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We find that the HNC J=3-2 emission is brighter than the HCN 3-2 emission by factors of 1.5 to 2.3 in the ultraluminous mergers Arp220 and Mrk231 and the luminous IR galaxy NGC4418. We furthermore report the detection of HNC J=4-3 in Mrk231. Overluminous HNC emission is unexpected in warm molecular gas in ultraluminous galaxies since I(HNC)>I(HCN) is usually taken as a signature of cold (10 - 20 K) dark clouds. Since the molecular gas of the studied galaxies is warm (T_k > 40 K) we present two alternative explanations to the overluminous HNC: a) HNC excitation is affected by pumping of the rotational levels through the mid-infrared continuum and b) XDRs (X-ray Dominated Regions) influence the abundances of HNC. HNC may become pumped at 21.5 micron brightness temperatures of 50 K, suggesting that HNC-pumping could be common in warm, ultraluminous galaxies with compact IR-nuclei.On the other hand, all three galaxies are either suspected of having buried AGN - or the presence of AGN is clear (Mrk231) - indicating that X-rays may affect the ISM chemistry. We conclude that both the pumping and XDR alternatives imply molecular cloud ensembles distinctly different from those of typical starforming regions in the Galaxy, or the ISM of less extreme starburst galaxies. The HNC molecule shows the potential of becoming an additional important tracer of extreme nuclear environments., 9 pages, six figures, uses aa.cls, accepted for Astronomy and Astrophysics

Monthly Notices of the Royal Astronomical Society, 366 (1), ISSN:0035-8711, ISSN:1365-2966, ISSN:1365-8711

Results from numerical simulations of normal isolated late-type spiral galaxies are presented; specifically, the galaxy NGC 628 is used as a template. The method employs a TREESPH code including stellar particles, gas particles, cooling and heating of the gas, star formation according to a Jeans criterion and supernova feedback. A regular spiral disc can be generated as an equilibrium situation of two opposing actions: on the one hand, cooling and dissipation of the gas; on the other hand, gas heating by the far-ultraviolet field of young stars and supernova mechanical forcing. The disc exhibits small- and medium-scale spiral structure of which the multiplicity increases as a function of radius. The theory of swing amplification can explain, both qualitatively and quantitatively, the emerging spiral structure. In addition, swing amplification predicts that the existence of a grand-design m=2 spiral is only possible if the disc is massive. The simulations show that the galaxy is then unstable to bar formation, confirming the result of Ostriker & Peebles. The occurrence of this bar instability is further investigated. A general criterion is derived for the transition between a stable and an unstable bar, depending on the disc mass contribution and the on-disc thickness. It seems that bar stability barely depends on the presence of gas. A detailed quantitative analysis is made of the emerging spiral structure and a comparison is made with observations. This demonstrates that the structure of the numerical isolated galaxies is not as strong and has a larger multiplicity compared with the structure of some exemplary real galaxies. It is argued that the suggestion of Kormendy & Norman holds, i.e. that a grand design can only be generated by a central bar or by tidal forces resulting from an encounter with another galaxy.

The new World Wide Web site "GOLDMine" (Galaxy On Line Database Milano Network) (http://goldmine.mib.infn.it) contains a multiwavelength data-base of an optically selected sample of 3267 galaxies in the Virgo cluster and in the Coma Supercluster. It is designed for professional astronomers who wish to find data and images for these galaxies. Data, gathered in 15 years of observational campaigns by the authors or taken from the literature include general parameters (catalogue names, celestial coordinates, morphological type, recessional velocity etc.); multiwavelength continuum photometry (total UV, U, B, V, J, H, K, FIR and radio magnitudes/flux densities); line photometry (HI, H_2, H_alpha); dynamical parameters (rotational velocity from the HI and H_alpha lines, velocity dispersion) and structural parameters (light concentration index, effective radius and brightness, asymptotic magnitude) in the optical (B and V) and Near Infrared (H or K) bands. Images include finding charts, optical (B and V), H_alpha, Near Infrared (H and/or K) and true color RGB frames (when available). Radial light profiles obtained from the B, V, H or K band images are also available. Integrated optical spectra along with broad Spectral Energy Distributions (SED) from the UV to the radio domain are given. All images can be obtained in JPG format, but the original (reduced) FITS images can be downloaded as well. The database will be updated regularly and will be extended to other local clusters and superclusters. Astronomers who wish to have their images included in GOLDMine are strongly encouraged to send us their material., Comment: 5 pages, 2 figures, accepted for pubblication in Astronomy & Astrophysics

We present spectroscopic data of ionized gas in the disk-halo regions of three edge-on galaxies, NGC 891, NGC 4631, and NGC 3079, covering a wavelength range from [O II] 3727 Angstrom to [S II] 6716.4 Angstrom. The inclusion of the [O II] emission provides new constraints on the properties of the diffuse ionized gas, in particular the origin of the observed spatial variations in the line-intensity ratios. We used three different methods to derive electron temperatures, abundances, and ionization fractions along the slit. The increase in the [O II]/H alpha line ratio toward the halo in all three galaxies requires an increase in either electron temperature or oxygen abundance. Keeping the oxygen abundance constant yields the most reasonable results for temperature, abundances, and ionization fractions. Since a constant oxygen abundance seems to require an increase in temperature toward the halo, we conclude that gradients in the electron temperature play a significant role in the observed variations in the optical line ratios from extraplanar diffuse ionized gas in these three spiral galaxies.

Very Large Array(7) neutral hydrogen observations of the shell elliptical galaxy NGC 3656 reveal an edge-on, warped minor-axis gaseous disk (M (H I) similar to 2 x 10(9) M-circle dot) extending 7 kpc. H I is also found outside the optical image, on two complexes to the northeast and northwest that seem to trace one or two tidal tails, or possibly an outer broken H I disk or ring. These complexes link with the outer edges of the inner disk and appear displaced with respect to the two optical tails in the galaxy. The disk kinematics is strongly lopsided, suggesting recent or ongoing accretion. Integral-field optical fiber spectroscopy at the region of the bright southern shell of NGC 3656 has provided a determination of the stellar velocities of the shell. The shell, at 9 kpc from the center, has traces of H I with velocities bracketing the stellar velocities, providing evidence for a dynamical association of H I and stars at the shell. Within the errors the stars have systemic velocity, suggesting a possible phase-wrapping origin for the shell. We probed a region of 40' x 40' (480 kpc x 480 kpc) x 1160 km s(-1) down to an H I mass sensitivity (6 sigma) of 3 x 10(7) M-circle dot and detect five dwarf galaxies with H I masses ranging from 2 x 10(8) to 2 x 10(9) M-circle dot, all within 180 kpc of NGC 3656 and all within the velocity range (450 km s(-1)) of the H I of NGC 3656. The dwarfs had been previously cataloged, but none had a known redshift. For the NGC 3656 group to be bound requires a total mass of (3-7.4) x 10(12) M-circle dot, yielding a mass-to-light ratio from 125 to 300. The overall H I picture presented by NGC 3656 supports the hypothesis of a disk-disk merger origin or possibly an ongoing process of multiple mergers with nearby dwarfs.

We have collected a large body of near-IR (H band), UV (2000 Angstrom) and H alpha measurements of late-type galaxies galaxies. These are used jointly with spectral evolutionary synthesis models to study the initial mass function (IMF) in the mass range m > 2 M.. For spirals (Sa-Sd), Magellanic irregulars (Im), and blue compact dwarfs, our determination is consistent with a Salpeter IMF with an upper mass cutoff M-up similar to 80 M.. The history of star formation and the amount of total gas (per unit mass) of galaxies are found to depend primarily on their total masses (as traced by the H-band luminosities) and, only secondarily, on morphological type. The present star formation activity of massive spirals is up to 100 times smaller than that averaged over their lifetime, while in low-mass galaxies it is comparable to or higher than that at earlier epochs. Dwarf galaxies presently have larger gas reservoirs per unit mass than massive spirals. The efficiency in transforming gas into stars and the timescale for gas depletion (similar to 10 Gyr) are independent of the luminosity and morphological type. This evidence is consistent with the idea that galaxies are coeval systems, that they evolved as closed boxes, forming stars following a simple, universal star formation law whose characteristic timescale is small (tau similar to 1 Gyr) in massive spirals and large (tau > 10 Gyr) in low-mass galaxies. A similar conclusion was drawn by Gavazzi & Scodeggio in 1996 to explain the color-magnitude relation of late-type galaxies. The consequences of this interpretation on the evolution of the star formation rate and the gas density per comoving volume of the universe with look-back time are discussed.

The spatial distribution of stars in a disk of a galaxy can be described by a radial scale length and a vertical scale height. The ratio of these two scale parameters contains information on the axis ratio of the velocity ellipsoid, i.e. the ratio of the vertical to radial stellar velocity dispersions of the stars, at least at some fiducial distance from the center. The radial velocity dispersion correlates well with the amplitude of the rotation curve and:with the disk integrated magnitude, as was found by Bottema (1993). These relations can be understood as the result of the stellar disk being (marginally) stable against local instabilities at all length scales. This is expressed by Toomre's well-known criterion, which relates the sheer in the rotation to a minimum value that the radial stellar velocity dispersion should have for stability for a given surface density. Via the Tully-Fisher (1977) relation, the velocity dispersion then becomes related to the integrated magnitude and hence to the scale length. The vertical velocity dispersion relates directly to the scale height through hydrostatic equilibrium. It can be shown that the ratio of the two length scales relates to the axis ratio of the velocity ellipsoid only through the Toomre parameter Q and in particular does not require a choice of the mass-to-light ratio or a distance scale. We have applied this to the statistically complete sample of edge-on galaxies, for which de Grijs (1997) has performed surface photometry and has determined the length scales in the stellar light distribution.

The observed stellar velocity dispersions of galactic discs show that the maximum rotation of a disc is on average 63% of the observed maximum rotation. This criterion can, however, not be applied to small or low surface brightness (LSB) galaxies because such systems show, in general, a continuously rising rotation curve until the outermost measured radial position. That is why a general relation has been derived, giving the maximum rotation for a disc depending on the luminosity, surface brightness, and colour of the disc. As a physical basis of this relation serves an adopted fixed mass-to-light ratio as a function of colour. That functionality is consistent with results from population synthesis models and its absolute value is determined from the observed stellar velocity dispersions. The derived maximum disc rotation is compared with a number of observed maximum rotations, clearly demonstrating the need for appreciable amounts of dark matter in the disc region and even more so for LSB galaxies. Matters have been illustrated for two examples; the galaxy NGC 6503 and LSB galaxy NGC 1560.

Polar ring galaxies are ideal objects with which to study the three-dimensional shapes of galactic gravitational potentials since two rotation curves can be measured in two perpendicular planes. Observational studies have uncovered systematically larger rotation velocities in the extended polar rings than in the associated host galaxies. In the dark matter context, this can only be explained through dark haloes that are systematically flattened along the polar rings. Here, we point out that these objects can also be used as very effective tests of gravity theories, such as those based on Milgromian dynamics (also known as Modified Newtonian Dynamics or MOND). We run a set of polar ring models using both Milgromian and Newtonian dynamics to predict the expected shapes of the rotation curves in both planes, varying the total mass of the system, the mass of the ring with respect to the host and the size of the hole at the centre of the ring. We find that Milgromian dynamics not only naturally leads to rotation velocities being typically higher in the extended polar rings than in the hosts, as would be the case in Newtonian dynamics without dark matter, but that it also gets the shape and amplitude of velocities correct. Milgromian dynamics thus adequately explains this particular property of polar ring galaxies.

Edge-on spiral galaxies often have stellar disks with relatively sharp truncations and warped HI-layers in the outer parts. Warps appear to start preferentially near the optical boundaries of the disks. Here we make a comparative study of warps and truncations in edge-on galaxies. The Garc\'��a-Ruiz et al. (2002) sample with extensive HI-mapping is complemented with luminosity distributions from the Sloan Digital Sky Survey. The method to identify truncations has been tested using the sample of edge-on galaxies of van der Kruit & Searle. Results are: (i.) The majority (17 out of 23) of the galaxies show evidence for truncations. (ii.) When an HI-warp is present it starts at 1.1 truncation radii. (iii.) This supplements the rules for warps formulated by Briggs (1990), if the Holmberg radius is replaced for edge-on systems with the truncation radius. (iv.) The truncation radius and the onset of the warps coincide radially with features in the rotation curve and the HI surface density. The latter is also true for less inclined systems. (v.) Inner disks are very flat and the onset of the warp just beyond the truncation radius is abrupt and discontinuous. These findings suggest that the inner flat disk and the outer warped disk are distinct components with quite different formation histories, probably involving quite different epochs. The inner disk forms initially and the warped outer disk forms as a result of much later infall of gas with a higher angular momentum in a different orientation. In an appendix the Holmberg radius is discussed. Contrary to what is often assumed Holmberg radii are not corrected for inclination., Accepted for pubication by Astronomy & Astrophysics

Monthly Notices of the Royal Astronomical Society, 380 (3), ISSN:0035-8711, ISSN:1365-2966, ISSN:1365-8711

We show that fluid stationary models are able to reproduce the observed, negative vertical gradient of the rotation velocity of the extra-planar gas in spiral galaxies. We have constructed models based on the simple condition that the pressure of the medium does not depend on density alone (baroclinic instead of barotropic solutions: isodensity and isothermal surfaces do not coincide). As an illustration, we have successfully applied our method to reproduce the observed velocity gradient of the lagging gaseous halo of NGC 891. The fluid stationary models discussed here can describe a hot homogeneous medium as well as a "gas" made of discrete, cold HI clouds with an isotropic velocity dispersion distribution. Although the method presented here generates a density and velocity field consistent with observational constraints, the stability of these configurations remains an open question., 12 pages, 9 figures. Accepted for publication in Astronomy and Astrophysics

Monthly Notices of the Royal Astronomical Society, 349 (2), ISSN:0035-8711, ISSN:1365-2966, ISSN:1365-8711

Results are presented of numerical simulations of normal isolated late type spiral galaxies. Specifically the galaxy NGC 628 is used as a template. The method employs a TREESPH code including stellar particles, gas particles, cooling and heating of the gas, star formation according to a Jeans criterion, and Supernova feedback. A regular spiral disc can be generated as an equilibrium situation of two opposing actions. On the one hand cooling and dissipation of the gas, on the other hand gas heating by the FUV field of young stars and SN mechanical forcing. The disc exhibits small and medium scale spiral structure of which the multiplicity increases as a function of radius. The theory of swing amplification can explain, both qualitatively and quantitatively, the emerging spiral structure. In addition, swing amplification predicts that the existence of a grand design m=2 spiral is only possible if the disc is massive. The simulations show that the galaxy is then unstable to bar formation. A general criterion is derived for the transition between bar stable and unstable, depending on disc mass contribution and on disc thickness. It seems that bar stability hardly depends on the presence of gas. A detailed quantitative analysis is made of the emerging spiral structure and a comparison is made with observations. That demonstrates that the structure of the numerical isolated galaxies is not as strong and has a larger multiplicity compared to the structure of some exemplary real galaxies. It is argued that a grand design can only be generated by a central bar or by tidal forces resulting from an encounter with another galaxy., Accepted for publication in: Monthly Notices of the Royal Astron. Soc. The astro-ph copy has a few figures with degraded resolution. A copy with high quality graphics (4281 kb) can be downloaded from the kapteyn institute weg page at http://www.astro.rug.nl (Goto preprints and preprints 2003)

International audience; We present the new catalog of principal galaxies (PGC2003). It constitutes the framework of the HYPERLEDA database that supersedes the LEDA one, with more data and more capabilities. The catalog is still restricted to confirmed galaxies, i.e. about one million galaxies, brighter than similar to 18 B-mag.In order to provide the best possible identification for each galaxy we give: accurate coordinates ( typical accuracy better than 2 arcsec), diameter, axis ratio and position angle. Diameters and axis ratios have been homogenized to the RC2 system at the limiting surface brightness of 25B - mag arcsec(-2), using a new method, the EPIDEMIC method. In order to provide the best designation for each galaxy, we collected the names from 50 catalogues. The compatibility of the spelling is tested against NED and SIMBAD, and, as far as possible we used a spelling compatible with both. For some cases, where no consensus exists between NED, SIMBAD and LEDA, we propose some changes that could make the spelling of names fully compatible.The full catalog is distributed through the CDS and can be extracted from HYPERLEDA.

International audience; After a compilation of HI data from 611 references and new observations made in Nancay, we produce a catalog of homogenized HI data for 16781 galaxies. The homogenization is made using the EPIDEMIC method from which all data are progressively converted into the adopted standard. The result is a catalog giving: 1) the logarithm of twice the maximum rotation velocity, log 2V(M)(sini) M, converted to the system of Mathewson et al. (1996). This quantity is given without correction for inclination; 2) the HI magnitude, m(21), ( area of the 21-cm line width expressed in magnitude) converted to the flux system of Theureau et al. (1998); 3) the HI velocity, V-HI, expressed with the optical definition (i.e., using wavelengths instead frequencies). The typical uncertainties are: 0.04 for log 2V(M)(sin i) 0.25 mag for m(21) and 9 km s(-1) for V-HI.

We present a calibration of the massive star formation rate vs. [CII] luminosity relation based on a sample of nearby, late-type galaxies observed with ISO-LWS and imaged in the Halpha+[NII] line. The relation holds for far-IR luminosities 10^8 < L_{FIR} < 10^{10.5} L (solar units). The derived star formation rates have an uncertainty of about a factor of 10. Part of this uncertainty is due to the different mix of contributions to the [CII] emission from the different components of the interstellar medium in individual galaxies, as discussed in an appendix., Accepted for publication on Astronomy & Astrophysics

We present spectroscopic data of ionized gas in the disk-halo regions of three edge-on galaxies, NGC 891, NGC 4631, and NGC 3079, covering a wavelength range from [O II] 3727 Angstrom to [S II] 6716.4 Angstrom. The inclusion of the [O II] emission provides new constraints on the properties of the diffuse ionized gas, in particular the origin of the observed spatial variations in the line-intensity ratios. We used three different methods to derive electron temperatures, abundances, and ionization fractions along the slit. The increase in the [O II]/H alpha line ratio toward the halo in all three galaxies requires an increase in either electron temperature or oxygen abundance. Keeping the oxygen abundance constant yields the most reasonable results for temperature, abundances, and ionization fractions. Since a constant oxygen abundance seems to require an increase in temperature toward the halo, we conclude that gradients in the electron temperature play a significant role in the observed variations in the optical line ratios from extraplanar diffuse ionized gas in these three spiral galaxies.

Very Large Array(7) neutral hydrogen observations of the shell elliptical galaxy NGC 3656 reveal an edge-on, warped minor-axis gaseous disk (M (H I) similar to 2 x 10(9) M-circle dot) extending 7 kpc. H I is also found outside the optical image, on two complexes to the northeast and northwest that seem to trace one or two tidal tails, or possibly an outer broken H I disk or ring. These complexes link with the outer edges of the inner disk and appear displaced with respect to the two optical tails in the galaxy. The disk kinematics is strongly lopsided, suggesting recent or ongoing accretion. Integral-field optical fiber spectroscopy at the region of the bright southern shell of NGC 3656 has provided a determination of the stellar velocities of the shell. The shell, at 9 kpc from the center, has traces of H I with velocities bracketing the stellar velocities, providing evidence for a dynamical association of H I and stars at the shell. Within the errors the stars have systemic velocity, suggesting a possible phase-wrapping origin for the shell. We probed a region of 40' x 40' (480 kpc x 480 kpc) x 1160 km s(-1) down to an H I mass sensitivity (6 sigma) of 3 x 10(7) M-circle dot and detect five dwarf galaxies with H I masses ranging from 2 x 10(8) to 2 x 10(9) M-circle dot, all within 180 kpc of NGC 3656 and all within the velocity range (450 km s(-1)) of the H I of NGC 3656. The dwarfs had been previously cataloged, but none had a known redshift. For the NGC 3656 group to be bound requires a total mass of (3-7.4) x 10(12) M-circle dot, yielding a mass-to-light ratio from 125 to 300. The overall H I picture presented by NGC 3656 supports the hypothesis of a disk-disk merger origin or possibly an ongoing process of multiple mergers with nearby dwarfs.

The spatial distribution of stars in a disk of a galaxy can be described by a radial scale length and a vertical scale height. The ratio of these two scale parameters contains information on the axis ratio of the velocity ellipsoid, i.e. the ratio of the vertical to radial stellar velocity dispersions of the stars, at least at some fiducial distance from the center. The radial velocity dispersion correlates well with the amplitude of the rotation curve and with the disk integrated magnitude, as was found by Bottema (1993). These relations can be understood as the result of the stellar disk being (marginally) stable against local instabilities at all length scales. This is expressed by Toomre's well-known criterion, which relates the sheer in the rotation to a minimum value that the radial stellar velocity dispersion should have for stability for a given surface density. Via the Tully-Fisher (1977) relation, the velocity dispersion then becomes related to the integrated magnitude and hence to the scale length. The vertical velocity dispersion relates directly to the scale height through hydrostatic equilibrium. It can be shown that the ratio of the two length scales relates to the axis ratio of the velocity ellipsoid only through the Toomre parameter Q and in particular does not require a choice of the mass-to-light ratio or a distance scale. We have applied this to the statistically complete sample of edge-on galaxies, for which de Grijs (1997) has performed surface photometry and has determined the length scales in the stellar light distribution., Comment: 10 pages A&A LaTeX, including 3 postscript figures, A&A in press

The global structure of galaxy clusters and its evolution are tested within a large set of TREESPH simulations, so to allow a fair statistical comparison with available X-ray data. Structure tests are based on the "power ratios", introduced by Buote & Tsai. Cosmological models considered are CDM, LCDM (Omega_L=0.7) and CHDM (1 mass.neu., Omega_h = 0.2). All models are normalized to provide a fair number density of clusters. For each model we run a P3M simulation in a large box, where we select the most massive 40 clusters. Going back to the initial redshift we run a hydro-TREESPH simulation for each of them. In this way we perform a statistical comparison of the global morphology of clusters, for each cosmological model, with ROSAT data, using Student t-test, F-test and K-S test. The last test and its generalization to 2--D distributions are also used to compare the joint distributions of 2 or 3 power ratios. We find that, using DM distribution, instead of gas, as done by some authors, leads to biased results, as baryons are distributed in a less structured way than DM. We also find that the cosmological models considered have different behaviours in these tests: LCDM has the worst performance. CDM and our CHDM have similar scores. The general trend of power ratio distributions is already fit by these models, but a further improvement is expected either from a different DM mix or a non-flat CDM model.

The observed stellar velocity dispersions of galactic discs show that the maximum rotation of a disc is on average 63% of the observed maximum rotation. This criterion can, however, not be applied to small or low surface brightness (LSB) galaxies because such systems show, in general, a continuously rising rotation curve until the outermost measured radial position. That is why a general relation has been derived, giving the maximum rotation for a disc depending on the luminosity, surface brightness, and colour of the disc. As a physical basis of this relation serves an adopted fixed mass-to-light ratio as a function of colour. That functionality is consistent with results from population synthesis models and its absolute value is determined from the observed stellar velocity dispersions. The derived maximum disc rotation is compared with a number of observed maximum rotations, clearly demonstrating the need for appreciable amounts of dark matter in the disc region and even more so for LSB galaxies. Matters have been illustrated for two examples; the galaxy NGC 6503 and LSB galaxy NGC 1560., Latex, 10 pages, 7 Postscript figures, uses l-aa.sty

We present a study of large-scale bars in the local universe, based on a large sample of 3692 galaxies, with 18.5