2,528 research outputs found
What shapes the far-infrared spectral energy distributions of galaxies?
To explore the connection between the global physical properties of galaxies
and their far-infrared (FIR) spectral energy distributions (SEDs), we study the
variation in the FIR SEDs of a set of hydrodynamically simulated galaxies that
are generated by performing dust radiative transfer in post-processing. Our
sample includes both isolated and merging systems at various stages of the
merging process and covers infrared (IR) luminosities and dust masses that are
representative of both low- and high-redshift galaxies. We study the FIR SEDs
using principle component analysis (PCA) and find that 97\% of the variance in
the sample can be explained by two principle components (PCs). The first PC
characterizes the wavelength of the peak of the FIR SED, and the second encodes
the breadth of the SED. We find that the coefficients of both PCs can be
predicted well using a double power law in terms of the IR luminosity and dust
mass, which suggests that these two physical properties are the primary
determinants of galaxies' FIR SED shapes. Incorporating galaxy sizes does not
significantly improve our ability to predict the FIR SEDs. Our results suggest
that the observed redshift evolution in the effective dust temperature at fixed
IR luminosity is not driven by geometry: the SEDs of ultraluminous
IR galaxies (ULIRGs) are cooler than those of local ULIRGs not because the
high-redshift galaxies are more extended but rather because they have higher
dust masses at fixed IR luminosity. Finally, based on our simulations, we
introduce a two-parameter set of SED templates that depend on both IR
luminosity and dust mass.Comment: Submitted to ApJ, comments welcom
Extragalactic Background Light and Gamma-Ray Attenuation
Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN)
and gamma ray bursts (GRBs) give upper limits on the extragalactic background
light (EBL) from the UV to the mid-IR that are only a little above the lower
limits from observed galaxies. These upper limits now rule out some EBL models
and purported observations, with improved data likely to provide even stronger
constraints. We present EBL calculations both based on multiwavelength
observations of thousands of galaxies and also based on semi-analytic models,
and show that they are consistent with these lower limits from observed
galaxies and with the gamma-ray upper limit constraints. Such comparisons
"close the loop" on cosmological galaxy formation models, since they account
for all the light, including that from galaxies too faint to see. We compare
our results with those of other recent works, and discuss the implications of
these new EBL calculations for gamma ray attenuation. Catching a few GRBs with
groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov
detectors could provide important new constraints on the high-redshift star
formation history of the universe.Comment: 12 pages, 8 multi-panel figures, Invited talk at the 25th Texas
Symposium on Relativistic Astrophysics, Heidelberg December 6-10, 201
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
The initial mass function (IMF) is a key ingredient in many studies of galaxy
formation and evolution. Although the IMF is often assumed to be universal,
there is continuing evidence that it is not universal. Spectroscopic studies
that derive the IMF of the unresolved stellar populations of a galaxy often
assume that this spectrum can be described by a single stellar population
(SSP). To alleviate these limitations, in this paper we have developed a unique
hierarchical Bayesian framework for modelling composite stellar populations
(CSPs). Within this framework we use a parameterized IMF prior to regulate a
direct inference of the IMF. We use this new framework to determine the number
of SSPs that is required to fit a set of realistic CSP mock spectra. The CSP
mock spectra that we use are based on semi-analytic models and have an IMF that
varies as a function of stellar velocity dispersion of the galaxy. Our results
suggest that using a single SSP biases the determination of the IMF slope to a
higher value than the true slope, although the trend with stellar velocity
dispersion is overall recovered. If we include more SSPs in the fit, the
Bayesian evidence increases significantly and the inferred IMF slopes of our
mock spectra converge, within the errors, to their true values. Most of the
bias is already removed by using two SSPs instead of one. We show that we can
reconstruct the variable IMF of our mock spectra for signal-to-noise ratios
exceeding 75.Comment: Accepted for publication in MNRAS, 16 pages, 8 figure
Steadily Increasing Star Formation Rates in Galaxies Observed at 3 <~ z <~ 5 in the CANDELS/GOODS-S Field
We investigate the star formation histories (SFHs) of high redshift (3 <~ z
<~ 5) star-forming galaxies selected based on their rest-frame ultraviolet (UV)
colors in the CANDELS/GOODS-S field. By comparing the results from the
spectral-energy-distribution-fitting analysis with two different assumptions
about the SFHs --- i.e., exponentially declining SFHs as well as increasing
ones, we conclude that the SFHs of high-redshift star-forming galaxies increase
with time rather than exponentially decline. We also examine the correlations
between the star formation rates (SFRs) and the stellar masses. When the
galaxies are fit with rising SFRs, we find that the trend seen in the data
qualitatively matches the expectations from a semi-analytic model of galaxy
formation. The mean specific SFR is shown to increase with redshift, also in
agreement with the theoretical prediction. From the derived tight correlation
between stellar masses and SFRs, we derive the mean SFH of star-forming
galaxies in the redshift range of 3 <~ z <~ 5, which shows a steep power-law
(with power alpha = 5.85) increase with time. We also investigate the formation
timescales and the mean stellar population ages of these star-forming galaxies.
Our analysis reveals that UV-selected star-forming galaxies have a broad range
of the formation redshift. The derived stellar masses and the stellar
population ages show positive correlation in a sense that more massive galaxies
are on average older, but with significant scatter. This large scatter implies
that the galaxies' mass is not the only factor which affects the growth or star
formation of high-redshift galaxies.Comment: 31 pages, 8 figures, 2 table
Diffuse Extragalactic Background Radiation
Attenuation of high--energy gamma rays by pair--production with UV, optical
and IR background photons provides a link between the history of galaxy
formation and high--energy astrophysics. We present results from our latest
semi-analytic models (SAMs), based upon a CDM hierarchical structural
formation scenario and employing all ingredients thought to be important to
galaxy formation and evolution, as well as reprocessing of starlight by dust to
mid- and far-IR wavelengths. Our models also use results from recent
hydrodynamic galaxy merger simulations. These latest SAMs are successful in
reproducing a large variety of observational constraints such as number counts,
luminosity and mass functions, and color bimodality. We have created 2 models
that bracket the likely ranges of galaxy emissivities, and for each of these we
show how the optical depth from pair--production is affected by redshift and
gamma-ray energy. We conclude with a discussion of the implications of our
work, and how the burgeoning science of gamma-ray astronomy will continue to
help constrain cosmology.Comment: 12 pages, 8 figures, to be published in the Proceedings of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, held
July 2008 in Heidelberg, German
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