575 research outputs found
Star formation in the central regions of galaxies
Massive star formation in the central regions of spiral galaxies plays an
important role in the dynamical and secular evolution of their hosts. Here, we
summarise a number of recent investigations of the star formation history and
the physical conditions of the gas in circumnuclear regions, to illustrate not
only the detailed results one can achieve, but also the potential of using
state-of-the-art spectroscopic and analysis techniques in researching the
central regions of galaxies in general. We review how the star formation
history of nuclear rings confirms that they are long-lived and stable
configurations. Gas flows in from the disk, through the bar, and into the ring,
where successive episodes of massive star formation occur. Analysing the ring
in NGC 7742 in particular, we determine the physical conditions of the line
emitting gas using a combination of ionisation and stellar population
modelling, concluding that the origin of the nuclear ring in this non-barred
galaxy lies in a recent minor merger with a small gas-rich galaxy.Comment: Invited contribution, to appear in "Mapping the Galaxy and other
galaxies", Eds. K. Wada and F. Combes, Springer, in pres
A subarcsecond resolution near-infrared study of Seyfert and `normal' galaxies: II. Morphology
We present a detailed study of the bar fraction in the CfA sample of Seyfert
galaxies, and in a carefully selected control sample of non-active galaxies, to
investigate the relation between the presence of bars and of nuclear activity.
To avoid the problems related to bar classification in the RC3, e.g.,
subjectivity, low resolution and contamination by dust, we have developed an
objective bar classification method, which we conservatively apply to our new
sub-arcsecond resolution near-infrared imaging data set (Peletier et al. 1999).
We are able to use stringent criteria based on radial profiles of ellipticity
and major axis position angle to determine the presence of a bar and its axial
ratio. Concentrating on non-interacting galaxies in our sample for which
morphological information can be obtained, we find that Seyfert hosts are
barred more often (79% +/- 7.5%) than the non-active galaxies in our control
sample (59% +/- 9%), a result which is at the 2.5 sigma significance level. The
fraction of non-axisymmetric hosts becomes even larger when interacting
galaxies are taken into account. We discuss the implications of this result for
the fueling of central activity by large-scale bars. This paper improves on
previous work by means of imaging at higher spatial resolution and by the use
of a set of stringent criteria for bar presence, and confirms that the use of
NIR is superior to optical imaging for detection of bars in disk galaxies.Comment: Latex, 3 figures, includes aaspptwo.sty, accepted for publication in
the Astrophysical Journa
The extinction by dust in the outer parts of spiral galaxies
To investigate the distribution of dust in Sb and Sc galaxies we have
analyzed near-infrared and optical surface photometry for an unbiased sample of
37 galaxies. Since light in the -band is very little affected by extinction
by dust, the colour is a good indicator of the amount of extinction, and
using the colour-inclination relation we can statistically determine the
extinction for an average Sb/Sc galaxy. We find in general a considerable
amount of extinction in spiral galaxies in the central regions, all the way out
to their effective radii. In the outer parts, at D, or at 3 times the
typical exponential scale lengths of the stellar distribution , we find a
maximum optical depth of 0.5 in for a face-on galaxy. If we impose the
condition that the dust is distributed in the same way as the stars, this upper
limit would go down to 0.1.Comment: 4 pages, postscript, gzip-compressed, uuencoded, includes 2 figures.
Accepted for publication in Astronomy & Astrophysics, Letter
Dynamics of Inner Galactic Disks: The Striking Case of M100
We investigate gas dynamics in the presence of a double inner Lindblad
resonance within a barred disk galaxy. Using an example of a prominent spiral,
M100, we reproduce the basic central morphology, including four dominant
regions of star formation corresponding to the compression maxima in the gas.
These active star forming sites delineate an inner boundary (so-called nuclear
ring) of a rather broad oval detected in the near infrared. We find that
inclusion of self-gravitational effects in the gas is necessary in order to
understand its behavior in the vicinity of the resonances and its subsequent
evolution. The self-gravity of the gas is also crucial to estimate the effect
of a massive nuclear ring on periodic orbits in the stellar bar.Comment: 11 pages, postscript, compressed, uuencoded. Paper and 4 figures
available at ftp://pa.uky.edu/shlosman/nobel or at
http://www.pa.uky.edu/~shlosman/ . Invited talk at the Centennial Nobel
Symposium on "Barred Galaxies and Circumnuclear Activity," A.Sandquist et al.
(Eds.), Springer-Verlag, in pres
Evidence for the Large-Scale Dissociation of Molecular Gas in the Inner Spiral Arms of M81
We compare the detailed distributions of HI, H alpha, and 150 nm far-UV
continuum emission in the spiral arms of M81 at a resolution of 9" (linear
resolution 150 pc at 3.7 Mpc distance). The bright H alpha emission peaks are
always associated with peaks in the far-UV emission. The converse is not always
true; there are many regions of far-UV emission with little corresponding H
alpha. The HI and the far-UV are always closely associated, in the sense that
the HI is often brightest around the edges of the far-UV emission. The effects
of extinction on the morphology are small, even in the far-UV. Extensive far-UV
emission, often with little corresponding H alpha, indicates the presence of
many ``B-stars'', which produce mostly non-ionizing UV photons. These far-UV
photons dissociate a small fraction of an extensive layer of H_2 into HI. The
observed morphology can be understood if ``chimneys'' are common in the spiral
arms of M81, where holes are blown out of the galactic disk, exposing the
bright HII regions and the corresponding far-UV associated with vigorous star
formation. These ``naked'' star-forming regions show little obscuration. H_2 is
turned into HI by UV photons impinging on the interior surfaces of these
chimneys. The intensity of the far-UV radiation measured by UIT can dissociate
the underlying H_2 with a typical density of ~10 H nucleii cm**-3 to produce
the observed amount of HI in the spiral arms of M81. Except for thin surface
layers locally heated in these photo-dissociation regions close to the far-UV
sources, the bulk of the molecular gas in the inner disk of M81 is apparently
too cold to produce much 12CO(1-0) emission.Comment: 12 pages, Latex. 8 postscript files. Better quality versions of the
figures available from ftp://star.herts.ac.uk/pub/Knapen/m81uv . Accepted,
Ap
The conceptual design of SeamFrame
This project deliverable provides the underlying architecture of a concept for linking models and databases and it provides the design of SeamFrame, delivering its architecture to provide an integration framework for models and simulation algorithms, supported by procedures for data handling and spatial representation, quality control, output visualization and documentatio
A New Probe of the Molecular Gas in Galaxies: Application to M101
Recent studies of nearby spiral galaxies suggest that photodissociation
regions (PDRs) are capable of producing much of the observed HI in galaxy
disks. In that case, measurements of the HI column density and the
far-ultraviolet (FUV) photon flux provide a new probe of the volume density of
the local underlying H_2. We develop the method and apply it to the giant Scd
spiral M101 (NGC 5457). We find that, after correction for the best-estimate
gradient of metallicity in the ISM of M101 and for the extinction of the
ultraviolet emission, molecular gas with a narrow range of density from 30-1000
cm^-3 is found near star- forming regions at all radii in the disk of M101 out
to a distance of 12' (approximately 26 kpc), close to the photometric limit of
R_25 = 13.5'.
In this picture, the ISM is virtually all molecular in the inner parts of
M101. The strong decrease of the HI column density in the inner disk of the
galaxy at R_G < 10 kpc is a consequence of a strong increase in the dust-to-gas
ratio there, resulting in an increase of the H_2 formation rate on grains and a
corresponding disappearance of hydrogen in its atomic form.Comment: accepted for publication in The Astrophysical Journal (1 August
2000); 29 pages including 20 figures (7 gif); AAS LaTex; contact authors for
full resolution versions of gif figure
- …