202 research outputs found
Disentangling star formation and merger growth in the evolution of luminous red galaxies
We introduce a novel technique for empirically understanding galaxy
evolution. We use empirically determined stellar evolution models to predict
the past evolution of the Sloan Digital Sky Survey (SDSS-II) Luminous Red
Galaxy (LRG) sample without any a-priori assumption about galaxy evolution. By
carefully contrasting the evolution of the predicted and observed number and
luminosity densities we test the passive evolution scenario for galaxies of
different luminosity, and determine minimum merger rates. We find that the LRG
population is not purely coeval, with some of galaxies targeted at z<0.23 and
at z>0.34 showing different dynamical growth than galaxies targeted throughout
the sample. Our results show that the LRG population is dynamically growing,
and that this growth must be dominated by the faint end. For the most luminous
galaxies, we find lower minimum merger rates than required by previous studies
that assume passive stellar evolution, suggesting that some of the dynamical
evolution measured previously was actually due to galaxies with non-passive
stellar evolution being incorrectly modelled. Our methodology can be used to
identify and match coeval populations of galaxies across cosmic times, over one
or more surveys.Comment: 10 pages, 5 figures, re-submitted to MNRAS after addressing referee's
report - clarifications and references added, sections expanded and typos
fixed. Conclusions unchange
Recovering galaxy star formation and metallicity histories from spectra using VESPA
We introduce VErsatile SPectral Analysis (VESPA): a new method which aims to
recover robust star formation and metallicity histories from galactic spectra.
VESPA uses the full spectral range to construct a galaxy history from synthetic
models. We investigate the use of an adaptative parametrization grid to recover
reliable star formation histories on a galaxy-by-galaxy basis. Our goal is
robustness as opposed to high resolution histories, and the method is designed
to return high time resolution only where the data demand it. In this paper we
detail the method and we present our findings when we apply VESPA to synthetic
and real Sloan Digital Sky Survey (SDSS) spectroscopic data. We show that the
number of parameters that can be recovered from a spectrum depends strongly on
the signal-to-noise, wavelength coverage and presence or absence of a young
population. For a typical SDSS sample of galaxies, we can normally recover
between 2 to 5 stellar populations. We find very good agreement between VESPA
and our previous analysis of the SDSS sample with MOPED.Comment: In press MNRAS, minor revisions to match accepted version by the
journa
Modelling the galaxy-halo connection with semi-recurrent neural networks
We present an artificial neural network design in which past and present-day
properties of dark matter halos and their local environment are used to predict
time-resolved star formation histories and stellar metallicity histories of
central and satellite galaxies. Using data from the IllustrisTNG simulations,
we train a TensorFlow-based neural network with two inputs: a standard layer
with static properties of the dark matter halo, such as halo mass and starting
time; and a recurrent layer with variables such as overdensity and halo mass
accretion rate, evaluated at multiple time steps from .
The model successfully reproduces key features of the galaxy halo connection,
such as the stellar-to-halo mass relation, downsizing, and colour bimodality,
for both central and satellite galaxies. We identify mass accretion history as
crucial in determining the geometry of the star formation history and trends
with halo mass such as downsizing, while environmental variables are important
indicators of chemical enrichment. We use these outputs to compute optical
spectral energy distributions, and find that they are well matched to the
equivalent results in IllustrisTNG, recovering observational statistics such as
colour bimodality and mass-magnitude diagrams.Comment: 22 pages, excluding references. 25 figures. Submitted to MNRA
Analysing Observables in Structure Formation Theories
This thesis presents an exploration of various aspects relating to the formation and
evolution of structure in the Universe. It focuses on two main observables which provide
information on two distinct epochs of the Universe: Part I analyses the Cosmic
Microwave Background (CMB) which is used to test early Universe theories and validate
current methods for cosmological parameters estimation; Part II analyses the
distribution, history and content of local galaxies with a view to learn about type Ia
supernovae progenitors, assembly of stellar mass in galaxies and galaxy evolution.
In Part I, a search for signs of non-Gaussianity in theWilkinson Microwave Anisotropy
Probe is conducted, using the two-point correlation function of peaks (hot and cold
spots) in the temperature field. A clear deviation from Gaussianity is found in both
data releases, which is associated with cold spots, the southern hemisphere, large-scales
and the galactic plane. The results indicate that the presence of un-subtracted foregrounds
in the data are a more likely explanation for this signal than a cosmological
origin, but the latter cannot be excluded. Part I further explores the two-point correlation
function of temperature peaks as an estimator to constrain fNL, a specific type
of non-Gaussianity. Using sets of non-Gaussian simulated maps with the correct cosmology
and resolution, this thesis explores how accurately one can hope to constrain
fNL when data from the upcoming CMB experiment Planck is available.
Part II presents a novel method developed to extract the star formation history of
a galaxy from its spectrum: VErsatile SPectral Analysis (VESPA). VESPA dynamically
adapts the number of parameters it recovers from each spectrum to each galaxy,
only recovering as much information as the data warrant. This insures the recovered
solutions are dominated by the signal, not the noise, and allows robust recovery of star
formation and metallicity histories and up to two dust extinction values per galaxy.
VESPA was applied to the fifth data release of the Sloan Digital Sky Survey (SDSS)
to construct a catalogue of histories for nearly half a million galaxies. Part II also
explores how this catalogue can provide important information about the formation of
structure in the local Universe. Specifically, this thesis: shows evidence for the presence
of a short-lived progenitor to SNIa and explores VESPA’s potential role in future
Dark Energy SNIa experiments; explores how changes in the Initial Mass Function affect
stellar mass estimates and its consequences; and by using estimates of the galaxy
mass function as a function of redshift (calculated using both the spectral fossil record
and instantaneous star formation rate methods) explores the possibility of putting constraints
on the merger history of present-day galaxies
Decoupling the rotation of stars and gas - I::the relationship with morphology and halo spin
Funding: UK Science and Technology Funding Council (STFC) via an PhD studentship (grant number ST/N504427/1).We use a combination of data from the MaNGA survey and MaNGA-like observations in IllustrisTNG100 to determine the prevalence of misalignment between the rotational axes of stars and gas. This census paper outlines the typical characteristics of misaligned galaxies in both observations and simulations to determine their fundamental relationship with morphology and angular momentum. We present a sample of ˜4500 galaxies from MaNGA with kinematic classifications which we use to demonstrate that the prevalence of misalignment is strongly dependent on morphology. The misaligned fraction sharply increases going to earlier morphologies (28 ± 3 per cent of 301 early-type galaxies, 10 ± 1 per cent of 677 lenticulars, and 5.4 ±0.6 per cent of 1634 pure late-type galaxies). For early-types, aligned galaxies are less massive than the misaligned sample whereas this trend reverses for lenticulars and pure late-types. We also find that decoupling depends on group membership for early-types with centrals more likely to be decoupled than satellites. We demonstrate that misaligned galaxies have similar stellar angular momentum to galaxies without gas rotation, much lower than aligned galaxies. Misaligned galaxies also have a lower gas mass than the aligned, indicative that gas loss is a crucial step in decoupling star-gas rotation. Through comparison to a mock MaNGA sample, we find that the strong trends with morphology and angular momentum hold true in IllustrisTNG100. We demonstrate that the lowered angular momentum is, however, not a transient property and that the likelihood of star-gas misalignment at z= 0 is correlated with the spin of the dark matter halo going back to z= 1.Publisher PDFPeer reviewe
The Ages of Type Ia Supernova Progenitors
Using light curves and host galaxy spectra of 101 Type Ia supernovae (SNe Ia)
with redshift from the SDSS Supernova Survey (SDSS-SN), we
derive the SN Ia rate as a function of progenitor age (the delay time
distribution, or DTD). We use the VESPA stellar population synthesis algorithm
to analyze the SDSS spectra of all galaxies in the field searched by SDSS-SN,
giving us a reference sample of 77,000 galaxies for our SN Ia hosts. Our method
does not assume any a priori shape for the DTD and therefore is minimally
parametric. We present the DTD in physical units for high stretch (luminous,
slow declining) and low stretch (subluminous, fast declining) supernovae in
three progenitor age bins. We find strong evidence of two progenitor channels:
one that produces high stretch SNe Ia Myr after the birth of the
progenitor system, and one that produces low stretch SNe Ia with a delay
Gyr. We find that each channel contributes roughly half of the
Type Ia rate in our reference sample. We also construct the average spectra of
high stretch and low stretch SN Ia host galaxies, and find that the difference
of these spectra looks like a main sequence B star with nebular emission lines
indicative of star formation. This supports our finding that there are two
populations of SNe Ia, and indicates that the progenitors of high stretch SNe
are at the least associated with very recent star formation in the last few
tens of Myr. Our results provide valuable constraints for models of Type Ia
progenitors and may help improve the calibration of SNe Ia as standard candles.Comment: 15 pages, 8 figures, 3 tables, AJ accepted. Moderate changes to match
accepted version, including a table of all SNe use
Extragalactic Constraints on the Initial Mass Function
The local stellar mass density is observed to be significantly lower than the
value obtained from integrating the cosmic star formation history (SFH),
assuming that all the stars formed with a Salpeter initial mass function (IMF).
Even other favoured IMFs, more successful in reconciling the observed
stellar mass density with that inferred from the SFH, have difficulties in
reproducing the stellar mass density observed at higher redshift. In this study
we investigate to what extent this discrepancy can be alleviated for any
universal power-law IMF. We find that an IMF with a high-mass slope shallower
(2.15) than the Salpeter slope (2.35) reconciles the observed stellar mass
density with the cosmic star formation history, but only at low redshifts. At
higher redshifts we find that observed stellar mass densities are
systematically lower than predicted from the cosmic star formation history, for
any universal power-law IMF.Comment: 6 pages, 5 figures, accepted to MNRA
Testing homegeneity with Galaxy Star formation histories
Observationally confirming spatial homogeneity on sufficiently large cosmological scales is of importance to test
one of the underpinning assumptions of cosmology, and is also imperative for correctly interpreting dark energy.
A challenging aspect of this is that homogeneity must be probed inside our past light cone, while observations take
place on the light cone. The star formation history (SFH) in the galaxy fossil record provides a novel way to do this.
We calculate the SFH of stacked luminous red galaxy (LRG) spectra obtained from the Sloan Digital Sky Survey.
We divide the LRG sample into 12 equal-area contiguous sky patches and 10 redshift slices (0.2 < z < 0.5),
which correspond to 120 blocks of volume ∼0.04 Gpc3. Using the SFH in a time period that samples the history
of the universe between look-back times 11.5 and 13.4 Gyr as a proxy for homogeneity, we calculate the posterior
distribution for the excess large-scale variance due to inhomogeneity, and find that the most likely solution is no
extra variance at all. At 95% credibility, there is no evidence of deviations larger than 5.8%.Department of HE and Training approved lis
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