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Search Results: 1 - 10 of 410837 matches for " Stephen M. Wilkins "
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Exploring the Dawn of Galaxies
Stephen M. Wilkins,Elizabeth Stanway
Physics , 2015, DOI: 10.1093/astrogeo/atv092
Abstract: 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.
New Star Forming Galaxies at z\approx 7 from WFC3 Imaging
Stephen M. Wilkins,Andrew J. Bunker,Silvio Lorenzoni,Joseph Caruana
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17626.x
Abstract: The addition of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) has led to a dramatic increase in our ability to study the z>6 Universe. The increase in the near-infrared (NIR) sensitivity of WFC3 over previous instruments has enabled us to reach apparent magnitudes approaching 29 (AB). This allows us to probe the rest-frame ultraviolet (UV) continuum, redshifted into the NIR at $z>6$. Taking advantage of the large optical depths at this redshift, resulting in the Lyman-alpha break, we use a combination of WFC3 imaging and pre-existing Advanced Camera for Surveys (ACS) imaging to search for z approx 7 over 4 fields. Our analysis reveals 29 new z approx 7 star forming galaxy candidates in addition to 16 pre-existing candidates already discovered in these fields. The improved statistics from our doubling of the robust sample of z-drop candidates confirms the previously observed evolution of the bright end of the luminosity function.
High-redshift galaxies and low-mass stars
Stephen M. Wilkins,Elizabeth R. Stanway,Malcolm N. Bremer
Physics , 2014, DOI: 10.1093/mnras/stu029
Abstract: 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.
Predictions for the intrinsic UV continuum properties of star forming galaxies and the implications for inferring dust extinction
Stephen M. Wilkins,Violeta Gonzalez-Perez,Cedric G. Lacey,Carlton M. Baugh
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.21344.x
Abstract: The observed ultraviolet continuum (UVC) slope is potentially a powerful diagnostic of dust obscuration in star forming galaxies. However, the intrinsic slope is also sensitive to the form of the stellar initial mass function (IMF) and to the recent star formation and metal enrichment histories of a galaxy. Using the galform semi-analytical model of galaxy formation, we investigate the intrinsic distribution of UVC slopes. For star-forming galaxies, we find that the intrinsic distribution of UVC slopes at z=0, parameterised by the power law index beta, has a standard deviation of sigma_beta=0.30. This suggests an uncertainty on the inferred UV attenuation of A_fuv=0.7$ (assuming a Calzetti attenuation curve) for an individual object, even with perfect photometry. Furthermore, we find that the intrinsic UVC slope correlates with star formation rate, intrinsic UV luminosity, stellar mass and redshift. These correlations have implications for the interpretation of trends in the observed UVC slope with these quantities irrespective of the sample size or quality of the photometry. Our results suggest that in some cases the attenuation by dust has been incorrectly estimated.
The accuracy of the UV continuum as an indicator of the star formation rate in galaxies
Stephen M. Wilkins,Violeta Gonzalez-Perez,Cedric G. Lacey,Carlton M. Baugh
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.22092.x
Abstract: The rest-frame intrinsic UV luminosity is often used as an indicator of the instantaneous star formation rate (SFR) in a galaxy. While it is in general a robust indicator of the ongoing star formation activity, the precise value of the calibration relating the UV luminosity to the SFR ($B_{\nu}$), is sensitive to various physical properties, such as the recent star formation and metal enrichment histories, along with the choice of stellar initial mass function. The distribution of these properties for the star-forming galaxy population then suggests that the adoption of a single calibration is not appropriate unless properly qualified with the uncertainties on the calibration. We investigate, with the aid of the {\sc galform} semi-analytic model of galaxy formation, the distribution of UV-SFR calibrations obtained using realistic star formation and metal enrichment histories. At $z=0$, we find that when the initial mass function is fixed (to the Kennicutt IMF), the median calibration is $B_{\rm fuv}=0.9$ where ${\rm SFR}/[{\rm M_{\odot}\,yr^{-1}}]=B_{\nu}\times 10^{-28}\times L_{\nu}/[{\rm ergs\,s^{-1}\,Hz^{-1}}]$. However, the width of the distribution $B_{\rm fuv}$ suggests that for a single object there is around a 20% {\em intrinsic} uncertainty (at $z=0$, rising to $\simeq 30%$ at $z=6$) on the star formation rate inferred from the FUV luminosity without additional constraints on the star formation history or metallicity. We also find that the median value of the calibration $B_{\rm fuv}$ is correlated with the star formation rate and redshift (at $z>3$) raising implications for the correct determination of the star formation rate from the UV.
Single Colour Diagnostics of the Mass-to-light Ratio: Predictions from Galaxy Formation Models
Stephen M. Wilkins,Violeta Gonzalez-Perez,Carlton M. Baugh,Cedric G. Lacey,Joe Zuntz
Physics , 2013, DOI: 10.1093/mnras/stt192
Abstract: 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 ($\log_{10}\Gamma_{V}=\log_{10}[(M/M_{\odot})/(L_{V}/L_{V\odot})]$) with a precision of $\sigma_{lg\Gamma}\simeq 0.06$ 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 $(m_{a}-m_{b})-\log_{10}\Gamma_{V}$ 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
The UV Properties of Star Forming Galaxies I: HST WFC3 Observations of Very-high Redshift Galaxies
Stephen M. Wilkins,Andrew J. Bunker,Elizabeth Stanway,Silvio Lorenzoni,Joseph Caruana
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19315.x
Abstract: The acquisition of deep Near-IR imaging with Wide Field Camera 3 on the Hubble Space Telescope has provided the opportunity to study the very-high redshift Universe. For galaxies up to $z\approx 7.7$ sufficient wavelength coverage exists to probe the rest-frame ultraviolet (UV) continuum without contamination from either Lyman-$\alpha$ emission or the Lyman-$\alpha$ break. In this work we use Near-IR imaging to measure the rest-frame UV continuum colours of galaxies at $4.7
Interpreting the observed UV continuum slopes of high-redshift galaxies
Stephen M. Wilkins,Andrew Bunker,William Coulton,Rupert Croft,Tiziana Di Matteo,Nishikanta Khandai,Yu Feng
Physics , 2013, DOI: 10.1093/mnras/stt096
Abstract: The observed UV continuum slope of star forming galaxies is strongly affected by the presence of dust. Its observation is then a potentially valuable diagnostic of dust attenuation, particularly at high-redshift where other diagnostics are currently inaccesible. Interpreting the observed UV continuum slope in the context of dust attenuation is often achieved assuming the empirically calibrated Meurer et al. (1999) relation. Implicit in this relation is the assumption of an intrinsic UV continuum slope ($\beta=-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 $\beta$, combined with the Meurer relation, suggest dust attenuation to be virtually zero in all but the most luminous systems.
Confronting predictions of the galaxy stellar mass function with observations at high-redshift
Stephen M. Wilkins,Tiziana Di Matteo,Rupert Croft,Nishikanta Khandai,Yu Feng,Andrew Bunker,William Coulton
Physics , 2013, DOI: 10.1093/mnras/sts480
Abstract: We investigate the evolution of the galaxy stellar mass function at high-redshift ($z\ge 5$) using a pair of large cosmological hydrodynamical simulations: {\em MassiveBlack} and {\em MassiveBlack-II}. By combining these simulations we can study the properties of galaxies with stellar masses greater than $10^{8}\,{\rm M_{\odot}}\,h^{-1}$ and (co-moving) number densities of $\log_{10}(\phi\, [{\rm Mpc^{-3}\,dex^{-1}}\,h^{3}])>-8$. Observational determinations of the galaxy stellar mass function at very-high redshift typically assume a relation between the observed UV luminosity and stellar mass-to-light ratio which is applied to high-redshift samples in order to estimate stellar masses. This relation can also be measured from the simulations. We do this, finding two significant differences with the usual observational assumption: it evolves strongly with redshift and has a different shape. Using this relation to make a consistent comparison between galaxy stellar mass functions we find that at $z=6$ and above the simulation predictions are in good agreement with observed data over the whole mass range. Without using the correct UV luminosity and stellar mass-to-light ratio, the discrepancy would be up to two orders of magnitude for large galaxies $>10^{10}\,{\rm M_{\odot}}\,h^{-1}$. At $z=5$, however the stellar mass function for low mass $<10^{9}\,{\rm M_{\odot}}\,h^{-1}$ galaxies is overpredicted by factors of a few, consistent with the behaviour of the UV luminosity function, and perhaps a sign that feedback in the simulation is not efficient enough for these galaxies.
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
Renske Smit,Rychard J. Bouwens,Ivo Labbé,Marijn Franx,Stephen M. Wilkins,Pascal A. Oesch
Physics , 2015,
Abstract: We derive H{\alpha} 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 H{\alpha} flux is inferred based on the offset between the IRAC 3.6 {\mu}m flux and that predicted from the best-fit SED. We demonstrate that the H{\alpha} 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_H{\alpha}/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_H{\alpha}/SFR ratio). Using H{\alpha} as a SFR indicator, we find a higher normalization of the star formation main sequence compared to 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 H{\alpha}-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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