2,225 research outputs found
Galaxy bulges and their black holes: a requirement for the quenching of star formation
One of the central features of the last 8 to 10 billion years of cosmic
history has been the emergence of a well-populated red sequence of
non-star-forming galaxies. A number of models of galaxy formation and evolution
have been devised to attempt to explain this behavior. Most current models
require feedback from supermassive black holes (AGN feedback) to quench star
formation in galaxies in the centers of their dark matter halos (central
galaxies). Such models make the strong prediction that all quenched central
galaxies must have a large supermassive black hole (and, by association, a
prominent bulge component). I show using data from the Sloan Digital Sky Survey
that the observations are consistent with this prediction. Over 99.5% of red
sequence galaxies with stellar masses in excess of 10^{10} M_{\sun} have a
prominent bulge component (as defined by having a Sersic index n above 1.5).
Those very rare red sequence central galaxies with little or no bulge (n<1.5)
usually have detectable star formation or AGN activity; the fraction of truly
quenched bulgeless central galaxies is <0.1% of the total red sequence
population. I conclude that a bulge, and by implication a supermassive black
hole, is an absolute requirement for full quenching of star formation in
central galaxies. This is in agreement with the most basic prediction of the
AGN feedback paradigm.Comment: 6 pages, 4 color figures (figure 1 is of slightly degraded quality).
To appear in August 1 edition of the Astrophysical Journa
Do the low PN velocity dispersions around elliptical galaxies imply that these lack dark matter?
While kinematical modelling of the low PN velocity dispersions observed in
the outer regions of elliptical galaxies suggest a lack of dark matter around
these galaxies, we report on an analysis of a suite of -body simulations
(with gas) of major mergers of spiral galaxies embedded in dark matter halos,
and find that the outer velocity dispersions are as low as observed for the
PNe. The inconsistency between our dynamical modelling and previous kinematical
modelling is caused by very radial stellar orbits and projection effects when
viewing face-on oblate ellipticals. Our simulations (weakly) suggest the youth
of PNe around ellipticals, and we propose that the universality of the PN
luminosity function may be explained if the bright PNe in ellipticals are
formed after the regular accretion of very low mass gas-rich galaxies.Comment: Contributed talk at meeting, "Planetary Nebulae as astronomical
tools", Gdansk, Poland, June-July 2005, ed. R. Szczerba, G. Stasi\'nska, and
S. K. G\'orny, AIP Conference Proceedings, Melville, New York, 2005. 4 or 5
pages, 6 figure
Evidence for a Positive Cosmological Constant from Flows of Galaxies and Distant Supernovae
Recent observations of high-redshift supernovae seem to suggest that the
global geometry of the Universe may be affected by a `cosmological constant',
which acts to accelerate the expansion rate with time. But these data by
themselves still permit an open universe of low mass density and no
cosmological constant. Here we derive an independent constraint on the lower
bound to the mass density, based on deviations of galaxy velocities from a
smooth universal expansion. This constraint rules out a low-density open
universe with a vanishing cosmological constant, and together the two favour a
nearly flat universe in which the contributions from mass density and the
cosmological constant are comparable. This type of universe, however, seems to
require a degree of fine tuning of the initial conditions that is in apparent
conflict with `common wisdom'.Comment: 8 pages, 1 figure. Slightly revised version. Letter to Natur
Is the Redshift Clustering of Long-Duration Gamma-Ray Bursts Significant?
The 26 long-duration gamma-ray bursts (GRBs) with known redshifts form a
distinct cosmological set, selected differently than other cosmological probes
such as quasars and galaxies. Since the progenitors are now believed to be
connected with active star-formation and since burst emission penetrates dust,
one hope is that with a uniformly-selected sample, the large-scale redshift
distribution of GRBs can help constrain the star-formation history of the
Universe. However, we show that strong observational biases in ground-based
redshift discovery hamper a clean determination of the large-scale GRB rate and
hence the connection of GRBs to the star formation history. We then focus on
the properties of the small-scale (clustering) distribution of GRB redshifts.
When corrected for heliocentric motion relative to the local Hubble flow, the
observed redshifts appear to show a propensity for clustering: 8 of 26 GRBs
occurred within a recession velocity difference of 1000 km/s of another GRB.
That is, 4 pairs of GRBs occurred within 30 h_65^-1 Myr in cosmic time, despite
being causally separated on the sky. We investigate the significance of this
clustering. Comparison of the numbers of close redshift pairs expected from the
simulation with that observed shows no significant small-scale clustering
excess in the present sample; however, the four close pairs occur only in about
twenty percent of the simulated datasets (the precise significance of the
clustering is dependent upon the modeled biases). We conclude with some
impetuses and suggestions for future precise GRB redshift measurements.Comment: Published in the Astronomical Journal, June 2003: see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003AJ....125.2865
Cluster versus POTENT Density and Velocity Fields: Cluster Biasing and Omega
The density and velocity fields as extracted from the Abell/ACO clusters are
compared to the corresponding fields recovered by the POTENT method from the
Mark~III peculiar velocities of galaxies. In order to minimize non-linear
effects and to deal with ill-sampled regions we smooth both fields using a
Gaussian window with radii ranging between 12 - 20\hmpc. The density and
velocity fields within 70\hmpc exhibit similarities, qualitatively consistent
with gravitational instability theory and a linear biasing relation between
clusters and mass. The random and systematic errors are evaluated with the help
of mock catalogs. Quantitative comparisons within a volume containing
independent samples yield
\betac\equiv\Omega^{0.6}/b_c=0.22\pm0.08, where is the cluster biasing
parameter at 15\hmpc. If , as indicated by the cluster
correlation function, our result is consistent with .Comment: 18 pages, latex, 2 ps figures 6 gif figures. Accepted for
pubblications in MNRA
Omega from the skewness of the cosmic velocity divergence
We propose a method for measuring the cosmological density parameter
from the statistics of the divergence field, , the
divergence of peculiar velocity, expressed in units of the Hubble constant, . The velocity field is spatially smoothed over to remove strongly nonlinear effects. Assuming weakly-nonlinear
gravitational evolution from Gaussian initial fluctuations, and using
second-order perturbative analysis, we show that \propto
-\Omega^{-0.6} ^2. The constant of proportionality depends on the
smoothing window. For a top-hat of radius R and volume-weighted smoothing, this
constant is , where . If the
power spectrum is a power law, , then . A Gaussian
window yields similar results. The resulting method for measuring is
independent of any assumed biasing relation between galaxies and mass.
The method has been successfully tested with numerical simulations. A
preliminary application to real data, provided by the POTENT recovery procedure
from observed velocities favors . However, because of an
uncertain sampling error, this result should be treated as an assessment of the
feasibility of our method rather than a definitive measurement of .Comment: 16 pages + 2 figures, uuencoded postscript file, also available by
anonymous ftp from ftp.cita.utoronto.ca in directory
/cita/francis/div_skewness, CITA 94-1
Lost and found dark matter in elliptical galaxies
The kinematical properties of elliptical galaxies formed during the mergers
of equal mass, stars+gas+dark matter spiral galaxies are compared to the
observed low velocity dispersions found for planetary nebulae on the outskirts
of ellipticals, which have been interpreted as pointing to a lack of dark
matter in ellipticals (which poses a problem for the standard model of galaxy
formation). We find that the velocity dispersion profiles of the stars in the
simulated ellipticals match well the observed ones. The low outer stellar
velocity dispersions are mainly caused by the radial orbits of the outermost
stars, which, for a given binding energy must have low angular momentum to
reach their large radial distances, usually driven out along tidal tails.Comment: Talk presented at 21st IAP meeting, Mass Profiles andShapes of
Cosmological Structures. Ed. G. A. Mamon, F. Combes, C. Deffayet & B. Fort
(Paris: EDP), 4 pages, 3 figures (4 plots
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