107 research outputs found
Population Synthesis and the Diagnostics of High-redshift Galaxies
The effect of redshift on the observation of distant galaxies is briefly
discussed emphasizing the possible sources of bias in the interpretation of
high-z data. A general energetic criterion to assess physical self-consistency
of evolutionary population synthesis models is also proposed, for a more
appropriate use of this important tool to investigate distinctive properties of
primeval galaxies.Comment: 8 pages and 6 color figures. Invited talk at the conference "New
Quests in Stellar Astrophysics: The link between Stars and Cosmology", 26-30
March, 2001, Puerto Vallarta, Mexico, eds. M. Chavez et al., to be published
by Kluwe
Galaxy Disks
The formation and evolution of galactic disks is particularly important for
understanding how galaxies form and evolve, and the cause of the variety in
which they appear to us. Ongoing large surveys, made possible by new
instrumentation at wavelengths from the ultraviolet (GALEX), via optical (HST
and large groundbased telescopes) and infrared (Spitzer) to the radio are
providing much new information about disk galaxies over a wide range of
redshift. Although progress has been made, the dynamics and structure of
stellar disks, including their truncations, are still not well understood. We
do now have plausible estimates of disk mass-to-light ratios, and estimates of
Toomre's parameter show that they are just locally stable. Disks are mostly
very flat and sometimes very thin, and have a range in surface brightness from
canonical disks with a central surface brightness of about 21.5 -mag
arcsec down to very low surface brightnesses. It appears that galaxy
disks are not maximal, except possibly in the largest systems. Their HI layers
display warps whenever HI can be detected beyond the stellar disk, with
low-level star formation going on out to large radii. Stellar disks display
abundance gradients which flatten at larger radii and sometimes even reverse.
The existence of a well-defined baryonic Tully-Fisher relation hints at an
approximately uniform baryonic to dark matter ratio. Thick disks are common in
disk galaxies and their existence appears unrelated to the presence of a bulge
component; they are old, but their formation is not yet understood. Disk
formation was already advanced at redshifts of , but at that epoch
disks were not yet quiescent and in full rotational equilibrium. Downsizing is
now well-established. The formation and history of star formation in S0s is
still not fully understood.Comment: This review has been submitted for Annual Reviews of Astronomy &
Astrophysics, vol. 49 (2011); the final printed version will have fewer
figures and a somewhat shortened text. A pdf-version of this preprint with
high-resolution figures is available from
http://www.astro.rug.nl/~vdkruit/jea3/homepage/disks-ph.pdf. (table of
contents added; 71 pages, 24 figures, 529 references
Stellar Population Diagnostics of Elliptical Galaxy Formation
Major progress has been achieved in recent years in mapping the properties of
passively-evolving, early-type galaxies (ETG) from the local universe all the
way to redshift ~2. Here, age and metallicity estimates for local cluster and
field ETGs are reviewed as based on color-magnitude, color-sigma, and
fundamental plane relations, as well as on spectral-line indices diagnostics.
The results of applying the same tools at high redshifts are then discussed,
and their consistency with the low-redshift results is assessed. Most low- as
well as high-redshift (z~1) observations consistently indicate 1) a formation
redshift z>~3 for the bulk of stars in cluster ETGs, with their counterparts in
low-density environments being on average ~1-2 Gyr younger, i.e., formed at
z>~1.5-2, 2) the duration of the major star formation phase anticorrelates with
galaxy mass, and the oldest stellar populations are found in the most massive
galaxies. With increasing redshift there is evidence for a decrease in the
number density of ETGs, especially of the less massive ones, whereas existing
data appear to suggest that most of the most-massive ETGs were already fully
assembled at z~1. Beyond this redshift, the space density of ETGs starts
dropping significantly, and as ETGs disappear, a population of massive,
strongly clustered, starburst galaxies progressively becomes more and more
prominent, which makes them the likely progenitors to ETGs.Comment: To appear on Annual Review of Astronomy & Astrophysics, Vol. 44
(2006). 46 pages with 16 figures. Replaced version includes updated
references, few typos less, and replaces Fig. 11 and Fig. 16 which had been
skrewed u
The Cosmic Infrared Background: Measurements and Implications
The cosmic infrared background records much of the radiant energy released by
processes of structure formation that have occurred since the decoupling of
matter and radiation following the Big Bang. In the past few years, data from
the Cosmic Background Explorer mission provided the first measurements of this
background, with additional constraints coming from studies of the attenuation
of TeV gamma-rays. At the same time there has been rapid progress in resolving
a significant fraction of this background with the deep galaxy counts at
infrared wavelengths from the Infrared Space Observatory instruments and at
submillimeter wavelengths from the Submillimeter Common User Bolometer Array
instrument. This article reviews the measurements of the infrared background
and sources contributing to it, and discusses the implications for past and
present cosmic processes.Comment: 61 pages, incl. 9 figures, to be published in Annual Reviews of
Astronomy and Astrophysics, 2001, Vol. 3
Gas accretion as the origin of chemical abundance gradients in distant galaxies
It has recently been suggested that galaxies in the early Universe can grow
through the accretion of cold gas, and that this may have been the main driver
of star formation and stellar mass growth. Because the cold gas is essentially
primordial, it has a very low abundance of elements heavier than helium
(metallicity). As it is funneled to the centre of a galaxy, it will lead the
central gas having an overall lower metallicity than gas further from the
centre, because the gas further out has been enriched by supernovae and stellar
winds, and not diluted by the primordial gas. Here we report chemical
abundances across three rotationally-supported star-forming galaxies at z~3,
only 2 Gyr after the Big Bang. We find an 'inverse' gradient, with the central,
star forming regions having a lower metallicity than less active ones, opposite
to what is seen in local galaxies. We conclude that the central gas has been
diluted by the accretion of primordial gas, as predicted by 'cold flow' models.Comment: To Appear in Nature Oct 14, 2010; Supplementary Information included
her
Far-Ultraviolet Radiation from Elliptical Galaxies
Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in
elliptical galaxies and early-type spiral bulges. It is the most variable
photometric feature associated with old stellar populations. Recent
observational and theoretical evidence shows that it is produced mainly by
low-mass, small-envelope, helium-burning stars in extreme horizontal branch and
subsequent phases of evolution. These are probably descendents of the dominant,
metal rich population of the galaxies. Their lifetime UV outputs are remarkably
sensitive to their physical properties and hence to the age and the helium and
metal abundances of their parents. UV spectra are therefore exceptionally
promising diagnostics of old stellar populations, although their calibration
requires a much improved understanding of giant branch mass loss, helium
enrichment, and atmospheric diffusion.Comment: 46 pages; includes LaTeX text file, 9 PS figures, 1 JPG figure, 2
style files. Full resolution figures and PS version available at
http://www.astro.virginia.edu/~rwo/araa99/. Article to appear in Annual
Reviews of Astronomy & Astrophysics, 199
Star Formation in Galaxies Along the Hubble Sequence
Observations of star formation rates (SFRs) in galaxies provide vital clues
to the physical nature of the Hubble sequence, and are key probes of the
evolutionary properties of galaxies. The focus of this review is on the broad
patterns in the star formation properties of galaxies along the Hubble
sequence, and their implications for understanding galaxy evolution and the
physical processes that drive the evolution. Star formation in the disks and
nuclear regions of galaxies are reviewed separately, then discussed within a
common interpretive framework. The diagnostic methods used to measure SFRs are
also reviewed, and a self-consistent set of SFR calibrations is presented as an
aid to workers in the field.Comment: 41 pages, with 9 figures. To appear in Volume 36 of the Annual Review
of Astronomy and Astrophysic
Fundamental Strings, Holography, and Nonlinear Superconformal Algebras
We discuss aspects of holography in the AdS_3 \times S^p near string geometry
of a collection of straight fundamental heterotic strings. We use anomalies and
symmetries to determine general features of the dual CFT. The symmetries
suggest the appearance of nonlinear superconformal algebras, and we show how
these arise in the framework of holographic renormalization methods. The
nonlinear algebras imply intricate formulas for the central charge, and we show
that in the bulk these correspond to an infinite series of quantum gravity
corrections. We also makes some comments on the worldsheet sigma-model for
strings on AdS_3\times S^2, which is the holographic dual geometry of parallel
heterotic strings in five dimensions.Comment: 25 page
The Galaxy Structure-Redshift Relationship
There exists a gradual, but persistent, evolutionary effect in the galaxy
population such that galaxy structure and morphology change with redshift. This
galaxy structure-redshift relationship is such that an increasingly large
fraction of all bright and massive galaxies at redshifts 2 < z < 3 are
morphologically peculiar at wavelengths from rest-frame ultraviolet to
rest-frame optical. There are however examples of morphologically selected
spirals and ellipticals at all redshifts up to z ~ 3. At lower redshift, the
bright galaxy population smoothly transforms into normal ellipticals and
spirals. The rate of this transformation strongly depends on redshift, with the
swiftest evolution occurring between 1 < z < 2. This review characterizes the
galaxy structure-redshift relationship, discusses its various physical causes,
and how these are revealing the mechanisms responsible for galaxy formation.Comment: 20 pages, 8 figures. Invited Review to appear in "Penetrating Bars
Through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes A New Note", ed.
D. Block et a
Lack of phenotypic and evolutionary cross-resistance against parasitoids and pathogens in Drosophila melanogaster
BackgroundWhen organisms are attacked by multiple natural enemies, the evolution of a resistance mechanism to one natural enemy will be influenced by the degree of cross-resistance to another natural enemy. Cross-resistance can be positive, when a resistance mechanism against one natural enemy also offers resistance to another; or negative, in the form of a trade-off, when an increase in resistance against one natural enemy results in a decrease in resistance against another. Using Drosophila melanogaster, an important model system for the evolution of invertebrate immunity, we test for the existence of cross-resistance against parasites and pathogens, at both a phenotypic and evolutionary level.MethodsWe used a field strain of D. melanogaster to test whether surviving parasitism by the parasitoid Asobara tabida has an effect on the resistance against Beauveria bassiana, an entomopathogenic fungus; and whether infection with the microsporidian Tubulinosema kingi has an effect on the resistance against A. tabida. We used lines selected for increased resistance to A. tabida to test whether increased parasitoid resistance has an effect on resistance against B. bassiana and T. kingi. We used lines selected for increased tolerance against B. bassiana to test whether increased fungal resistance has an effect on resistance against A. tabida.Results/ConclusionsWe found no positive cross-resistance or trade-offs in the resistance to parasites and pathogens. This is an important finding, given the use of D. melanogaster as a model system for the evolution of invertebrate immunity. The lack of any cross-resistance to parasites and pathogens, at both the phenotypic and the evolutionary level, suggests that evolution of resistance against one class of natural enemies is largely independent of evolution of resistance against the other
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