21 research outputs found
Star formation and the evolution of massive galaxies across cosmic time
This thesis investigates the evolution of massive galaxies throughout the last 11 billion years using measured stellar masses and star formation rates. Firstly, we present a study of the resolved star-forming properties of a sample of distant massive (M > 10{11} M) galaxies in the GOODS NICMOS Survey (GNS) within the redshift range 1.5 < z < 3 in order to measure the spatial location of ongoing star formation (SF). We find that the SFRs present in different regions of a galaxy reflect the already existent stellar mass density, i.e. high density regions have higher SFRs than lower density regions, on average. We find that these massive galaxies fall into three broad classifications of SF distributions. These different SF distributions increase the effective radii to z=0, by ~16 plus-minus 5 % , with little change in the Sersic index (n), with an average delta n = -0.9 plus-minus 0.9, after evolution. These results are not in agreement with the observed change in the effective radius and n between z ~2.5 and z ~0. We conclude that SF and stellar migration alone cannot account for the observed change in structural parameters for this galaxy population, implying that other mechanisms must additionally be at work to produce the evolution, such as merging.
In Chapter 2, we present a study of the stellar mass growth of the progenitors of local massive galaxies at number densities of n 1.5 SF is the dominant form of stellar mass growth, while at z<1.5 mergers become the dominant form with minor mergers the dominant form of growth at z<1.0. We also explore the implication of these results on other galaxy formation processes such as the cold gas accretion rate of the progenitors of most massive galaxies over the same redshift range. We find that the gas accretion rate decreases with redshift with an average gas accretion rate of ~65 M yr{-1} over the redshift range of 1.5<z<3.0.
Finally, we investigate the evolution of the properties of local massive galaxies over the redshift range 0.31.7 and transitioning to high n objects at z2.5 are passive low n systems, possibly implying that local massive galaxies were passive disk-like systems at early cosmic times
Star formation and the evolution of massive galaxies across cosmic time
This thesis investigates the evolution of massive galaxies throughout the last 11 billion years using measured stellar masses and star formation rates. Firstly, we present a study of the resolved star-forming properties of a sample of distant massive (M > 10{11} M) galaxies in the GOODS NICMOS Survey (GNS) within the redshift range 1.5 < z < 3 in order to measure the spatial location of ongoing star formation (SF). We find that the SFRs present in different regions of a galaxy reflect the already existent stellar mass density, i.e. high density regions have higher SFRs than lower density regions, on average. We find that these massive galaxies fall into three broad classifications of SF distributions. These different SF distributions increase the effective radii to z=0, by ~16 plus-minus 5 % , with little change in the Sersic index (n), with an average delta n = -0.9 plus-minus 0.9, after evolution. These results are not in agreement with the observed change in the effective radius and n between z ~2.5 and z ~0. We conclude that SF and stellar migration alone cannot account for the observed change in structural parameters for this galaxy population, implying that other mechanisms must additionally be at work to produce the evolution, such as merging.
In Chapter 2, we present a study of the stellar mass growth of the progenitors of local massive galaxies at number densities of n 1.5 SF is the dominant form of stellar mass growth, while at z<1.5 mergers become the dominant form with minor mergers the dominant form of growth at z<1.0. We also explore the implication of these results on other galaxy formation processes such as the cold gas accretion rate of the progenitors of most massive galaxies over the same redshift range. We find that the gas accretion rate decreases with redshift with an average gas accretion rate of ~65 M yr{-1} over the redshift range of 1.5<z<3.0.
Finally, we investigate the evolution of the properties of local massive galaxies over the redshift range 0.31.7 and transitioning to high n objects at z2.5 are passive low n systems, possibly implying that local massive galaxies were passive disk-like systems at early cosmic times
Minor versus major mergers: the stellar mass growth of massive galaxies from z=3 using number density selection techniques
We present a study on the stellar mass growth of the progenitors of local massive galaxies with a variety of number density selections with n≤1×10−4 Mpc−3 (corresponding to M*=1011.24 M⊙ at z=0.3) in the redshift range 0.3<z<3.0. We select the progenitors of massive galaxies using a constant number density selection, and one which is adjusted to account for major mergers. We find that the progenitors of massive galaxies grow by a factor of 4 in total stellar mass over this redshift range. On average the stellar mass added via the processes of star formation, major and minor mergers account for 24±8, 17±15 and 34±14per cent, respectively, of the total galaxy stellar mass at z=0.3. Therefore 51±20per cent of the total stellar mass in massive galaxies at z=0.3 is created externally to their z=3 progenitors. We explore the implication of these results on the cold gas accretion rate and size evolution of the progenitors of most massive galaxies over the same redshift range. We find an average gas accretion rate of∼66±32 M⊙ yr−1 over the redshift range of 1.5<z<3.0. We find that the size evolution of a galaxy sample selected this way is on average lower than the findings of other investigation
The evolution of galaxies at constant number density: a less biased view of star formation, quenching, and structural formation
Due to significant galaxy contamination and impurity in stellar mass selected samples (up to 95 per cent from z = 0–3), we examine the star formation history, quenching time-scales, and structural evolution of galaxies using a constant number density selection with data from the United Kingdom Infra-Red Deep Sky Survey Ultra-Deep Survey field. Using this methodology, we investigate the evolution of galaxies at a variety of number densities from z= 0–3. We find that samples chosen at number densities ranging from 3 × 10−4 to 10−5 galaxies Mpc−3 (corresponding to z ∼ 0.5 stellar masses of M∗ = 1010.95−11.6 M0) have a star-forming blue fraction of ∼50 per cent at z ∼ 2.5, which evolves to a nearly 100 per cent quenched red and dead population by z ∼ 1. We also see evidence for number density downsizing, such that the galaxies selected at the lowest densities (highest masses) become a homogeneous red population before those at higher number densities. Examining the evolution of the colours for these systems furthermore shows that the formation redshift of galaxies selected at these number densities is zform > 3. The structural evolution through size and S´ersic index fits reveal that while there remains evolution in terms of galaxies becoming larger and more concentrated in stellar mass at lower redshifts, the magnitude of the change is significantly smaller than for a mass-selected sample. We also find that changes in size and structure continues at z < 1, and is coupled strongly to passivity evolution.We conclude that galaxy structure is driving the quenching of galaxies, such that galaxies become concentrated before they become passive
The mass evolution of the first galaxies: stellar mass functions and star formation rates at in the CANDELS GOODS-South field
We measure new estimates for the galaxy stellar mass function and star
formation rates for samples of galaxies at using data in
the CANDELS GOODS South field. The deep near-infrared observations allow us to
construct the stellar mass function at directly for the first time.
We estimate stellar masses for our sample by fitting the observed spectral
energy distributions with synthetic stellar populations, including nebular line
and continuum emission. The observed UV luminosity functions for the samples
are consistent with previous observations, however we find that the observed
- M relation has a shallow slope more consistent with a constant
mass to light ratio and a normalisation which evolves with redshift. Our
stellar mass functions have steep low-mass slopes (),
steeper than previously observed at these redshifts and closer to that of the
UV luminosity function. Integrating our new mass functions, we find the
observed stellar mass density evolves from at to at . Finally, combining the measured UV continuum
slopes () with their rest-frame UV luminosities, we calculate dust
corrected star-formation rates (SFR) for our sample. We find the specific
star-formation rate for a fixed stellar mass increases with redshift whilst the
global SFR density falls rapidly over this period. Our new SFR density
estimates are higher than previously observed at this redshift.Comment: 28 pages, 23 figures, 2 appendices. Accepted for publication in
MNRAS, August 7 201
A Deep Probe of the Galaxy Stellar Mass Functions at z~1-3 with the GOODS NICMOS Survey
We use a sample of 8298 galaxies observed in the HST GOODS NICMOS Survey
(GNS) to construct the galaxy stellar mass function as a function of both
redshift and stellar mass up to z=3.5 and down to masses of Mstar=10^8.5 Msun
at z~1. We discover that a significant fraction of all massive Mstar>10^11 Msun
galaxies are in place up to the highest redshifts we probe, with a decreasing
fraction of lower mass galaxies present at all redshifts. This is an example of
`galaxy mass downsizing', and is the result of massive galaxies forming before
lower mass ones, and not just simply ending their star formation earlier as in
traditional downsizing scenarios. We find that the faint end slope is
significantly steeper than what is found in previous investigations. We
demonstrate that this steeper mass function better matches the stellar mass
added due to star formation, thereby alleviating some of the mismatch between
these two measures of the evolution of galaxy mass. We furthermore examine the
stellar mass function divided into blue/red systems, as well as for star
forming and non-star forming galaxies. We find a similar mass downsizing
present for both blue/red and star-forming/non-star forming galaxies, and that
the low mass galaxies are mostly all blue, and are therefore creating the steep
mass functions. We furthermore show that, although there is a downsizing such
that high mass galaxies are nearer their z=0 values at high redshift, this
turns over at masses Mstar~10^10 Msun, such that the lowest mass galaxies are
more common than galaxies at slight higher masses, creating a `dip' in the
observed galaxy mass function. We argue that the galaxy assembly process may be
driven by different mechanisms at low and high masses, and that the efficiency
of the galaxy formation process is lowest at masses Mstar~10^10 Msun at 1<z<3.
(Abridged)Comment: 16 pages, 11 figures, MNRAS, accepte
Deconstructing the galaxy stellar mass function with UKIDSS and CANDELS: the impact of colour, structure and environment
We combine photometry from the Ultra Deep Survey (UDS), Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) UDS and CANDELS the Great Observatories Origins Deep Survey-South (GOODS-S) surveys to construct the galaxy stellar mass function probing both the low- and high-mass end accurately in the redshift range 0.326.0), affording us robust measures of structural parameters. We construct stellar mass functions for the entire sample as parametrized by the Schechter function, and find that there is a decline in the values of ϕ and of α with higher redshifts, and a nearly constant M* up to z∼3. We divide the galaxy stellar mass function by colour, structure, and environment and explore the links between environmental overdensity, morphology, and the quenching of star formation. We find that a double Schechter function describes galaxies with high Sérsic index (n>2.5), similar to galaxies which are red or passive. The low-mass end of the n>2.5 stellar mass function is dominated by blue galaxies, whereas the high-mass end is dominated by red galaxies. This shows that there is a possible link between morphological evolution and star formation quenching in high mass galaxies, which is not seen in lower mass systems. This in turn suggests that there are strong mass-dependent quenching mechanisms. In addition, we find that the number density of high-mass systems is elevated in dense environments, suggesting that an environmental process is building up massive galaxies quicker in over densities than in lower densitie
The redshift and mass dependence on the formation of the Hubble sequence at z > 1 from CANDELS/UDS
In this paper we present a detailed study of the structures and morphologies of a sample of 1188 massive galaxies with M-* >= 10(10) M-circle dot between redshifts z = 1 and 3 within the Ultra Deep Survey (UDS) region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. Using this sample we determine how galaxy structure and morphology evolve with time, and investigate the nature of galaxy structure at high redshift. We visually classify our sample into discs, ellipticals and peculiar systems and correct for redshift effects on these classifications through simulations. We find significant evolution in the fractions of galaxies at a given visual classification as a function of redshift. The peculiar population is dominant at z > 2 with a substantial spheroid population, and a negligible disc population. We compute the transition redshift, z(trans), where the combined fraction of spheroidal and disc galaxies is equal to that of the peculiar population, as z(trans) = 1.86 +/- 0.62 for galaxies in our stellar mass range. We find that this transition changes as a function of stellar mass, with Hubble-type galaxies becoming dominant at higher redshifts for higher mass galaxies (z(trans) = 2.22 +/- 0.82), than for the lower mass galaxies (z(trans) = 1.73 +/- 0.57). Higher mass galaxies become morphologically settled before their lower mass counterparts, a form of morphological downsizing. We furthermore compare our visual classifications with the Sersic index, the concentration, asymmetry and clumpiness (CAS) parameters, star formation rate and rest-frame U - B colour. We find links between the colour of a galaxy, its star formation rate and how extended or peculiar it appears. Finally, we discuss the negligible z > 2 disc fraction based on visual morphologies and speculate that this is an effect of forming disc appearing peculiar through processes such as violent disc instabilities or mergers. We conclude that to properly define and measure high-redshift morphology and structure a new and more exact classification scheme is needed
Deconstructing the Galaxy Stellar Mass Function with UKIDSS and CANDELS: the Impact of Colour, Structure and Environment
We combine photometry from the UDS, and CANDELS UDS and CANDELS GOODS-S
surveys to construct the galaxy stellar mass function probing both the low and
high mass end accurately in the redshift range 0.3<z<3. The advantages of using
a homogeneous concatenation of these datasets include meaningful measures of
environment in the UDS, due to its large area (0.88 deg^2), and the high
resolution deep imaging in CANDELS (H_160 > 26.0), affording us robust measures
of structural parameters. We construct stellar mass functions for the entire
sample as parameterised by the Schechter function, and find that there is a
decline in the values of phi and of alpha with higher redshifts, and a nearly
constant M* up to z~3. We divide the galaxy stellar mass function by colour,
structure, and environment and explore the links between environmental
over-density, morphology, and the quenching of star formation. We find that a
double Schechter function describes galaxies with high Sersic index (n>2.5),
similar to galaxies which are red or passive. The low-mass end of the n>2.5
stellar mass function is dominated by blue galaxies, whereas the high-mass end
is dominated by red galaxies. This hints that possible links between
morphological evolution and star formation quenching are only present in
high-mass galaxies. This is turn suggests that there are strong mass dependent
quenching mechanisms. In addition, we find that the number density of high mass
systems is elevated in dense environments, suggesting that an environmental
process is building up massive galaxies quicker in over densities than in lower
densities.Comment: 26 pages, 14 figures, Accepted for publication in MNRA
The Redshift and Mass Dependence on the Formation of the Hubble Sequence at \u3cem\u3ez\u3c/em\u3e \u3e 1 from CANDELS/UDS
In this paper we present a detailed study of the structures and morphologies of a sample of 1188 massive galaxies with M* ≥ 1010 M⊙between redshifts z = 1 and 3 within the Ultra Deep Survey (UDS) region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. Using this sample we determine how galaxy structure and morphology evolve with time, and investigate the nature of galaxy structure at high redshift. We visually classify our sample into discs, ellipticals and peculiar systems and correct for redshift effects on these classifications through simulations. We find significant evolution in the fractions of galaxies at a given visual classification as a function of redshift. The peculiar population is dominant at z \u3e 2 with a substantial spheroid population, and a negligible disc population. We compute the transition redshift, ztrans, where the combined fraction of spheroidal and disc galaxies is equal to that of the peculiar population, as ztrans = 1.86 ± 0.62 for galaxies in our stellar mass range. We find that this transition changes as a function of stellar mass, with Hubble-type galaxies becoming dominant at higher redshifts for higher mass galaxies (ztrans = 2.22 ± 0.82), than for the lower mass galaxies (ztrans = 1.73 ± 0.57). Higher mass galaxies become morphologically settled before their lower mass counterparts, a form of morphological downsizing. We furthermore compare our visual classifications with the Sérsic index, the concentration, asymmetry and clumpiness (CAS) parameters, star formation rate and rest-frame U − B colour. We find links between the colour of a galaxy, its star formation rate and how extended or peculiar it appears. Finally, we discuss the negligible z \u3e 2 disc fraction based on visual morphologies and speculate that this is an effect of forming disc appearing peculiar through processes such as violent disc instabilities or mergers. We conclude that to properly define and measure high-redshift morphology and structure a new and more exact classification scheme is needed