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. The implied dark matter mass, within the total gaseous extent, is a factor 5 higher than the visible baryonic mass. Dark matter density profiles with an inner cusp as steep as R^{-1}, are compatible with M33 data. The dark matter concentrations required for fitting the M33 rotation curve are very low but marginally consistent with halos 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 spheroid 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 disk 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 H-alpha surface brightness if a shear rate criterion is used. The classical Toomre criterion predicts correctly the size of the unstable region only when the stellar or dark halo gravity, derived in this paper, is added to that of the gaseous disk.Comment: 14 pages, MNRAS in pres

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.Comment: 11 pages, 10 figures, MNRAS latex style, accepted by MNRA

Testing MOND with Local Group spiral galaxies

The rotation curves and the relative mass distributions of the two nearby Local Group spiral galaxies, M31 and M33, show discrepancies with Modified Newtonian dynamic (MOND) predictions. In M33 the discrepancy lies in the kinematics of the outermost regions. It can be alleviated by adopting tilted ring models compatible with the 21-cm datacube but different from the one that best fits the data. In M31 MOND fails to fit the falling part of the rotation curve at intermediate radii, before the curve flattens out in the outermost regions. Newtonian dynamics in a framework of a stellar disc embedded in a dark halo can explain the complex rotation curve profiles of these two galaxies, while MOND has some difficulties. However, given the present uncertainties in the kinematics of these nearby galaxies, we cannot address the success or failure of MOND theory in a definite way. More sensitive and extended observations around the critical regions, suggested by MOND fits discussed in this paper, may lead to a definite conclusion.Comment: 6 pages, 4 figures. To be published in MNRA

The Onset of the Cold HI Phase in Disks of Protogalaxies

We discuss a possible delay experienced by protogalaxies with low column density of gas in forming stars over large scales. After the hydrogen has recombined, as the external ionizing UV flux decreases and the metal abundance $Z$ increases, the HI, initially in the warm phase (T\simgt 5000 K), makes a transition to the cool phase (T\simlt 100 K). The minimum abundance $Z_{min}$ for which this phase transition takes place in a small fraction of the Hubble time decreases rapidly with increasing gas column density. Therefore in the ``anemic'' disk galaxies, where $N_{HI}$ is up to ten times smaller than for normal large spirals, the onset of the cool HI phase is delayed. The onset of gravitational instability is also delayed, since these objects are more likely to be gravitationally stable in the warm phase than progenitors of today's large spiral galaxies. The first substantial burst of star formation may occur only as late as at redshifts $z \sim 0.5$ and give a temporary high peak luminosity, which may be related to the ``faint blue objects". Galaxy disks of lower column density tend to have lower escape velocities and a starburst/galactic fountain instability which decreases the gas content of the inner disk drastically.Comment: TeX file, 24 pages, 4 figures available upon request from [email protected], to appear in The Astrophysical J. (Sept. 1

Instabilities in Photoionized Interstellar Gas

We present a linear analysis of acoustic and thermo-reactive instabilities in a diffuse gas, photoionized and heated by a radiation field, cooled by collisional excitation of hydrogen and metal lines. The hydrogen recombination reaction has a stabilizing effect on the thermal mode found by Field (1965) since the condensation instability occurs in a narrower region of the parameter space and grows on longer time scales due to its oscillatory character. This effect is stronger when the mean photon energy is not much larger than the hydrogen ionization energy. Moreover, for fixed values of thermal pressure and photoionization rate, there are thermo- reactive unstable equilibria for which no transition to a stable phase is possible. By extending our analysis of the thermo-reactive modes to the nonlinear regime we show that when no phase transition is possible the medium evolves through a series of nonequilibrium states characterized by large amplitude, nonlinear periodic oscillations of temperature, density and hydrogen ionization fraction. We find also unstable acoustic waves which, for solar metal abundances, are the fastest growing modes in two temperature intervals: around $T\sim 100$~K and $T\sim 8000$~K (i.e. cold and warm phase, respectively), independent of the mean photon energy. Possible implications for the interstellar medium and intergalactic medium are briefly outlined.Comment: TeX file, 24 pages, 6 figures available upon request from [email protected], to appear in The Astrophysical J. (July 10

Radio emission during the formation of stellar clusters in M33

We investigate thermal and non-thermal radio continuum associated with the early formation and evolution of Young Stellar Clusters (YSCs) selected by their MIR emission in M33. For the first time in an external galaxy it has been possible to identify radio counterparts to more than 300 star forming regions. We proof the nature of candidate YSCs fully embedded in molecular clouds, by recovering their associated faint radio continuum luminosities. Using the Halpha line to identify free-free radio emission at 5 GHz in the more evolved, partially exposed YSCs, we retrieve information on the relevance of magnetic fields and cosmic rays across the M33 disk at 25 pc spatial scales. A cross-correlation of MIR and radio continuum luminosities is established from bright to very faint YSCs, with MIR-to-radio emission ratio showing a gradual decline towards the outer disk, while the magnetic field is pervasive at all radii. We establish and discuss the tight relation between radio continuum and other star formation indicators, such as Halpha. This relation holds for individual YSCs over four orders of magnitude as well as for molecular clouds hosting YSCs. On average about half of radio emission at 5 GHz in YSCs is non-thermal. For exposed but compact YSCs the non-thermal radio fraction increases with source brightness, while for large HII regions the fraction is lower and shows no clear trend. This has been found for YSCs with and without identified SNRs and underlines the possible role of massive stars in triggering particle acceleration through winds and shocks: these particles diffuse throughout the native molecular cloud prior to cloud dispersal.Comment: 14 pages, 10 figures, accepted for publication in A&

Gas and Star Formation in M33: An Artistic Pathway

M33 is the closest blue, star forming, flocculent spiral galaxy for which it has been possible to combine an overwhelming quantity of multiwavelength high resolution data to shed light on its assembly and star formation across cosmic time. I will summarize some of the key ingredients related to the formation and evolution of this galaxy, such as its dark matter, the baryonic distribution and the metallicity gradients. M33 is a pure disk galaxy with a lower baryonic fraction than M31, of order 0.02, and a dark matter profile typical of structure growth in ΛCDM cosmology. Disk dynamics and the growth of perturbations can be visualized in a detailed 2-D map. The consequent star forming sites across the disk, analyzed using mid-infrared observations, points out young stellar clusters spanning 4 orders of magnitude in luminosity. This database has allowed to study IMF sampling at the high mass end and the concept of a cluster birthline. Stars and gas, present beyond 2-optical radii, point out to the occurrence of possible cosmic gas infall fueling star formation. Bruce Elmegreen’s outstanding contribution to science becomes evident in the analysis of M33, here underlined also through an artistic pathway

The lifecycle of molecular clouds and the extragalactic side of Francesco

I outline the interest of Francesco for star formation in nearby galaxies and the main results of the latest project Francesco was interested in and actively working at. This involves the association between giant molecular clouds (GMCs) and young stellar cluster candidates (YSCCs) in M33. The GMCs have a remarkable spatial correlation with infrared selected YSCCs, stronger than with Halpha or optically selected sources, with a typical separation of 17 pc. Through age estimates of the YSCCs, and a classification of GMCs according to various evolutionary stages, we estabish that 14 Myrs is a typical lifetime of a GMC in M33 with the inactive and embedded phases lasting about 4 and 2 Myrs respectively