116 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
Reevaluating old stellar populations
Determining the properties of old stellar populations (those with age >1 Gyr) has long involved the comparison of their integrated light, either in the form of photometry or spectroscopic indexes, with empirical or synthetic templates. Here we reevaluate the properties of old stellar populations using a new set of stellar population synthesis models, designed to incorporate the effects of binary stellar evolution pathways as a function of stellar mass and age. We find that single-aged stellar population models incorporating binary stars, as well as new stellar evolution and atmosphere models, can reproduce the colours and spectral indices observed in both globular clusters and quiescent galaxies. The best fitting model populations are often younger than those derived from older spectral synthesis models, and may also lie at slightly higher metallicities
A star-forming galaxy at z= 5.78 in the Chandra Deep Field South
We report the discovery of a luminous z = 5.78 star-forming galaxy in the Chandra Deep Field South. This galaxy was selected as an ‘i-drop’ from the GOODS public survey imaging with the Hubble Space Telescope/Advanced Camera for Surveys (object 3 in the work of Stanway, Bunker & McMahon 2003). The large colour of (i′−z′)AB = 1.6 indicated a spectral break consistent with the Lyman α forest absorption shortward of Lyman α at z≈ 6. The galaxy is very compact (marginally resolved with ACS with a half-light radius of 0.08 arcsec, so rhl 5. Our spectroscopic redshift for this object confirms the validity of the i′-drop technique of Stanway et al. to select star-forming galaxies atz≈ 6
Binary fraction indicators in resolved stellar populations and supernova- type ratios
The binary fraction of a stellar population can have pronounced effects on its properties, and in particular the number counts of different massive star types, and the relative subtype rates of the supernovae which end their lives. Here we use binary population synthesis models with a binary fraction that varies with initial mass to test the effects on resolved stellar pops and supernovae, and ask whether these can constrain the poorly-known binary fraction in different mass and metallicity regimes. We show that Wolf-Rayet star subtype ratios are valuable binary diagnostics, but require large samples to distinguish by models. Uncertainties in which stellar models would be spectroscopically classified as Wolf-Rayet stars are explored. The ratio of thermonuclear, stripped envelope and other core-collapse supernovae may prove a more accessible test and upcoming surveys will be sufficient to constrain both the high mass and low mass binary fraction in the z < 1 galaxy population
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