111 research outputs found
The star formation histories of early-type galaxies: insights from the rest-frame ultra-violet
Our current understanding of the star formation histories of early-type
galaxies is reviewed, in the context of recent observational studies of their
ultra-violet (UV) properties. Combination of UV and optical spectro-photometric
data indicates that the bulk of the stellar mass in the early-type population
forms at high redshift (z > 2), typically over short timescales (< 1 Gyr).
Nevertheless, early-types of all luminosities form stars over the lifetime of
the Universe, with most luminous (-23 < M(V) < -21) systems forming 10-15% of
their stellar mass after z = 1 (with a scatter to higher value), while their
less luminous (M(V) > -21) counterparts form 30-60% of their mass in the same
redshift range. The large scatter in the (rest-frame) UV colours in the
redshift range 0 < z < 0.7 indicates widespread low-level star formation in the
early-type population over the last 8 billion years. The mass fraction of young
(< 1 Gyr old) stars in luminous early-type galaxies varies between 1% and 6% at
z~0 and is in the range 5-13% at z~0.7. The intensity of recent star formation
and the bulk of the UV colour distribution is consistent with what might be
expected from minor mergers (mass ratios < 1:6) in an LCDM cosmology.Comment: Brief Review, Mod. Phys. Lett.
AGN in dwarf galaxies: frequency, triggering processes and the plausibility of AGN feedback
© 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical SocietyWhile active galactic nuclei (AGN) are considered to be key drivers of the evolution of massive galaxies, their potentially significant role in the dwarf-galaxy regime (M ∗ < 10 9 M ⊙) remains largely unexplored. We combine optical and infrared data, from the Hyper Suprime-Cam (HSC) and the Wide-field Infrared Explorer, respectively, to explore the properties of ∼800 AGN in dwarfs at low redshift (z < 0.3). Infrared-selected AGN fractions are ∼10-30 per cent in dwarfs, which, for reasonable duty cycles, indicates a high black hole (BH)-occupation fraction. Visual inspection of the deep HSC images indicates that the merger fraction in dwarf AGN (∼6 per cent) shows no excess compared to a control sample of non-AGN, suggesting that the AGN-triggering processes are secular in nature. Energetic arguments indicate that, in both dwarfs and massive galaxies, bolometric AGN luminosities (L AGN) are significantly greater than supernova luminosities (L SN). L AGN/L SN is, in fact, higher in dwarfs, with predictions from simulations suggesting that this ratio only increases with redshift. Together with the potentially high BH-occupation fraction, this suggests that if AGN feedback is an important driver of massive-galaxy evolution, the same is likely to be true in the dwarf regime, contrary to our classical thinking.Peer reviewedFinal Published versio
Morphology in the Era of Large Surveys
The study of galaxies has changed dramatically over the past few decades with
the advent of large-scale astronomical surveys. These large collaborative
efforts have made available high-quality imaging and spectroscopy of hundreds
of thousands of systems, providing a body of observations which has
significantly enhanced our understanding not only of cosmology and large-scale
structure in the universe but also of the astrophysics of galaxy formation and
evolution. Throughout these changes, one thing that has remained constant is
the role of galaxy morphology as a clue to understanding galaxies. But
obtaining morphologies for large numbers of galaxies is challenging; this
topic, "Morphology in the era of large surveys", was the subject of a recent
discussion meeting at the Royal Astronomical Society, and this "Astronomy and
Geophysics" article is a report on that meeting.Comment: Meeting Report article published in the October 2013 issue of the
Royal Astronomical Society journal Astronomy and Geophysics. 4 page pdf with
colour image
How the spectral energy distribution and galaxy morphology constrain each other, with application to morphological selection using galaxy colours
© 2022 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.We introduce an empirical methodology to study how the spectral energy distribution (SED) and galaxy morphology constrain each other and implement this on 8000 galaxies from the HST CANDELS survey in the GOODS-South field. We show that the SED does constrain morphology and present a method that quantifies the strength of the link between these two quantities. Two galaxies with very similar SEDs are around three times more likely to also be morphologically similar, with SED constraining morphology most strongly for relatively massive red ellipticals. We apply our methodology to explore likely upper bounds on the efficacy of morphological selection using colour. We show that, under reasonable assumptions, colour selection is relatively ineffective at separating homogeneous morphologies. Even with the use of up to six colours for morphological selection, the average purity in the resultant morphological classes is only around 60 per cent. While the results can be improved by using the whole SED, the gains are not significant, with purity values remaining around 70 per cent or below.Peer reviewedFinal Published versio
Identifying the progenitor set of present-day early-type galaxies: a view from the standard model
We present a comprehensive theoretical study, using a semi-analytical model
within the standard LCDM framework, of the photometric properties of the
progenitors of present-day early-type galaxies in the redshift range 0<z<1. We
explore progenitors of all morphologies and study their characteristics as a
function of the luminosity and local environment of the early-type remnant at
z=0. In agreement with previous studies, we find that, while larger early-types
are generally assembled later, their luminosity-weighted stellar ages are
typically older. In dense cluster-like environments, 70% of early-type systems
are `in place' by z=1 and evolve without interactions thereafter, while in the
field the corresponding value is 30%. Averaging across all environments at z~1,
less than 50% of the stellar mass which ends up in early-types today is
actually in early-type progenitors at this redshift, in agreement with recent
observational work. We develop probabilistic prescriptions which provide a
means of including spiral (i.e. non early-type) progenitors at intermediate and
high redshifts, based on their luminosity and optical colours. For example, we
find that, at intermediate redshifts (z~0.5), large (M_V0.7)
spirals have 75-95% chance of being an early-type progenitor, while the
corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is
50-75%. The prescriptions developed here can be used to address, from the
perspective of the standard model, the issue of `progenitor bias', whereby the
exclusion of late-type progenitors in observational studies can lead to
inaccurate conclusions regarding the evolution of the early-type population
over cosmic time. (abridged)Comment: Published in A&A, 2009, 503, 445. The article can be downloaded at:
http://adsabs.harvard.edu/abs/2009A%26A...503..445
The Role of Environment on the Formation of Early-Type Galaxies
(Abridged) We present a detailed study of the stellar populations of a
volume-limited sample of early-type galaxies from SDSS, across a range of
environments -- defined as the mass of the host dark matter halo. The stellar
populations are explored through the SDSS spectra, via projection onto a set of
two spectral vectors determined from Principal Component Analysis. We find the
velocity dispersion of the galaxy to be the main driver behind the different
star formation histories of early-type galaxies. However, environmental effects
are seen to play a role (although minor). Galaxies populating the lowest mass
halos have stellar populations on average ~1Gyr younger than the rest of the
sample. The fraction of galaxies with small amounts of recent star formation is
also seen to be truncated when occupying halos more massive than 3E13Msun. The
sample is split into satellite and central galaxies for a further analysis of
environment. Satellites are younger than central galaxies of the same stellar
mass. The younger satellite galaxies in 6E12Msun halos have stellar populations
consistent with the central galaxies found in the lowest mass halos of our
sample (i.e. 1E12Msun). This result is indicative of galaxies in lower mass
halos being accreted into larger halos.Comment: 11 pages, 10 figures. Accepted for publication in MNRA
The rise and fall of stellar discs across the peak of cosmic star formation history: mergers versus smooth accretion
Building galaxy merger trees from a state-of-the-art cosmological
hydrodynamics simulation, Horizon-AGN, we perform a statistical study of how
mergers and smooth accretion drive galaxy morphologic properties above .
More specifically, we investigate how stellar densities, effective radii and
shape parameters derived from the inertia tensor depend on mergers of different
mass ratios. We find strong evidence that smooth accretion tends to flatten
small galaxies over cosmic time, leading to the formation of disks. On the
other hand, mergers, and not only the major ones, exhibit a propensity to puff
up and destroy stellar disks, confirming the origin of elliptical galaxies. We
also find that elliptical galaxies are more susceptible to grow in size through
mergers than disc galaxies with a size-mass evolution r \prop M^{1.2} instead
of r \prop M^{-0.5} - M^{0.5} depending on the merger mass ratio. The gas
content drive the size-mass evolution due to merger with a faster size growth
for gas-poor galaxies r \prop M^2 than for gas-rich galaxies r \prop M.Comment: 16 pages, 19 figures, submitted to MNRA
Eigengalaxies: describing galaxy morphology using principal components in image space
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical SocietyWe demonstrate how galaxy morphologies can be represented by weighted sums of "eigengalaxies" and how eigengalaxies can be used in a probabilistic framework to enable principled and simplified approaches in a variety of applications. Eigengalaxies can be derived from a Principal Component Analysis (PCA) of sets of single- or multi-band images. They encode the image space equivalent of basis vectors that can be combined to describe the structural properties of large samples of galaxies in a massively reduced manner. As an illustration, we show how a sample of 10,243 galaxies in the Hubble Space Telescope CANDELS survey can be represented by just 12 eigengalaxies. We show in some detail how this image space may be derived and tested. We also describe a probabilistic extension to PCA (PPCA) which enables the eigengalaxy framework to assign probabilities to galaxies. We present four practical applications of the probabilistic eigengalaxy framework that are particularly relevant for the next generation of large imaging surveys: we (i) show how low likelihood galaxies make for natural candidates for outlier detection (ii) demonstrate how missing data can be predicted (iii) show how a similarity search can be performed on exemplars (iv) demonstrate how unsupervised clustering of objects can be implemented.Peer reviewe
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