47 research outputs found
Observations of extended and counterrotating disks of ionized gas in S0 galaxies
While many E/S0 galaxies have been found to show emission line spectra in their nuclear regions, the question of the presence and nature of extended disks of ionized gas in these galaxies has been addressed only in recent years. Typically the ionized gas is detected in the inner region on a scale of approx. 1 kpc (e.g., Phillips et al. 1986, Caldwell 1984). Here researchers present evidence that the disks of ionized gas of at least some S0 galaxies are much more extended than previously believed. In addition, with the detection of the counterrotation of gas and stars in NGC 7007 they strengthen the basis for arguments that the source of gas in S0 galaxies is externa
The multicomponent structure of bulges
The morphology of disk galaxies is usually described by two major components, the centrally concentrated spheroidal component, called the bulge, and an oblate disk. The ratio of there contribution to the total luminosity - the bulge-to-disk ratio - is one of the parameters characterizing the Hubble sequence. Following de Vaucouleurs (1948), for most galaxies the radial distribution of the outer spheroid is fairly well described by the r exp 1/4 law I(r)=I(sub 0) exp(-(alpha)r), whereas the radial luminosity distribution of the disk follows an exponential law: I(r)=I(sub 0) exp(-alpha(r exp 1/4)) (Freeman 1970), with r the radial distance from the center. I(sub 0) and alpha are characteristic constants for each individual galaxy. Parameters for the structural properties of these components give important constraints for models of the formation and evolution of galaxies. Therefore we have tried to decompose disk and bulge components from high S/N CCD observations of a sample of edge-on disk galaxies. A common procedure for the decomposition is to model one component in a region where it dominates and subtract it from the combined light distribution. This technique was successfully carried out e.g. by van der Kruit & Earl (1981, 1982) and Wakamatsu & Hamabe (1984, 1989). Here we present two more examples of bulge-dominated edge-on SO galaxies, namely ESO 506-G33 and NGC 7123, which show an additional small and concentrated central component besides disk and 'bulge'
Photometric parameters of edge-on galaxies from 2MASS observations
To analyze the vertical structure of edge-on galaxies, we have used images of
a large uniform sample of flat galaxies that have been taken during the 2MASS
all-sky survey. The photometric parameters, such as the radial scale length,
the vertical scale height, and the deprojected central surface brightness of
galactic disks have been obtained. We find a strong correlation between the
central surface brightness and the ratio of the vertical scale height to the
vertical scale length: the thinner the galaxy, the lower the central surface
brightness of its disk. The vertical scale height does not increase
systematically with the distance from the galaxy center in the frames of this
sample.Comment: 7 pages, 5 postscript figures. To be published in Astronomy &
Astrophysics, v. 389, p.795, 200
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
The stellar disk thickness of LSB galaxies
We present surface photometry results for a sample of eleven edge-on galaxies
observed with the 6m telescope at the Special Astrophysical Observatory
(Russia). The photometric scale length, scale height, and central surface
brightness of the stellar disks of our sample galaxies are estimated. We show
that four galaxies in our sample, which are visually referred as objects of the
lowest surface brightness class in the Revised Flat Galaxies Catalog, have bona
fide low surface brightness (LSB) disks. We find from the comparison of
photometric scales that the stellar disks of LSB galaxies are thinner than
those of high surface brightness (HSB) ones. There is a clear correlation
between the central surface brightness of the stellar disk and its vertical to
radial scale ratio. The masses of spherical subsystems (dark halo + bulge) and
the dark halo masses are obtained for the sample galaxies based on the
thickness of their stellar disks. The LSB galaxies tend to harbor more massive
spherical subsystems than the HSB objects, whereas no systematic difference in
the dark halo masses between LSB and HSB galaxies is found. At the same time,
the inferred mass-to-luminosity ratio for the LSB disks appears to be
systematically higher than for HSB disks.Comment: 33 pages with 17 Postscript figures, uses aastex.cls, accepted by Ap
The puzzle about the radial cut-off in galactic disks
The stellar disk in a spiral galaxy is believed to be truncated physically
because the disk surface brightness is observed to fall faster than that for an
exponential in the outer, faint regions. We review the literature associated
with this phenomenon and find that a number of recent observations contradict
the truncation picture. Hence we question the very existence of a physical
outer cut-off in stellar disks. We show, in this paper, that the observed drop
in the surface brightness profiles in fact corresponds to a negligible decrease
in intensity, and that this minor change at the faint end appears to be
exaggerated on a log-normal plot. Since minor deviations from a perfect
exponential are common throughout the disk, we suggest that such a deviation at
the faint end could easily give rise to the observed sharp drop.Comment: 4 pages, 3 .eps figures, Astron. & Astrophys Letters, In pres
Radial Profiles of Star Formation in the Far Outer Regions of Galaxy Disks
Star formation in galaxies is triggered by a combination of processes,
including gravitational instabilities, spiral wave shocks, stellar compression,
and turbulence compression. Some of these persist in the far outer regions
where the column density is far below the threshold for instabilities, making
the outer disk cutoff somewhat gradual. We show that in a galaxy with a single
exponential gas profile the star formation rate can have a double exponential
with a shallow one in the inner part and a steep one in the outer part. Such
double exponentials have been observed recently in the broad-band intensity
profiles of spiral and dwarf Irregular galaxies. The break radius in our model
occurs slightly outside the threshold for instabilities provided the Mach
number for compressive motions remains of order unity to large radii. The ratio
of the break radius to the inner exponential scale length increases for higher
surface brightness disks because the unstable part extends further out. This is
also in agreement with observations. Galaxies with extended outer gas disks
that fall more slowly than a single exponential, such as 1/R, can have their
star formation rate scale approximately as a single exponential with radius,
even out to 10 disk scale lengths. Halpha profiles should drop much faster than
the star formation rate as a result of the rapidly decreasing ambient density.Comment: To appear in ApJ. Available from
ftp.lowell.edu/pub/dah/papers/sfouterdisks
Stellar Disk Truncations: Where do we stand ?
In the light of several recent developments we revisit the phenomenon of
galactic stellar disk truncations. Even 25 years since the first paper on outer
breaks in the radial light profiles of spiral galaxies, their origin is still
unclear. The two most promising explanations are that these 'outer edges'
either trace the maximum angular momentum during the galaxy formation epoch, or
are associated with global star formation thresholds. Depending on their true
physical nature, these outer edges may represent an improved size
characteristic (e.g., as compared to D_25) and might contain fossil evidence
imprinted by the galaxy formation and evolutionary history. We will address
several observational aspects of disk truncations: their existence, not only in
normal HSB galaxies, but also in LSB and even dwarf galaxies; their detailed
shape, not sharp cut-offs as thought before, but in fact demarcating the start
of a region with a steeper exponential distribution of starlight; their
possible association with bars; as well as problems related to the
line-of-sight integration for edge-on galaxies (the main targets for truncation
searches so far). Taken together, these observations currently favour the
star-formation threshold model, but more work is necessary to implement the
truncations as adequate parameters characterising galactic disks.Comment: LaTeX, 10 pages, 6 figures, presented at the "Penetrating Bars
through Masks of Cosmic Dust" conference in South Africa, proceedings
published by Kluwer, and edited by Block, D.L., Freeman, K.C., Puerari, I., &
Groess, R; v3 to match published versio
Are rotation curves in NGC 6946 and the Milky Way magnetically supported?
Following the model of magnetically supported rotation of spiral galaxies,
the inner disk rotation is dominated by gravity but magnetism is not negligible
at radii where the rotation curve becomes flat, and indeed becomes dominant at
very large radii. Values of the order of 1 G, or even less, produce a
centripetal force when the absolute value of the slope of the curve [,
R] (azimuthal field strength versus radius) is less than . The
-profile is called the critical profile. From this hypothesis, the
following is to be expected: at large radii, a ``subcritical'' profile (slope
flatter than ); at still larger radii a -profile becoming
asymptotically critical as the density becomes asymptotically vanishing. Recent
observations of magnetic fields in NGC 6946 and the Milky Way are in very good
agreement with these predictions. This magnetic alternative requires neither
galactic dark matter (DM) nor modification of fundamental laws of physics, but
it is not in conflict with these hypotheses, especially with the existence of
cosmological cold dark matter (CDM).Comment: 11 pages, 2 figures, accepted for publication in Astron. Astrophy
Baryonic Collapse within Dark Matter Halos and the Formation of Gaseous Galactic Disks
This paper constructs an analytic framework for calculating the assembly of
galactic disks from the collapse of gas within dark matter halos, with the goal
of determining the surface density profiles. Gas parcels (baryons) fall through
the potentials of dark matter halos on nearly ballistic, zero energy orbits and
collect in a rotating disk. The dark matter halos have a nearly universal form,
as determined previously through numerical simulations. The calculation is
first carried out for a variety of pre-collapse mass distributions and rotation
profiles, including polytropic spheres in hydrostatic equilibrium with the halo
potential. The resulting disk surface density profiles have nearly power-law
forms, with well-defined edges. This idealized scenario is generalized to
include non-spherical starting states and multiple accretion events (due to gas
being added to the halo via merger events). This latter complication is
explored in detail and considers a log-normal distribution for the angular
momenta of the pre-collapse states. If this distribution is wide, then the
composite surface density approaches a universal power-law form, independent of
the shape of the constituent profiles. When the angular momentum distribution
has an intermediate width, the composite surface density attains a nearly
exponential form, roughly consistent with profiles of observed galaxies.Comment: 47 pages including 12 figures, accepted to Ap