arXiv subjects

Elizabeth Stanway

Publications and source records attributed to Elizabeth Stanway.

18 recordsLinked to original sources

arXiv2022

Can neutron star mergers alone explain the r-process enrichment of the Milky Way?

Comparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if at low metallicities such mergers have very short delay times and significant ejecta masses that are facilitated by the masses of the compact objects. Namely, black hole-neutron star mergers, depending on the black-hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this paper, i.e., COMPAS, StarTrack, Brussels, ComBinE, and BPASS, can currently reproduce the elemental abundance observations. The predictions are problematic not only for neutron star mergers, but also for Type Ia supernovae, which may point to shortcomings in binary evolution models.

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arXiv2020

Machine Learning for Transient Recognition in Difference Imaging With Minimum Sampling Effort

The amount of observational data produced by time-domain astronomy is exponentially in-creasing. Human inspection alone is not an effective way to identify genuine transients fromthe data. An automatic real-bogus classifier is needed and machine learning techniques are commonly used to achieve this goal. Building a training set with a sufficiently large number of verified transients is challenging, due to the requirement of human verification. We presentan approach for creating a training set by using all detections in the science images to be thesample of real detections and all detections in the difference images, which are generated by the process of difference imaging to detect transients, to be the samples of bogus detections. This strategy effectively minimizes the labour involved in the data labelling for supervised machine learning methods. We demonstrate the utility of the training set by using it to train several classifiers utilizing as the feature representation the normalized pixel values in 21-by-21pixel stamps centered at the detection position, observed with the Gravitational-wave Optical Transient Observer (GOTO) prototype. The real-bogus classifier trained with this strategy can provide up to 95% prediction accuracy on the real detections at a false alarm rate of 1%.

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arXiv2020

Hoki: Making BPASS accessible through Python

We now know that a large number of stars are born in multiple systems. Additionally, more than 70% of massive stars are found in close binary systems, meaning that they will interact over the course of their lifetime. This has strong implications for their evolution as well as the transients (e.g supernovae) and the potential gravitational wave progenitors they produce. Therefore, in order to understand and correctly interpret astronomical observations of stellar populations, we must use theoretical models able to account for the effects of binary stars. This is the case of the Binary Population and Spectral Synthesis code (BPASS), which has been a staple of the field for over 10 years. As is the case for most other theoretical models, the data products of BPASS are large, varied and complex. As a result, their use requires a level of expertise that is not immediately accessible to a wider community that may hold key observational data. The goal of hoki is to bridge the gap between observation and theory, by providing a set of tools to make BPASS data easily accessible and facilitate analysis. The use of Python is deliberate as it is a ubiquitous language within Astronomy. This allows BPASS results to be used naturally within the pre-existing workflow of most astronomers.

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arXiv2018

The second closest gamma-ray burst: sub-luminous GRB 111005A with no supernova in a super-solar metallicity environment

We report the detection of the radio afterglow of a long gamma-ray burst (GRB) 111005A at 5-345 GHz, including the very long baseline interferometry observations with the positional error of 0.2 mas. The afterglow position is coincident with the disk of a galaxy ESO 580-49 at z= 0.01326 (~1" from its center), which makes GRB 111005A the second closest GRB known to date, after GRB 980425. The radio afterglow of GRB 111005A was an order of magnitude less luminous than those of local low-luminosity GRBs, and obviously than those of cosmological GRBs. The radio flux was approximately constant and then experienced an unusually rapid decay a month after the GRB explosion. Similarly to only two other GRBs, we did not find the associated supernovae (SN), despite deep near- and mid-infrared observations 1-9 days after the GRB explosion, reaching ~20 times fainter than other SNe associated with GRBs. Moreover, we measured twice solar metallicity for the GRB location. The low gamma-ray and radio luminosities, rapid decay, lack of a SN, and super-solar metallicity suggest that GRB 111005A represents a different rare class of GRBs than typical core-collapse events. We modelled the spectral energy distribution of the GRB 111005A host finding that it is a dwarf, moderately star-forming galaxy, similar to the host of GRB 980425. The existence of two local GRBs in such galaxies is still consistent with the hypothesis that the GRB rate is proportional to the cosmic star formation rate (SFR) density, but suggests that the GRB rate is biased towards low SFRs. Using the far-infrared detection of ESO 580-49, we conclude that the hosts of both GRBs 111005A and 980425 exhibit lower dust content than what would be expected from their stellar masses and optical colours.

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arXiv2018

Investigating a population of infrared-bright gamma-ray burst host galaxies

We identify and explore the properties of an infrared-bright gamma-ray burst (GRB) host population. Candidate hosts are selected by coincidence with sources in WISE, with matching to random coordinates and a false alarm probability analysis showing that the contamination fraction is approx 0.5. This methodology has already identified the host galaxy of GRB 080517. We combine survey photometry from Pan-STARRS, SDSS, APASS, 2MASS, GALEX and WISE with our own WHT/ACAM and VLT/X- shooter observations to classify the candidates and identify interlopers. Galaxy SED fitting is performed using MAGPHYS, in addition to stellar template fitting, yielding 13 possible IR-bright hosts. A further 7 candidates are identified from previously published work. We report a candidate host for GRB 061002, previously unidentified as such. The remainder of the galaxies have already been noted as potential hosts. Comparing the IR-bright population properties including redshift z, stellar mass M*, star formation rate SFR and V-band attenuation Av to GRB host catalogues in the literature, we find that the infrared-bright population is biased toward low z, high M* and high Av. This naturally arises from their initial selection - local and dusty galaxies are more likely to have the required IR flux to be detected in WISE. We conclude that while IR-bright GRB hosts are not a physically distinct class, they are useful for constraining existing GRB host populations, particularly for long GRBs.

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arXiv2018

Emission-line diagnostics of nearby HII regions including interacting binary populations

We present numerical models of the nebular emission from H II regions around young stellar populations over a range of compositions and ages. The synthetic stellar pop- ulations include both single stars and interacting binary stars. We compare these models to the observed emission lines of 254 HII regions of 13 nearby spiral galax- ies and 21 dwarf galaxies drawn from archival data. The models are created using the combination of the Binary Population and Spectral Synthesis (bpass) code with the photoionization code cloudy to study the differences caused by the inclusion of interacting binary stars in the stellar population. We obtain agreement with the observed emission line ratios from the nearby star-forming regions and discuss the effect of binary star evolution pathways on the nebular ionization of H II regions. We find that at population ages above 10 Myr, single-star models rapidly decrease in flux and ionization strength, while binary-star model still produce strong flux and high O III/H\b{eta} ratios. Our models can reproduce the metallicity of H II regions from spiral galaxies but we find higher metallicities than previously estimated for the H II regions from dwarf galaxies. Comparing the equivalent width of H\b{eta} emission between mod- els and observations, we find that accounting for ionizing photon leakage can affect age estimates for HII regions. When it is included, the typical age derived for HII regions is 5Myr from single-star models, and up to 10Myr with binary-star models. This is due to the existence of binary-star evolution pathways which produce more hot WR and helium stars at older ages. For future reference, we calculate new BPASS binary maximal starburst lines as a function of metallicity, and for the total model population, and present these in Appendix A.

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arXiv2017

Emission-line Diagnostics of Nearby HII Regions Including Supernova Hosts

We present a new model of the optical nebular emission from HII regions by combin- ing the results of the Binary Population and Spectral Synthesis (bpass) code with the photoion- ization code cloudy (Ferland et al. 1998). We explore a variety of emission-line diagnostics of these star-forming HII regions and examine the effects of metallicity and interacting binary evo- lution on the nebula emission-line production. We compare the line emission properties of HII regions with model stellar populations, and provide new constraints on their stellar populations and supernova progenitors. We find that models including massive binary stars can successfully match all the observational constraints and provide reasonable age and mass estimation of the HII regions and supernova progenitors.

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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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arXiv2011

The UV Properties of Star Forming Galaxies I: HST WFC3 Observations of Very-high Redshift Galaxies

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-$α$ emission or the Lyman-$α$ break. In this work we use Near-IR imaging to measure the rest-frame UV continuum colours of galaxies at $4.7<z<7.7$. We are have carefully defined a colour-colour selection to minimise any inherent bias in the measured UV continuum slope for the drop-out samples. For the highest-redshift sample ($6.7<z<7.7$), selected as $z_{f850lp}$-band dropouts, we find mean UV continuum colours approximately equal to zero (AB), consistent with a dust-free, solar metallicity, star forming population (or a moderately dusty population of low metallicity). At lower-redshift we find that the mean UV continuum colours of galaxies (over the same luminosity range) are redder, and that galaxies with higher luminosities are also slightly redder on average. One interpretation of this is that lower-redshift and more luminous galaxies are dustier, however this interpretation is complicated by the effects of the star formation history and metallicity and potentially the initial mass function on the UV continuum colours.

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arXiv2011

Candidate z~8-9 Galaxies from WFC3 Imaging

We present a search for galaxies at 7.6<z<9.8 using the latest HST WFC3 near-infrared data, based on the Lyman-break technique. We search for galaxies which have large (Y-J) colours (the "Y-drops") on account of the Lyman-alpha forest absorption, and with (J-H) colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift z~8-9 (15 are robust and a further 9 more marginal but consistent with being high redshift) over an area of ~50 square arcminutes. Previous searches for Y-drops with WFC3 have focussed only on the Hubble Ultra Deep Field (HUDF), and our larger survey (involving two other nearby deep fields and a wider area survey) has trebelled the number of robust Y-drop candidates. For the first time, we have sufficient z~8-9 galaxies to fit both phi^* and M^* of the UV Schechter luminosity function. There is evidence for evolution in this luminosity function from z=6-7 to z=8-9, in the sense that there are fewer UV-bright galaxies at z~8-9, consistent with an evolution mainly in M^*. The candidate z~8-9 galaxies we detect have insufficient ionizing flux to reionize the Universe, and it is probable that galaxies below our detection limit provide a significant UV contribution. The faint-end slope, alpha, is not well constrained. However, adopting a similiar faint-end slope to that determined at z=3-6 (alpha=-1.7) and a Salpeter initial mass function, then the ionizing photon budget still falls short if f_escape<0.5, even integrating down to M(UV)=-8. A steeper faint end slope or a low-metallicity population (or a top-heavy IMF) might still provide sufficient photons for star-forming galaxies to reionize the Universe, but confirmation of this might have to await the James Webb Space Telescope.

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arXiv2009

Galaxies at High Redshift and Reionization

The quest to discover the most distant galaxies has developed rapidly in the last decade. We are now exploring redshifts of 6 and beyond, when the Universe was less than a billion years old, an epoch when the previously-neutral intergalactic medium was reionized. The continuing discovery of galaxies at progressively higher and higher redshifts has been driven by the availability of large telescopes on the ground and in space, improvements in detector technology, and new search strategies. Over the past 4 years, the Lyman break technique has been shown to be effective in isolating z~6 star-forming i'-drop galaxies through spectroscopic confirmation with large ground-based telescopes (Keck, Gemini and the ESO VLTs). Narrow-band imaging, notably with the wide field of the Subaru telescope, has also produced samples of Lyman-alpha emitters at these redshifts. A The discovery of this i'-drop galaxy population has been used to infer the global star formation rate density at this epoch (z~6), and we are now beginning to constrain the contribution to reionization of the UV flux from these galaxies. Infrared data from the Spitzer Space Telescope has been used to determine the spectral energy distributions (SEDs) from the rest-frame UV to the optical, and constrain the previous star formation histories, masses and ages. The indications are that much of the stellar mass of these galaxies might have formed in vigorous bursts at z>6. The next big advances would be to test the population synthesis modelling of these z~6 galaxies through spectroscopy of the rest-frame optical (rather than crude broad-band SEDs), and also to push the observational horizon for galaxies further to directly explore star formation during the reionization epoch. JWST is likely to have a profound impact on realising these goals.

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arXiv2007

On Contamination and Completeness in z>5 Lyman Break Galaxy Surveys

A large population of z>5 Lyman break galaxies has been identified in recent years. However, the high redshift galaxies selected by different surveys are subject to a variety of selection effects - some overt, others more subtle. We present an analysis of sample completeness and contamination issues in high redshift surveys, focusing on surveys at z=5 and using a spectroscopically-confirmed low redshift sample from the DEEP2 survey in order to characterise contaminant galaxies. We find that most surveys underestimate their contamination from highly clustered galaxies at z=1 and stars. We consider the consequences of this for both the rest-frame ultraviolet luminosity function and the clustering signal from z=5 galaxies. We also find that sources with moderate strength Lyman-alpha emission lines can be omitted from dropout surveys due to their blue colours, again effecting the derived luminosity functions. We discuss the points of comparison between different samples, and the applicability of survey-specific results to the population at z>5 in general.

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arXiv2006

The Stellar Mass Density at z~6 from Spitzer Imaging of i-drop Galaxies

We measure the ages, stellar masses, and star formation histories of z~6 galaxies, observed within 1Gyr of the Big Bang. We use imaging from HST and Spitzer from the public "Great Observatories Origins Deep Survey", coupled with ground-based near-infrared imaging, to measure the spectral energy distributions from 0.8-5microns, spanning the rest-frame UV and optical. From our sample of ~50 i-drop Lyman-break star-forming galaxies in GOODS-South with z'(AB)<27mag, we focus on ~30 with reliable photometric or spectroscopic redshifts. Half of these are confused with foreground sources at Spitzer resolution, but from the 16 with clean photometry we find that a surprisingly large fraction (40%) have evidence for substantial Balmer-breaks. This indicates the presence of old underlying stellar populations that dominate the stellar masses. For these objects, we find ages of 200-700Myr, implying formation redshifts of 7 7 could have played a major role in reionizing the universe.

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arXiv2005

Star Forming Galaxies at z~6 and Reionization

We determine the abundance of i'-band drop-outs in the HST/ACS GOODS surveys and the Hubble Ultra Deep Field (UDF). The majority of these sources are likely to be z~6 galaxies whose flux decrement arises from Lyman-alpha absorption. We have shown with Keck/DEIMOS spectroscopy that this technique does indeed select high redshift galaxies, and we discovered Lyman-alpha emission for about a third of the galaxies with z'_AB<25.6 The increased depth of UDF enables us to reach a ~10sigma limiting magnitude of z'_AB=28.5 (equivalent to 1.5M_sun/yr at z=6.1, or 0.1L*_UV for the z~3 U-drop population). The star formation rate at z~6 was approximately x6 LESS than at z~3. This declining comoving star formation rate poses an interesting challenge for models which suggest that L_UV>0.1L* star forming galaxies at z~6 reionized the universe. The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z>>6. We have recently discovered evidence of an early epoch of star formation in some of the i'-drops at z~6. Spitzer images with IRAC at 3.6-4.5microns show evidence of the age-sensitive Balmer/4000Ang break, dominated by stars older than 100Myr (and most probably 400Myr old). This pushes the formation epoch for these galaxies to z_form=7.5-13.5. There are at least some galaxies already assembled with stellar masses ~3x10^10M_sun (equivalent to 0.2M* today) within the first billion years. The early formation of such systems may have played a key role in reionizing the Universe at z~10.

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arXiv2005

Spitzer Imaging of i'-drop Galaxies: Old Stars at z~6

We present new evidence for mature stellar populations with ages >100Myr in massive galaxies (M_stellar>10^10M_sun) seen at a time when the Universe was less than 1Gyr old. We analyse the prominent detections of two z~6 star-forming galaxies (SBM03#1 & #3) made at wavelengths corresponding to the rest-frame optical using the IRAC camera onboard the Spitzer Space Telescope. We had previously identified these galaxies in HST/ACS GOODS images of Chandra Deep Field South through the "i-drop" Lyman break technique, and subsequently confirmed spectroscopically with the Keck telescope. The new Spitzer photometry reveals significant Balmer/4000Ang discontinuities, indicative of dominant stellar populations with ages >100Myr. Fitting a range of population synthesis models (for normal initial mass functions) to the HST/Spitzer photometry yields ages of 250-650Myr and implied formation redshifts z~7.5-13.5 in presently-accepted world models. Remarkably, our sources have best-fit stellar masses of 1.3-3.8x10^10M_sun (95% confidence) assuming a Salpeter initial mass function. This indicates that at least some galaxies with stellar masses >20% of those of a present-day L* galaxy had already assembled within the first Gyr after the Big Bang. We also deduce that the past average star formation rate must be comparable to the current observed rate (SFR_UV~5-30M_sun/yr), suggesting that there may have been more vigorous episodes of star formation in such systems at higher redshifts. Although a small sample, limited primarily by Spitzer's detection efficiency, our result lends support to the hypothesis advocated in our earlier analyses of the Ultra Deep Field and GOODS HST/ACS data. The presence of established systems at z~6 suggests long-lived sources at earlier epochs (z>7) played a key role in reionizing the Universe.

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arXiv2005

Near-Infrared Properties of I-Drop Galaxies in the Hubble Ultra Deep Field

We analyse near-infrared HST/NICMOS F110W (J) and F160W (H) band photometry of a sample of 27 i'-drop candidate z~6 galaxies in the central region of the HST/ACS Ultra Deep Field (HUDF). The infrared colours of the 20 objects not affected by near neighbours are consistent with a high redshift interpretation. This suggests that the low redshift contamination of this i'-drop sample is smaller than that observed at brighter magnitudes where values of 10-40% have been reported. The J-H colours are consistent with a slope flat in f_nu (f_lam \propto lam^{-2}), as would be expected for an unreddened starburst. There is, however, evidence for a marginally bluer spectral slope (f_lam propto lam^{-2.2) which is perhaps indicative of an extremely young starburst (~10 Myr old) or a top heavy initial mass function and little dust. The low levels of contamination, median photometric redshift of z~6.0 and blue spectral slope, inferred using the near-infrared data, supports the validity of the assumptions in our earlier work in estimating the star formation rates and, that the majority of the i-drop candidates galaxies lie at z~6.

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arXiv2004

HST imaging and Keck Spectroscopy of z~6 I-band Drop-Out Galaxies in the ACS GOODS Fields

We measure the surface density of i'-band dropout galaxies at z~6 through wide field HST/ACS imaging and ultra-deep Keck/DEIMOS spectroscopy. Using deep HST/ACS SDSS-i' (F775W) and SDSS-z' (F850LP) imaging from GOODS-N (200 arcmin^2), we identify 9 i'-drops satisfying an (i'-z')_AB>1.5 selection criterion to a depth of z'_AB=25.6 (corresponding to L*_UV at z~3-4). We use HK' imaging data to improve the fidelity of our sample, discriminating against lower redshift red galaxies and cool Galactic stars. Three i'-drops are consistent with M/L/T dwarf stars. We present ultra-deep Keck/DEIMOS spectroscopy of 10 objects from our combined GOODS-N and GOODS-S i'-drop sample. We detect Lyman-alpha emission at z=5.83 from one object in the GOODS-S field, which lies only 8arcmin away (i.e. 3Mpc/h_70) from the z=5.78 object already confirmed by Bunker et al. (2003). One possible Lyman-alpha emitter at z=6.24 is found in the GOODS-N field (although identification of this spatially-offset emission line is ambiguous). Using the rest-frame UV continuum from our 6 candidate z~6 galaxies from the GOODS-N field, we determine a lower limit to the unobscured volume-averaged global star formation rate at z~6 of (5.4+/-2.2)x10^-4 h_70 M_sun/yr/Mpc^3. We find that the cosmic star formation density in galaxies with unobscured star formation rates 15M_sun/yr/h_70^2 falls by a factor of 8 between z~3 and z~6. Hence the luminosity function of LBGs must evolve in this redshift interval: a constant integrated star formation density at $z>3$ requires a much steeper faint-end slope, or a brighter characteristic luminosity. This result is in agreement with our previous measurement from the Chandra Deep Field South (Stanway et al. 2003), indicating that cosmic variance is not a dominant source of uncertainty.

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arXiv2003

Lyman Break Galaxies and the Star Formation Rate of the Universe at z~6

We determine the space density of UV-luminous star-burst galaxies at z~6 using deep HST ACS SDSS-i' (F775W) and SDSS-z' (F850LP) and VLT ISAAC J and K_s band imaging of the Chandra Deep Field South. We find 8 galaxies and one star with (i'-z')>1.5 to a depth of z'{AB}= 25.6 (an 8-sigma detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of ~15M_sun/yr/h{70}^2 at z=5.8, equivalent to L* for the Lyman break population at z =3-4 (Omega_{Lambda}=0.7, Omega_M=0.3). We are sensitive to star forming galaxies at 5.6<z<7.0 with an effective comoving volume of \~1.8x10^5 Mpc^3/h{70}^3 after accounting for incompleteness at the higher redshifts due to luminosity bias. This volume should encompass the primeval sub-galactic scale fragments of the progenitors of about a thousand L* galaxies at the current epoch. We determine a volume-averaged global star formation rate of (6.7 +/- 2.7)x10^{-4}h{70}M_sun/yr/Mpc^3 at z~6 from rest-frame UV selected star-bursts at the bright end of the luminosity function: this is a lower limit because of dust obscuration and galaxies below our sensitivity limit. This measurement shows that at z~6 the star formation density at the bright end is a factor of ~6 times less than that determined by Steidel et al. (1999) for a comparable sample of UV selected galaxies at z=3-4, and so extends our knowledge of the star formation history of the Universe to earlier times than previous work and into the epoch where reionization may have occurred.

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