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

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

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

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

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

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 $BVIJ$ 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

Nucler star formation in NGC 6240

We have made use of archival HST BVIJH photometry to constrain the nature of the three discrete sources, A1, A2 and B1, identified in the double nucleus of NGC 6240. STARBURST99 models have been fitted to the observed colours, under the assumption, first, that these sources can be treated as star clusters (i.e. single, instantaneous episodes of star formation), and subsequently as star-forming regions (i.e. characterised by continuous star formation). For both scenarios, we estimate ages as young as 4 million years, integrated masses ranging between 7x10^6 Msun (B1) and 10^9 Msun (A1) and a rate of 1 supernova per year, which, together with the stellar winds, sustains a galactic wind of 44 Msun/yr. In the case of continuous star formation, a star-formation rate has been derived for A1 as high as 270 Msun/yr, similar to what is observed for warm Ultraluminous Infrared Galaxies (ULIRGs) with a double nucleus. The A1 source is characterised by a mass density of about 1200 Msun/pc^3 which resembles the CO molecular mass density measured in cold ULIRGs and the stellar density determined in ``elliptical core'' galaxies. This, together with the recent discovery of a supermassive binary black hole in the double nucleus of NGC 6240, might indicate that the ongoing merger could shape the galaxy into a core elliptical.Comment: 14 pages, 5 postscript figures, accepted by A&

Mass segregation in very young open clusters -- A case study of NGC 2244 and NGC 6530

We derive the proper motions, membership probabilities, and velocity dispersions of stars in the regions of the young (about 2-4 Myr-old) open clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC 6530 (the dominant cluster in the Sgr OB1 association) from photographic plate material obtained at Shanghai Astronomical Observatory, with time baselines of 34 and 87 years, respectively. Both clusters show clear evidence of mass segregation, but they do not exhibit any significant velocity-mass (or, equivalently, a velocity-luminosity) dependence. This provides strong support for the suggestion that the observed mass segregation is -- at least partially -- due to the way in which star formation has proceeded in these complex star-forming regions (``primordial'' mass segregation). Based on arguments related to the clusters' published initial mass functions, in conjunction with our new measurements of their internal velocity dispersions (35 and 8 km/s for NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of the dissolution of NGC 2244 on very short time-scales, while we speculate that NGC 6530 may be more stable against the effects of internal two-body relaxation. However, this latter object may well be destroyed by the strong tidal field prevalent at its location in the Galactic plane in the direction of the Galactic Center.Comment: 36 pages, 10 figures, accepted to A

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