258 research outputs found
De-noising of galaxy optical spectra with autoencoders
Optical spectra contain a wealth of information about the physical properties
and formation histories of galaxies. Often though, spectra are too noisy for
this information to be accurately retrieved. In this study, we explore how
machine learning methods can be used to de-noise spectra and increase the
amount of information we can gain without having to turn to sample averaging
methods such as spectral stacking. Using machine learning methods trained on
noise-added spectra - SDSS spectra with Gaussian noise added - we investigate
methods of maximising the information we can gain from these spectra, in
particular from emission lines, such that more detailed analysis can be
performed. We produce a variational autoencoder (VAE) model, and apply it on a
sample of noise-added spectra. Compared to the flux measured in the original
SDSS spectra, the model values are accurate within 0.3-0.5 dex, depending on
the specific spectral line and S/N. Overall, the VAE performs better than a
principle component analysis (PCA) method, in terms of reconstruction loss and
accuracy of the recovered line fluxes. To demonstrate the applicability and
usefulness of the method in the context of large optical spectroscopy surveys,
we simulate a population of spectra with noise similar to that in galaxies at
observed by the Dark Energy Spectroscopic Instrument (DESI). We show
that we can recover the shape and scatter of the MZR in this "DESI-like"
sample, in a way that is not possible without the VAE-assisted de-noising.Comment: 14 pages, 10 figures, 6 tables, accepted for publication in MNRA
The Far-Infrared, UV and Molecular Gas Relation in Galaxies up to z=2.5
We use the infrared excess (IRX) FIR/UV luminosity ratio to study the
relation between the effective UV attenuation (A_IRX) and the UV spectral slope
(beta) in a sample of 450 1<z<2.5 galaxies. The FIR data is from very deep
Herschel observations in the GOODS fields that allow us to detect galaxies with
SFRs typical of galaxies with log(M)>9.3. Thus, we are able to study galaxies
on and even below the main SFR-stellar mass relation (main sequence). We find
that main sequence galaxies form a tight sequence in the IRX--beta plane, which
has a flatter slope than commonly used relations. This slope favors a SMC-like
UV extinction curve, though the interpretation is model dependent. The scatter
in the IRX-beta plane, correlates with the position of the galaxies in the
SFR-M plane. Using a smaller sample of galaxies with CO gas masses, we study
the relation between the UV attenuation and the molecular gas content. We find
a very tight relation between the scatter in the IRX-beta plane and the
specific attenuation (S_A), a quantity that represents the attenuation
contributed by the molecular gas mass per young star. S_A is sensitive to both
the geometrical arrangement of stars and dust, and to the compactness of the
star forming regions. We use this empirical relation to derive a method for
estimating molecular gas masses using only widely available integrated
rest-frame UV and FIR photometry. The method produces gas masses with an
accuracy between 0.12-0.16 dex in samples of normal galaxies between z~0 and
z~1.5. Major mergers and sub-millimeter galaxies follow a different S_A
relation.Comment: 11 pages, 6 pages appendix, 11 figures, accepted to Ap
The GALEX Arecibo SDSS Survey. VI. Second Data Release and Updated Gas Fraction Scaling Relations
We present the second data release from the GALEX Arecibo SDSS Survey (GASS),
an ongoing large Arecibo program to measure the HI properties for an unbiased
sample of ~1000 galaxies with stellar masses greater than 10^10 Msun and
redshifts 0.025<z<0.05. GASS targets are selected from the Sloan Digital Sky
Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging
surveys, and are observed until detected or until a gas mass fraction limit of
a few per cent is reached. This second data installment includes new Arecibo
observations of 240 galaxies, and marks the 50% of the complete survey. We
present catalogs of the HI, optical and ultraviolet parameters for these
galaxies, and their HI-line profiles. Having more than doubled the size of the
sample since the first data release, we also revisit the main scaling relations
of the HI mass fraction with galaxy stellar mass, stellar mass surface density,
concentration index, and NUV-r color, as well as the gas fraction plane
introduced in our earlier work.Comment: 30 pages, 12 figures. Accepted for publication in A&A. Version with
complete Appendix A available at http://www.mpa-garching.mpg.de/GASS/pubs.php
. GASS released data can be found at
http://www.mpa-garching.mpg.de/GASS/data.ph
Geometrical tests of cosmological models. III. The cosmology-evolution diagram at z=1
The rotational velocity of distant galaxies, when interpreted as a size
(luminosity) indicator, may be used as a tool to select high redshift standard
rods (candles) and probe world models and galaxy evolution via the classical
angular diameter-redshift or Hubble diagram tests. We implement the proposed
testing strategy using a sample of 30 rotators spanning the redshift range
0.2<z<1 with high resolution spectra and images obtained by the VIMOS/VLT Deep
Redshift Survey (VVDS) and the Great Observatories Origins Deep Survey (GOODs).
We show that by applying at the same time the angular diameter-redshift and
Hubble diagrams to the same sample of objects (i.e. velocity selected galactic
discs) one can derive a characteristic chart, the cosmology-evolution diagram,
mapping the relation between global cosmological parameters and local
structural parameters of discs such as size and luminosity. This chart allows
to put constraints on cosmological parameters when general prior information
about discs evolution is available. In particular, by assuming that equally
rotating large discs cannot be less luminous at z=1 than at present (M(z=1) <
M(0)), we find that a flat matter dominated cosmology (Omega_m=1) is excluded
at a confidence level of 2sigma and an open cosmology with low mass density
(Omega_m = 0.3) and no dark energy contribution is excluded at a confidence
level greater than 1 sigma. Inversely, by assuming prior knowledge about the
cosmological model, the cosmology-evolution diagram can be used to gain useful
insights about the redshift evolution of the structural parameters of baryonic
discs hosted in dark matter halos of nearly equal masses.Comment: 14 pages and 11 figures. A&A in pres
Dust temperature and CO-to-H2 conversion factor variations in the SFR-M* plane
Deep Herschel imaging and 12CO(2-1) line luminosities from the IRAM PdBI are
combined for a sample of 17 galaxies at z>1 from the GOODS-N field. The sample
includes galaxies both on and above the main sequence (MS) traced by
star-forming galaxies in the SFR-M* plane. The far-infrared data are used to
derive dust masses, Mdust. Combined with an empirical prescription for the
dependence of the gas-to-dust ratio on metallicity (GDR), the CO luminosities
and Mdust values are used to derive for each galaxy the CO-to-H2 conversion
factor, alpha_co. Like in the local Universe, the value of alpha_co is a factor
of ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We
also uncover a relation between alpha_co and dust temperature (Tdust; alpha_co
decreasing with increasing Tdust) as obtained from modified blackbody fits to
the far-infrared data. While the absolute normalization of the alpha_co(Tdust)
relation is uncertain, the global trend is robust against possible systematic
biases in the determination of Mdust, GDR or metallicity. Although we cannot
formally distinguish between a step and a smooth evolution of alpha_co with the
dust temperature, we can conclude that in galaxies of near-solar metallicity, a
critical value of Tdust=30K can be used to determine whether the appropriate
alpha_co is closer to the starburst value (1.0 Msun(K kms pc^2)^-1, if
Tdust>30K) or closer to the Galactic value (4.35 Msun (K kms pc^2)^-1, if
Tdust<30K). This indicator has the great advantage of being less subjective
than visual morphological classifications of mergers/SFGs, which can be
difficult at high z because of the clumpy nature of SFGs. In the absence of
far-infrared data, the offset of a galaxy from the main sequence (i.e.,
log[SSFR(galaxy)/SSFR_MS(M*,z)]) can be used to identify galaxies requiring the
use of an alpha_co conversion factor lower than the Galactic value.Comment: Accepted for publication in Astronomy and Astrophysics (A&A); 15
pages, 6 figures; V2: updated reference lis
Deriving a multivariate CO-to-H conversion function using the [CII]/CO(1-0) ratio and its application to molecular gas scaling relations
We present Herschel PACS observations of the [CII] 158 micron emission line
in a sample of 24 intermediate mass (9<logM/M<10) and low
metallicity (0.4< Z/Z<1.0) galaxies from the xCOLD GASS survey.
Combining them with IRAM CO(1-0) measurements, we establish scaling relations
between integrated and molecular region [CII]/CO(1-0) luminosity ratios as a
function of integrated galaxy properties. A Bayesian analysis reveals that only
two parameters, metallicity and offset from the star formation main sequence,
MS, are needed to quantify variations in the luminosity ratio;
metallicity describes the total dust content available to shield CO from UV
radiation, while MS describes the strength of this radiation field. We
connect the [CII]/CO luminosity ratio to the CO-to-H conversion factor and
find a multivariate conversion function , which can be used up to
z~2.5. This function depends primarily on metallicity, with a second order
dependence on MS. We apply this to the full xCOLD GASS and PHIBSS1
surveys and investigate molecular gas scaling relations. We find a flattening
of the relation between gas mass fraction and stellar mass at
logM/M<10. While the molecular gas depletion time varies with
sSFR, it is mostly independent of mass, indicating that the low L/SFR
ratios long observed in low mass galaxies are entirely due to photodissociation
of CO, and not to an enhanced star formation efficiency.Comment: Submitted to MNRAS, this version after referee comments. 21 page
The Arecibo Galaxy Environment Survey III: Observations Toward the Galaxy Pair NGC 7332/7339 and the Isolated Galaxy NGC 1156
Two 5 square degree regions around the NGC 7332/9 galaxy pair and the
isolated galaxy NGC 1156 have been mapped in the 21-cm line of neutral hydrogen
(HI) with the Arecibo L-band Feed Array out to a redshift of ~0.065
km/s) as part of the Arecibo Galaxy Environment Survey. One of the aims of this
survey is to investigate the environment of galaxies by identifying dwarf
companions and interaction remnants; both of these areas provide the potential
for such discoveries. The neutral hydrogen observations were complemented by
optical and radio follow-up observations with a number of telescopes. A total
of 87 galaxies were found, of which 39 (45 per cent) were previously cataloged
and 15 (17 per cent) have prior redshifts. Two dwarf galaxies have been
discovered in the NGC 7332 group and a single dwarf galaxy in the vicinity NGC
1156 . A parallel optical search of the area revealed one further possible
dwarf galaxy near NGC 7332.Comment: 18 pages, 17 figures, uses emulateap
The evolution of the dust temperatures of galaxies in the SFR–M∗plane up to z ~ 2
We study the evolution of the dust temperature of galaxies in the SFR−M ∗ plane up to z ∼ 2 using far-infrared and submillimetre observations from the Herschel Space Observatory taken as part of the PACS Evolutionary Probe (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time key programmes. Starting from a sample of galaxies with reliable star-formation rates (SFRs), stellar masses (M ∗ ) and redshift estimates, we grid the SFR−M ∗ parameter space in several redshift ranges and estimate the mean dust temperature (T dust ) of each SFR–M ∗ −z bin. Dust temperatures are inferred using the stacked far-infrared flux densities (100–500μm) of our SFR–M ∗ −z bins. At all redshifts, the dust temperature of galaxies smoothly increases with rest-frame infrared luminosities (L IR ), specific SFRs (SSFR; i.e., SFR/M ∗ ), and distances with respect to the main sequence (MS) of the SFR−M ∗ plane (i.e., Δlog(SSFR) MS = log[SSFR(galaxy)/SSFR MS (M ∗ ,z)]). The T dust −SSFR and T dust – Δlog(SSFR) MS correlations are statistically much more significant than the T dust −L IR one. While the slopes of these three correlations are redshift-independent, their normalisations evolve smoothly from z = 0 and z ∼ 2. We convert these results into a recipe to derive T dust from SFR, M ∗ and z, valid out to z ∼ 2 and for the stellar mass and SFR range covered by our stacking analysis. The existence of a strong T dust −Δlog(SSFR) MS correlation provides us with several pieces of information on the dust and gas content of galaxies. Firstly, the slope of the T dust −Δlog(SSFR) MS correlation can be explained by the increase in the star-formation efficiency (SFE; SFR/M gas ) with Δlog(SSFR) MS as found locally by molecular gas studies. Secondly, at fixed Δlog(SSFR) MS , the constant dust temperature observed in galaxies probing wide ranges in SFR and M ∗ can be explained by an increase or decrease in the number of star-forming regions with comparable SFE enclosed in them. And thirdly, at high redshift, the normalisation towards hotter dust temperature of the T dust −Δlog(SSFR) MS correlation can be explained by the decrease in the metallicities of galaxies or by the increase in the SFE of MS galaxies. All these results support the hypothesis that the conditions prevailing in the star-forming regions of MS and far-above-MS galaxies are different. MS galaxies have star-forming regions with low SFEs and thus cold dust, while galaxies situated far above the MS seem to be in a starbursting phase characterised by star-forming regions with high SFEs and thus hot dust
Smooth(er) Stellar Mass Maps in CANDELS: Constraints on the Longevity of Clumps in High-redshift Star-forming Galaxies
We perform a detailed analysis of the resolved colors and stellar populations
of a complete sample of 323 star-forming galaxies at 0.5 < z < 1.5, and 326
star-forming galaxies at 1.5 < z < 2.5 in the ERS and CANDELS-Deep region of
GOODS-South. Galaxies were selected to be more massive than 10^10 Msun and have
specific star formation rates above 1/t_H. We model the 7-band optical ACS +
near-IR WFC3 spectral energy distributions of individual bins of pixels,
accounting simultaneously for the galaxy-integrated photometric constraints
available over a longer wavelength range. We analyze variations in rest-frame
color, stellar surface mass density, age, and extinction as a function of
galactocentric radius and local surface brightness/density, and measure
structural parameters on luminosity and stellar mass maps. We find evidence for
redder colors, older stellar ages, and increased dust extinction in the nuclei
of galaxies. Big star-forming clumps seen in star formation tracers are less
prominent or even invisible on the inferred stellar mass distributions.
Off-center clumps contribute up to ~20% to the integrated SFR, but only 7% or
less to the integrated mass of all massive star-forming galaxies at z ~ 1 and z
~ 2, with the fractional contributions being a decreasing function of
wavelength used to select the clumps. The stellar mass profiles tend to have
smaller sizes and M20 coefficients, and higher concentration and Gini
coefficients than the light distribution. Our results are consistent with an
inside-out disk growth scenario with brief (100 - 200 Myr) episodic local
enhancements in star formation superposed on the underlying disk.
Alternatively, the young ages of off-center clumps may signal inward clump
migration, provided this happens efficiently on the order of an orbital
timescale.Comment: Accepted by The Astrophysical Journal, 27 pages, 1 table, 16 figure
- …