511 research outputs found
First Detection of CO in a Low Surface Brightness Galaxy
We report on the first attempts at searching for CO in red low surface
brightness galaxies, and the first detection of molecular gas in a low surface
brightness (mu_B(0)_{obs} > 23 mag arcsec^{-2}) galaxy. Using the IRAM 30m
telescope, CO(1-0) and CO(2-1) lines were searched for in four galaxies --
P06-1, P05-5, C05-3, & C04-2. In three of the galaxies no CO was detected, to
T_{MB} ~ 1.8mK (at the 3 sigma level). In the fourth galaxy, P06-1, both lines
were detected. Comparing our findings with previous studies shows P06-1 to have
a molecular-to-atomic mass ratio considerably lower than is predicted using
theoretical models based on high surface brightness galaxy studies. This
indicates the N(H_2)/(int{T(CO)dv}) conversion factor for low surface
brightness galaxies may currently be consistently underestimated by a factor of
3 - 20.Comment: 8 pages, 2 figures, accepted by the ApJ
Molecular hydrogen beyond the optical edge of an isolated spiral galaxy
We know little about the outermost portions of galaxies because there is
little light coming from them. We do know that in many cases atomic hydrogen
(HI) extends well beyond the optical radius \cite{Casertano91}. In the centers
of galaxies, however, molecular hydrogen (H2) usually dominates by a large
factor, raising the question of whether H2 is abundant also in the outer
regions but hitherto unseen.Here we report the detection of emission from
carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius
of the nearby galaxy NGC 4414. The molecular clouds probably formed in the
regions of relatively high HI column density and in the absence of spiral
density waves. The relative strength of the lines from the two lowest
rotational levels indicates that both the temperature and density of the H2 are
quite low compared to conditions closer to the center. The inferred surface
density of the molecular material continues the monotonic decrease from the
inner regions. We conclude that while molecular clouds can form in the outer
region of this galaxy, there is little mass associated with them.Comment: 3 Nature page
Children's working understanding of the knowledge gained from seeing and feeling
In three Experiments, (N = 48 3- to 4-year olds; 100 3- to 5-year olds; 54 4-yearolds), children who could see or feel a target toy, recognized when they had sufficient information to answer “Which one is it?” and when they needed additional access. They were weaker at taking the informative modality of access when the choice was between
seeing more of a partially visible toy and feeling it; at doing so when the target was completely hidden; and at reporting seeing or feeling as their source of knowledge of the target’s identity having experienced both. Working understanding of the knowledge gained from seeing and feeling (identifying the target efficiently) was not necessarily in advance of explicit understanding (reporting the informative source)
Where Do Cooling Flows Cool?
Although only about 5 percent of the total baryonic mass in luminous
elliptical galaxies is in the form of cooled interstellar gas, it is
concentrated within the optical effective radius r_e where it influences the
local dynamical mass. The mass of cooled gas must be spatially distributed
since it greatly exceeds the masses of central black holes. We explore here the
proposition that a population of low mass, optically dark stars is created from
the cooled gas. We consider a wide variety of radial distributions for the
interstellar cooling, but only a few are consistent with observed X-ray surface
brightness profiles. In a region of concentrated interstellar cooling, the
X-ray emission can exceed that observed, suggesting the presence of additional
support by magnetic stresses or non-thermal pressure. In general we find that
the mass of cooled gas contributes significantly to stellar dynamical mass to
light ratios which vary with galactic radius. If the stars formed from cooled
interstellar gas are optically luminous, their influence on the the mass to
light ratio would be reduced. The mass of cooled gas inside r_e is sensitive to
the rate that old stars lose mass, which is nearly independent of the initial
mass function of the old stellar population.Comment: 18 pages with 6 figures; accepted by Astrophysical Journa
Chandra Detection of Massive Black Holes in Galactic Cooling Flows
Anticipating forthcoming observations with the Chandra X-ray telescope, we
describe the continuation of interstellar cooling flows deep into the cores of
elliptical galaxies. Interstellar gas within about r = 50 parsecs from the
massive black hole is heated to T > 1 keV and should be visible unless thermal
heating is diluted by non-thermal pressure. Since our flows are subsonic near
the massive black holes, distributed cooling continues within 300 pc from the
center. Dark, low mass stars formed in this region may be responsible for some
of the mass attributed to central black holes.Comment: 6 pages with 3 figures; accepted by Astrophysical Journal Letter
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.
A Molecular Tidal Tail in the Medusa Minor Merger
We have detected CO 1-0 emission along the tidal tail of the NGC 4194 (the
Medusa) merger. It is the first CO detection in the optical tail of a minor
merger. Emission is detected both in the centre of the tail and at its tip. The
molecular mass in the 33'' Onsala 20m beam is estimated to be >= 8.5 x 10^7
M_{sun} which is at least 4% of the total molecular mass measured so far in
this system. We suggest that the emission is a molecular tidal tail which is
part of the extended structure of the main body, and that the molecular gas was
thrown out by the collision instead of having formed in situ from condensing
atomic material. We find it unlikely that the emission is associated with a
tidal dwarf galaxy (even if the future formation of such an object is
possible), but high resolution HI, CO and optical observations are necessary to
resolve the issue. The Medusa is very likely the result of an elliptical+spiral
collison and our detection supports the notion that molecular gas in minor
mergers can be found at great distances from the merger centre.Comment: 4 Pages, 2 figures included, accepted for A&A letter
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
Far Infrared and Submillimeter Emission from Galactic and Extragalactic Photo-Dissociation Regions
Photodissociation Region (PDR) models are computed over a wide range of
physical conditions, from those appropriate to giant molecular clouds
illuminated by the interstellar radiation field to the conditions experienced
by circumstellar disks very close to hot massive stars. These models use the
most up-to-date values of atomic and molecular data, the most current chemical
rate coefficients, and the newest grain photoelectric heating rates which
include treatments of small grains and large molecules. In addition, we examine
the effects of metallicity and cloud extinction on the predicted line
intensities. Results are presented for PDR models with densities over the range
n=10^1-10^7 cm^-3 and for incident far-ultraviolet radiation fields over the
range G_0=10^-0.5-10^6.5, for metallicities Z=1 and 0.1 times the local
Galactic value, and for a range of PDR cloud sizes. We present line strength
and/or line ratio plots for a variety of useful PDR diagnostics: [C II] 158
micron, [O I] 63 and 145 micron, [C I] 370 and 609 micron, CO J=1-0, J=2-1,
J=3-2, J=6-5 and J=15-14, as well as the strength of the far-infrared
continuum. These plots will be useful for the interpretation of Galactic and
extragalactic far infrared and submillimeter spectra observable with ISO,
SOFIA, SWAS, FIRST and other orbital and suborbital platforms. As examples, we
apply our results to ISO and ground based observations of M82, NGC 278, and the
Large Magellenic Cloud.Comment: 54 pages, 20 figures, accepted for publication in The Astrophysical
Journa
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