1,243 research outputs found
Light Dark Matter: Models and Constraints
We study the direct detection prospects for a representative set of
simplified models of sub-GeV dark matter (DM), accounting for existing
terrestrial, astrophysical and cosmological constraints. We focus on dark
matter lighter than an MeV, where these constraints are most stringent, and
find three scenarios with accessible direct detection cross sections: (i) DM
interacting via an ultralight kinetically mixed dark photon, (ii) a DM
sub-component interacting with nucleons or electrons through a light scalar or
vector mediator, and (iii) DM coupled with nucleons via a mediator heavier than
~ 100 keV.Comment: 44 pages, 13 figures, reference added and minor updates to some of
the constraints, conclusions unchange
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
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
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
- …