138 research outputs found
Applications of Stellar Population Synthesis in the Distant Universe
Comparison with artificial galaxy models is essential for translating the
incomplete and low signal-to-noise data we can obtain on astrophysical stellar
populations to physical interpretations which describe their composition,
physical properties, histories and internal conditions. In particular, this is
true for distant galaxies, whose unresolved light embeds clues to their
formation and evolution as well as their impact on their wider environs.
Stellar population synthesis models are now used as the foundation of analysis
at all redshifts, but are not without their problems. Here we review the use of
stellar population synthesis models, with a focus on applications in the
distant Universe.Comment: 32 page review, published in Galaxies special issue, "Star Formation
in the UV", ed. Jorick Vin
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<z<0.25) Universe, which are tuned to match the luminosities and star
formation volume densities observed in z>~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.Comment: 13 pages, MNRAS accepte
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 ≈ 7 and beyond. The existing Hubble Wide Field Camera 3 (WFC3) and Visible and Infrared Survey Telescope for Astronomy (VISTA) Infrared Camera (VIRCam) instruments allow deep surveys to be undertaken well beyond 1 μm – a capability that will be further extended with the launch and commissioning of the 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 Hubble/WFC3 and VISTA/VIRCam Y-drop selections targeting galaxies at z ∼ 7.5 are vulnerable to contamination from T- and Y-class stars. Future observations using JWST, targeting the z ∼ 7 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
Core-collapse supernovae ages and metallicities from emission-line diagnostics of nearby stellar populations
Massive stars are the main objects that illuminate H II regions and they
evolve quickly to end their lives in core-collapse supernovae (CCSNe). Thus it
is important to investigate the association between CCSNe and H II regions. In
this paper, we present emission line diagnostics of the stellar populations
around nearby CCSNe, that include their host H II regions, from the PMAS/PPAK
Integral-field Supernova hosts COmpilation (PISCO). We then use BPASS stellar
population models to determine the age, metallicity and gas parameters for H II
regions associated with CCSNe, contrasting models that consider either single
star evolution alone or incorporate interacting binaries. We find binary-star
models, that allow for ionizing photon loss, provide a more realistic fit to
the observed CCSN hosts with metallicities that are closer to those derived
from the oxygen abundance in O3N2. We also find that type II and type Ibc SNe
arise from progenitor stars of similar age, mostly from 7 to 45 Myr, which
corresponds to stars with masses < 20 solar mass . However these two types SNe
have little preference in their host environment metallicity measured by oxygen
abundance or in progenitor initial mass. We note however that at lower
metallicities supernovae are more likely to be of type II.Comment: 22 pages, 19 Figures, 6 Tables. Accepted by MNRAS. Comments welcom
BPASS predictions for binary black hole mergers
Using the Binary Population and Spectral Synthesis code, BPASS, we have calculated the rates, time-scales and mass distributions for binary black hole (BH) mergers as a function of metallicity. We consider these in the context of the recently reported first Laser Interferometer Gravitational-Wave Observatory (LIGO) event detection. We find that the event has a very low probability of arising from a stellar population with initial metallicity mass fraction above Z = 0.010 (Z ≳ 0.5 Z⊙). Binary BH merger events with the reported masses are most likely in populations below 0.008 (Z ≲ 0.4 Z⊙). Events of this kind can occur at all stellar population ages from 3 Myr up to the age of the Universe, but constitute only 0.1–0.4 per cent of binary BH mergers between metallicities of Z = 0.001 and 0.008. However at metallicity Z = 10−4, 26 per cent of binary BH mergers would be expected to have the reported masses. At this metallicity, the progenitor merger times can be close to ≈10 Gyr and rotationally mixed stars evolving through quasi-homogeneous evolution, due to mass transfer in a binary, dominate the rate. The masses inferred for the BHs in the binary progenitor of GW 150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections
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 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.Comment: to appear in ApJ; replaced with accepted versio
Changing conceptions of solar system habitability in Dan Dare between the 1950s and the 2010s
In this article, I consider how Dan Dare has articulated our changing understanding of Solar System habitability, with particular reference to the first three Dan Dare audio dramas and the 1950s comic strips on which they were based: 'Voyage to Venus' (1950-51), 'The Red Moon Mystery' (1951-52) and 'Marooned on Mercury' (1952-53). Of special interest is the degree to which the adaptations have balanced the need for action with the original strip's pedagogical mission, and the extent to which that mediates the concerns of a changing demographic within their audience. By the time of the 2016 audio dramatization, an averagely science-literate audience was no longer willing to accept an Earth-like Venusian ecosystem. [...]the Treens are now described as colonizing Venus from elsewhere and, instead of living on the hostile surface, they inhabit platforms suspended in the cloud layer; the surface and other Venusian races no longer feature. With the still-debated discovery of phosphine, a potential chemical marker of biological activity or 'biosignature' in the Venusian cloud deck (Greaves et al 2020), the status of Venus as a habitable planet is once more the subject of intense scrutiny, and the audio updating of this key setting has never appeared more appropriate. [...]it condensed water on its surface
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