1,131 research outputs found
Faint counts as a function of morphological type in a hierarchical merger model
The unprecedented resolution of the refurbished Wide Field and Planetary
Camera 2 (WFPC2) on the Hubble Space Telescope (HST) has led to major advances
in our understanding of galaxy formation. The high image quality in the Medium
Deep Survey and Hubble Deep Field has made it possible, for the first time, to
classify faint distant galaxies according to morphological type. These
observations have revealed a large population of galaxies classed as irregulars
or which show signs of recent merger activity. Their abundance rises steeply
with apparent magnitude, providing a likely explanation for the large number of
blue galaxies seen at faint magnitudes. We demonstrate that such a population
arises naturally in a model in which structure forms hierarchically and which
is dynamically dominated by cold dark matter. The number counts of irregular,
spiral and elliptical galaxies as a function of magnitude seen in the HST data
are well reproduced in this model.We present detailed predictions for the
outcome of spectroscopic follow-up observations of the HST surveys. By
measuring the redshift distributions of faint galaxies of different
morphological types, these programmes will provide a test of the hierarchical
galaxy formation paradigm and might distinguish between models with different
cosmological parameters.Comment: 5 pages, 3 postscript figures included. To be published as a Letter
in Monthly Notices of the RAS. Postscript version available at
http://star-www.dur.ac.uk/~cmb/counts.htm
Recursive Approximation of the High Dimensional max Function
An alternative smoothing method for the high dimensional max functionhas been studied. The proposed method is a recursive extension of thetwo dimensional smoothing functions. In order to analyze the proposedmethod, a theoretical framework related to smoothing methods has beendiscussed. Moreover, we support our discussion by considering someapplication areas. This is followed by a comparison with analternative well-known smoothing method.n dimensional max function;recursive approximation;smoothing methods;vertical linear complementarity (VLCP)
GALAXY DYNAMICS IN CLUSTERS
We use high resolution simulations to study the formation and distribution of
galaxies within a cluster which forms hierarchically. We follow both dark
matter and baryonic gas which is subject to thermal pressure, shocks and
radiative cooling. Galaxy formation is identified with the dissipative collapse
of the gas into cold, compact knots. We examine two extreme representations of
galaxies during subsequent cluster evolution --- one purely gaseous and the
other purely stellar. The results are quite sensitive to this choice.
Gas-galaxies merge efficiently with a dominant central object while
star-galaxies merge less frequently. Thus, simulations in which galaxies remain
gaseous appear to suffer an ``overmerging'' problem, but this problem is much
less severe if the gas is allowed to turn into stars. We compare the kinematics
of the galaxy population in these two representations to that of dark halos and
of the underlying dark matter distribution. Galaxies in the stellar
representation are positively biased (\ie over-represented in the cluster) both
by number and by mass fraction. Both representations predict the galaxies to be
more centrally concentrated than the dark matter, whereas the dark halo
population is more extended. A modest velocity bias also exists in both
representations, with the largest effect, , found for the more massive star-galaxies. Phase diagrams show that the
galaxy population has a substantial net inflow in the gas representation, while
in the stellar case it is roughly in hydrostatic equilibrium. Virial mass
estimators can underestimate the true cluster mass by up to a factor of 5. The
discrepancy is largest if only the most massive galaxies are used, reflecting
significant mass segregation.Comment: 30 pages, self-unpacking (via uufiles) postscript file without
figures. Eighteen figures (and slick color version of figure 3) and entire
paper available at ftp://oahu.physics.lsa.umich.edu/groups/astro/fews Total
size of paper with figures is ~9.0 Mb uncompressed. Submitted to Ap.J
Cosmological Reionization
In popular cosmological scenarios, some time beyond a redshift of 10, stars
within protogalaxies created the first heavy elements; these systems, together
perhaps with an early population of quasars, generated the ultraviolet
radiation and mechanical energy that reheated and reionized the cosmos. The
history of the Universe during and soon after these crucial formative stages is
recorded in the all-pervading intergalactic medium (IGM), which contains most
of the ordinary baryonic material left over from the big bang. Throughout the
epoch of structure formation, the IGM becomes clumpy and acquires peculiar
motions under the influence of gravity, and acts as a source for the gas that
gets accreted, cools, and forms stars within galaxies, and as a sink for the
metal enriched material, energy, and radiation which they eject.Comment: LateX, 13 pages, 4 figures, slightly revised version (corrected
several typos), to appear in Phil. Trans. R. Soc. London A (2000) 35
A Comparison of Semi-Analytic and Smoothed Particle Hydrodynamics Galaxy Formation
We compare the statistical properties of galaxies found in two different
models of hierarchical galaxy formation: the semi-analytic model of Cole et al.
and the smoothed particle hydrodynamics (SPH) simulations of Pearce et al.
Using a `stripped-down' version of the semi-analytic model which mimics the
resolution of the SPH simulations and excludes physical processes not included
in them, we find that the two models produce an ensemble of galaxies with
remarkably similar properties, although there are some differences in the gas
cooling rates and in the number of galaxies that populate halos of different
mass. The full semi-analytic model, which has effectively no resolution limit
and includes a treatment of star formation and supernovae feedback, produces
somewhat different (but readily understandable) results. Agreement is
particularly good for the present-day global fractions of hot gas, cold dense
(i.e. galactic) gas and uncollapsed gas, for which the SPH and stripped-down
semi-analytic calculations differ by at most 25%. In the most massive halos,
the stripped-down semi-analytic model predicts, on the whole, up to 50% less
gas in galaxies than is seen in the SPH simulations. The two techniques
apportion this cold gas somewhat differently amongst galaxies in a given halo.
This difference can be tracked down to the greater cooling rate in massive
halos in the SPH simulation compared to the semi-analytic model. (abridged)Comment: 19 pages, 13 figures, to appear in MNRAS. Significantly extended to
explore galaxy progenitor distributions and behaviour of models at high
redshift
The Role of Starbursts in the Formation of Galaxies & Active Galactic Nuclei
Starbursts are episodes of intense star-formation in the central regions of
galaxies, and are the sites of roughly 25% of the high-mass star-formation in
the local universe. In this contribution I review the role starbursts play in
the formation and evolution of galaxies, the intergalactic medium, and active
galactic nuclei. Four major conclusions are drawn. 1) Starburst galaxies are
good analogues (in fact, the only plausible local analogues) to the known
population of star-forming galaxies at high-redshift. 2) Integrated over cosmic
time, supernova-driven galactic-winds (`superwinds') play an essential role in
the evolution of galaxies and the inter-galactic medium. 3) Circumnuclear
starbursts are an energetically-significant component of the Seyfert
phenomenon. 4) The evolution of the population of the host galaxies of
radio-quiet quasars is significantly different than that of powerful radio
galaxies, and is at least qualitatively consistent with the standard picture of
the hierarchical assembly of massive galaxies at relatively late times.Comment: 16 pages, 4 figures, Royal Society discussion meeting `The formation
of galaxies
Bulge Globular Clusters in Spiral Galaxies
There is now strong evidence that the metal-rich globular clusters (GC) near
the center of our Galaxy are associated with the Galactic bulge rather than the
disk as previously thought. Here we extend the concept of bulge GCs to the GC
systems of nearby spiral galaxies. In particular, the kinematic and metallicity
properties of the GC systems favor a bulge rather than a disk origin. The
number of metal-rich GCs normalized by the bulge luminosity is roughly constant
(i.e. bulge S_N ~ 1) in nearby spirals, and this value is similar to that for
field ellipticals when only the red (metal--rich) GCs are considered. We argue
that the metallicity distributions of GCs in spiral and elliptical galaxies are
remarkably similar, and that they obey the same correlation of mean GC
metallicity with host galaxy mass. We further suggest that the metal-rich GCs
in spirals are the direct analogs of the red GCs seen in ellipticals. The
formation of a bulge/spheroidal stellar system is accompanied by the formation
of metal-rich GCs. The similarities between GC systems in spiral and elliptical
galaxies appear to be greater than the differences.Comment: 5 pages, Latex, 2 figures, 1 table, Accepted for publication in ApJ
Letter
A G1-like globular cluster in NGC 1023
The structure of a very bright (MV = -10.9) globular cluster in NGC 1023 is
analyzed on two sets of images taken with the Hubble Space Telescope. From
careful modeling of King profile fits to the cluster image, a core radius of
0.55+/-0.1 pc, effective radius 3.7+/-0.3 pc and a central V-band surface
brightness of 12.9+/-0.5 mag / square arcsec are derived. This makes the
cluster much more compact than Omega Cen, but very similar to the brightest
globular cluster in M31, G1 = Mayall II. The cluster in NGC 1023 appears to be
very highly flattened with an ellipticity of about 0.37, even higher than for
Omega Cen and G1, and similar to the most flattened clusters in the Large
Magellanic Cloud.Comment: 14 pages, 3 figures, 1 table. Accepted for AJ, Oct 200
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