261 research outputs found
A dust component 2 kpc above the plane in NGC 891
The halo of NGC 891 has been the subject of studies for more than a decade.
One of its most striking features is the large asymmetry in H emission.
In this letter, we will take a quantitative look at this asymmetry at different
wavelengths for the first time. We suggest that NGC 891 is intrinsically almost
symmetric and the large asymmetry in H emission is mostly due to dust
attenuation. We will quantify the additional optical depth needed to cause the
observed asymmetry in this model. By comparing large strips on the North East
side of the galaxy with strips covering the same area in the South West we can
quantify and analyze the asymmetry in the different wavelengths. From the 24
m emission we find that the intrinsic asymmetry in star formation in NGC
891 is small i.e., . The additional asymmetry in H is modeled
as additional symmetric dust attenuation which extends up to 40\arcsec
(1.9 kpc) above the plane of the galaxy with a mid-plane value of =0.8
and a scale height of 0.5 kpcComment: A&A in press. 5 pages, 3 figure
The Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. I. Observations
The non-circular streaming motions in barred galaxies are sensitive to the
mass of the bar and can be used to lift the degeneracy between disk and dark
matter halo encountered when fitting axisymmetric rotation curves of disk
galaxies. In this paper, we present photometric and kinematic observations of
NGC 4123, a barred galaxy of modest size (V_rot = 130 km/sec, L = 0.7 L_*),
which reveal strong non-circular motions. The bar has straight dust lanes and
an inner Lindblad resonance. The disk of NGC 4123 has no sign of truncation out
to 10 scale lengths, and star-forming regions are found well outside R_25. A
Fabry-Perot H-alpha velocity field shows velocity jumps of >100 km/sec at the
location of the dust lanes within the bar, indicating shocks in the gas flow.
VLA observations yield the velocity field of the H I disk. Axisymmetric mass
models yield good fits to the rotation curve outside the bar regionfor disk
I-band M/L of 2.25 or less, and dark halos with either isothermal or power-law
profiles can fit the data well. In a companion paper, we model the full 2-D
velocity field, including non-circular motions, to determine the stellar M/L
and the mass of the dark halo.Comment: accepted by ApJ, 16 pages, 9 figures (1 color), uses emulateapj.sty,
onecolfloat.st
The Outer Disks of Early-Type Galaxies. I. Surface-Brightness Profiles of Barred Galaxies
We present a study of 66 barred, early-type (S0-Sb) disk galaxies, focused on
the disk surface brightness profile outside the bar region and the nature of
Freeman Type I and II profiles, their origins, and their possible relation to
disk truncations. This paper discusses the data and their reduction, outlines
our classification system, and presents -band profiles and classifications
for all galaxies in the sample.
The profiles are derived from a variety of different sources, including the
Sloan Digital Sky Survey (Data Release 5). For about half of the galaxies, we
have profiles derived from more than one telescope; this allows us to check the
stability and repeatability of our profile extraction and classification. The
vast majority of the profiles are reliable down to levels of mu_R ~ 27 mag
arcsec^-2; in exceptional cases, we can trace profiles down to mu_R > 28. We
can typically follow disk profiles out to at least 1.5 times the traditional
optical radius R_25; for some galaxies, we find light extending to ~ 3 R_25.
We classify the profiles into three main groups: Type I (single-exponential),
Type II (down-bending), and Type III (up-bending). The frequencies of these
types are approximately 27%, 42%, and 24%, respectively, plus another 6% which
are combinations of Types II and III. We further classify Type II profiles by
where the break falls in relation to the bar length, and in terms of the
postulated mechanisms for breaks at large radii ("classical trunction" of star
formation versus the influence of the Outer Lindblad Resonance of the bar). We
also classify the Type III profiles by the probable morphology of the outer
light (disk or spheroid). Illustrations are given for all cases. (Abridged)Comment: 41 pages, 26 PDF figures. To appear in the Astronomical Journal.
Version with full-resolution figures available at
http://www.mpe.mpg.de/~erwin/research
A Pressure Anomaly for HII Regions in Irregular Galaxies
The pressures of giant HII regions in 6 dwarf Irregular galaxies are a factor
of ~10 larger than the average pressures of the corresponding galaxy disks,
obtained from the stellar and gaseous column densities. Either the visible HII
regions in these dwarfs are all so young that they are still expanding, or
there is an unexpected source of disk self-gravity that increases the
background pressure. We consider the possibility that the additional
self-gravity comes from disk dark matter, but suggest this is unlikely because
the vertical scale heights inferred for Irregular galaxies are consistent with
the luminous matter alone. Some of the HII region overpressure is probably the
result of local peaks in the gravitational field that come from large gas
concentrations, many of which are observed directly. These peaks also explain
the anomalously low average column density thresholds for star formation that
were found earlier for Irregular galaxies, and they permit the existence of a
cool HI phase as the first step toward dense molecular cores. Many of the HII
regions could also be so strongly over-pressured that they will expand for a
long time. In this case, the observed population would be only 7% of the total,
and the aging HII regions, now too faint to see, should occupy nearly the
entire dwarf galaxy volume. Such prolonged HII region expansion would explain
the origin of the giant HI shells that are seen in these galaxies, and account
for the lack of bright central clusters inside these shells.Comment: 27 pages, 4 figures, Astrophysical Journal, 540, Sep 10, 2000, in
pres
Inner Molecular Rings in Barred Galaxies: BIMA SONG CO Observations
Although inner star-forming rings are common in optical images of barred
spiral galaxies, observational evidence for the accompanying molecular gas has
been scarce. In this paper we present images of molecular inner rings, traced
using the CO (1-0) emission line, from the
Berkeley-Illinois-Maryland-Association Survey of Nearby Galaxies (BIMA SONG).
We detect inner ring CO emission from all five SONG barred galaxies classified
as inner ring (type (r)). We also examine the seven SONG barred galaxies
classified as inner spiral (type (s)); in one of these, NGC 3627, we find
morphological and kinematic evidence for a molecular inner ring. Inner ring
galaxies have been classified as such based on optical images, which emphasize
recent star formation. We consider the possibility that there may exist inner
rings in which star formation efficiency is not enhanced. However, we find that
in NGC 3627 the inner ring star formation efficiency is enhanced relative to
most other regions in that galaxy. We note that the SONG (r) galaxies have a
paucity of CO and H alpha emission interior to the inner ring (except near the
nucleus), while NGC 3627 has relatively bright bar CO and H alpha emission; we
suggest that galaxies with inner rings such as NGC 3627 may be misclassified if
there are significant amounts of gas and star formation in the bar.Comment: To be published in the Astrophysical Journal, July 2002 A version of
the paper with full resolution figures is available at:
http://www.astro.umd.edu/~mregan/ms.ps.g
Stellar populations across the NGC4244 truncated galactic disk
We use HST/ACS to study the resolved stellar populations of the nearby,
nearly edge-on galaxy NGC4244 across its outer disk surface density break. The
stellar photometry allows us to study the distribution of different stellar
populations and reach very low equivalent surface brightnesses. We find that
the break occurs at the same radius for young, intermediate age, and old stars.
The stellar density beyond the break drops sharply by a factor of at least 600
in 5 kpc. The break occurs at the same radius independent of height above the
disk, but is sharpest in the midplane and nearly disappears at large heights.
These results make it unlikely that truncations are caused by a star formation
threshold alone: the threshold would have to keep the same radial position from
less than 100 Myr to 10 Gyr ago, in spite of potential disturbances such as
infall and redistribution of gas by internal processes. A dynamical
interpretation of truncation formation is more likely such as due to angular
momentum redistribution by bars or density waves, or heating and stripping of
stars caused by the bombardment of dark matter sub-halos. The latter
explanation is also in quantitative agreement with the small diffuse component
we see around the galaxy.Comment: ApJ Letters, in press. Five pages, 2 figure
The cold gaseous halo of NGC 891
We present HI observations of the edge-on galaxy NGC 891. These are among the
deepest ever performed on an external galaxy. They reveal a huge gaseous halo,
much more extended than seen previously and containing almost 30 % of the HI.
This HI halo shows structures on various scales. On one side, there is a
filament extending (in projection) up to 22 kpc vertically from the disk. Small
(M_HI ~ 10^6 Msol) halo clouds, some with forbidden (apparently
counter-rotating) velocities, are also detected. The overall kinematics of the
halo gas is characterized by differential rotation lagging with respect to that
of the disk. The lag, more pronounced at small radii, increases with height
from the plane. There is evidence that a significant fraction of the halo is
due to a galactic fountain. Accretion from intergalactic space may also play a
role in building up the halo and providing low angular momentum material needed
to account for the observed rotation lag. The long HI filament and the
counter-rotating clouds may be direct evidence of such accretion.Comment: Accepted for publication in The Astronomical Journal. High-resolution
version available at http://www.astron.nl/~oosterlo/n891Pape
A Dust-Penetrated Classification Scheme for Bars as Inferred from their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spirals is a
fundamental aspect of the Hubble galaxy classification system. This ``tuning
fork'' view was revised by de Vaucouleurs, whose classification volume
recognized apparent ``bar strength'' (SA, SAB, SB) as a continuous property of
galaxies called the ``family''. However, the SA, SAB, and SB families are
purely visual judgments that can have little bearing on the actual bar strength
in a given galaxy. Until very recently, published bar judgments were based
exclusively on blue light images, where internal extinction or star formation
can either mask a bar completely or give the false impression of a bar in a
nonbarred galaxy. Near-infrared camera arrays, which principally trace the old
stellar populations in both normal and barred galaxies, now facilitate a
quantification of bar strength in terms of their gravitational potentials and
force fields. In this paper, we show that the maximum value, Qb, of the ratio
of the tangential force to the mean radial force is a quantitative measure of
the strength of a bar. Qb does not measure bar ellipticity or bar shape, but
rather depends on the actual forcing due to the bar embedded in its disk. We
show that a wide range of true bar strengths characterizes the category ``SB'',
while de Vaucouleurs category ``SAB'' corresponds to a much narrower range of
bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar
classes into a dust-penetrated classification system for spiral galaxies.Comment: Accepted for publication in the Astrophysical Journal (LaTex, 30
pages + 3 figures); Figs. 1 and 3 are in color and are also available at
http://bama.ua.edu/~rbuta/bars
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