Edvige Corbelli

The Molecular Gas Distribution and Schmidt Law in M33

The relationship between the star formation rate and surface density of neutral gas within the disk of M33 is examined with new imaging observations of 12CO J = 1-0 emission gathered with the Five College Radio Astronomy Observatory (FCRAO) 14 m telescope and IRAS HiRes images of the 60 and 100 μm emission. The Schmidt law, ΣSFR ~ Σ, is constructed using radial profiles of the H I 21 cm, CO, and far-infrared emission. We identify a strong correlation between the star formation rate and molecular gas surface density. This suggests that the condensation of giant molecular clouds (GMCs) is the limiting step to star formation within the M33 disk. The corresponding molecular Schmidt index, nmol, is 1.36 ± 0.08. The star formation rate has a steep dependence on total mass gas surface density, (Σ + Σ), owing to the shallow radial profile of the atomic gas that dominates the total gas surface density for most radii. The disk pressure of the gas is shown to play a prominent role in regulating the molecular gas fraction in M33.

Dark matter and visible baryons in M33

In this paper we present new measurements of the gas kinematics in M33 using the CO J = 1 - 0 line. The resulting rotational velocities complement previous 21-cm line data for a very accurate and extended rotation curve of this nearby galaxy. The implied dark matter mass, within the total gaseous extent, is a factor of 5 higher than the visible baryonic mass. Dark matter density profiles with an inner cusp as steep as R - 1 , suggested by some numerical simulation of structures formation, are compatible with the actual data. The dark matter concentrations required for fitting the M33 rotation curve are very low but still marginally consistent with haloes forming in a standard cold dark matter cosmology. The M33 virialized dark halo is at least 50 times more massive than the visible baryons and its size is comparable with the M33-M31 separation. Inner cusps as steep as R - 1 . 5 are ruled out, while halo models with a large size core of constant density are consistent with the M33 data. A central excess of stars is needed and we evaluate its dynamical mass range. Using accurate rotational velocity gradients and the azimuthally averaged baryonic surface densities, we show that a disc instability can regulate the star formation activity in M33. Considering the gaseous surface density alone, the predicted outer star formation threshold radius is consistent with the observed drop of the Ha surface brightness if a shear rate criterion is used with the lowest possible value of velocity dispersion. The classical Toomre criterion predicts the size of the unstable region correctly only when the stellar or dark halo gravity, derived in this paper, is added to that of the gaseous disc.

The dust scaling relations of the Herschel Reference Survey

We combine new Herschel/SPIRE sub-millimeter observations with existing multiwavelength data to investigate the dust scaling relations of the Herschel Reference Survey, a magnitude-, volume-limited sample of similar to 300 nearby galaxies in different environments. We show that the dust-to-stellar mass ratio anti-correlates with stellar mass, stellar mass surface density and NUV - r colour across the whole range of parameters covered by our sample. Moreover, the dust-to-stellar mass ratio decreases significantly when moving from late-to early-type galaxies. These scaling relations are similar to those observed for the Hi gas-fraction, supporting the idea that the cold dust is tightly coupled to the cold atomic gas component in the interstellar medium. We also find a weak increase of the dust-to-Hi mass ratio with stellar mass and colour but no trend is seen with stellar mass surface density. By comparing galaxies in different environments we show that, although these scaling relations are followed by both cluster and field galaxies, Hi-deficient systems have, at fixed stellar mass, stellar mass surface density and morphological type systematically lower dust-to-stellar mass and higher dust-to-Hi mass ratios than Hi-normal/field galaxies. This provides clear evidence that dust is removed from the star-forming disk of cluster galaxies but the effect of the environment is less strong than what is observed in the case of the Hi disk. Such effects naturally arise if the dust disk is less extended than the Hi and follows more closely the distribution of the molecular gas phase, i.e., if the dust-to-atomic gas ratio monotonically decreases with distance from the galactic center.

A WIDE-FIELD HIGH-RESOLUTION H I MOSAIC OF MESSIER 31. I. OPAQUE ATOMIC GAS AND STAR FORMATION RATE DENSITY

We have undertaken a deep, wide-field H I imaging survey of M31, reaching a maximum resolution of about 50 pc and 2 km s–1 across a 95 × 48 kpc region. The H I mass and brightness sensitivity at 100 pc resolution for a 25 km s–1 wide spectral feature is 1500 M ☉ and 0.28 K. Our study reveals ubiquitous H I self-opacity features, discernible in the first instance as filamentary local minima in images of the peak H I brightness temperature. Local minima are organized into complexes of more than kpc length and are particularly associated with the leading edge of spiral arm features. Just as in the Galaxy, there is only patchy correspondence of self-opaque features with CO(1-0) emission. We have produced images of the best-fit physical parameters: spin temperature, opacity-corrected column density, and nonthermal velocity dispersion, for the brightest spectral feature along each line of sight in the M31 disk. Spectroscopically opaque atomic gas is organized into filamentary complexes and isolated clouds down to 100 pc. Localized opacity corrections to the column density exceed an order of magnitude in many cases and add globally to a 30% increase in the atomic gas mass over that inferred from the integrated brightness under the usual assumption of negligible self-opacity. Opaque atomic gas first increases from 20 to 60 K in spin temperature with radius to 12 kpc but then declines again to 20 K beyond 25 kpc. We have extended the resolved star formation law down to physical scales more than an order of magnitude smaller in area and mass than has been possible previously. The relation between total gas mass and star formation rate density is significantly tighter than that with molecular mass and is fully consistent in both slope and normalization with the power-law index of 1.56 found in the molecule-dominated disk of M51 at 500 pc resolution. Below a gas mass density of about 5 M ☉ pc–2, there is a downturn in star formation rate density which may represent a real local threshold for massive star formation at a cloud mass of about 5 × 104 M ☉.

The extended rotation curve and the dark matter halo of M33

We present the 21-cm rotation curve of the nearby galaxy M33 out to a galactocentric distance of 16 kpc (13 disk scale-lengths). The rotation curve keeps rising out to the last measured point and implies a dark halo mass larger than 5 10^{10} solar masses. The stellar and gaseous disks provide virtually equal contributions to the galaxy gravitational potential at large galactocentric radii but no obvious correlation is found between the radial distribution of dark matter and the distribution of stars or gas. Results of the best fit to the mass distribution in M33 picture a dark halo which controls the gravitational potential from 3 kpc outward, with a matter density which decreases radially as R^{-1.3}. The density profile is consistent with the theoretical predictions for structure formation in hierarchical clustering cold dark matter models but mass concentrations are lower than those expected in the standard cosmogony.

A Warped Disk Model for M33 and the 21 Centimeter Line Width in Spiral Galaxies

To determine the actual H I distribution and the velocity field in the outermost disk of the spiral galaxy M33, a tilted-ring model is fitted to 21 cm line data taken with the Arecibo telescope. Since M33 is one of the main calibrators for the extragalactic distance scale derived through the Tully-Fisher relation, the outer disk warping is of interest for a correct determination and deprojection of the galaxys line width. Even though our best model predicts small effects on the observed line width of M33, we show that similar outer disk warping in galaxies oriented differently along our line of sight could affect the widths considerably. Therefore there may be systematic effects in the determination of the rotation velocities and dynamic masses of spiral galaxies, whose exact value depends also on which method is used for measuring the galaxys total line width.

The Herschel Virgo Cluster Survey - II. Truncated dust disks in H I-deficient spirals

By combining Herschel-SPIRE observations obtained as part of the Herschel Virgo Cluster Survey with 21 cm Hi data from the literature, we investigate the role of the cluster environment on the dust content of Virgo spiral galaxies. We show for the first time that the extent of the dust disk is significantly reduced in Hi-deficient galaxies, following remarkably well the observed “truncation” of the Hi disk. The ratio of the submillimetre-to-optical diameter correlates with the Hi-deficiency, suggesting that the cluster environment is able to strip dust as well as gas. These results provide important insights not only into the evolution of cluster galaxies but also into the metal enrichment of the intra-cluster medium.

A wide-field H I mosaic of Messier 31 - II. The disk warp, rotation, and the dark matter halo

Aims. We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M 31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466 pc resolution. Methods. We fit a tilted ring model to the HI data from 8 to 37 kpc and establish conclusively the presence of a dark halo and its density distribution via dynamical analysis of the rotation curve. Results. The disk of M 31 warps from 25 kpc outwards and becomes more inclined with respect to our line of sight. Newtonian dynamics without a dark matter halo provide a very poor fit to the rotation curve. In the framework of modified Newtonian dynamic (MOND) however the 21-cm rotation curve is well fitted by the gravitational potential traced by the baryonic matter density alone. The inclusion of a dark matter halo with a density profile as predicted by hierarchical clustering and structure formation in a ACDM cosmology makes the mass model in newtonian dynamic compatible with the rotation curve data. The dark halo concentration parameter for the best fit is C = 12 and its total mass is 1.2 x 10 12 M ⊙ . If a dark halo model with a constant-density core is considered, the core radius has to be larger than 20 kpc in order for the model to provide a good fit to the data. We extrapolate the best-fit ACDM and constant-density core mass models to very large galactocentric radii, comparable to the size of the dark matter halo. A comparison of the predicted mass with the M 31 mass determined at such large radii using other dynamical tracers, confirms the validity of our results. In particular the ACDM dark halo model which best fits the 21-cm data well reproduces the mass of M 31 traced out to 560 kpc. Our best estimate for the total mass of M 31 is 1.3 x 10 12 M ⊙ , with 12% baryonic fraction and only 6% of the baryons in the neutral gas phase.

The Herschel Virgo Cluster Survey – VIII. The Bright Galaxy Sample★

We describe the Herschel Virgo Cluster Survey and the first data that cover the complete survey area (four 4 x 4 deg2 regions). We use these data to measure and compare the global far-infrared properties of 78 optically bright galaxies that are selected at 500 mu m and detected in all five far-infrared bands. We show that our measurements and calibration are broadly consistent with previous data obtained by the IRAS, ISO, Spitzer and Planck. We use SPIRE and PACS photometry data to produce 100-, 160-, 250-, 350- and 500-mu m cluster luminosity distributions. These luminosity distributions are not power laws, but peaked, with small numbers of both faint and bright galaxies. We measure a cluster 100500 mu m far-infrared luminosity density of 1.6(7.0) +/- 0.2 x 10(9) L Mpc(-3). This compares to a cluster 0.42.5 mu m optical luminosity density of 5.0(20.0) x 10(9) L Mpc(-3), some 3.2(2.9) times larger than the far-infrared. A typical photon originates from an optical depth of 0.4 +/- 0.1. Most of our sample galaxies are well fitted by a single modified blackbody (beta= 2), leading to a mean dust mass of log M-Dust= 7.31 M and temperature of 20.0 K. We also derive both stellar and atomic hydrogen masses from which we calculate mean values for the star-to-gas (atomic) and gas (atomic)-to-dust mass ratios of 15.1 and 58.2, respectively. Using our derived dust, atomic gas and stellar masses, we estimate cluster mass densities of 8.6(27.8) x 106, 4.6(13.9) x 108 and 7.8(29.7) x 109 M Mpc-3 for dust, atomic gas and stars, respectively. These values are higher than those derived for field galaxies by factors of 39(126), 6(18) and 34(129), respectively. In the above, the luminosity/mass densities are given using the whole sample with the values in brackets using just those galaxies that lie between 17 and 23 Mpc. We provide a data table of flux densities in all the Herschel bands for all 78 bright Virgo Cluster galaxies.

Sharp H I edges in the outskirts of disk galaxies

Observations indicate that some extended outer disks have a sharp cutoff in the surface density of neutral hydrogen when this approaches the value of ∼2×10 19 cm −2 . In this paper we model these H I edges as places where the ratio of neutral to ionized hydrogen drops rapidly due to ionizing radiation. We use two different models for the vertical distribution of gas above the outer Galactic plane: in the first model we derive the density from the ideal gas law, while in the second model we insert a macroscopic pressure term and derive the density as for an isothermal slab