654 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
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
MUSE-AO view of the starburst-AGN connection: NGC 7130
We present the discovery of a small kinematically decoupled core of
0.2 (60 pc) in radius as well as an outflow jet in the
archetypical AGN-starburst "composite" galaxy NGC 7130 from integral field data
obtained with the adaptive optics-assisted MUSE-NFM instrument on the VLT.
Correcting the already good natural seeing at the time of our science
verification observations with the four-laser GALACSI AO system, we reach an
unprecedented spatial resolution at optical wavelengths of around
0.15. We confirm the existence of star-forming knots arranged
in a ring of 0.58 (185 pc) in radius around the nucleus,
previously observed from UV and optical Hubble Space Telescope and CO(6-5) ALMA
imaging. We determine the position of the nucleus as the location of a peak in
gas velocity dispersion. A plume of material extends towards the NE from the
nucleus until at least the edge of our field of view at 2 (640
pc) radius which we interpret as an outflow jet originating in the AGN. The
plume is not visible morphologically, but is clearly characterised in our data
by emission-line ratios characteristic of AGN emission, enhanced gas velocity
dispersion, and distinct non-circular gas velocities. Its orientation is
roughly perpendicular to the line of nodes of the rotating host galaxy disc. A
circumnuclear area of positive and negative velocities of 0.2
in radius indicates a tiny inner disc, which can only be seen after combining
the integral field spectroscopic capabilities of MUSE with adaptive optics.Comment: Accepted for publication in A&A letter
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
Chandra X-Ray Point Sources, including Supernova 1979C, in the Spiral Galaxy M100
Six x-ray point sources, with luminosities of in the 0.4--7 keV band, were detected in Chandra
observations of the spiral galaxy M100. One source is identified with supernova
SN 1979C and appears to have roughly constant x-ray flux for the period 16--20
years after the outburst. The x-ray spectrum is soft, as would be expected if
the x-ray emission is due to the interaction of supernova ejecta with
circumstellar matter. Most of the other sources are variable either within the
Chandra observation or when compared to archival data. None are coincident with
the peak of the radio emission at the nucleus. These sources have harder
spectra than the supernova and are likely x-ray binaries. M100 has more bright
x-ray sources than typical for spiral galaxies of its size. This is likely
related to active star formation occurring in the galaxy.Comment: accepted by the Astrophysical Journal, 7 page
Evidence for globular cluster collapse after a dwarf-dwarf merger: A potential nuclear star cluster in formation
Direct observational evidence for the creation of nuclear star clusters
(NSCs) is needed to support the proposed scenarios for their formation. We
analysed the dwarf galaxy UGC 7346, located in the peripheral regions of the
Virgo Cluster, to highlight a series of properties that indicate the formation
of a NSC caught in its earlier stages. First, we report on remnants of a past
interaction in the form of diffuse streams or shells, suggesting a recent
merging of two dwarf galaxies with a 1:5 stellar mass ratio. Second, we
identify a number of globular cluster (GC) candidates that are broadly
compatible in colour with the main component that is both more extended and
more massive. Strikingly, we find these GCs candidates to be highly
concentrated towards the centre of the galaxy (R = 0.41 R). We
suggest that the central concentration of the GCs is likely produced by the
dynamical friction of this merger. This would make UGC 7346 a unique case of a
galaxy caught in the earlier stages of NSC formation. The formation of NSCs due
to collapse of GCs by dynamical friction in dwarf mergers would provide a
natural explanation of the environmental correlations found for the nucleation
fraction for early-type dwarf galaxies, whereby denser environments host
galaxies with a higher nucleation fraction.Comment: Matching published version in A&A Letter
Walking the tightrope with an e-portfolio:imbalance between support and autonomy hampers self-directed learning
Teacher coaching is essential to support self-directed learning, but requires a lot of time and energy. This mixed-methods study investigated the effects of using an e-portfolio with a self-coaching protocol and limited teacher coaching on the development of self-directed learning skills and motivation. With regard to self-directed learning, students' overestimation of performance on learning tasks grew over time, while their ability to formulate points for improvement did not change significantly. With regard to motivation, students' controlled (i.e. largely extrinsic) motivation increased while their autonomous (i.e. largely intrinsic) motivation decreased. Thematic analysis of interviews demonstrated students needed more support and feedback from their teacher. The use of suboptimal cues for performance assessment and an imperfect balance between autonomy and support hampered self-directed learning and motivation. We recommend providing just-in-time feedback about performance on learning tasks and giving students some autonomy over the choice of learning tasks, to enhance reflection and motivation
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