420 research outputs found
Uncovering Spiral Structure in Flocculent Galaxies
We present K'(2.1 micron) observations of four nearby flocculent spirals,
which clearly show low-level spiral structure and suggest that kiloparsec-scale
spiral structure is more prevalent in flocculent spirals than previously
supposed. In particular, the prototypical flocculent spiral NGC 5055 is shown
to have regular, two-arm spiral structure to a radius of 4 kpc in the near
infrared, with an arm-interarm contrast of 1.3. The spiral structure in all
four galaxies is weaker than that in grand design galaxies. Taken in unbarred
galaxies with no large, nearby companions, these data are consistent with the
modal theory of spiral density waves, which maintains that density waves are
intrinsic to the disk. As an alternative, mechanisms for driving spiral
structure with non-axisymmetric perturbers are also discussed. These
observations highlight the importance of near infrared imaging for exploring
the range of physical environments in which large-scale dynamical processes,
such as density waves, are important.Comment: 12 pages AASTeX; 3 compressed PS figures can be retrieved from
ftp://ftp.astro.umd.edu/pub/michele as file thornley.tar (1.6Mbytes).
Accepted to Ap.J. Letters.(Figures now also available here, and from
ftp://ftp.astro.umd.edu/pub/michele , in GIF format.
Molecular Gas in Elliptical Galaxies: Distribution and Kinematics
I present interferometric images (approx. 7" resolution) of CO emission in
five elliptical galaxies and nondetections in two others. These data double the
number of elliptical galaxies whose CO emission has been fully mapped. The
sample galaxies have 10^8 to 5x10^9 solar masses of molecular gas distributed
in mostly symmetric rotating disks with diameters of 2 to 12 kpc. Four out of
the five molecular disks show remarkable alignment with the optical major axes
of their host galaxies. The molecular masses are a few percent of the total
dynamical masses which are implied if the gas is on circular orbits. If the
molecular gas forms stars, it will make rotationally supported stellar disks
which will be very similar in character to the stellar disks now known to be
present in many ellipticals. Comparison of stellar kinematics to gas kinematics
in NGC 4476 implies that the molecular gas did not come from internal stellar
mass loss because the specific angular momentum of the gas is about three times
larger than that of the stars.Comment: 47 pages, 6 tables, 27 figures. Accepted by AJ, scheduled for August
200
Molecular gas in nearby powerful radio galaxies
We report the detection of CO(1-0) and CO(2-1) emission from the central
region of nearby 3CR radio galaxies (z 0.03). Out of 21 galaxies, 8 have
been detected in, at least, one of the two CO transitions. The total molecular
gas content is below 10 \msun. Their individual CO emission exhibit, for 5
cases, a double-horned line profile that is characteristic of an inclined
rotating disk with a central depression at the rising part of its rotation
curve. The inferred disk or ring distributions of the molecular gas is
consistent with the observed presence of dust disks or rings detected optically
in the cores of the galaxies. We reason that if their gas originates from the
mergers of two gas-rich disk galaxies, as has been invoked to explain the
molecular gas in other radio galaxies, then these galaxies must have merged a
long time ago (few Gyr or more) but their remnant elliptical galaxies only
recently (last 10 years or less) become active radio galaxies. Instead, we
argue the the cannibalism of gas-rich galaxies provide a simpler explanation
for the origin of molecular gas in the elliptical hosts of radio galaxies (Lim
et al. 2000). Given the transient nature of their observed disturbances, these
galaxies probably become active in radio soon after the accretion event when
sufficient molecular gas agglomerates in their nuclei.Comment: 6 pages, including 2 figures,in "QSO Hosts and Their Environments",
ed. I. Marquez, in pres
Spitzer Uncovers Active Galactic Nuclei Missed by Optical Surveys in 7 Late-type Galaxies
We report the discovery using Spitzers high resolution spectrograph of 7
Active Galactic Nuclei (AGN) in a sample of 32 late-type galaxies that show no
definitive signatures of AGN in their optical spectra. Our observations suggest
that the AGN detection rate in late-type galaxies is possibly 4 times larger
than what optical spectroscopic observations alone suggest. We demonstrate
using photoionization models with an input AGN and an extreme EUV-bright
starburst ionizing radiation field that the observed mid-infrared line ratios
cannot be replicated unless an AGN contribution, in some cases as little as 10%
of the total galaxy luminosity, is included. These models show that when the
fraction of the total luminosity due to the AGN is low, optical diagnostics are
insensitive to the presence of the AGN. In this regime of parameter space, the
mid-infrared diagnostics offer a powerful tool for uncovering AGN missed by
optical spectroscopy. The AGN bolometric luminosities in our sample range from
~3 X 10^41 - ~2 X 10^43 ergs s^-1, which, based on the Eddington limit,
corresponds to a lower mass limit for the black hole that ranges from ~3 X
10^3Mdot to as high as ~1.5 X 10^5Mdot. These lower mass limits however do not
put a strain on the well-known relationship between the black hole mass and the
host galaxy's stellar velocity dispersion established in predominantly
early-type galaxies. Our findings add to the growing evidence that black holes
do form and grow in low-bulge environments and that they are significantly more
common than optical studies indicate.Comment: 8 figures, 17 pages, astro-ph\0801.2766 (Abel & Satyapal 2008; ApJ
accepted) and this posting designed to form a two-part investigatio
Non-linear Dependence of L(B) on L(FIR) and M(H2) among Spiral Galaxies and Effects of Tidal Interaction
Through the study of a carefully selected sample of isolated spiral galaxies,
we have established that two important global physical quantities for tracing
star forming activities, L(FIR) and M(H2), have non-linear dependence on
another commonly cited global quantity L(B). Furthermore we show that simple
power law relations can effectively describe these non-linear relations for
spiral galaxies spanning four orders of magnitude in FIR and M(H2) and nearly
three orders of magnitude in L(B). While the existence of non-linear dependence
of M(H2) (assuming a constant CO-to-H2 conversion) and L(FIR) on optical
luminosity L(B) has been previously noted in the literature, an improper
normalization of simple scaling by L(B) has been commonly used in many previous
studies to claim enhanced molecular gas content and induced activities among
tidally interacting and other types of galaxies. We remove these non-linear
effects using the template relations derived from the isolated galaxy sample
and conclude that strongly interacting galaxies do not have enhanced molecular
gas content, contrary to previous claims. With these non-linear relations among
L(B), L(FIR) and M(H2) properly taken into account, we confirm again that the
FIR emission and the star formation efficiency L(FIR)/M(H2) are indeed enhanced
by tidal interactions. Virgo galaxies show the same level of M(H2) and L(FIR)
as isolated galaxies. We do not find any evidence for enhanced star forming
activity among barred galaxies.Comment: 19 pages and 5 figures, requires AAS style files, ApJ, accepte
Multi-line Herschel/HIFI observations of water reveal infall motions and chemical segregation around high-mass protostars
(Abridged) We use HIFI maps of the 987 GHz H2O 2(02)-1(11) emission to
measure the sizes and shapes of 19 high-mass protostellar envelopes. To
identify infall, we use HIFI spectra of the optically thin C18O 9-8 and H2O-18
1(11)-0(00) lines. The high-J C18O line traces the warm central material and
redshifted H2O-18 1(11)-0(00) absorption indicates material falling onto the
warm core. We probe small-scale chemical differentiation by comparing H2O 752
and 987 GHz spectra with those of H2O-18.
Our measured radii of the central part of the H2O 2(02)-1(11) emission are
30-40% larger than the predictions from spherical envelope models, and axis
ratios are <2, which we consider good agreement. For 11 of the 19 sources, we
find a significant redshift of the H2O-18 1(11)-0(00) line relative to C18O
9-8. The inferred infall velocities are 0.6-3.2 km/s, and estimated mass inflow
rates range from 7e-5 to 2e-2 M0/yr, with the highest mass inflow rates
occurring toward the sources with the highest masses, and possibly the youngest
ages. The other sources show either expanding motions or H2O-18 lines in
emission. The H2O-18 1(11)-0(00) line profiles are remarkably similar to the
"differences" between the H2O 2(02)-1(11) and 2(11)-2(02) profiles, suggesting
that the H2O-18 line and the H2O 2(02)-1(11) absorption originate just inside
the radius where water evaporates from grains, typically 1000-5000 au from the
center. In some sources, the H2O-18 line is detectable in the outflow, where no
C18O emission is seen.
Together, the H2O-18 absorption and C18O emission profiles show that the
water abundance around high-mass protostars has at least three levels: low in
the cool outer envelope, high within the 100 K radius, and very high in the
outflowing gas. Thus, despite the small regions, the combination of lines
presented here reveals systematic inflows and chemical information about the
outflows.Comment: Accepted for publication in Astronomy & Astrophysics; 10 pages body +
10 pages appendi
Spitzer Observations of Low Luminosity Isolated and Low Surface Brightness Galaxies
We examine the infrared properties of five low surface brightness galaxies
(LSBGs) and compare them with related but higher surface brightness galaxies,
using Spitzer Space Telescope images and spectra. All the LSBGs are detected in
the 3.6 and 4.5um bands, representing the stellar population. All but one are
detected at 5.8 and 8.0um, revealing emission from hot dust and aromatic
molecules, though many are faint or point-like at these wavelengths. Detections
of LSBGs at the far-infrared wavelengths, 24, 70, and 160um, are varied in
morphology and brightness, with only two detections at 160um, resulting in
highly varied spectral energy distributions. Consistent with previous
expectations for these galaxies, we find that detectable dust components exist
for only some LSBGs, with the strength of dust emission dependent on the
existence of bright star forming regions. However, the far-infrared emission
may be relatively weak compared with normal star-forming galaxies.Comment: 20 pages, 8 figures, accepted to Ap
Why Are Rotating Elliptical Galaxies Less Elliptical at X-ray Frequencies?
If mass and angular momentum were conserved in cooling flows associated with
luminous, slowly rotating elliptical galaxies, the inflowing hot gas would spin
up, resulting in disks of cold gas and X-ray images that are highly flattened
along the equatorial plane out to several effective radii. Such X-ray
flattening is not observed at the spatial resolution currently available to
X-ray observations. Evidently mass and angular momentum are not in fact
conserved. If cooling flows are depleted by localized radiative cooling at
numerous sites distributed throughout the flows, then disks of cooled gas do
not form and the X-ray images appear nearly circular. However, the distribution
of young stars formed from the cooled gas is still somewhat flattened relative
to the stellar light. X-ray images of galactic cooling flows can also be
circularized by the turbulent diffusion of angular momentum away from the axis
of rotation, but the effective viscosity of known processes -- stellar mass
loss, supernovae, cooling site evolution, etc. -- is insufficient to
appreciably circularize the X-ray images. Radial gradients in the interstellar
iron abundance are unaffected by the expected level of interstellar turbulence
since these gradients are continuously re-established by Type Ia supernovae.Comment: 17 pages with 6 figures; accepted by Astrophysical Journa
First Detection of Millimeter/Submillimeter Extragalactic H2O Maser Emission
We report the first detection of an extragalactic millimeter wavelength H2O
maser at 183 GHz towards NGC 3079 using the Submillimeter Array (SMA), and a
tentative submillimeter wave detection of the 439 GHz maser towards the same
source using the James Clerk Maxwell Telescope (JCMT). These H2O transitions
are known to exhibit maser emission in star-forming regions and evolved stars.
NGC 3079 is a well-studied nuclear H2O maser source at 22 GHz with a
time-variable peak flux density in the range 3 -- 12 Jy. The 183 GHz H2O maser
emission, with peak flux density 0.5 Jy (7 detection), also
originates from the nuclear region of NGC 3079 and is spatially coincident with
the dust continuum peak at 193 GHz (53 mJy integrated). Peak emission at both
183 and 439 GHz occurs in the same range of velocity as that covered by the 22
GHz spectrum. We estimate the gas to dust ratio of the nucleus of NGC 3079 to
be 150, comparable to the Galactic value of 160. Discovery of maser
emission in an active galactic nucleus beyond the long-known 22 GHz transition
opens the possibility of future position-resolved radiative transfer modeling
of accretion disks and outflows pc from massive black holes.Comment: 12 pages, 3 figures, ApJ Letters accepte
- âŠ