815 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
The central region of M83: Massive star formation, kinematics, and the location and origin of the nucleus
We report new near-IR integral field spectroscopy of the central starburst
region of the barred spiral galaxy M83 obtained with CIRPASS on Gemini-S, which
we analyse in conjunction with GHaFaS Fabry-Perot data, an AAT IRIS2 Ks-band
image, and near- and mid-IR imaging from the Hubble and Spitzer space
telescopes. The bulk of the current star formation activity is hidden from
optical view by dust extinction, but is seen in the near- and mid-IR to the
north of the nucleus. This region is being fed by inflow of gas through the bar
of M83, traced by the prominent dust lane entering into the circumnuclear
region from the north. An analysis of stellar ages confirms that the youngest
stars are indeed in the northwest. A gradual age gradient, with older stars
further to the south, characterises the well-known star-forming arc in the
central region of M83. Detailed analyses of the Pa beta ionised gas kinematics
and near-IR imaging confirm that the kinematic centre coincides with the
photometric centre of M83, and that these are offset significantly, by about 3
arcsec or 60 pc, from the visible nucleus of the galaxy. We discuss two
possible options, the first of which postulates that the kinematic and
photometric centre traces a galaxy nucleus hidden by a substantial amount of
dust extinction, in the range A_V=3-10 mag. By combining this information with
kinematic results and using arguments from the literature, we conclude that
such a scenario is, however, unlikely, as is the existence of other "hidden"
nuclei in M83. We thus concur with recent authors and favour a second option,
in which the nucleus of the galaxy is offset from its kinematic and photometric
centre. This is presumably a result of some past interaction, possibly related
to the event which lies at the origin of the disturbance of the outer disk of
the galaxy. (Abridged)Comment: MNRAS, in press; 16 pages latex, 15 figure
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
A composite HII region luminosity function in H alpha of unprecedented statistical weight
Context. Statistical properties of HII region populations in disk galaxies
yield important clues to the physics of massive star formation.
Aims. We present a set of HII region catalogues and luminosity functions for
a sample of 56 spiral galaxies in order to derive the most general form of
their luminosity function.
Methods. HII region luminosity functions are derived for individual galaxies
which, after photometric calibration, are summed to form a total luminosity
function comprising 17,797 HII regions from 53 galaxies.
Results. The total luminosity function, above its lower limit of
completeness, is clearly best fitted by a double power law with a significantly
steeper slope for the high luminosity portion of the function. This change of
slope has been reported in the literature for individual galaxies, and occurs
at a luminosity of log L = 38.6\pm0.1 (L in erg/s) which has been termed the
Stromgren luminosity. A steep fall off in the luminosity function above log L =
40 is also noted, and is related to an upper limit to the luminosities of
underlying massive stellar clusters. Detailed data are presented for the
individual sample galaxies.
Conclusions. The luminosity functions of HII regions in spiral galaxies show
a two slope power law behaviour, with a significantly steeper slope for the
high luminosity branch. This can be modelled by assuming that the high
luminosity regions are density bounded, though the scenario is complicated by
the inhomogeneity of the ionized interstellar medium. The break, irrespective
of its origin, is of potential use as a distance indicator for disc galaxies.Comment: Accepted for publication as a Letter in Astronomy & Astrophysics.
Latex with postscript figures. Online-only tables and figures are included in
this preprint. The HII region catalogues for 56 galaxies will be published
electronically on the CDS but are available also on request from the author
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