676 research outputs found
An N-body/SPH Study of Isolated Galaxy Mass Density Profiles
We investigate the evolution of mass density profiles in secular disk galaxy
models, paying special attention to the development of a two-component profile
from a single initial exponential disk free of cosmological evolution (i.e., no
accretion or interactions). As the source of density profile variations, we
examine the parameter space of the spin parameter, halo concentration, virial
mass, disk mass and bulge mass, for a total of 162 simulations in the context
of a plausible model of star formation and feedback (GADGET-2). The evolution
of the galaxy mass density profile, including the development of a
two-component profile with an inner and outer segment, is controlled by the
ratio of the disk mass fraction, , to the halo spin parameter,
. The location of the break between the two components and speed at
which it develops is directly proportional to ; the amplitude of
the transition between the inner and outer regions is however controlled by the
ratio of halo concentration to virial velocity. The location of the divide
between the inner and outer profile does not change with time. (Abridged)Comment: 27 pages, 31 figures. Accepted for publication at MNRAS. A
high-resolution version of the paper with figures can be found here
http://www.mpia-hd.mpg.de/~foyle/papers/MN-07-1491-MJ.R1.pd
Cosmic Evolution of Stellar Disk Truncations: From z~1 to the Local Universe
We have conducted the largest systematic search so far for stellar disk
truncations in disk-like galaxies at intermediate redshift (z<1.1), using the
Great Observatories Origins Deep Survey South (GOODS-S) data from the Hubble
Space Telescope - ACS. Focusing on Type II galaxies (i.e. downbending profiles)
we explore whether the position of the break in the rest-frame B-band radial
surface brightness profile (a direct estimator of the extent of the disk where
most of the massive star formation is taking place), evolves with time. The
number of galaxies under analysis (238 of a total of 505) is an order of
magnitude larger than in previous studies. For the first time, we probe the
evolution of the break radius for a given stellar mass (a parameter well suited
to address evolutionary studies). Our results suggest that, for a given stellar
mass, the radial position of the break has increased with cosmic time by a
factor 1.3+/-0.1 between z~1 and z~0. This is in agreement with a moderate
inside-out growth of the disk galaxies in the last ~8 Gyr. In the same period
of time, the surface brightness level in the rest-frame B-band at which the
break takes place has increased by 3.3+/-0.2 mag/arcsec^2 (a decrease in
brightness by a factor of 20.9+/-4.2). We have explored the distribution of the
scale lengths of the disks in the region inside the break, and how this
parameter relates to the break radius. We also present results of the
statistical analysis of profiles of artificial galaxies, to assess the
reliability of our results.Comment: 22 pages, 14 figures, accepted for publication in ApJ. Figures 1, 3
and 6 have somehow downgraded resolution to match uploading requirement
A photodissociation model for the morphology of the HI near OB associations in M33
We present an approach for analysing the morphology and physical properties
of Hi features near giant OB asso- ciations in M33, in the context of a model
whereby the Hi excess arises from photodissociation of the molecular gas in
remnants of the parent Giant Molecular Clouds (GMCs). Examples are presented
here in the environs of NGC604 and CPSDPZ204, two prominent Hii regions in M33.
These are the first results of a detailed analysis of the environs of a large
number of OB associations in that galaxy. We present evidence for "diffusion"
of the far-UV radiation from the OB association through a clumpy remnant GMC,
and show further that enhanced CO(1-0) emission appears preferentially
associated with GMCs of higher volume density.Comment: Accepted to Ap
Star formation thresholds and galaxy edges: why and where
We study global star formation thresholds in the outer parts of galaxies by
investigating the stability of disk galaxies embedded in dark halos. The disks
are self-gravitating, contain metals and dust, and are exposed to UV radiation.
We find that the critical surface density for the existence of a cold
interstellar phase depends only weakly on the parameters of the model and
coincides with the empirically derived surface density threshold for star
formation. Furthermore, it is shown that the drop in the thermal velocity
dispersion associated with the transition from the warm to the cold gas phase
triggers gravitational instability on a wide range of scales. The presence of
strong turbulence does not undermine this conclusion if the disk is
self-gravitating. Models based on the hypothesis that the onset of thermal
instability determines the star formation threshold in the outer parts of
galaxies can reproduce many observations, including the threshold radii, column
densities, and the sizes of stellar disks as a function of disk scale length
and mass. Finally, prescriptions are given for implementing star formation
thresholds in (semi-)analytic models and three-dimensional hydrodynamical
simulations of galaxy formation.Comment: 16 pages, 6 figures, accepted for publication in the Astrophysical
Journal. Version 2: text significantly revised (major improvements), physics
unchanged. Version 3: minor correction
Kinematics and dynamics of the "superthin" edge-on disk galaxy IC 5249
We present spectroscopic observations of the stellar motions in the disk of
the superthin edge-on spiral galaxy IC 5249 and re-analyse synthesis
observations of the HI. We find that the HI rotation curve rises initially to
about 90-100 km/s, but contrary to the conclusion of Abe et al. (1999) flattens
well before the edge of the optical disk. Over most part of the optical disk we
have been able to establish that the (tangential) stellar velocity dispersion
is 25-30 km/s. From earlier surface photometry we adopt a value for the radial
scalelength of the disk of 7 +/- 1 kpc, a vertical scaleheight of 0.65 +/- 0.05
kpc and a disk truncation radius of 17 +/- 1 kpc. The very thin appearance of
IC 5249 on the sky is the result of a combination of a low (face-on) surface
brightness, a long scalelength and a a sharp truncation at only about 2.5
scalelengths. From various arguments we derive the stellar velocity dispersions
at a position one radial scalelength as sigma_R about 35 km/s, sigma_{theta}
about 30 km/s and sigma_z about 20 km/s. This is comparable to the values for
the disk of our Galaxy in the solar neighborhood.Comment: 11 pages and 8 figures. Accepted for Astronomy and Astrophysics
(September 2001
Deep CCD Surface Photometry of the Edge-On Spiral NGC 4244
We have obtained deep surface photometry of the edge-on spiral galaxy NGC
4244. Our data reliably reach 27.5 R magnitude arcsec^{-2}, a significant
improvement on our earlier deep CCD surface photometry of other galaxies. NGC
4244 is a nearby Scd galaxy whose total luminosity is approximately one
magnitude fainter than the peak of the Sc luminosity function. We find that it
has a simple structure: a single exponential disk, with a scale height h_Z =
246 +/- 2 pc, a scale length h_R = 1.84 +/- 0.02 kpc and a disk cutoff at a
radius R(max) = 10.0 kpc (5.4 scale lengths). We confirm a strong cutoff in the
stellar disk at R(max), which happens over only 1 kpc. We do not see any
statistically significant evidence for disk flaring with radius. Unlike the
more luminous Sc galaxies NGC 5907 and M 33, NGC 4244 does not show any
evidence for a second component, such as a thick disk or halo, at mu(R) < 27.5
magnitude arcsec^{-2}.Comment: 36 pages, including 12 figures; accepted for publication in Sept 99
A
The Shape of Dark Matter Haloes II. The Galactus HI Modelling & Fitting Tool
We present a new HI modelling tool called \textsc{Galactus}. The program has
been designed to perform automated fits of disc-galaxy models to observations.
It includes a treatment for the self-absorption of the gas. The software has
been released into the public domain. We describe the design philosophy and
inner workings of the program. After this, we model the face-on galaxy NGC2403,
using both self-absorption and optically thin models, showing that
self-absorption occurs even in face-on galaxies. It is shown that the maximum
surface brightness plateaus seen in Paper I of this series are indeed signs of
self-absorption. The apparent HI mass of an edge-on galaxy can be drastically
lower compared to that same galaxy seen face-on. The Tully-Fisher relation is
found to be relatively free from self-absorption issues.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-II.pd
NGC 300: an extremely faint, outer stellar disk observed to 10 scale lengths
We have used the Gemini Multi-object Spectrograph (GMOS) on the Gemini South
8m telescope in exceptional conditions (0.6" FWHM seeing) to observe the outer
stellar disk of the Sculptor group galaxy NGC 300 at two locations. At our
point source detection threshold of r' = 27.0 (3-sigma) mag, we trace the
stellar disk out to a radius of 24', or 2.2 R_25 where R_25 is the 25
mag/arcsec**2 isophotal radius. This corresponds to about 10 scale lengths in
this low-luminosity spiral (M_B = -18.6), or about 14.4 kpc at a cepheid
distance of 2.0 +/- 0.07 Mpc. The background galaxy counts are derived in the
outermost field, and these are within 10% of the mean survey counts from both
Hubble Deep Fields. The luminosity profile is well described by a nucleus plus
a simple exponential profile out to 10 optical scale lengths. We reach an
effective surface brightness of 30.5 mag/arcsec**2 (2-sigma) at 55%
completeness which doubles the known radial extent of the optical disk. These
levels are exceedingly faint in the sense that the equivalent surface
brightness in B or V is about 32 mag/arcsec**2. We find no evidence for
truncation of the stellar disk. Only star counts can be used to reliably trace
the disk to such faint levels, since surface photometry is ultimately limited
by nonstellar sources of radiation. In the Appendix, we derive the expected
surface brightness of one such source: dust scattering of starlight in the
outer disk.Comment: ApJ accepted -- 30 pages, 13 figures -- see
ftp://www.aao.gov.au/pub/local/jbh/astro-ph/N300 for full resolution figures
and preprin
Structural Parameters of Thin and Thick Disks in Edge-On Disk Galaxies
We analyze the global structure of 34 late-type, edge-on, undisturbed, disk
galaxies spanning a wide range of mass. We measure structural parameters for
the galaxies using two-dimensional least-squares fitting to our -band
photometry. The fits require both a thick and a thin disk to adequately fit the
data. The thick disks have larger scale heights and longer scale lengths than
the embedded thin disks, by factors of ~2 and ~1.25, respectively. The observed
structural parameters agree well with the properties of thick and thin disks
derived from star counts in the Milky Way and from resolved stellar populations
in nearby galaxies. We find that massive galaxies' luminosities are dominated
by the thin disk. However, in low mass galaxies (Vc < 120 km/s), thick disk
stars contribute nearly half of the luminosity and dominate the stellar mass.
Thus, although low mass dwarf galaxies appear blue, the majority of their stars
are probably quite old.
Our data are most easily explained by a formation scenario where the thick
disk is assembled through direct accretion of stellar material from merging
satellites while the thin disk is formed from accreted gas. The baryonic
fraction in the thin disk therefore constrains the gas-richness of the merging
pre-galactic fragments. If we include the mass in HI as part of the thin disk,
the thick disk contains <10% of the baryons in high mass galaxies, and ~25-30%
of the baryons in low-mass galaxies. We discuss how our trends can be explained
by supernova-driven outflow at early times as well as the possibilities for
predicting abundance trends in thick disks, and for removing discrepancies
between semi-analytic galaxy formation models and the observed colors of low
mass galaxies. (abstract abridged)Comment: 25 pages, 24 figures, accepted for publication in A
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