of metals more massive galaxies are predicted to have higher dust attenuation resulting in a significant steepening of the observed far-UV luminosity function at high luminosities. The contribution of active SMBHs to the UV luminosities of galaxies with stellar masses $10^{9-10}\,{\rm M_{\odot}}$ is around $3\%$ on average. Approximately $25\%$ of galaxies with $M_{*}\approx 10^{10}\,{\rm M_{\odot}}$ are predicted to have active SMBH which contribute $>10\%$ of the total UV luminosity.

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arXiv2016

Inferred Hα Flux as a Star-Formation Rate Indicator at z ~ 4-5: Implications for Dust Properties, Burstiness, and the z = 4-8 Star-Formation-Rate Functions

We derive Hα fluxes for a large spectroscopic and photometric-redshift-selected sample of sources over GOODS-North and South in the redshift range z = 3.8-5.0 with deep HST, Spitzer/IRAC, and ground-based observations. The Hα flux is inferred based on the offset between the IRAC 3.6 μm flux and that predicted from the best-fit SED. We demonstrate that the Hα flux correlates well with dust- corrected UV star-formation rate (SFR) and therefore can serve as an independent SFR indicator. However, we also find a systematic offset in the SFR_Hα/SFR_UV ratios for z ~ 4-5 galaxies relative to local relations (assuming the same dust corrections for nebular regions and stellar light). We show that we can resolve the modest tension in the inferred SFRs by assuming bluer intrinsic UV slopes (increasing the dust correction), a rising star-formation history or assuming a low metallicity stellar population with a hard ionizing spectrum (increasing the L_Hα/SFR ratio). Using Hα as a SFR indicator, we find a normalization of the star formation main sequence in good agreement with recent SED-based determinations and also derive the SFR functions at z ~ 4-8. In addition, we assess for the first time the burstiness of star formation in z ~ 4 galaxies on <100 Myr time scales by comparing UV and Hα-based sSFRs; their one-to-one relationship argues against significantly bursty star-formation histories. Further progress will be made on these results, by incorporating new results from ALMA to constrain the dust-obscured star formation in high-redshift UV-selected samples.

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arXiv2016

Forecasts for the WFIRST High Latitude Survey using the BlueTides Simulation

We use the BlueTides simulation to predict the properties of the high-$z$ galaxy and active galactic nuclei (AGN) populations for the planned 2200deg$^2$ Wide-Field Infrared Survey Telescope's (WFIRST)-AFTA High Latitude Survey (HLS). BlueTides is a cosmological hydrodynamic simulation, which incorporates a variety of baryon physics in a $(400h^{-1} \mathrm{Mpc})^3$ volume evolved to $z=8$ with 0.7 trillion particles. The galaxy luminosity functions in the simulation show good agreement with all the current observational constraints (up to $z=11$) and predicts an enhanced number of UV bright galaxies. At the proposed depth of the HLS ($m < 26.75$), BlueTides predicts $10^6$ galaxies at $z=8$ with a few up to $z\sim 15$ due to the enhanced bright end of the galaxy luminosity function. At $z=8$, galaxies in the mock HLS have specific star formation rates of $\sim 10 {\rm Gyr}^{-1}$ and ages of $\sim 80 {\rm Myr}$ (both evolving linearly with redshift) and a non-evolving mass-metallicity relation. BlueTides also predicts $\sim 10^4$ AGN in WFIRST HLS from $z=8$ out to $z\sim 14$. These AGN host black holes of $M\sim 10^6-10^8 M_\odot$ accreting close to their Eddington luminosity. Galaxies and AGN have host halo masses of $M_{halo}\sim 10^{11-12} M_\odot$ and a linear bias $b\approx 13-20$. Given the expected galaxy space densities, their high bias and large volume probed we speculate that it may be feasible for WFIRST HLS detect the Baryon Acoustic Oscillation peak in the galaxy power spectrum out to $z=8-9$.

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arXiv2016

The Photometric Properties of Galaxies in the Early Universe

We use the large cosmological hydro-dynamic simulation BlueTides to predict the photometric properties of galaxies during the epoch of reionisation ($z=8-15$). These properties include the rest-frame UV to near-IR broadband spectral energy distributions, the Lyman continuum photon production, the UV star formation rate calibration, and intrinsic UV continuum slope. In particular we focus on exploring the effect of various modelling assumptions, including the assumed choice of stellar population synthesis model, initial mass function, and the escape fraction of Lyman continuum photons, upon these quantities. We find that these modelling assumptions can have a dramatic effect on photometric properties leading to consequences for the accurate determination of physical properties from observations. For example, at $z=8$ we predict that nebular emission can account for up-to $50\%$ of the rest-frame $R$-band luminosity, while the choice of stellar population synthesis model can change the Lyman continuum production rate up to a factor of $\times 2$.

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arXiv2016

The Lyman-continuum photon production efficiency in the high-redshift Universe

The Lyman Continuum photon production efficiency ($ξ_{\rm ion}$) is a critical ingredient for inferring the number of photons available to reionise the intergalactic medium. To estimate the theoretical production efficiency in the high-redshift Universe we couple the BlueTides cosmological hydrodynamical simulation with a range of stellar population synthesis models. We find Lyman Continuum photon production efficiencies of $\log_{10}(ξ_{\rm ion}/{\rm erg^{-1}\, Hz})\approx 25.1-25.5$ depending on the choice of stellar population synthesis model. These results are broadly consistent with recent observational constraints at high-redshift though favour a model incorporating the effects of binary evolution

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arXiv2015

Galaxy And Mass Assembly (GAMA): Panchromatic Data Release (far-UV --- far-IR) and the low-z energy budget

We present the GAMA Panchromatic Data Release (PDR) constituting over 230deg$^2$ of imaging with photometry in 21 bands extending from the far-UV to the far-IR. These data complement our spectroscopic campaign of over 300k galaxies, and are compiled from observations with a variety of facilities including: GALEX, SDSS, VISTA, WISE, and Herschel, with the GAMA regions currently being surveyed by VST and scheduled for observations by ASKAP. These data are processed to a common astrometric solution, from which photometry is derived for 221,373 galaxies with r<19.8 mag. Online tools are provided to access and download data cutouts, or the full mosaics of the GAMA regions in each band. We focus, in particular, on the reduction and analysis of the VISTA VIKING data, and compare to earlier datasets (i.e., 2MASS and UKIDSS) before combining the data and examining its integrity. Having derived the 21-band photometric catalogue we proceed to fit the data using the energy balance code MAGPHYS. These measurements are then used to obtain the first fully empirical measurement of the 0.1-500$μ$m energy output of the Universe. Exploring the Cosmic Spectral Energy Distribution (CSED) across three time-intervals (0.3-1.1Gyr, 1.1-1.8~Gyr and 1.8---2.4~Gyr), we find that the Universe is currently generating $(1.5 \pm 0.3) \times 10^{35}$ h$_{70}$ W Mpc$^{-3}$, down from $(2.5 \pm 0.2) \times 10^{35}$ h$_{70}$ W Mpc$^{-3}$ 2.3~Gyr ago. More importantly, we identify significant and smooth evolution in the integrated photon escape fraction at all wavelengths, with the UV escape fraction increasing from 27(18)% at z=0.18 in NUV(FUV) to 34(23)% at z=0.06. The GAMA PDR will allow for detailed studies of the energy production and outputs of individual systems, sub-populations, and representative galaxy samples at $z<0.5$. The GAMA PDR can be found at: http://gama-psi.icrar.org/

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arXiv2015

Exploring the Dawn of Galaxies

Some few hundred million years after the big bang the Universe was illuminated by the first stars and galaxies thereby bringing an end to the cosmological dark ages. Since the installation of WFC3 on the Hubble Space Telescope our ability to probe this critical period of the Universe's history has dramatically changed with thousands of objects now identified within the first billion years of the Universe's history. Our understanding of this period of the Universe's history will further grow thanks to both the Atacama Large millimetre/sub-millimetre Array and the James Webb Space Telescope.

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arXiv2015

Quantifying the UV continuum slopes of galaxies to z~10 using deep Hubble and Spitzer/IRAC observations

Measurements of the UV-continuum slopes provide valuable information on the physical properties of galaxies forming in the early universe, probing the dust reddening, age, metal content, and even the escape fraction. While constraints on these slopes generally become more challenging at higher redshifts as the UV continuum shifts out of the Hubble Space Telescope bands (particularly at z>7), such a characterisation actually becomes abruptly easier for galaxies in the redshift window z=9.5-10.5 due to the Spitzer/IRAC 3.6um-band probing the rest-UV continuum and the long wavelength baseline between this Spitzer band and the Hubble H-band. Higher S/N constraints on the UV slope are possible at z~10 than at z=8. Here we take advantage of this opportunity and five recently discovered bright z=9.5-10.5 galaxies to present the first measurements of the mean slope for a multi-object sample of galaxy candidates at z~10. We find the measured observed slopes of these candidates are $-2.1\pm0.3\pm0.2$ (random and systematic), only slightly bluer than the measured slopes at 3.5<z<7.5 for galaxies of similar luminosities. Small increases in the stellar ages, metallicities, and dust content of the galaxy population from z~10 to z~7 could easily explain the apparent evolution in slopes.

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arXiv2015

Luminosity function of [OII] emission-line galaxies in the MassiveBlack-II simulation

We examine the luminosity function (LF) of [OII] emission-line galaxies in the high-resolution cosmological simulation MassiveBlack-II (MBII). From the spectral energy distribution of each galaxy, we select a sub-sample of star-forming galaxies at $0.06 \le z \le 3.0$ using the [OII] emission line luminosity L([OII]). We confirm that the specific star formation rate matches that in the GAMA survey. We show that the [OII] LF at z=1.0 from the MBII shows a good agreement with the LFs from several surveys below L([OII])=$10^{43.0}$ erg/s while the low redshifts ($z \le 0.3$) show an excess in the prediction of bright [OII] galaxies, but still displaying a good match with observations below L([OII])=$10^{41.6}$ erg/s. Based on the validity in reproducing the properties of [OII] galaxies at low redshift ($z \le 1$), we forecast the evolution of the [OII] LF at high redshift ($z \le 3$), which can be tested by upcoming surveys such as the HETDEX and DESI. The slopes of the LFs at bright and faint ends range from -3 to -2 showing minima at z=2. The slope of the bright end evolves approximately as 1/(z+1) at z=2 while the faint end evolves as ~3/(z+1) at $0.6 \le z \le 2$. In addition, a similar analysis is applied for the evolution of [OIII] LFs, which is to be explored in the forthcoming survey WFIRST-AFTA. Finally, we show that the auto-correlation function of [OII] and [OIII] emitting galaxies shows a rapid evolution from z=2 to 1.

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arXiv2014

Galaxy And Mass Assembly (GAMA): mass-size relations of z$<$0.1 galaxies subdivided by Sérsic index, colour and morphology

We use data from the Galaxy And Mass Assembly (GAMA) survey in the redshift range 0.01$<$z$<$0.1 (8399 galaxies in $g$ to $K_s$ bands) to derive the stellar mass $-$ half-light radius relations for various divisions of 'early' and 'late'-type samples. We find the choice of division between early and late (i.e., colour, shape, morphology) is not particularly critical, however, the adopted mass limits and sample selections (i.e., the careful rejection of outliers and use of robust fitting methods) are important. In particular we note that for samples extending to low stellar mass limits ($<10^{10}\mathcal{M_{\odot}}$) the Sérsic index bimodality, evident for high mass systems, becomes less distinct and no-longer acts as a reliable separator of early- and late-type systems. The final set of stellar mass $-$ half-light radius relations are reported for a variety of galaxy population subsets in 10 bands ($ugrizZYJHK_s$) and are intended to provide a comprehensive low-z benchmark for the many ongoing high-z studies. Exploring the variation of the stellar mass $-$ half-light radius relations with wavelength we confirm earlier findings that galaxies appear more compact at longer wavelengths albeit at a smaller level than previously noted: at $10^{10}\mathcal{M_{\odot}}$ both spiral systems and ellipticals show a decrease in size of 13% from $g$ to $K_s$ (which is near linear in log wavelength). Finally we note that the sizes used in this work are derived from 2D Sérsic light profile fitting (using GALFIT3), i.e., elliptical semi-major half light radii, improving on earlier low-z benchmarks based on circular apertures.

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arXiv2014

Spectroscopy of z ~ 7 candidate galaxies: Using Lyman-alpha to constrain the neutral fraction of hydrogen in the high-redshift universe

Following our previous spectroscopic observations of $z>7$ galaxies with Gemini/GNIRS and VLT/XSHOOTER, which targeted a total of 8 objects, we present here our results from a deeper and larger VLT/FORS2 spectroscopic sample of Wide Field Camera 3 selected $z>7$ candidate galaxies. With our FORS2 setup we cover the 737-1070nm wavelength range, enabling a search for Lyman-$α$ in the redshift range spanning 5.06 - 7.80. We target 22 $z$-band dropouts and find no evidence of Lyman-$α$ emission, with the exception of a tentative detection ($<5σ$, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman-$α$ flux and the broad-band magnitudes are used to constrain the rest-frame Equivalent Widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame Equivalent Width emitters follows the trend seen at $z=3-6.5$ is inconsistent with our non-detections at $z\sim7.8$ at the 96\% confidence level. This may indicate that a significant neutral HI fraction in the intergalactic medium suppresses Lyman-$α$, with an estimated neutral fraction $χ_{HI}\sim0.5$, in agreement with other estimates.

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arXiv2014

The MassiveBlack-II Simulation: The Evolution of Halos and Galaxies to z~0

(Abridged for arXiv)We investigate the properties of halos, galaxies and blackholes to z=0 in the high resolution hydrodynamical simulation MassiveBlack-II (MBII) which evolves a LCDM cosmology in a comoving volume Vbox=100(Mpc/h)^3. MBII is the highest resolution simulation of this size which includes a self-consistent model for star formation, black hole accretion and associated feedback. We provide a simulation browser web application which enables interactive search and tagging of halos, subhalos and their properties and publicly release our galaxy catalogs. Our analysis of the halo mass function (MF) in MBII reveals that baryons have strong effects, with changes in the halo abundance of 20-35% below the knee of the MF (Mhalo < 10^13.2 Msun/h at z=0) when compared to fits based on dark matter only simulations. We provide a fitting function for the halo MF out to redshift z=11 and discuss how the onset of non-universality in the MF limits the accuracy of our fit. We study the halo occupation distribution and clustering of galaxies, in particular the evolution and scale dependence of stochasticity and bias finding reasonable agreement with observational data. The shape of the cosmic spectral energy distribution predicted by MBII is consistent with observations, but lower in amplitude. The Galaxy Stellar Mass Function (GSMF) function is broadly consistent with observations at z>=2. At z<2, the population of passive low mass (for M*<10^9 Msun) galaxies in MBII makes the GSMF too steep compared to observations whereas at the high mass end (M*>10^11 Msun) galaxies hosting bright AGNs make significant contributions to the GSMF. The quasar bolometric luminosity function is also largely consistent with observations. We note however that more efficient AGN feedback (beyond simple thermal coupling used here) is likely necessary for the largest, rarest objects/clusters at low redshifts.

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arXiv2014

High-redshift galaxies and low-mass stars

The sensitivity available to near-infrared surveys has recently allowed us to probe the galaxy population at $z\approx 7$ and beyond. The existing {\em Hubble} Wide Field Camera 3 (WFC3) and the Visible and Infrared Survey Telescope for Astronomy (VISTA) Infrared Camera (VIRCam) instruments allow deep surveys to be undertaken well beyond one micron - a capability that will be further extended with the launch and commissioning of the {\em James Webb Space Telescope (JWST)}. As new regions of parameter space in both colour and depth are probed new challenges for distant galaxy surveys are identified. In this paper we present an analysis of the colours of L and T dwarf stars in widely used photometric systems. We also consider the implications of the newly-identified Y dwarf population - stars that are still cooler and less massive than T dwarfs for both the photometric selection and spectroscopic follow-up of faint and distant galaxies. We highlight the dangers of working in the low-signal-to-noise regime, and the potential contamination of existing and future samples. We find that {\em Hubble}/WFC3 and VISTA/VIRCam $Y$-drop selections targeting galaxies at $z\sim7.5$ are vulnerable to contamination from T and Y class stars. Future observations using {\em JWST}, targeting the $z\sim7$ galaxy population, are also likely to prove difficult without deep medium-band observations. We demonstrate that single emission line detections in typical low signal-to-noise spectroscopic observations may also be suspect, due to the unusual spectral characteristics of the cool dwarf star population.

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arXiv2013

Theoretical predictions for the effect of nebular emission on the broad band photometry of high-redshift galaxies

By combining optical and near-IR observations from the Hubble Space Telescope with NIR photometry from the Spitzer Space Telescope it is possible to measure the rest-frame UV-optical colours of galaxies at z=4-8. The UV-optical spectral energy distribution of star formation dominated galaxies is the result of several different factors. These include the joint distribution of stellar masses, ages, and metallicities, and the subsequent reprocessing by dust and gas in the ISM. Using a large cosmological hydrodynamical simulation we investigate the predicted spectral energy distributions of galaxies at high-redshift with a particular emphasis on assessing the potential contribution of nebular emission. We find that the average pure stellar UV-optical colour correlates with both luminosity and redshift such that galaxies at lower-redshift and higher-luminosity are typically redder. Assuming the escape fraction of ionising photons is close to zero, the effect of nebular emission is to redden the UV-optical 1500-V_w colour by, on average, 0.4 mag at z=8 declining to 0.25 mag at z=4. Young and low-metallicity stellar populations, which typically have bluer pure stellar UV-optical colours, produce larger ionising luminosities and are thus more strongly affected by the reddening effects of nebular emission. This causes the distribution of 1500-V_w colours to narrow and the trends with luminosity and redshift to weaken. The strong effect of nebular emission leaves observed-frame colours critically sensitive to the source redshift. For example, increasing the redshift by 0.1 can result in observed frame colours changing by up to ~0.6. These predictions reinforce the need to include nebular emission when modelling the spectral energy distributions of galaxies at high-redshift and also highlight the difficultly in interpreting the observed colours of individual galaxies without precise redshifts.

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arXiv2013

Single Colour Diagnostics of the Mass-to-light Ratio: Predictions from Galaxy Formation Models

Accurate galaxy stellar masses are crucial to better understand the physical mechanisms driving the galaxy formation process. We use synthetic star formation and metal enrichment histories predicted by the {\sc galform} galaxy formation model to investigate the precision with which various colours $(m_{a}-m_{b})$ can alone be used as diagnostics of the stellar mass-to-light ratio. As an example, we find that, at $z=0$, the {\em intrinsic} (B$_{f435w}-$V$_{f606w}$) colour can be used to determine the intrinsic rest-frame $V$-band stellar mass-to-light ratio ($\log_{10}Γ_{V}=\log_{10}[(M/M_{\odot})/(L_{V}/L_{V\odot})]$) with a precision of $σ_{lgΓ}\simeq 0.06$ when the initial mass function and redshift are known beforehand. While the presence of dust, assuming a universal attenuation curve, can have a systematic effect on the inferred mass-to-light ratio using a single-colour relation, this is typically small as it is often possible to choose a colour for which the dust reddening vector is approximately aligned with the $(m_{a}-m_{b})-\log_{10}Γ_{V}$ relation. The precision with which the stellar mass-to-light ratio can be recovered using a single colour diagnostic rivals implementations of SED fitting using more information but in which simple parameterisations of the star formation and metal enrichment histories are assumed. To facilitate the wide use of these relations, we provide the optimal observer frame colour to estimate the stellar mass-to-light ratio, along with the associated parameters, as a function of redshift ($0<z<1.5$) for two sets of commonly used filters.

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arXiv2013

Interpreting the observed UV continuum slopes of high-redshift galaxies

The observed UV continuum slope of star forming galaxies is strongly affected by the presence of dust. Its observation is then a potentially valuable diagnostic of dust attenuation, particularly at high-redshift where other diagnostics are currently inaccesible. Interpreting the observed UV continuum slope in the context of dust attenuation is often achieved assuming the empirically calibrated Meurer et al. (1999) relation. Implicit in this relation is the assumption of an intrinsic UV continuum slope ($β=-2.23$). However, results from numerical simulations suggest that the intrinsic UV continuum slopes of high-redshift star forming galaxies are bluer than this, and moreover vary with redshift. Using values of the intrinsic slope predicted by numerical models of galaxy formation combined with a Calzetti et al. (2000) reddening law we infer UV attenuations ($A_{1500}$) $0.35-0.5\,{\rm mag}$ ($A_{V}$: $0.14-0.2\,{\rm mag}$ assuming Calzetti et al. 2000) greater than simply assuming the Meurer relation. This has significant implications for the inferred amount of dust attenuation at very-high ($z\approx 7$) redshift given current observational constraints on $β$, combined with the Meurer relation, suggest dust attenuation to be virtually zero in all but the most luminous systems.

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