234 research outputs found
Bulgeless Giant Galaxies Challenge our Picture of Galaxy Formation by Hierarchical Clustering
We dissect giant Sc-Scd galaxies with Hubble Space Telescope photometry and
Hobby-Eberly Telescope spectroscopy. We use HET's High Resolution Spectrograph
(resolution = 15,000) to measure stellar velocity dispersions in the nuclear
star clusters and pseudobulges of the pure-disk galaxies M33, M101, NGC 3338,
NGC 3810, NGC 6503, and NGC 6946. We conclude: (1) Upper limits on the masses
of any supermassive black holes are MBH <= (2.6+-0.5) * 10**6 M_Sun in M101 and
MBH <= (2.0+-0.6) * 10**6 M_Sun in NGC 6503. (2) HST photometry shows that the
above galaxies contain tiny pseudobulges that make up <~ 3 % of the stellar
mass but no classical bulges. We inventory a sphere of radius 8 Mpc centered on
our Galaxy to see whether giant, pure-disk galaxies are common or rare. In this
volume, 11 of 19 galaxies with rotation velocity > 150 km/s show no evidence
for a classical bulge. Four may contain small classical bulges that contribute
5-12% of the galaxy light. Only 4 of the 19 giant galaxies are ellipticals or
have classical bulges that contribute 1/3 of the galaxy light. So pure-disk
galaxies are far from rare. It is hard to understand how they could form as the
quiescent tail of a distribution of merger histories. Recognition of
pseudobulges makes the biggest problem with cold dark matter galaxy formation
more acute: How can hierarchical clustering make so many giant, pure-disk
galaxies with no evidence for merger-built bulges? This problem depends
strongly on environment: the Virgo cluster is not a puzzle, because >2/3 of its
stellar mass is in merger remnants.Comment: 28 pages, 16 Postscript figures, 2 tables; requires emulateapj.sty
and apjfonts.sty; accepted for publication in ApJ; for a version with full
resolution figures, see http://chandra.as.utexas.edu/~kormendy/kdbc.pd
The connection between star formation and stellar mass: Specific star formation rates to redshift one
We investigate the contribution of star formation to the growth of stellar
mass in galaxies over the redshift range 0.5 < z < 1.1 by studying the redshift
evolution of the specific star formation rate (SSFR), defined as the star
formation rate per unit stellar mass. We use an I-band selected sample of 6180
field galaxies from the Munich Near-Infrared Cluster Survey (MUNICS) with
spectroscopically calibrated photometric redshifts. The SSFR decreases with
stellar mass at all redshifts. The low SSFRs of massive galaxies indicates that
star formation does not significantly change their stellar mass over this
redshift range: The majority of massive galaxies have assembled the bulk of
their mass before redshift unity. Furthermore, these highest mass galaxies
contain the oldest stellar populations at all redshifts. The line of maximum
SSFR runs parallel to lines of constant star formation rate. With increasing
redshift, the maximum SFR is generally increasing for all stellar masses, from
SFR ~ 5 M_sun/yr at z = 0.5 to SFR ~ 10 M_sun/yr at z = 1.1. We also show that
the large SSFRs of low-mass galaxies cannot be sustained over extended periods
of time. Finally, our results do not require a substantial contribution of
merging to the growth of stellar mass in massive galaxies over the redshift
range probed. We note that highly obscured galaxies which remain undetected in
our sample do not affect these findings for the bulk of the field galaxy
population.Comment: 5 pages, 3 colour figures, accepted for publication in MNRAS Letter
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Measuring Dark Matter Profiles Non-Parametrically In Dwarf Spheroidals: An Application To Draco
We introduce a novel implementation of orbit-based (or Schwarzschild) modeling that allows dark matter density profiles to be calculated non-parametrically in nearby galaxies. Our models require no assumptions to be made about velocity anisotropy or the dark matter profile. The technique can be applied to any dispersion-supported stellar system, and we demonstrate its use by studying the Local Group dwarf spheroidal galaxy (dSph) Draco. We use existing kinematic data at larger radii and also present 12 new radial velocities within the central 13 pc obtained with the VIRUS-W integral field spectrograph on the 2.7 m telescope at McDonald Observatory. Our non-parametric Schwarzschild models find strong evidence that the dark matter profile in Draco is cuspy for 20 = 20 pc is well fit by a power law with slope alpha = -1.0 +/- 0.2, consistent with predictions from cold dark matter simulations. Our models confirm that, despite its low baryon content relative to other dSphs, Draco lives in a massive halo.NSF-0908639Astronom
The Molecular Gas Density in Galaxy Centers and How It Connects to Bulges
In this paper we present gas density, star formation rate, stellar masses,
and bulge disk decompositions for a sample of 60 galaxies. Our sample is the
combined sample of BIMA SONG, CARMA STING, and PdBI NUGA surveys. We study the
effect of using CO-to-H_2 conversion factors that depend on the CO surface
brightness, and also that of correcting star formation rates for diffuse
emission from old stellar populations. We estimate that star formation rates in
bulges are typically lower by 20% when correcting for diffuse emission. We find
that over half of the galaxies in our sample have molecular gas surface density
>100 M_sun pc^-2. We find a trend between gas density of bulges and bulge
Sersic index; bulges with lower Sersic index have higher gas density. Those
bulges with low Sersic index (pseudobulges) have gas fractions that are similar
to that of disks. We also find that there is a strong correlation between
bulges with the highest gas surface density and the galaxy being barred.
However, we also find that classical bulges with low gas surface density can be
barred as well. Our results suggest that understanding the connection between
the central surface density of gas in disk galaxies and the presence of bars
should also take into account the total gas content of the galaxy and/or bulge
Sersic index. Indeed, we find that high bulge Sersic index is the best
predictor of low gas density inside the bulge (not barredness of the disk).
Finally, we show that when using the corrected star formation rates and gas
densities, the correlation between star formation rate surface density and gas
surface density of bulges is similar to that of disks.Comment: Accepted for publication in Ap
Specific star formation rates to redshift 5 from the FORS Deep Field and the GOODS-S Field
We explore the build-up of stellar mass in galaxies over a wide redshift
range 0.4 < z < 5.0 by studying the evolution of the specific star formation
rate (SSFR), defined as the star formation rate per unit stellar mass, as a
function of stellar mass and age. Our work is based on a combined sample of ~
9000 galaxies from the FORS Deep Field and the GOODS-S field, providing high
statistical accuracy and relative insensitivity against cosmic variance. As at
lower redshifts, we find that lower-mass galaxies show higher SSFRs than higher
mass galaxies, although highly obscured galaxies remain undetected in our
sample. Furthermore, the highest mass galaxies contain the oldest stellar
populations at all redshifts, in principle agreement with the existence of
evolved, massive galaxies at 1 < z < 3. It is remarkable, however, that this
trend continues to very high redshifts of z ~ 4. We also show that with
increasing redshift the SSFR for massive galaxies increases by a factor of ~
10, reaching the era of their formation at z ~ 2 and beyond. These findings can
be interpreted as evidence for an early epoch of star formation in the most
massive galaxies, and ongoing star-formation activity in lower mass galaxies.Comment: Accepted for publication in ApJL; 4 pages, 2 color figures, uses
emulateapj.cl
The contribution of star formation and merging to stellar mass buildup in galaxies
We present a formalism to infer the presence of merging by comparing the time
derivative of the observed galaxy stellar mass function (MF) to the change of
the MF expected from the star formation rate (SFR) in galaxies as a function of
mass and time. We present the SFR in as a function of stellar mass and time
spanning 9=3 the average SFR, is a power
law of stellar mass (SFR~M^0.6). The average SFR in the most massive objects at
this redshift is 100-500 Msun/yr. At z~3, the SFR starts to drop at the high
mass end. As z decreases further, the SFR drops at progressively lower masses
(downsizing), dropping most rapidly for high mass (logM>11) galaxies. The mass
at which the SFR starts to deviate from the power-law form (break mass)
progresses smoothly from logM~13 at z~5 to logM~10.9 at z~0.5. The break mass
evolves with redshift as M(z)=2.7x10^10 (1+z)^2.1. We directly observe a
relationship between SFH and mass. More massive galaxies have steeper and
earlier onsets of SF, their SFR peaks earlier and higher, and the following
decay has a shorter e-folding time. The SFR observed in high mass galaxies at
z~4 is sufficient to explain their rapid increase in number density. Within
large uncertainties, at most 0.8 major mergers per Gyr are consistent with the
high-z data, yet enough to transform most high mass objects into ellipticals
contemporaneously with their major star formation episode. In contrast, at
z11, mergers contribute 0.1-0.2 Gyr^-1 to the relative
increase in number density (~1 major merger per massive object at 1.5>z>0). At
10<logM<11, galaxies are being preferably destroyed in mergers at high z, while
at later times the change in their numbers turns positive. This shows the
top-down buildup of the red sequence suggested by other observations.Comment: Accepted for publication in Ap
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