59 research outputs found
Galaxy physical properties from population model fitting
In the last two decades astronomers carried out a large number of galaxy surveys tuned towards the study of galaxy formation and evolution. With the ever improving technology, increasing telescope sizes of ground-based telescopes and the development of space-based telescopes it has become possible to detect galaxies at a time when the Universe was only a few hundred million years old. However, for the majority of galaxies a detailed spectroscopic analysis is not possible due to their distance and limited telescope time. Thus, many surveys rely on photometric data alone to help unveil the properties of galaxies. One of the most important areas of study within galaxy formation and evolution is the analysis of the galaxy stellar population parameters as these can provide us with information about the star formation histories of galaxies and when and possibly how they assembled their mass. A popular approach in the literature is the fitting of synthetic spectral energy distributions inferred from stellar population modelling to the multi-wavelength photometry of galaxies. However, this approach comes with a large number of fitting parameters all of which are essentially user-dependent and will bias the result in one way or another. The aim of this thesis is to investigate the accuracy and efficiency of spectral energy distribution fitting as derivation technique for the galaxy physical properties, such as age, stellar mass, dust reddening, etc., as a function of the fitting parameters, such as star formation histories, age grids, metallicity, initial mass function, dust reddening, reddening law, filter setup and wavelength coverage and stellar population model, and to find the setup of parameters that recovers the properties best. In particular, we investigate in detail the dependence of the derived properties on the assumed wavelength coverage and exact filter setups. Mock galaxies with known properties serve as test particles for this exercise. The synthetic spectral energy distributions used in this thesis are based on the Maraston (2005) stellar population models. Literature results which investigate similar problems are obtained using the models of Bruzual & Charlot (2003). Firstly, the fitting is carried out under the assumption that galaxy redshifts are known mimicking surveys for which galaxy redshifts are derived spectroscopically. Then we study the case in which the redshift is not known and needs to be determined alongside the galaxy physical properties which is the case for most photometric surveys. In general, we find that - using normal template star formation histories as widely used in the literature - ages and stellar masses of star-forming galaxies are underestimated, reddening and star formation rates are overestimated. This is due to a mismatch in star formation history and the overshining effect. The addition of the rest-frame near-IR appears to be crucial for the derivation of robust results. For aged galaxies with little or no on-going star formation we find that a setup covering a wide range of star formation histories and metallicities works best when the fit is carried out excluding dust reddening. For high redshift star-forming galaxies we find that a new type of star formation history (inverted-� models which start forming stars at high redshift) recovers stellar masses and star formation rates best. The parameters of truly passive galaxies are much better determined. In order to ease the comparison of literature data that was analysed with different fitting parameter setups we provide scaling relations for the transformation of stellar masses between different setups. Our results concerning the importance of the wavelength coverage in the fitting are particularly useful for the planning of future surveys and observation proposals. We apply our findings from the study of mock galaxies to various samples of real galaxies which cover different redshift ranges and galaxy types. We derive the stellar population properties for a sample of star-forming galaxies at z ∼ 2 from the GOODS-S survey using inverted-� models (with high formation redshifts) and show that the obtained dust reddening and star formation histories are in excellent agreement with those derived from other methods. We also show how the wrong set of fitting parameters can lead to unrealistically young ages, low stellar masses and high star formation rates which are a pure artefact from the fit. Furthermore, we study a sample of low redshift, predominantly passive galaxies from the SDSS-III/BOSS survey for which we use the spectral model of Maraston et al. (2009) that is tuned to the needs of this particular type of galaxies. We find that BOSS galaxies are mostly passive, old and massive at each redshift in the range 0 < z < 0.7. Finally, we complement the study of SDSS-III/BOSS galaxies by deriving stellar masses for the SDSS-I/II galaxies in a similar fashion. We conclude that the simultaneous derivation of stellar population properties of galaxies from spectral energy distribution fitting is difficult but that these properties can be very well derived provided the right setup and wavelength coverage are used in the fitting. We also conclude that more work is needed to better match star formation histories of aged galaxies with little on-going star formation in order to improve estimates of stellar population parameters.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
An Application of Multi-band Forced Photometry to One Square Degree of SERVS: Accurate Photometric Redshifts and Implications for Future Science
We apply The Tractor image modeling code to improve upon existing multi-band
photometry for the Spitzer Extragalactic Representative Volume Survey (SERVS).
SERVS consists of post-cryogenic Spitzer observations at 3.6 and 4.5 micron
over five well-studied deep fields spanning 18 square degrees. In concert with
data from ground-based near-infrared (NIR) and optical surveys, SERVS aims to
provide a census of the properties of massive galaxies out to z ~ 5. To
accomplish this, we are using The Tractor to perform "forced photometry." This
technique employs prior measurements of source positions and surface brightness
profiles from a high-resolution fiducial band from the VISTA Deep Extragalactic
Observations (VIDEO) survey to model and fit the fluxes at lower-resolution
bands. We discuss our implementation of The Tractor over a square degree test
region within the XMM-LSS field with deep imaging in 12 NIR/optical bands. Our
new multi-band source catalogs offer a number of advantages over traditional
position-matched catalogs, including 1) consistent source cross-identification
between bands, 2) de-blending of sources that are clearly resolved in the
fiducial band but blended in the lower-resolution SERVS data, 3) a higher
source detection fraction in each band, 4) a larger number of candidate
galaxies in the redshift range 5 < z < 6, and 5) a statistically significant
improvement in the photometric redshift accuracy as evidenced by the
significant decrease in the fraction of outliers compared to spectroscopic
redshifts. Thus, forced photometry using The Tractor offers a means of
improving the accuracy of multi-band extragalactic surveys designed for galaxy
evolution studies. We will extend our application of this technique to the full
SERVS footprint in the future.Comment: accepted to ApJ, 22 pages, 12 figure
Breaking the Curve with CANDELS: A Bayesian Approach to Reveal the Non-Universality of the Dust-Attenuation Law at High Redshift
Dust attenuation affects nearly all observational aspects of galaxy
evolution, yet very little is known about the form of the dust-attenuation law
in the distant Universe. Here, we model the spectral energy distributions
(SEDs) of galaxies at z = 1.5--3 from CANDELS with rest-frame UV to near-IR
imaging under different assumptions about the dust law, and compare the amount
of inferred attenuated light with the observed infrared (IR) luminosities. Some
individual galaxies show strong Bayesian evidence in preference of one dust law
over another, and this preference agrees with their observed location on the
plane of infrared excess (IRX, ) and UV slope
(). We generalize the shape of the dust law with an empirical model,
where
is the dust law of Calzetti et al. (2000), and show that there
exists a correlation between the color excess and tilt with
+ . Galaxies with high
color excess have a shallower, starburst-like law, and those with low color
excess have a steeper, SMC-like law. Surprisingly, the galaxies in our sample
show no correlation between the shape of the dust law and stellar mass,
star-formation rate, or . The change in the dust law with color excess
is consistent with a model where attenuation is caused by by scattering, a
mixed star-dust geometry, and/or trends with stellar population age,
metallicity, and dust grain size. This rest-frame UV-to-near-IR method shows
potential to constrain the dust law at even higher () redshifts.Comment: 20 pages, 18 figures, resubmitted to Ap
SN Ia host galaxy properties from Sloan Digital Sky Survey-II spectroscopy
We study the stellar populations of Type Ia supernova (SN Ia) host galaxies using Sloan
Digital Sky Survey (SDSS)-II spectroscopy. The main focus is on the relationships of SN Ia
properties with stellar velocity dispersion and the stellar population parameters age, metallicity
and element abundance ratios. We concentrate on a sub-sample of 84 SNe Ia from the
SDSS-II Supernova Survey and find that SALT2 stretch factor values show the strongest dependence
on stellar population age. Hence, more luminous SNe Ia appear in younger stellar
progenitor systems. No statistically significant trends in the Hubble residual with any of the
stellar population parameters studied are found. Moreover, the method of photometric stellar
mass derivation affects the Hubble residual–mass relationship. For an extended sample (247
objects), including SNe Ia with SDSS host galaxy photometry only, the Hubble residual–mass
relationship behaves as a sloped step function. In the high-mass regime, probed by our host
spectroscopy sample, this relationship is flat. Below a stellar mass of ∼2 × 1010M , i.e. close
to the evolutionary transition mass of low-redshift galaxies, the trend changes dramatically
such that lower mass galaxies possess lower luminosity SNe Ia after light-curve corrections.
The sloped step function of the Hubble residual–mass relationship should be accounted for
when using stellar mass as a further parameter for minimizing the Hubble residuals.Department of HE and Training approved lis
Photometric redshifts for galaxies in the Spitzer Extragalactic Representative Volume Survey (SERVS)
CANDELS Sheds Light on the Environmental Quenching of Low-mass Galaxies
We investigate the environmental quenching of galaxies, especially those with
stellar masses (M*), beyond the local universe. Essentially
all local low-mass quenched galaxies (QGs) are believed to live close to
massive central galaxies, which is a demonstration of environmental quenching.
We use CANDELS data to test {\it whether or not} such a dwarf QG--massive
central galaxy connection exists beyond the local universe. To this purpose, we
only need a statistically representative, rather than a complete, sample of
low-mass galaxies, which enables our study to . For each low-mass
galaxy, we measure the projected distance () to its nearest massive
neighbor (M*) within a redshift range. At a given redshift
and M*, the environmental quenching effect is considered to be observed if the
distribution of QGs () is significantly skewed toward
lower values than that of star-forming galaxies (). For galaxies
with , such a difference between
and is detected up to . Also, about 10\%
of the quenched galaxies in our sample are located between two and four virial
radii () of the massive halos. The median projected distance from
low-mass QGs to their massive neighbors, , decreases with
satellite M* at , but increases with satellite M*
at . This trend suggests a smooth, if any,
transition of the quenching timescale around at
.Comment: 8 pages, 5 figures. ApJL accepted. Typos correcte
A WFC3 Grism Emission Line Redshift Catalog in the GOODS-South Field
We combine HST/WFC3 imaging and G141 grism observations from the CANDELS and
3D-HST surveys to produce a catalog of grism spectroscopic redshifts for
galaxies in the CANDELS/GOODS-South field. The WFC3/G141 grism spectra cover a
wavelength range of 1.1<lambda<1.7 microns with a resolving power of R~130 for
point sources, thus providing rest-frame optical spectra for galaxies out to
z~3.5. The catalog is selected in the H-band (F160W) and includes both galaxies
with and without previously published spectroscopic redshifts. Grism spectra
are extracted for all H-band detected galaxies with H<24 and a CANDELS
photometric redshift z_phot > 0.6. The resulting spectra are visually inspected
to identify emission lines and redshifts are determined using cross-correlation
with empirical spectral templates. To establish the accuracy of our redshifts,
we compare our results against high-quality spectroscopic redshifts from the
literature. Using a sample of 411 control galaxies, this analysis yields a
precision of sigma_NMAD=0.0028 for the grism-derived redshifts, which is
consistent with the accuracy reported by the 3D-HST team. Our final catalog
covers an area of 153 square arcmin and contains 1019 redshifts for galaxies in
GOODS-S. Roughly 60% (608/1019) of these redshifts are for galaxies with no
previously published spectroscopic redshift. These new redshifts span a range
of 0.677 < z < 3.456 and have a median redshift of z=1.282. The catalog
contains a total of 234 new redshifts for galaxies at z>1.5. In addition, we
present 20 galaxy pair candidates identified for the first time using the grism
redshifts in our catalog, including four new galaxy pairs at z~2, nearly
doubling the number of such pairs previously identified.Comment: 25 Pages, 9 Figures, submitted to A
Star formation rates and masses of z ~ 2 galaxies from multicolour photometry
Fitting synthetic spectral energy distributions (SED) to the multi-band
photometry of galaxies to derive their star formation rates (SFR), stellar
masses, ages, etc. requires making a priori assumptions about their star
formation histories (SFH). A widely adopted parameterization of the SFH, the
so-called tau-models where SFR goes as e^{-t/tau) is shown to lead to
unrealistically low ages when applied to star forming galaxies at z ~ 2, a
problem shared by other SFHs when the age is left as a free parameter in the
fitting. This happens because the SED of such galaxies, at all wavelengths, is
dominated by their youngest stellar populations, which outshine the older ones.
Thus, the SED of such galaxies conveys little information on the beginning of
star formation. To cope with this problem, we explore a variety of SFHs, such
as constant SFR and inverted-tau models - with SFR as e^{+t/tau) - along with
various priors on age, including assuming that star formation started at high
redshift in all the galaxies. We find that inverted-tau models with such latter
assumption give SFRs and extinctions in excellent agreement with the values
derived using only the UV part of the SED. These models are also shown to
accurately recover the SFRs and masses of mock galaxies at z ~ 2 constructed
from semi-analytic models. All other explored SFH templates do not fulfil these
two test. In particular, direct-tau models with unconstrained age in the
fitting procedure overstimate SFRs and underestimate stellar mass, and would
exacerbate an apparent mismatch between the cosmic evolution of the volume
densities of SFR and stellar mass. We conclude that for high-redshift star
forming galaxies an exponentially increasing SFR with a high formation redshift
is preferable to other forms of the SFH so far adopted in the literature.Comment: 19 pages, 28 figures, Monthly Notices of the Royal Astronomical
Society in pres
A Critical Assessment of Stellar Mass Measurement Methods
In this paper we perform a comprehensive study of the main sources of random
and systematic errors in stellar mass measurement for galaxies using their
Spectral Energy Distributions (SEDs). We use mock galaxy catalogs with
simulated multi-waveband photometry (from U-band to mid-infrared) and known
redshift, stellar mass, age and extinction for individual galaxies. Given
different parameters affecting stellar mass measurement (photometric S/N
ratios, SED fitting errors, systematic effects, the inherent degeneracies and
correlated errors), we formulated different simulated galaxy catalogs to
quantify these effects individually. We studied the sensitivity of stellar mass
estimates to the codes/methods used, population synthesis models, star
formation histories, nebular emission line contributions, photometric
uncertainties, extinction and age. For each simulated galaxy, the difference
between the input stellar masses and those estimated using different simulation
catalogs, , was calculated and used to identify the most
fundamental parameters affecting stellar masses. We measured different
components of the error budget, with the results listed as follows: (1). no
significant bias was found among different codes/methods, with all having
comparable scatter; (2). A source of error is found to be due to photometric
uncertainties and low resolution in age and extinction grids; (3). The median
of stellar masses among different methods provides a stable measure of the mass
associated with any given galaxy; (4). The deviations in stellar mass strongly
correlate with those in age, with a weaker correlation with extinction; (5).
the scatter in the stellar masses due to free parameters are quantified, with
the sensitivity of the stellar mass to both the population synthesis codes and
inclusion of nebular emission lines studied.Comment: 33 pages, 20 Figures, Accepted for publication in Astrophysical
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