539 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
The stars and gas in outer parts of galaxy disks: Extended or truncated -- flat or warped?
I review observations of truncations of stellar disks and models for their
origin, compare observations of truncations in moderately inclined galaxies to
those in edge-on systems and discuss the relation between truncations and
HI-warps and their systematics and origin. Truncations are a common feature in
edge-on stellar disks, but the relation of truncations in face-on to those in
edge-on galaxies needs further clarification. The origin of truncations is most
likely related to a maximum in the specific angular momentum in the material
that formed the stellar disks, but this model does probably require some
redistribution of angular momentum. HI-warps start just beyond the truncation
radius and disks and warps appear distinct components. This suggests that inner
disks form initially and settle as rigid, very flat structures, while HI-warps
result from later infall of gas with a different orientation of the angular
momentum.Comment: Invited Review at the Vatican Symposium: Formation and Evolution of
Galaxy Disks, October 2007, proceeding editors Jose G. Funes, SJ and Enrico
M. Corsin
Lessons from the Milky Way: the Kapteyn Universe
Jacobus Cornelius Kapteyn (1851-1922) presented a model for the distribution
of stars in space together with a dynamical interpretation in terms of an
equilibrium between the gravitational field of the stars and their random
motion and rotation. In the vertical direction Kapteyn's results are
substantially correct. Usually the Kapteyn Universe is described as being
flawed due to neglect of interstellar absorption. Kapteyn was led to adopt this
on the basis of widely accepted evidence by Shapley on an absence of reddening
of stars in globular clusters. But another, equally important misconception was
Kapteyn's interpretation of the two Star Streams as manifestations of two
groups of stars rotating around a center in opposite directions. This was
supported by the observation of very different mixes in stellar types in the
two streams. Had Kapteyn adopted the absorption as he himself had determined it
he would not have been able to arrive at a consistent picture.Comment: To appear in LESSONS FROM THE LOCAL GROUP: A Conference in Honour of
David Block and Bruce Elmegreen, eds. Freeman, K.C., Elmegreen, B.G., Block,
D.L.and Woolway, M., Springer: New Yor
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
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
Truncations of stellar disk and warps of HI-layers in edge-on spiral galaxies
Edge-on spiral galaxies often have stellar disks with relatively sharp
truncations and warped HI-layers in the outer parts. Warps appear to start
preferentially near the optical boundaries of the disks. Here we make a
comparative study of warps and truncations in edge-on galaxies. The
Garc\'{\i}a-Ruiz et al. (2002) sample with extensive HI-mapping is complemented
with luminosity distributions from the Sloan Digital Sky Survey. The method to
identify truncations has been tested using the sample of edge-on galaxies of
van der Kruit & Searle. Results are: (i.) The majority (17 out of 23) of the
galaxies show evidence for truncations. (ii.) When an HI-warp is present it
starts at 1.1 truncation radii. (iii.) This supplements the rules for warps
formulated by Briggs (1990), if the Holmberg radius is replaced for edge-on
systems with the truncation radius. (iv.) The truncation radius and the onset
of the warps coincide radially with features in the rotation curve and the HI
surface density. The latter is also true for less inclined systems. (v.) Inner
disks are very flat and the onset of the warp just beyond the truncation radius
is abrupt and discontinuous. These findings suggest that the inner flat disk
and the outer warped disk are distinct components with quite different
formation histories, probably involving quite different epochs. The inner disk
forms initially and the warped outer disk forms as a result of much later
infall of gas with a higher angular momentum in a different orientation. In an
appendix the Holmberg radius is discussed. Contrary to what is often assumed
Holmberg radii are not corrected for inclination.Comment: Accepted for pubication by Astronomy & Astrophysic
A tale of two galaxies: light and mass in NGC891 and NGC7814
The two edge-on galaxies NGC891 and NGC7814 are representative of two extreme
morphologies: the former is disk-dominated while the latter is almost entirely
bulge-dominated. It has been argued (van der Kruit 1983) that since the two
galaxies, which are optically so different, have similar rotation curves their
total mass distributions cannot be related in any way to the light
distributions. This would lead to the conclusion that dark matter is the
dominating component of the mass. We have derived new rotation curves from
recent, high-sensitivity HI observations and have found that the shapes of the
rotation curves are significantly different for the two galaxies. They indicate
that in NGC7814 the mass is more concentrated to the centre as compared to
NGC891. This reflects the distribution of light which is more centrally
concentrated in NGC7814 than in NGC891. Mass and light do seem to be closely
related. This is confirmed by the analysis of the rotation curves in mass
components: solutions close to the maximum light (bulge + disk) do provide
excellent fits. In NGC891 bulge and disk can explain the rotation curve without
any need for dark matter out to ~15 kpc. In NGC7814 the bulge dominates in the
inner parts; further out the rotation curve is well reproduced by a maximum
disk but its M/L ratio is excessively high. A substantial dark matter
contribution, closely coupled to the luminous component, seems, therefore,
necessary.Comment: 10 pages, 9 figures, accepted for publication in A&
Anti-Truncation of Disks in Early-Type Barred Galaxies
The disks of spiral galaxies are commonly thought to be truncated: the radial
surface brightness profile steepens sharply beyond a certain radius (3--5
inner-disk scale lengths). Here we present the radial brightness profiles of a
number of barred S0--Sb galaxies with the opposite behavior: their outer
profiles are distinctly shallower in slope than the main disk profile. We term
these "anti-truncations"; they are found in at least 25% of a larger sample of
barred S0--Sb galaxies. There are two distinct types of anti-truncations. About
one-third show a fairly gradual transition and outer isophotes which are
progressively rounder than the main disk isophotes, suggestive of a disk
embedded within a more spheroidal outer zone -- either the outer extent of the
bulge or a separate stellar halo. But the majority of the profiles have rather
sharp surface-brightness transitions to the shallower, outer exponential
profile and, crucially, outer isophotes which are not significantly rounder
than the main disk; in the Sab--Sb galaxies, the outer isophotes include
visible spiral arms. This suggests that the outer light is still part of the
disk. A subset of these profiles are in galaxies with asymmetric outer
isophotes (lopsided or one-armed spirals), suggesting that interactions may be
responsible for at least some of the disklike anti-truncations.Comment: LaTeX, 5 pages, 4 EPS figures. To appear in Astrophysical Journal
Letters. Minor typos fixed; proper acknowledgments adde
Stellar disk truncations at high-z: probing inside-out galaxy formation
We have conducted a systematic search for stellar disk truncations in
disk-like galaxies at intermediate redshift (z<1.1) using the Hubble Ultra Deep
Field (UDF) data. We use the position of the truncation as a direct estimator
of the size of the stellar disk. After accounting for the surface brightness
evolution of the galaxies, our results suggest that the radial position of the
truncations has increased with cosmic time by ~1-3 kpc in the last ~8 Gyr. This
result indicates a small to moderate (~25%) inside-out growth of the disk
galaxies since z~1.Comment: 2 figures, ApJL in pres
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