7,663 research outputs found
Evidence for a non-universal stellar initial mass function in low-redshift high-density early-type galaxies
We determine an absolute calibration of stellar mass-to-light ratios for the
densest \simeq 3% of early-type galaxies in the local universe (redshift
z\simeq 0.08) from SDSS DR7. This sample of \sim 4000 galaxies has, assuming a
Chabrier IMF, effective stellar surface densities, Sigma_e > 2500 M_sun/pc^2,
stellar population synthesis (SPS) stellar masses log_10(M_sps/M_sun)<10.8, and
aperture velocity dispersions of sigma_ap=168^{+37}_{-34} km/s (68% range). In
contrast to typical early-type galaxies, we show that these dense early-type
galaxies follow the virial fundamental plane, which suggests that
mass-follows-light. With the additional assumption that any dark matter does
not follow the light, the dynamical masses of dense galaxies provide a direct
measurement of stellar masses. Our dynamical masses (M_dyn), obtained from the
spherical Jeans equations, are only weakly sensitive to the choice of
anisotropy (\beta) due to the relatively large aperture of the SDSS fiber for
these galaxies: R_ap \simeq 1.5 R_e. Assuming isotropic orbits (\beta=0) we
find a median log_{10} (M_dyn/M_sps) = 0.233 \pm 0.003, consistent with a
Salpeter IMF, while more bottom heavy IMFs and standard Milky-Way IMFs are
strongly disfavored. Our results are consistent with, but do not require, a
dependence of the IMF on dynamical mass or velocity dispersion. We find
evidence for a color dependence to the IMF such that redder galaxies have
heavier IMFs with M_dyn/M_sps \propto (g-r)^{1.13\pm0.09}. This may reflect a
more fundamental dependence of the IMF on the age or metallicity of a stellar
population, or the density at which the stars formed.Comment: 5 pages, 6 figures, accepted to MNRAS Letters, minor changes to
previous versio
Galaxy alignment on large and small scales
Galaxies are not randomly distributed across the universe but showing
different kinds of alignment on different scales. On small scales satellite
galaxies have a tendency to distribute along the major axis of the central
galaxy, with dependence on galaxy properties that both red satellites and
centrals have stronger alignment than their blue counterparts. On large scales,
it is found that the major axes of Luminous Red Galaxies (LRGs) have
correlation up to 30Mpc/h. Using hydro-dynamical simulation with star
formation, we investigate the origin of galaxy alignment on different scales.
It is found that most red satellite galaxies stay in the inner region of dark
matter halo inside which the shape of central galaxy is well aligned with the
dark matter distribution. Red centrals have stronger alignment than blue ones
as they live in massive haloes and the central galaxy-halo alignment increases
with halo mass. On large scales, the alignment of LRGs is also from the
galaxy-halo shape correlation, but with some extent of mis-alignment. The
massive haloes have stronger alignment than haloes in filament which connect
massive haloes. This is contrary to the naive expectation that cosmic filament
is the cause of halo alignment.Comment: 4 pages, 3 figures, To appear in the proceedings of the IAU Symposium
308 "The Zeldovich Universe: Genesis and Growth of the Cosmic Web
The Tully-Fisher Zero Point Problem
A long standing problem for hierarchical disk galaxy formation models has
been the simultaneous matching of the zero point of the Tully-Fisher relation
and the galaxy luminosity function (LF). We illustrate this problem for a
typical disk galaxy and discuss three solutions: low stellar mass-to-light
ratios, low initial dark halo concentrations, and no halo contraction. We
speculate that halo contraction may be reversed through a combination of mass
ejection through feedback and angular momentum exchange brought about by
dynamical friction between baryons and dark matter during the disk formation
process.Comment: 4 pages, 1 figure, to appear in proceedings of "Formation and
Evolution of Galaxy Disks", Rome, October 2007, Eds. J.G. Funes, S.J. and
E.M. Corsin
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