arXiv subjects

Elizabeth R. Stanway

Publications and source records attributed to Elizabeth R. Stanway.

At least 37 records · Page 2Linked to original sources

arXiv2017

What can distant galaxies teach us about massive stars?

Observations of star-forming galaxies in the distant Universe (z > 2) are starting to confirm the importance of massive stars in shaping galaxy emission and evolution. Inevitably, these distant stellar populations are unresolved, and the limited data available must be interpreted in the context of stellar population synthesis models. With the imminent launch of JWST and the prospect of spectral observations of galaxies within a gigayear of the Big Bang, the uncertainties in modelling of massive stars are becoming increasingly important to our interpretation of the high redshift Universe. In turn, these observations of distant stellar populations will provide ever stronger tests against which to gauge the success of, and flaws in, current massive star models.

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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

Physical Properties of Local Star-Forming Analogues to z~5 Lyman Break Galaxies

Intense, compact, star-forming galaxies are rare in the local Universe but ubiquitous at high redshift. We interpret the 0.1-22 um spectral energy distributions (SED) of a sample of 180 galaxies at 0.05<z<0.25 selected for extremely high surface densities of inferred star formation in the ultraviolet. By comparison with well-established stellar population synthesis models we find that our sample comprises young (~ 60 - 400 Myrs), moderate mass (~ $6\times 10^9$ Msun) star-forming galaxies with little dust extinction (mean stellar continuum extinction $E_\mathrm{cont}$(B-V) ~ 0.1) and find star formation rates of a few tens of Solar masses per year. We use our inferred masses to determine a mean specific star formation rate for this sample of ~ $10^{-9}$ yr$^{-1}$, and compare this to the specific star formation rates in distant Lyman break galaxies (LBGs), and in other low redshift populations. We conclude that our sample's characteristics overlap significantly with those of the z~5 LBG population, making ours the first local analogue population well tuned to match those high redshift galaxies. We consider implications for the origin and evolution of early galaxies.

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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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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

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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arXiv2014

Establishing an analogue population for the most distant galaxies

Lyman break analogues (LBAs) are local galaxies selected to match a more distant (usually z~3) galaxy population in luminosity, UV-spectral slope and physical characteristics, and so provide an accessible laboratory for exploring their properties. However, as the Lyman break technique is extended to higher redshifts, it has become clear that the Lyman break galaxies (LBGs) at z~3 are more massive, luminous, redder, more extended and at higher metallicities than their z~5 counterparts. Thus extrapolations from the existing LBA samples (which match z=3 properties) have limited value for characterising z>5 galaxies, or inferring properties unobservable at high redshift. We present a new pilot sample of twenty-one compact star forming galaxies in the local (0.05 ~5 LBGs. Analysis of optical emission line indices suggests that these sources have typical metallicities of a few tenths Solar (again, consistent with the distant population). We also present radio continuum observations of a subset of this sample (13 sources) and determine that their radio fluxes are consistent with those inferred from the ultraviolet, precluding the presence of a heavily obscured AGN or significant dusty star formation.

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arXiv2013

VLT/XSHOOTER & Subaru/MOIRCS Spectroscopy of HUDF-YD3: No Evidence for Lyman-alpha Emission at z=8.55

We present spectroscopic observations with VLT/XSHOOTER and Subaru/MOIRCS of a relatively bright Y-band drop-out galaxy in the Hubble Ultra Deep Field, first selected by Bunker et al. (2010), McLure et al. (2010) and Bouwens et al. (2010) to be a likely z~8-9 galaxy on the basis of its colours in the HST ACS and WFC3 images. This galaxy, HUDF.YD3 (also known as UDFy-38135539) has been targetted for VLT/SINFONI integral field spectroscopy by Lehnert et al. (2010), who published a candidate Lyman-alpha emission line at z=8.55 from this source. In our independent spectroscopy using two different infrared spectrographs (5 hours with VLT/XSHOOTER and 11 hours with Subaru/MOIRCS) we are unable to reproduce this line. We do not detect any emission line at the spectral and spatial location reported in Lehnert et al. (2010), despite the expected signal in our combined MOIRCS & XSHOOTER data being 5-sigma. The line emission also seems to be ruled out by the faintness of this object in recently extremely deep F105W (Y-band) HST/WFC3 imaging from HUDF12; the line would fall within this filter and such a galaxy should have been detected at Y(AB)=28.6mag (~20 sigma) rather than the marginal Y(AB)~30mag observed in the Y-band image, >3 times fainter than would be expected if the emission lie was real. Hence it appears highly unlikely that the reported Lyman-alpha line emission at z>8 is real, meaning that the highest-redshift sources for which Lyman-alpha emission has been seen are at z=6.9-7.2. It is conceivable that Lyman-alpha does not escape galaxies at higher redshifts, where the Gunn-Peterson absorption renders the Universe optically thick to this line. However, deeper spectroscopy on a larger sample of candidate z>7 galaxies will be needed to test this.

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arXiv2012

Constraining the Bright-end of the UV Luminosity Function for z ~ 7 - 9 Galaxies: results from CANDELS/GOODS-South

The recent Hubble Space Telescope near-infrared imaging with the Wide-Field Camera #3 (WFC3) of the GOODS-South field in the CANDELS program covering nearly 100arcmin^2, along with already existing Advanced Camera for Surveys optical data, makes possible the search for bright galaxy candidates at redshift z ~ 7 - 9 using the Lyman-break technique. We present the first analysis of z'-drop z ~ 7 candidate galaxies in this area, finding 19 objects. We also analyse Y-drops at z ~ 8, trebling the number of bright (H_AB < 27 mag) Y-drops from our previous work, and compare our results with those of other groups based on the same data. The bright high redshift galaxy candidates we find serve to better constrain the bright end of the luminosity function at those redshift, and may also be more amenable to spectroscopic confirmation than the fainter ones presented in various previous work on the smaller fields (the Hubble Ultra Deep Field and the WFC 3 Early Release Science observations). We also look at the agreement with previous luminosity functions derived from WFC3 drop-out counts, finding a generally good agreement, except for the luminosity function of Yan et al. (2010) at z ~ 8, which is strongly ruled out.

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arXiv2012

No Evidence for Lyman-alpha Emission in Spectroscopy of z > 7 Candidate Galaxies

We present Gemini/GNIRS spectroscopic observations of 4 z-band (z~7) dropout galaxies and VLT/XSHOOTER observations of one z-band dropout and 3 Y-band (z~8-9) dropout galaxies in the Hubble Ultra Deep Field, which were selected with Wide Field Camera 3 imaging on the Hubble Space Telescope. We find no evidence of Lyman-alpha emission with a typical 5-sigma sensitivity of 5X10^-18erg/cm^2/s, and we use the upper limits on Lyman-alpha flux and the broad-band magnitudes to constrain the rest-frame equivalent widths for this line emission. Accounting for incomplete spectral coverage, we survey 3.0 z-band dropouts and 2.9 Y-band dropouts to a Lyman-alpha rest-frame equivalent width limit > 120Ang (for an unresolved emission line); for an equivalent width limit of 50Ang the effective numbers of drop-outs surveyed fall to 1.2 z-band drop-outs and 1.5 Y-band drop-outs. 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=7-9 at the ~ 1-sigma level for spectrally unresolved lines, which may indicate that a significant neutral HI fraction in the intergalactic medium suppresses the Lyman-alpha line in z-drop and Y-drop galaxies at z > 7.

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arXiv2011

The effect of stellar evolution uncertainties on the rest-frame ultraviolet stellar lines of CIV and HeII in high-redshift Lyman-break galaxies

Young, massive stars dominate the rest-frame ultraviolet spectra of star-forming galaxies. At high redshifts (z>2), these rest-UV features are shifted into the observed-frame optical and a combination of gravitational lensing, deep spectroscopy and spectral stacking analysis allows the stellar population characteristics of these sources to be investigated. We use our stellar population synthesis code BPASS to fit two strong rest-UV spectral features in published Lyman-break galaxy spectra, taking into account the effects of binary evolution on the stellar spectrum. In particular, we consider the effects of quasi-homogeneous evolution (arising from the rotational mixing of rapidly-rotating stars), metallicity and the relative abundance of carbon and oxygen on the observed strengths of HeII (1640 Angstroms) and CIV (1548,1551 Angstroms) spectral lines. We find that Lyman-break galaxy spectra at z=2-3 are best fit with moderately sub-solar metallicities, and with a depleted carbon-to-oxygen ratio. We also find that the spectra of the lowest metallicity sources are best fit with model spectra in which the HeII emission line is boosted by the inclusion of the effect of massive stars being spun-up during binary mass-transfer so these rapidly-rotating stars experiencing quasi-homogeneous evolution.

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arXiv2010

Constraining the Molecular Gas in the Environs of a z~8 Gamma Ray Burst Host Galaxy

GRB 090423 is the most distant spectroscopically-confirmed source observed in the universe. Using observations at 37.5 GHz, we place constraints on molecular gas emission in the CO(3-2) line from its host galaxy and immediate environs. The source was not detected either in line emission or in the rest-frame 850 micron continuum, yielding an upper limit of S_{8mm}=9.3 milli-Jy and M(H_2)<4.3x10^9 M_sun (3 sigma), applying standard conversions. This implies that the host galaxy of GRB 090423 did not possess a large reservoir of warm molecular gas but was rather modest either in star formation rate or in mass. It suggests that this was not an extreme starburst, and hence that gamma ray bursts at high redshift trace relatively modest star formation rates, in keeping with the behaviour seen at lower redshifts. We do, however, identify a millimetre emission line source in the field of GRB 090423. Plausible interpretations include a CO(1-0) emitting galaxy at z=2.1, CO(2-1) at z=5.2 and CO(3-2) at z=8.3. Efforts to identify a counterpart for the molecular line emitter and to further characterise this source are continuing.

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arXiv2010

Constraining the Thermal Dust Content of Lyman-Break Galaxies in an Overdense Field at z~5

We have carried out 870 micron observations in the J1040.7-1155 field, known to host an overdensity of Lyman break galaxies at z=5.16 +/- 0.05. We do not detect any individual source at the S(870)=3.0 mJy/beam (2 sigma) level. A stack of nine spectroscopically confirmed z>5 galaxies also yields a non-detection, constraining the submillimeter flux from a typical galaxy at this redshift to S(870)<0.85 mJy, which corresponds to a mass limit M(dust)<1.2x10^8 M_sun (2 sigma). This constrains the mass of thermal dust in distant Lyman break galaxies to less than one tenth of their typical stellar mass. We see no evidence for strong submillimeter galaxies associated with the ultraviolet-selected galaxy overdensity, but cannot rule out the presence of fainter, less massive sources.

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arXiv2009

Probing $\sim L_{*}$ Lyman-break Galaxies at $z\approx 7$ in GOODS-South with WFC3 on HST

We analyse recently acquired near-infrared Hubble Space Telescope imaging of the GOODS-South field to search for star forming galaxies at z~7.0. By comparing WFC 3 0.98 micron Y-band images with ACS z-band (0.85 micron) images, we identify objects with colours consistent with Lyman break galaxies at z~6.4-7.4. This new data covers an area five times larger than that previously reported in the WFC3 imaging of the Hubble Ultra Deep Field, and affords a valuable constraint on the bright end of the luminosity function. Using additional imaging of the region in the ACS B, V and i-bands from GOODS v2.0 and the WFC3 J-band we attempt to remove any low-redshift interlopers. Our selection criteria yields 6 candidates brighter than Y_AB = 27.0, of which all except one are detected in the ACS z-band imaging and are thus unlikely to be transients. Assuming all 6 candidates are at z~7 this implies a surface density of objects brighter than Y_AB = 27.0 of 0.30\pm0.12 arcmin-2, a value significantly smaller than the prediction from the z ~ 6 luminosity function. This suggests continued evolution of the bright end of the luminosity function between z = 6 to 7, with number densities lower at higher redshift.

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arXiv2009

Spectral synthesis including massive binaries

We have constructed a new code to produce synthetic spectra of stellar populations that includes massive binaries. We have tested this code against the broadband colours of unresolved young massive stellar clusters in nearby galaxies, the equivalent widths of the Red and Blue Wolf-Rayet bumps in star-forming SDSS galaxies and the UV and optical spectra of the star forming regions Tol-A and B in NGC5398. In each case we find a good agreement between our models and observations. We find that in general binary populations are bluer and have fewer red supergiants, and thus significantly less flux in the I-band and at longer wavelengths, than single star populations. Also we find that Wolf-Rayet stars occur over a wider range of ages up to 10^7 years in a stellar population including binaries, increasing the UV flux and Wolf-Rayet spectral features at later times. In addition we find that nebula emission contributes significantly to these observed properties and must be considered when comparing stellar models with observations of unresolved stellar populations. We conclude that incorporation of massive stellar binaries can improve the agreement between observations and synthetic spectral synthesis codes, particularly for systems with young stellar populations.

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arXiv2008

A Limit on the Number Density of Bright z~7 Galaxies

We present a survey of bright optical dropout sources in two deep, multiwavelength surveys comprising eleven widely-separated fields, aimed at constraining the galaxy luminosity function at z~7 for sources at 5-10L*(z=6). Our combined survey area is 225 arcmin^2 to a depth of J=24.2 (AB, 3 sigma) and 135 arcmin^2 to J=25.3 (AB, 4 sigma). We find that infrared data longwards of 2 microns is essential for classifying optical dropout sources, and in particular for identifying cool Galactic star contaminants. Our limits on the number density of high redshift sources are consistent with current estimates of the Lyman break galaxy luminosity function at z=6.

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arXiv2007

Discovery of a single faint AGN in a large sample of z>5 Lyman break galaxies

As part of a large spectroscopic survey of z>5 Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an AGN. Out of a sample of more than fifty spectroscopically-confirmed R-band dropout galaxies at z~5 and above, only J104048.6-115550.2 at z=5.44 shows evidence for a high ionisation-potential emission-line indicating the presence of a hard ionising continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Ly_alpha line. Uniquely within this sample of LBGs, emission from NV is also detected, a clear signature of AGN photo-ionisation. The object is spatially resolved in HST imaging. This, and the comparatively high Ly_alpha/NV flux ratio indicates that the majority of the Ly_alpha (and the UV continuum longward of it) originates from stellar photo-ionisation, a product of the ongoing starburst in the Lyman break galaxy. Even without the AGN emission, this object would have been photometrically-selected and spectroscopically-confirmed as a Lyman break in our survey. The measured optical flux (I(AB)=26.1) is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN are found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.

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arXiv2004

A Matched Catalogue i-dropout galaxies at z~6 in the Hubble Ultra Deep Field

A recent preprint by Yan & Windhorst (astro-ph/0407493) independently repeats our selection of candidate z~6 galaxies in the Hubble Ultra Deep Field (astro-ph/0403223). The agreement of this independent study with our original work is excellent, and most of the i-band drop-out galaxies are reproduced. To avoid confusion with various ID numbers for sources, we present the community with a matched list of i-band drop-outs in the Hubble Ultra Deep Field.

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