541 research outputs found
The global structure of galactic discs
A statistical study of global galaxy parameters can help to improve our
understanding of galaxy formation processes. In this paper we present the
analysis of global galaxy parameters based on optical and near-infrared
observations of a large sample of edge-on disc galaxies. We found a correlation
between the ratio of the radial to vertical scale parameter and galaxy type:
galaxies become systematically thinner when going from S0's to Sc's, whereas
the distribution seems to level off for later types.
The observed scale length ratios (and thus the radial colour gradients)
largely represent the galaxies' dust content. On average the colour gradients
indicated by the scale length ratios increase from type Sa to at least type Sc.
For galaxy types later than Sc, the average colour gradient seems to decrease
again.
The distribution of K-band (edge-on) disc central surface brightnesses is
rather flat, although with a large scatter. However, the latest-type sample
galaxies (T > 6) show an indication that their average disc central surface
brightnesses may be fainter than those of the earlier types. This effect is
probably not the result of dust extinction.Comment: 17 pages, LaTex, 11 figures, accepted for publication in MNRA
Binaries and the dynamical mass of star clusters
The total mass of a distant star cluster is often derived from the virial
theorem, using line-of-sight velocity dispersion measurements and half-light
radii, under the implicit assumption that all stars are single (although it is
known that most stars form part of binary systems). The components of binary
stars exhibit orbital motion, which increases the measured velocity dispersion,
resulting in a dynamical mass overestimation. In this article we quantify the
effect of neglecting the binary population on the derivation of the dynamical
mass of a star cluster. We find that the presence of binaries plays an
important role for clusters with total mass M < 10^5 Msun; the dynamical mass
can be significantly overestimated (by a factor of two or more). For the more
massive clusters, with Mcl > 10^5 Msun, binaries do not affect the dynamical
mass estimation significantly, provided that the cluster is significantly
compact (half-mass radius < 5 pc).Comment: Comments: 2 pages. Conference proceedings for IAUS246 'Dynamical
Evolution of Dense Stellar Systems', ed. E. Vesperini (Chief Editor), M.
Giersz, A. Sills, Capri, Sept. 200
Star and cluster formation in extreme environments
Current empirical evidence on the star-formation processes in the extreme,
high-pressure environments induced by galaxy encounters (mostly based on
high-resolution Hubble Space Telescope observations) strongly suggests that
star CLUSTER formation is an important and perhaps even the dominant mode of
star formation in such starburst events. The sizes, luminosities, and mass
estimates of the young massive star clusters (YMCs) are entirely consistent
with what is expected for young Milky Way-type globular clusters (GCs). Recent
evidence lends support to the scenario that GCs, which were once thought to be
the oldest building blocks of galaxies, are still forming today. Here, I
present a novel empirical approach to assess the shape of the
initial-to-current YMC mass functions, and hence their possible survival
chances for a Hubble time.Comment: 6 pages, LaTeX with Kluwer style files included; to appear in:
"Starbursts - from 30 Doradus to Lyman break galaxies" (Cambridge UK,
September 2004; talk summary), Astrophysics & Space Science Library, eds. de
Grijs R., Gonzalez Delgado R.M., Kluwer: Dordrech
Stellar Distributions and NIR Colours of Normal Galaxies
We discuss some results of a morphological study of edge-on galaxies, based
on optical and especially near-infrared surface photometry. We find that the
vertical surface brightness distributions of galaxies are fitted very well by
exponential profiles, much better than by isothermal distributions. We find
that in general the vertical scale height increases when going outward. This
increase is strong for early-type spiral galaxies and very small for late
types. We argue that it can be due to the presence of thick discs with scale
lengths larger than the galaxy's main disc. Finally we discuss the
colour-magnitude relation in I-K for spiral galaxies. We find that it is a
tight relation, for which the scatter is similar to the observational
uncertainties, with a steeper slope than for elliptical galaxies.Comment: Invited review, to appear in "Extragalactic Astronomy in the
Infrared", eds. G.A. Mamon, Trinh Xuan Thuan, and J. Tran Thanh Van, Editions
Frontieres, Gif-sur-Yvette. LaTeX2e, 10 pages, 6 postscript figures and
moriond.sty included. See also
ftp://kapteyn.astro.rug.nl/peletier/lesarcs.ps.g
Spiral galaxy distance indicators based on near-infrared photometry
We compare two methods of distance determination to spiral galaxies using
optical/near-infrared (NIR) observations, the (I-K) versus M_K colour -
absolute magnitude (CM) relation and the I and K-band Tully-Fisher relation
(TFR). Dust-free colours and NIR absolute magnitudes greatly enhance the
usefulness of the NIR CM relation as a distance indicator for moderately to
highly inclined_spiral_ galaxies_in the field_ (inclinations between ~ 80 and
90 deg); by avoiding contamination by dust the scatter in the CM relation is
significantly reduced, compared to similar galaxy samples published previously.
The CM relation can be used to determine distances to field spiral galaxies
with M_K > -25.5, to at least M_K ~ -20. Our results, supplemented with
previously published observations for which we can - to some degree - control
the effects of extinction, are consistent with a universal nature of the CM
relation for field spiral galaxies. High-resolution observations done with the
Hubble Space Telescope can provide a powerful tool to calibrate the relation
and extend the useful distance range by more than a factor of 2 compared to
ground-based observations. The intrinsic scatter in the NIR CM relation in the
absolute K-band magnitudes is ~0.5 mag, yielding a lower limit to the accuracy
of distance determinations on the order of 25%. Although we find an unusually
low scatter in the TFR (probably a statistical accident), a typical scatter in
the TFR would yield distances to our sample galaxies with uncertainties of only
about 15%. However, one of the main advantages of the use of the NIR CM
relation is that_we only need photometric data_ to obtain distance estimates;
use of the TFR requires additional kinematic data, although it can be used to
significantly greater distances.Comment: 12 pages, incl. 5 postscript files, LaTeX, accepted for publication
in MNRA
Star Cluster Evolution: From young massive star clusters to old globulars
Young, massive star clusters are the most notable and significant end
products of violent star-forming episodes triggered by galaxy collisions,
mergers, and close encounters. The question remains, however, whether or not at
least a fraction of the compact YMCs seen in abundance in extragalactic
starbursts, are potentially the progenitors of globular cluster (GC)-type
objects. However, because of the lack of a statistically significant sample of
similar nearby objects we need to resort to either statistical arguments or to
the painstaking approach of case by case studies of individual objects in more
distant galaxies. Despite the difficulties inherent to addressing this issue
conclusively, an ever increasing body of observational evidence lends support
to the scenario that GCs, which were once thought to be the oldest building
blocks of galaxies, are still forming today.Comment: 4 pages, to appear in "Globular Clusters - Guides to Galaxies", a
conference held in Concepcion, Chile, March 200
Chemical evolution of the M82 B fossil starburst
M82 B is an old starburst site located in the eastern part of the M82 disc.
We derive the distributions of age and metallicity of the star clusters located
in this region of M82 by using theoretical evolutionary population synthesis
models. Our analysis is based on the comparison of the photometry
obtained by de Grijs et al. (2001) with the colours of single-generation
stellar populations. We show that M82 B went through a chemical enrichment
phase up to super-solar metallicities around the time of the last close
encounter between M82 and its large neighbour galaxy M81. We date and confirm
the event triggering the enhanced cluster formation at about 1 Gyr ago. At
almost the same time an additional, distinct subpopulation of metal-poor
clusters formed in the part of M82 B nearest to the galactic centre. The
formation of these peculiar clusters may be related to infall of circumgalactic
gas onto M82 B.Comment: 14 pages, accepted for publication in MNRA
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