405 research outputs found
Stellar Population Effects on the Inferred Photon Density at Reionization
The relationship between stellar populations and the ionizing flux with which
they irradiate their surroundings has profound implications for the evolution
of the intergalactic medium. We quantify the ionizing flux arising from
synthetic stellar populations which incorporate the evolution of interacting
binary stars. We determine that these show ionizing flux boosted by 60 per cent
at 0.05 < Z < 0.3 Z_sun and a more modest 10-20 per cent at near-Solar
metallicities relative to star-forming populations in which stars evolve in
isolation. The relation of ionizing flux to observables such as 1500A continuum
and ultraviolet spectral slope is sensitive to attributes of the stellar
population including age, star formation history and initial mass function. For
a galaxy forming 1 M_sun yr^{-1}, observed at > 100 Myr after the onset of star
formation, we predict a production rate of photons capable of ionizing
hydrogen, N_ion = 1.4 x 10^{53} s^{-1} at Z = Z_sun and 3.5 x 10^{53} s^{-1} at
0.1 Z_sun, assuming a Salpeter-like initial mass function. We evaluate the
impact of these issues on the ionization of the intergalactic medium, finding
that the known galaxy populations can maintain the ionization state of the
Universe back to z ~ 9, assuming that their luminosity functions continue to
M_UV = -10, and that constraints on the intergalactic medium at z ~ 2 - 5 can
be satisfied with modest Lyman continuum photon escape fractions of 4 - 24 per
cent depending on assumed metallicity.Comment: 17 pages, accepted by MNRAS. BPASS models can be found at
http://bpass.auckland.ac.nz
Radio Observations of GRB Host Galaxies
We present 5.5 and 9.0 GHz observations of a sample of seventeen GRB host
galaxies at 0.5<z<1.4, using the radio continuum to explore their star
formation properties in the context of the small but growing sample of galaxies
with similar observations. Four sources are detected, one of those (GRB
100418A) likely due to lingering afterglow emission. We suggest that the
previously-reported radio afterglow of GRB 100621A may instead be due to host
galaxy flux. We see no strong evidence for redshift evolution in the typical
star formation rate of GRB hosts, but note that the fraction of `dark' bursts
with detections is higher than would be expected given constraints on the more
typical long GRB population. We also determine the average radio-derived star
formation rates of core collapse supernovae at comparable redshift, and show
that these are still well below the limits obtained for GRB hosts, and show
evidence for a rise in typical star formation rate with redshift in supernova
hosts.Comment: 15 pages, MNRAS accepte
Dissecting the complex environment of a distant quasar with MUSE
High redshift quasars can be used to trace the early growth of massive
galaxies and may be triggered by galaxy-galaxy interactions. We present MUSE
science verification data on one such interacting system consisting of the
well-studied z=3.2 PKS1614+051 quasar, its AGN companion galaxy and bridge of
material radiating in Lyalpha between the quasar and its companion. We find a
total of four companion galaxies (at least two galaxies are new discoveries),
three of which reside within the likely virial radius of the quasar host,
suggesting that the system will evolve into a massive elliptical galaxy by the
present day. The MUSE data are of sufficient quality to split the extended
Lyalpha emission line into narrow velocity channels. In these the gas can be
seen extending towards each of the three neighbouring galaxies suggesting that
the emission-line gas originates in a gravitational interaction between the
galaxies and the quasar host. The photoionization source of this gas is less
clear but is probably dominated by the two AGN. The quasar's Lyalpha emission
spectrum is double-peaked, likely due to absorbing neutral material at the
quasar's systemic redshift with a low column density as no damping wings are
present. The spectral profiles of the AGN and bridge's Lyalpha emission are
also consistent with absorption at the same redshift indicating this neutral
material may extend over > 50 kpc. The fact that the neutral material is seen
in the line of sight to the quasar and transverse to it, and the fact that we
see the quasar and it also illuminates the emission-line bridge, suggests the
quasar radiates isotropically and any obscuring torus is small. These results
demonstrate the power of MUSE for investigating the dynamics of interacting
systems at high redshift.Comment: 9 pages, 6 figures, published in MNRA
Evaluating the impact of binary parameter uncertainty on stellar population properties
Binary stars have been shown to have a substantial impact on the integrated light of stellar populations, particularly at low metallicity and early ages - conditions prevalent in the distant Universe. But the fraction of stars in stellar multiples as a function of mass, their likely initial periods and distribution of mass ratios are all known empirically from observations only in the local Universe. Each has associated uncertainties. We explore the impact of these uncertainties in binary parameters on the properties of integrated stellar populations, considering which properties and timescales are most susceptible to uncertainty introduced by binary fractions and whether observations of the integrated light might be sufficient to determine binary parameters. We conclude that the effects of uncertainty in the empirical binary parameter distributions are likely smaller than those introduced by metallicity and stellar population age uncertainties for observational data. We identify emission in the He II 1640Å emission line and continuum colour in the ultraviolet-optical as potential indicators of a high mass binary presence, although poorly constrained metallicity, dust extinction and degeneracies in plausible star formation history are likely to swamp any measurable signal
The detection of FIR emission from high redshift star-forming galaxies in the ECDF-S
ABRIDGED: We have used the LABOCA Survey of the ECDF-S (LESS) to investigate
rest-frame FIR emission from typical SF systems (LBGs) at redshift 3, 4, and 5.
We initially concentrate on LBGs at z~3 and select three subsamples on stellar
mass, extinction corrected SF and rest-frame UV-magnitude. We produce composite
870micron images of the typical source in our subsamples, obtaining ~4sigma
detections and suggesting a correlation between FIR luminosity and stellar
mass. We apply a similar procedure to our full samples at z~3, 4, 4.5 and 5 and
do not obtain detections - consistent with a simple scaling between FIR
luminosity and stellar mass. In order to constrain the FIR SED of these systems
we explore their emission at multiple wavelengths spanning the peak of dust
emission at z~3 using the Herschel SPIRE observations of the field. We obtain
detections at multiple wavelengths for both our stellar mass and UV-magnitude
selected samples, and find a best-fit SED with T_dust in the ~33-41K range. We
calculate L_FIR, obscured SFRs and M_dust, and find that a significant fraction
of SF in these systems is obscured. Interestingly, our extinction corrected SFR
sample does not display the large FIR fluxes predicted from its red UV-spectral
slope. This suggests that the method of assuming an intrinsic UV-slope and
correcting for dust attenuation may be invalid for this sample - and that these
are not in fact the most actively SF systems. All of our z~3 samples fall on
the `main sequence' of SF galaxies at z~3 and our detected subsamples are
likely to represent the high obscuration end of LBGs at their epoch. We compare
the FIR properties of our subsamples with various other populations, finding
that our stellar mass selected sample shows similar FIR characteristics to SMGs
at the same epoch and therefore potentially represents the low L_FIR end of the
high redshift FIR luminosity function.Comment: 18 pages, 10 figure, MNRAS accepted, corrected typos,
acknowledgements adde
Towards the origin of the radio emission in AR Sco, the first radio-pulsing white dwarf binary
The binary system AR Sco contains an M star and the only known radio-pulsing
white dwarf. The system shows emission from radio to X-rays, likely dominated
by synchrotron radiation. The mechanism that produces most of this emission
remains unclear. Two competing scenarios have been proposed: Collimated
outflows, and direct interaction between the magnetospheres of the white dwarf
and the M star. The two proposed scenarios can be tested via very long baseline
interferometric radio observations. We conducted a radio observation with the
Australian Long Baseline Array (LBA) on 20 Oct 2016 at 8.5 GHz to study the
compactness of the radio emission. Simultaneous data with the Australian
Telescope Compact Array (ATCA) were also recorded for a direct comparison of
the obtained flux densities. AR Sco shows radio emission compact on
milliarcsecond angular scales (, or $4\
\mathrm{R_{\odot}}\approx 6.5\ \mathrm{mJy}$. A comparison with the simultaneous ATCA
data shows that no flux is resolved out on mas scales, implying that the radio
emission is produced in this compact region. Additionally, the obtained radio
light curves on hour timescales are consistent with the optical light curve.
The radio emission in AR Sco is likely produced in the magnetosphere of the M
star or the white dwarf, and we see no evidence for a radio outflow or
collimated jets significantly contributing to the radio emission.Comment: 4 pages, 2 figures, accepted for publication in A&
Exploring the impact of IMF and binary parameter stochasticity with a binary population synthesis code
Low mass star formation regions are unlikely to fully populate their initial
mass functions, leading to a deficit of massive stars. In binary stellar
populations, the full range of binary separations and mass ratios will also be
underpopulated. To explore the effects of stochastic sampling in the integrated
light of stellar clusters, we calculate models at a broad range of cluster
masses, from 10^2 to 10^7 M_sun, using a binary stellar population synthesis
code. For clusters with stellar masses less than 10^5 M_sun, observable
quantities show substantial scatter and their mean properties reflect the
expected deficit of massive stars. In common with previous work, we find that
purely stochastic sampling of the initial mass function appears to
underestimate the mass of the most massive star in known clusters. However,
even with this constraint, the majority of clusters likely inject sufficient
kinetic energy to clear their birth clusters of gas. For quantities which
directly measure the impact of the most massive stars, such as N_{ion},
xi_{ion} and beta_{UV}, uncertainties due to stochastic sampling dominate over
those from the IMF shape or distribution of binary parameters, while stochastic
sampling has a negligible effect on the stellar continuum luminosity density.Comment: 15 pages and 8 page appendix. Accepted for publication in MNRA
Exploring the Evolution of Dust Temperature using Spectral Energy Distribution Fitting in a Large Photometric Survey
Panchromatic analysis of galaxy spectral energy distributions, spanning from
the ultraviolet to the far-infrared, probes not only the stellar population but
also the properties of interstellar dust through its extinction and
long-wavelength reemission. However little work has exploited the full power of
such fitting to constrain the redshift evolution of dust temperature in
galaxies. To do so, we simultaneously fit ultraviolet, optical and infrared
observations of stacked galaxy subsamples at a range of stellar masses and
photometric redshifts at 0<<5, using an energy-balance formalism. However,
we find UV-emission beyond the Lyman limit in some photometric redshift
selected galaxy subsamples, giving rise to the possibility of contaminated
observations. We carefully define a robust, clean subsample which extends to no
further than ~2. This has consistently lower derived temperatures by
K, relative to the full sample. We find a linear increase
in dust temperature with redshift, with K. Our inferred temperature evolution is consistent with a modest
rise in dust temperature with redshift, but inconsistent with some previous
analyses. We also find a majority of photometrically-selected subsamples at
>4.5 under-predict the IR emission while giving reasonable fits to the
UV-optical. This could be due to a spatial disconnect in the locations of the
UV and IR emission peaks, suggesting that an energy-balance formalism may not
always be applicable in the distant Universe.Comment: 16 Pages, 9 Figures + appendix, accepted for publication in MNRA
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