363 research outputs found
Shellflow. I. The Convergence of the Velocity Field at 6000 km/s
We present the first results from the Shellflow program, an all-sky
Tully-Fisher (TF) peculiar velocity survey of 276 Sb-Sc galaxies with redshifts
between 4500 and 7000 km/s. Shellflow was designed to minimize systematic
errors between observing runs and between telescopes, thereby removing the
possibility of a spurious bulk flow caused by data inhomogeneity. A fit to the
data yields a bulk flow amplitude V_bulk = 70{+100}{-70} km/s (1 sigma error)
with respect to the Cosmic Microwave Background, i.e., consistent with being at
rest. At the 95% confidence level, the flow amplitude is < 300 km/s. Our
results are insensitive to which Galactic extinction maps we use, and to the
parameterization of the TF relation. The larger bulk motion found in analyses
of the Mark III peculiar velocity catalog are thus likely to be due to
non-uniformities between the subsamples making up Mark III. The absence of bulk
flow is consistent with the study of Giovanelli and collaborators and flow
field predictions from the observed distribution of IRAS galaxies.Comment: Accepted version for publication in ApJ. Includes an epitaph for
Jeffrey Alan Willick (Oct 8, 1959 - Jun 18, 2000
OMEGA AND BIASING FROM OPTICAL GALAXIES VERSUS POTENT MASS
The mass density field in the local universe, recovered by the POTENT method
from peculiar velocities of 3000 galaxies, is compared with the density
field of optically-selected galaxies. Both density fields are smoothed with a
Gaussian filter of radius 12 Mpc. Under the assumptions of
gravitational instability and a linear biasing parameter b\sbo between
optical galaxies and mass, we obtain \beta\sbo \equiv \om^{0.6}/b\sbo = 0.74
\pm 0.13. This result is obtained from a regression of POTENT mass density on
optical density after correcting the mass density field for systematic biases
in the velocity data and POTENT method. The error quoted is just the
formal error estimated from the observed scatter in the density--density
scatterplot; it does not include the uncertainty due to cosmic scatter in the
mean density or in the biasing relation. We do not attempt a formal analysis of
the goodness of fit, but the scatter about the fit is consistent with our
estimates of the uncertainties.Comment: Final revised version (minor typos corrected). 13 pages, gzipped tar
file containing LaTeX and figures. The Postscript file is available at
ftp://dust0.dur.ac.uk/pub/mjh/potopt/potopt.ps.Z or (gzipped) at
ftp://xxx.lanl.gov/astro-ph/ps/9501/9501074.ps.gz or via WWW at
http://xxx.lanl.gov/ps/astro-ph/9501074 or as separate LaTeX text and
encapsulated Postscript figures in a compressed tar'd file at
ftp://dust0.dur.ac.uk/pub/mjh/potopt/latex/potopt.tar.
The Bulge-Halo Connection in Galaxies: A Physical Interpretation of the Vcirc-sigma_0 Relation
We explore the dependence of the ratio of a galaxy's circular velocity,
Vcirc, to its central velocity dispersion, sigma_0, on morphology, or
equivalently total light concentration. Such a dependence is expected if light
traces the mass. Over the full range of galaxy types, masses and brightnesses,
and assuming that the gas velocity traces the circular velocity, we find that
galaxies obey the relation log(Vcirc/sigma_0)= 0.63-0.11*C28 where
C28=5log(r80/r20) and the radii are measured at 80 percent and 20 percent of
the total light. Massive galaxies scatter about the Vcirc = sqrt(2)*sigma_0
line for isothermal stellar systems. Disk galaxies follow the simple relation
Vcirc/sigma_0=2(1-B/T), where B/T is the bulge-to-total light ratio. For pure
disks, C28~2.8, B/T -> 0, and Vcirc~=2*sigma_0. Self-consistent equilibrium
galaxy models from Widrow & Dubinski (2005) constrained to match the
size-luminosity and velocity-luminosity relations of disk galaxies fail to
match the observed Vcirc/sigma_0 distribution. Furthermore, the matching of
dynamical models for Vcirc(r)/sigma(r) with observations of dwarf and
elliptical galaxies suffers from limited radial coverage and relatively large
error bars; for dwarf systems, however, kinematical measurements at the galaxy
center and optical edge suggest Vcirc(Rmax) > 2*sigma_0 (in contrast with past
assumptions that Vcirc = sqrt(2)*sigma_0 for dwarfs.) The Vcirc-sigma_0-C28
relation has direct implications for galaxy formation and dynamical models,
galaxy scaling relations, the mass function of galaxies, and the links between
respective formation and evolution processes for a galaxy's central massive
object, bulge, and dark matter halo.Comment: Accepted for publication in ApJL. Current version matches ApJL page
requiremen
Fitting functions for a disk-galaxy model with different LCDM-halo profiles
We present an adaptation of the standard scenario of disk-galaxy formation to
the concordant LCDM cosmology aimed to derive analytical expressions for the
scale length and rotation speed of present-day disks that form within four
different, cosmologically motivated protogalactic dark matter halo-density
profiles. We invoke a standard galaxy-formation model that includes virial
equilibrium of spherical dark halos, specific angular momentum conservation
during gas cooling, and adiabatic halo response to the gas inflow. The mean
mass-fraction and mass-to-light ratio of the central stellar disk are treated
as free parameters whose values are tuned to match the zero points of the
observed size-luminosity and circular speed-luminosity relations of galaxies.
We supply analytical formulas for the characteristic size and rotation speed of
disks built inside Einasto r^{1/6}, Hernquist, Burkert, and Navarro-Frenk-White
dark matter halos. These expressions match simultaneously the observed zero
points and slopes of the different correlations that can be built in the RVL
space of disk galaxies from plausible values of the galaxy- and star-formation
efficiencies
The Tully-Fisher Relation for 25,000 SDSS Galaxies as Function of Environment
We construct Tully-Fisher relationships (TFRs) in the , , , and
bands and stellar mass TFRs (smTFRs) for a sample of late spiral
type galaxies (with ) from the Sloan Digital Sky Survey (SDSS)
and study the effects of environment on the relation. We use SDSS-measured
Balmer emission line widths, , as a proxy for disc circular
velocity, . A priori it is not clear whether we can construct
accurate TFRs given the small diameter of the fibres used for SDSS
spectroscopic measurements. However, we show by modelling the H
emission profile as observed through a aperture that for galaxies at
appropriate redshifts () the fibres sample enough of the disc to
obtain a linear relationship between and ,
allowing us to obtain a TFR and to investigate dependence on other variables.
We also develop a methodology for distinguishing between astrophysical and
sample bias in the fibre TFR trends. We observe the well-known steepening of
the TFR in redder bands in our sample. We divide the sample of galaxies into
four equal groups using projected neighbour density () quartiles and
find no significant dependence on environment, extending previous work to a
wider range of environments and a much larger sample. Having demonstrated that
we can construct SDSS-based TFRs is very useful for future applications because
of the large sample size available.Comment: accepted for publication in MNRAS. 15 pages, 12 figure
Bayesian Investigation of Isochrone Consistency Using the Old Open Cluster Ngc 188
This paper provides a detailed comparison of the differences in parameters derived for a star cluster from its color– magnitude diagrams (CMDs) depending on the filters and models used. We examine the consistency and reliability of fitting three widely used stellar evolution models to 15 combinations of optical and near-IR photometry for the old open cluster NGC 188. The optical filter response curves match those of theoretical systems and are thus not the source of fit inconsistencies. NGC 188 is ideally suited to this study thanks to a wide variety of high-quality photometry and available proper motions and radial velocities that enable us to remove non-cluster members and many binaries. Our Bayesian fitting technique yields inferred values of age, metallicity, distance modulus, and absorption as a function of the photometric band combinations and stellar models. We show that the historically favored three-band combinations of UBV and VRI can be meaningfully inconsistent with each other and with longer baseline data sets such as UBVRIJHKS. Differences among model sets can also be substantial. For instance, fitting Yi et al. (2001) and Dotter et al. (2008) models to UBVRIJHKS photometry for NGC 188 yields the following cluster parameters: age={5.78 ± 0.03, 6.45 ± 0.04} Gyr, [Fe/H] = {+0.125 ± 0.003, −0.077 ± 0.003} dex, (m - M)V = {11.441 ± 0.007, 11.525 ± 0.005} mag, and AV = {0.162 ± 0.003, 0.236 ± 0.003} mag, respectively. Within the formal fitting errors, these two fits are substantially and statistically different. Such differences among fits using different filters and models are a cautionary tale regarding our current ability to fit star cluster CMDs. Additional modeling of this kind, with more models and star clusters, and future Gaia parallaxes are critical for isolating and quantifying the most relevant uncertainties in stellar evolutionary models
M31's Heavy Element Distribution and Outer Disk
Hubble Space Telescope imaging of 11 fields in M31 were reduced to
color-magnitude diagrams. The fields were chosen to sample all galactocentric
radii to 50 kpc. Assuming that the bulk of the sampled stellar populations are
older than a few Gyr, the colors of the red giants map to an abundance
distribution with errors of order 0.1 dex in abundance. The radially sampled
abundance distributions are all about the same width, but show a mild abundance
gradient that flattens outside ~20 kpc. The various distributions were weighted
and summed with the aid of new surface brightness profile fits to obtain an
abundance distribution representative of the entirety of M31. M31 is a system
near chemical maturity. This ``observed closed box'' is compared to analytical
closed box models. M31 suffers from a lack of metal-poor stars and metal-rich
stars relative to the simplest closed-box model in the same way as the solar
neighborhood.Comparing to several simple chemical evolution models, neither
complete mixing of gas at all times nor zero mixing, inhomogeneous models give
the most convincing match to the data. As noted elsewhere, the outer disk of
M31 is a factor of ten more metal-rich than the Milky Way halo, ten times more
metal-rich than the dwarf spheroidals cospatial with it, and more metal-rich
than most of the globular clusters at the same galactocentric radius.
Difficulties of interpretation are greatly eased if we posit that the M31 disk
dominates over the halo at all radii out to 50 kpc. In fact, scaling from
current density models of the Milky Way, one should not expect to see halo
stars dominating over disk stars until beyond our 50 kpc limit. A corollary
conclusion is that most published studies of the M31 "halo" are actually
studies of its disk.Comment: 28 pages, 11 black-and-white figures, in press, Astrophysical Journa
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