874 research outputs found
Galactic Bulges
We review current knowledge on the structure, properties and evolution of
galactic bulges, considering particularly common preconceptions in the light of
recent observational results.Comment: in press, Annual Review Astron. Astrophys. 35 1997. Plain tex, 9
figures included. Also available by anonymous ftp at
ftp://ftp.ast.cam.ac.uk/pub/gil
Revealing velocity dispersion as the best indicator of a galaxy's color, compared to stellar mass, surface mass density or morphology
Using data of nearby galaxies from the Sloan Digital Sky Survey we
investigate whether stellar mass, central velocity dispersion, surface mass
density, or the Sersic n parameter is best correlated with a galaxy's
rest-frame color. Specifically, we determine how the mean color of galaxies
varies with one parameter when another is fixed. When the stellar mass is fixed
we see that strong trends remain with all other parameters, whereas residual
trends are weaker when surface mass density, n, or velocity dispersion are
fixed. Overall velocity dispersion is the best indicator of a galaxy's typical
color, showing the largest residual color dependence when any of the other
three parameters are fixed, and stellar mass is the poorest. Other studies have
indicated that both the halo and black hole properties are better correlated
with velocity dispersion than with stellar mass, surface mass density or Sersic
n. Therefore, our results are consistent with a picture where a galaxy's star
formation history and present star formation rate are determined to some
significant degree by the current properties and assembly history of its dark
matter halo and/or the feedback from its central super massive black hole.Comment: 7 pages, 5 figures, submitted to ApJ Letter
Near Infrared Observations of a Redshift 4.92 Galaxy: Evidence for Significant Dust Absorption
Near-infrared imaging and spectroscopy have been obtained of the
gravitationally lensed galaxy at z=4.92 discovered in HST images by Franx et
al. (1997). Images at 1.2, 1.6 and 2.2 microns show the same arc morphology as
the HST images. The spectrum with resolution \lambda / \Delta\lambda ~ 70 shows
no emission lines with equivalent width stronger than 100 A in the rest frame
wavelength range 0.34 to 0.40 microns. In particular, [OII]3727 A and
[NeIII]3869 A are not seen. The energy distribution is quite blue, as expected
for a young stellar population with the observed Ly alpha flux. The spectral
energy distribution can be fit satisfactorily for such a young stellar
population when absorption by dust is included. The models imply a reddening
0.1 mag < E(B-V) < 0.4 mag. The stellar mass of the lensed galaxy lies in the
range of 2 to 16 x 10^9 Msun. This is significantly higher than estimates based
on the HST data alone. Our data imply that absorption by dust is important to
redshifts of ~5.Comment: LaTeX with ApJ journal format, 2 postscript figures, ApJL, accepte
Spitzer IRAC confirmation of z_850-dropout galaxies in the Hubble Ultra Deep Field: stellar masses and ages at z~7
Using Spitzer IRAC mid-infrared imaging from the Great Observatories Origins
Deep Survey, we study z_850-dropout sources in the Hubble Ultra Deep Field.
After carefully removing contaminating flux from foreground sources, we clearly
detect two z_850-dropouts at 3.6 micron and 4.5 micron, while two others are
marginally detected. The mid-infrared fluxes strongly support their
interpretation as galaxies at z~7, seen when the Universe was only 750 Myr old.
The IRAC observations allow us for the first time to constrain the rest-frame
optical colors, stellar masses, and ages of the highest redshift galaxies.
Fitting stellar population models to the spectral energy distributions, we find
photometric redshifts in the range 6.7-7.4, rest-frame colors U-V=0.2-0.4,
V-band luminosities L_V=0.6-3 x 10^10 L_sun, stellar masses 1-10 x 10^9 M_sun,
stellar ages 50-200 Myr, star formation rates up to ~25 M_sun/yr, and low
reddening A_V<0.4. Overall, the z=7 galaxies appear substantially less massive
and evolved than Lyman break galaxies or Distant Red Galaxies at z=2-3, but
fairly similar to recently identified systems at z=5-6. The stellar mass
density inferred from our z=7 sample is rho* = 1.6^{+1.6}_{-0.8} x 10^6 M_sun
Mpc^-3 (to 0.3 L*(z=3)), in apparent agreement with recent cosmological
hydrodynamic simulations, but we note that incompleteness and sample variance
may introduce larger uncertainties. The ages of the two most massive galaxies
suggest they formed at z>8, during the era of cosmic reionization, but the star
formation rate density derived from their stellar masses and ages is not nearly
sufficient to reionize the universe. The simplest explanation for this
deficiency is that lower-mass galaxies beyond our detection limit reionized the
universe.Comment: 4 pages, 3 figures, emulateapj, Accepted for publication in ApJ
Letter
Measuring the evolution of the M/L ratio from the fundamental plane in CL 0024+16 at z=0.39
The existence of the Fundamental Plane of early-type galaxies implies that the M/L ratios of early-types are well behaved. It provides therefore an important tool to measure the evolution of the M/L ratio with redshift. These measurements, in combination with measurements of the evolution of the luminosity function, can be used to constrain the mass evolution of galaxies. We present the Fundamental Plane relation measured for galaxies in the rich cluster CL 0024+16 at z=0.391. The galaxies satisfy a tight Fundamental Plane, with relatively low scatter (15 %). The M/L is 31 +- 12 % lower than the M/L measured in Coma, which is consistent with simple evolutionary models. Hence, galaxies with very similar dynamical properties existed at a z=0.4. More, and deeper data are needed to measure the evolution of the slope and the scatter of the Fundamental Plane to higher accuracy. Furthermore, data on the richest nearby clusters would be valuable to test the hypothesis that the Fundamental Plane is independent of cluster environment
- …