53 research outputs found
Star-formation histories of massive quiescent galaxies
No full textThis thesis presents several related analyses designed to understand the star-formation
histories (SFHs) and quenching mechanisms of massive quiescent
galaxies across cosmic time. More generally, it contains research directed at
sophisticated modelling and Bayesian fitting of galaxy spectra. I firstly present
Bayesian Analysis of Galaxies for Physical Inference and Parameter EStimation,
or Bagpipes, a new, publicly available Python code that can be used to rapidly
generate complex model galaxy spectra and to fit these to arbitrary combinations
of spectroscopic and photometric data.
I then perform a detailed analysis of the SFHs of a sample of 9289 quiescent
galaxies from UltraVISTA with stellar masses, M∗> 1010M⊙ and observed
redshifts from 0:25 < z < 3:75. The majority of these galaxies exhibit SFHs
that rise gradually then quench relatively rapidly, over 1-2 Gyr. This behaviour
is consistent with recent cosmological hydrodynamic simulations, where AGN-driven
feedback in the low-accretion (jet) mode is the dominant quenching
mechanism. At z > 1, I also find a class of objects with SFHs that rise and
fall very rapidly, with quenching timescales of < 1 Gyr, consistent with quasar-mode
AGN feedback. Finally, at z < 1, I find a population with SFHs that quench
more slowly than they rise, over > 3 Gyr, consistent with other such analyses in
the local Universe. I confirm the trend towards earlier formation with increasing
stellar mass (downsizing) at fixed observed redshift, and a trend towards more
rapid quenching at higher stellar masses.
I then present a general investigation of the use of parametric SFH models
in spectral fitting analyses. Parametric models for galaxy SFHs are widely
used, though they are known to impose strong priors on physical parameters,
with consequences for measurements of the galaxy stellar-mass function, star-formation-
rate density (SFRD) and star-forming main sequence (SFMS). I
investigate the effects of the exponentially declining, delayed exponentially
declining, lognormal and double power law SFH models. I demonstrate that each
of these models imposes strong priors on specific star-formation rates (sSFRs),
potentially biasing the SFMS, and also imposes a strong prior preference for
young stellar populations. I show that stellar mass, SFR and mass-weighted
age inferences from high-quality mock photometry vary with the choice of SFH
model by at least 0.1, 0.3 and 0.2 dex respectively. However the biases with
respect to the true values depend more on the true SFH shape than the choice
of model. I also demonstrate that photometric data cannot discriminate between
SFH models, meaning it is important to perform independent tests to find well-motivated
priors. In response to this I finally fit a low-redshift, volume-complete
sample from the Galaxy and Mass Assembly (GAMA) Survey with each model.
I demonstrate that the inferred stellar masses and SFRs at redshift, z ~ 0:05
are consistent with other analyses. However, the inferred cosmic SFRDs peak
at z ~ 0:4, approximately 6 Gyr later than direct observations suggest, meaning
that mass-weighted ages are significantly underestimated. This makes the use of
parametric SFH models for understanding mass assembly in galaxies challenging.
I finally present a Bayesian full-spectral-fitting analysis of 75 massive (M∗>
1010:3M⊙) UVJ-selected galaxies at redshifts of 1:0 < z < 1:3, combining
extremely deep rest-frame ultraviolet spectroscopy from VANDELS with multi-wavelength
photometry by the use of a sophisticated physical plus systematic
uncertainties model. I constrain the stellar mass vs stellar age relationship,
finding a strong trend towards earlier formation with increasing stellar mass
(downsizing) of 1:48+0:34
≲0:39 Gyr per decade in mass. I show that this is consistent
with other spectroscopic studies from 0 < z < 2. This places strong constraints
on the AGN-feedback models used in cosmological simulations. I demonstrate
that, although the relationships predicted by the Simba and IllustrisTNG
simulations agree well with observations at z = 0:1, they are too shallow at
z = 1, predicting an evolution of . 0:5 Gyr per decade in mass. The majority
of the lowest-mass galaxies in the sample (M∗~ 1010:5M⊙) are consistent with
formation in recent (z < 2), intense starburst events, with timescales of ≲ 500
Myr. A second class of objects experience extended star-formation epochs before
rapidly quenching, passing through both green-valley and post-starburst phases.
The most massive galaxies in the sample are extreme systems: already old by
z = 1, they formed at z ~ 5 and quenched by z = 3. However, I find evidence
for their continued evolution through both AGN and rejuvenated star-formation
activity. To understand the detailed SFHs of these objects, similar studies must
be extended to the highest redshifts
A massive quiescent galaxy at redshift 4.658
No full textFunding: A. C. Carnall thanks the Leverhulme Trust for their support via a Leverhulme Early Career Fellowship. R. J. McLure, J. S. Dunlop, D. J. McLeod, V. Wild, R. Begley, C. T. Donnan and M. L. Hamadouche acknowledge the support of the Science and Technology Facilities Council. F. Cullen acknowledges support from a UKRI Frontier Research Guarantee Grant (grant reference EP/X021025/1). A. Cimatti acknowledges support from the grant PRIN MIUR 2017 - 20173ML3WW 001.The extremely rapid assembly of the earliest galaxies during the first billion years of cosmic history is a major challenge for our understanding of galaxy formation physics (1; 2; 3; 4; 5). The advent of JWST has exacerbated this issue by confirming the existence of galaxies in significant numbers as early as the first few hundred million years (6; 7; 8). Perhaps even more surprisingly, in some galaxies, this initial highly efficient star formation rapidly shuts down, or quenches, giving rise to massive quiescent galaxies as little as 1.5 billion years after the Big Bang (9; 10), however, due to their faintness and red colour, it has proven extremely challenging to learn about these extreme quiescent galaxies, or to confirm whether any exist at earlier times. Here we report the spectroscopic confirmation of a massive quiescent galaxy, GS-9209, at redshift, z = 4.658, just 1.25 billion years after the Big Bang, using JWST NIRSpec. From these data we infer a stellar mass of M∗ = 3.8 ± 0.2 × 1010 M⊙, which formed over a ≃ 200 Myr period before this galaxy quenched its star formation activity at z=6.5+0.2−0.5, when the Universe was ≃ 800 million years old. This galaxy is both a likely descendent of the highest-redshift submillimetre galaxies and quasars, and a likely progenitor for the dense, ancient cores of the most massive local galaxies.Peer reviewe
Constraining quenching mechanisms at high redshift: the sizes, masses and star-formation histories of massive galaxies
No full textObservations of the local and high-redshift Universe have revealed a clear
bi-modality in the galaxy population. This is apparent in galaxy colours,
morphologies and star-formation rates, with galaxies being categorised into two
distinct populations; star-forming and quiescent. The existence of this bimodality
means that one or more mechanisms must be able to cease, or quench,
star-formation in galaxies. One of the key questions in extragalactic astronomy
is to identify which quenching mechanisms are most important and on what
timescales they operate.
In this thesis, I use a combination of photometry and spectroscopy to examine
the evolution of the quiescent galaxy population at high redshift in an attempt
to piece together a coherent picture of their evolution over the past ten billion
years of cosmic history. Some of the foundational results that have shaped our
understanding of galaxy evolution are the relationships between galaxy stellar
mass, size and age. In addition to the bi-modality observed up to z ∼ 2, it has also
been shown that galaxies which formed earlier in cosmic time are more massive
than their later counterparts, a phenomenon often referred to as “downsizing”.
In this thesis, ultra-deep spectroscopic data along with HST and JWST imaging
from large-scale surveys is used to study the sizes, masses and star-formation
histories of massive galaxies from z = 0.25 to z = 2.25.
Firstly, I present a study exploring the most massive galaxies at 0.6 < z < 1.3
using ultra-deep spectroscopy and photometry from the VANDELS and LEGA-C
ESO public spectroscopic surveys. I investigate the relationships between galaxy
stellar mass, physical size and age (using a well-known proxy for age predominant
in quiescent galaxy spectra; the Dn4000 index), and demonstrate for the first time
that downsizing is clearly evident in both our quiescent samples. I present a toy
model to explain the size evolution of massive quiescent galaxies from z = 1.3 to
z = 0.6 based on minor mergers.
Next, I present the results of full spectral fitting of the VANDELS quiescent
sample at 1.0 < z < 1.3, examining in further detail the relationship between
stellar mass and age, this time using galaxy formation and quenching times.
Upon further investigation into these trends, it becomes apparent that there is a
sample of quiescent galaxies which appear to have experienced very short periods
of star-formation, followed by abrupt quenching. These galaxies indicate higher
levels of α-enhancement compared to the rest of the quiescent galaxies in my
sample. This implies that there is an important mechanism at work which can
end star-formation on very short timescales. One galaxy in my sample formed
less than a billion years after the Big Bang, experiencing a short episode of
star-formation before quickly becoming quiescent, based on star-formation and
quenching timescales.
Building on these key results at z ∼ 1, I investigate whether these trends can be
seen at higher redshift using data obtained from the JWST PRIMER and JADES
surveys. Due to the improved sensitivity and spatial resolution of JWST imaging,
it is possible to examine the sizes and morphologies of star-forming and quiescent
galaxies to lower stellar masses than previously possible. Over the redshift range
0.25 < z < 2.25, I find that low-mass quiescent galaxies in my sample exhibit
Sérsic indices and sizes qualitatively consistent with these galaxies quenching by
infall into cluster environments. Contrastingly, the sizes and morphologies of
more massive quiescent galaxies are consistent with having quenched by internal
feedback mechanisms.
In summary, this thesis aims to provide a quantitative investigation of the
evolution of quiescent galaxies over cosmic time. The unprecedented size and
quality of the datasets used throughout this work have led to many important
findings, enabling a deeper understanding of the properties of quiescent galaxies,
and the overall formation and evolution of galaxies across time
The diverse quenching pathways of post-starburst galaxies in SDSS-IV MaNGA
No full textFunding: HL and ACC acknowledge support from a UK Research and Innovation (UKRI) Frontier Research Guarantee grant (PI Carnall; grant reference EP/Y037065/1). VW acknowledges Science and Technologies Facilities Council (STFC) grants ST/V000861/1 and ST/Y00275X/1, and Leverhulme Research Fellowship RF-2024-589/4. YMC acknowledges support by the National Natural Science Foundation of China, NSFC Grant Nos. 12333002.The quenching of star formation in galaxies is an important aspect of galaxy evolution, but the physical mechanisms that drive it are still not understood. Measuring the spatial distribution of quenching can help determine these mechanisms. We present the star formation histories (SFHs) and stellar metallicity evolution of rapidly quenched regions in 86 local post-starburst (PSB) galaxies from the Mapping Nearby Galaxies at APO (MaNGA) integral field survey, obtained through Bayesian full spectral fitting of their rest-frame optical spectra. We found that regardless of spatial location, the PSB regions have similar past SFHs and chemical evolution, once radial metallicity gradients are accounted for. This suggests that all PSB regions are regulated by a common set of local scale processes in the interstellar medium, regardless of the broader triggering mechanism. We show that the centres of galaxies with outer PSB regions are also quenching. The central specific star formation rate has declined by ∼1.0 dex on average during the last 2 Gyr, a significantly steeper decline than main sequence galaxies over the same period (≈0.2 dex). This central quenching can be either synchronous, outside-in or inside-out, and slower or as fast as the outer regions, highlighting the diversity of quenching pathways for local galaxies. Our results imply a primary quenching mechanism that is both catastrophic and global in rapidly halting star formation in local galaxies. We suggest the predominant cause is galaxy mergers or interactions, with large scale feedback from a starburst or a central supermassive black hole playing a lesser role.Peer reviewe
High-velocity outflows in massive post-starburst galaxies at z > 1
No full textWe investigate the prevalence of galactic-scale outflows in post-starburst (PSB) galaxies at high redshift (1 1010M⊙) PSBs at z > 1, there is clear evidence for a strong blue-shifted component to the Mg ii absorption feature, indicative of high-velocity outflows (vout∼1150±160kms−1) in the interstellar medium. We conclude that such outflows are typical in massive PSBs at this epoch, and potentially represent the residual signature of a feedback process that quenched these galaxies. Using full spectral fitting, we also obtain a typical stellar velocity dispersion σ* for these PSBs of ∼200kms−1, which confirms they are intrinsically massive in nature (dynamical mass Md∼1011M⊙). Given that these high-z PSBs are also exceptionally compact (re ∼ 1–2kpc) and spheroidal (Sérsic index n ∼ 3), we propose that the outflowing winds may have been launched during a recent compaction event (e.g. major merger or disc collapse) that triggered either a centralized starburst or active galactic nuclei (AGN) activity. Finally, we find no evidence for AGN signatures in the optical spectra of these PSBs, suggesting they were either quenched by stellar feedback from the starburst itself, or that if AGN feedback is responsible, the AGN episode that triggered quenching does not linger into the post-starburst phase.Peer reviewe
The stellar metallicities of massive quiescent galaxies at 1.0 < z < 1.3 from KMOS+VANDELS
No full textWe present a rest-frame UV-optical stacked spectrum representative of massive quiescent galaxies at 1.0<z<1.3 with log(M∗/M⊙)>10.8. The stack is constructed using VANDELS survey data, combined with new KMOS observations. We apply two independent full-spectral-fitting approaches, measuring a total metallicity, [Z/H]=−0.13±0.08 with Bagpipes, and [Z/H]=0.04±0.14 with Alf, a fall of ∼0.2−0.3 dex compared with the local Universe. We also measure an iron abundance, [Fe/H] =−0.18±0.08, a fall of ∼0.15 dex compared with the the local Universe. We measure the alpha enhancement via the magnesium abundance, obtaining [Mg/Fe]=0.23±0.12, consistent with similar-mass galaxies in the local Universe, indicating no evolution in the average alpha enhancement of log(M∗/M⊙)=11 quiescent galaxies over the last ∼8 Gyr. This suggests the very high alpha enhancements recently reported for several bright z∼1−2 quiescent galaxies are due to their extreme masses, log(M∗/M⊙)≳11.5, rather than being typical of the z≳1 population. The metallicity evolution we observe with redshift (falling [Z/H], [Fe/H], constant [Mg/Fe]) is consistent with recent studies. We recover a mean stellar age of 2.5+0.6−0.4 Gyr, corresponding to a formation redshift, zform=2.4+0.6−0.3. Recent studies have obtained varying average formation redshifts for z≳1 massive quiescent galaxies, and, as these studies report consistent metallicities, we identify different star-formation-history models as the most likely cause. Larger spectroscopic samples from upcoming ground-based instruments will provide precise constraints on ages and metallicities at z≳1. Combining these with precise JWST z>2 quiescent-galaxy stellar-mass functions will provide an independent test of formation redshifts derived from spectral fitting
PRIMER & JADES reveal an abundance of massive quiescent galaxies at 2 < z < 5
No full textFunding: SDS, ACC, H-HL, and ET acknowledge support from a UKRI Frontier Research Guarantee Grant (PI Carnall; grant reference EP/Y037065/1). FC, TMS, KZA-C, and DS acknowledge support from a UKRI Frontier Research Guarantee Grant (PI Cullen; grant reference: EP/X021025/1). Support for Program number JWST-GO-03543.014 was provided through a grant from the STScI under NASA contract NAS5-03127. We gratefully acknowledge support from the NASA Astrophysics Data Analysis Program (ADAP) under grant 80NSSC23K0495.We select a mass-complete sample of 225 quiescent galaxies at z > 2 with M∗ > 1010 M☉ from PRIMER and JADES photometry spanning a total area of ≃ 320 sq. arcmin. Our analysis is restricted to only area with optical coverage in three Hubble Space Telescope (HST) ACS filters, which we show is important for selecting the most complete and clean samples. We investigate the contamination in our sample via James Webb Space Telescope (JWST) NIRSpec spectroscopy, Chandra X-ray imaging, and ALMA interferometry, calculating a modest contamination fraction of 12.9+4.0−3.1 per cent. The removal of HST data increases star-forming galaxy contamination by ≃ 10 per cent and results in a ≃ 20 per cent loss of candidates recovered from HST + JWST data combined. We calculate massive quiescent galaxy number densities at 2 3, up to ≃ 1 dex. We place 14 of our z > 3 massive quiescent galaxies on the BPT and WHaN diagrams using medium-resolution spectroscopic data from the EXCELS survey, finding a very high incidence of weak active galactic nucleus (≃ 50 per cent), consistent with recent results at cosmic noon. This is interesting in the context of ‘maintenance-mode’ feedback, which is invoked in many simulations to prevent the re-ignition of quenched galaxies. To properly characterize the evolution of early massive quiescent galaxies, greater coverage in optical filters and significantly larger spectroscopic samples will be required.Peer reviewe
How to Measure Galaxy Star Formation Histories. II. Nonparametric Models
No full textNonparametric star formation histories (SFHs) have long promised to be the "gold standard" for galaxy spectral energy distribution (SED) modeling as they are flexible enough to describe the full diversity of SFH shapes, whereas parametric models rule out a significant fraction of these shapes a priori. However, this flexibility is not fully constrained even with high-quality observations, making it critical to choose a well-motivated prior. Here, we use the SED-fitting code Prospector to explore the effect of different nonparametric priors by fitting SFHs to mock UV–IR photometry generated from a diverse set of input SFHs. First, we confirm that nonparametric SFHs recover input SFHs with less bias and return more accurate errors than do parametric SFHs. We further find that, while nonparametric SFHs robustly recover the overall shape of the input SFH, the primary determinant of the size and shape of the posterior star formation rate as a function of time (SFR(t)) is the choice of prior, rather than the photometric noise. As a practical demonstration, we fit the UV–IR photometry of ~6000 galaxies from the Galaxy and Mass Assembly survey and measure scatters between priors to be 0.1 dex in mass, 0.8 dex in SFR100 Myr, and 0.2 dex in mass-weighted ages, with the bluest star-forming galaxies showing the most sensitivity. An important distinguishing characteristic for nonparametric models is the characteristic timescale for changes in SFR(t). This difference controls whether galaxies are assembled in bursts or in steady-state star formation, corresponding respectively to (feedback-dominated/accretion-dominated) models of galaxy formation and to (larger/smaller) confidence intervals derived from SED fitting. High-quality spectroscopy has the potential to further distinguish between these proposed models of SFR(t)
Rest-frame UV properties of CIII/λ1909 emitters at z=2-4 with the ultra-deep VANDELS survey
No full textThe first 2-3 Gyr of cosmic history are key to understand reionization and how most present-day galaxies form and assemble. According to recent observations, normal galaxies at these early epochs show more extreme stellar and nebular properties than their lower redshift counterparts, but the connection between these properties and the physical mechanisms facilitating galaxy growth and the escape of ionizing photons used for sustaining cosmic reionization still need to be established. In this scenario, CIII]λ1909 emitters have been proved to be relevant to find out these connections because of their properties similar to primaeval galaxies. In this talk, we present the physical properties and chemical abundances of a large sample of CIII] emitters at z=2-4 selected from the unprecedentedly deep VIMOS/VANDELS spectroscopic survey. In particular, we discuss UV emission-line diagnostics and key scaling relations involving stellar metallicities, C/O abundances, and other global properties
Reading the Scottish Enlightenment : libraries, readers and intellectual culture in provincial Scotland c.1750-c.1820
No full textThe thesis explores the reception of the works of the Scottish Enlightenment in provincial Scotland, broadly defined, aiming to gauge their diffusion in the libraries of private book collectors and 'public' book-lending institutions, and to suggest the meanings and uses that contemporary Scottish readers assigned to major texts like Hume's History of England and Smith's Wealth of Nations. I thereby acknowledge the relevance of more traditional quantitative approaches to the history of reading (including statistical analysis of the holdings of contemporary book collections), but prioritise the study of sources that also allow us to access the 'hows' and 'whys' of individual reading practices and experiences. Indeed, the central thrust of my work has been the discovery and interrogation of large numbers of commonplace books, marginalia, diaries, correspondence and other documentary records which can be used to illuminate the reading experience itself in an explicit attempt to develop an approach to Scottish reading practices that can contribute in comparative terms to the burgeoning field of the history of reading. More particularly, such sources allow me to assess the impact that specific texts had on the lives, thought-processes and values of a wide range of contemporary readers, and to conclude that by reading these texts in their own endlessly idiosyncratic ways, consumers of literature in Scotland assimilated many of the prevalent attitudes and priorities of the literati in the major cities. Since many of the most important and pervasive manifestations of Enlightenment in Scotland were not particularly Scottish, however, I also cast doubt on the distinctive Scottishness of the prevailing 'cultural' definition of the Scottish Enlightenment, arguing that such behaviour might more appropriately be considered alongside cultural developments in Georgian England
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