arXiv subjects

Carlo Giovanardi

Publications and source records attributed to Carlo Giovanardi.

16 recordsLinked to original sources

arXiv2019

Rise and fall of molecular clouds across the M33 disk

We carried out deep searches for CO line emission in the outer disk of M33, at R>7 kpc, and examined the dynamical conditions that can explain variations in the mass distribution of the molecular cloud throughout the disk of M33. We used the IRAM-30~m telescope to search for CO lines in the outer disk toward 12 faint mid-infrared (MIR) selected sources and in an area of the southern outer disk hosting MA1, a bright HII region. We detect narrow CO lines at the location of two MIR sources at galactocentric distances of about 8 kpc that are associated with low-mass young stellar clusters, and at four locations in the proximity of MA1. The paucity of CO lines at the location of weak MIR-selected sources probably arises because most of them are not star-forming sites in M33, but background sources. Although very uncertain, the total molecular mass of the detected clouds around MA1 is lower than expected given the stellar mass of the cluster, because dispersal of the molecular gas is taking place as the HII region expands. The mean mass of the giant molecular clouds (GMCs) in M33 decreases radially by a factor 2 from the center out to 4 kpc, then it stays constant until it drops at R>7 kpc. We suggest that GMCs become more massive toward the center because of the fast rotation of the disk, which drives mass growth by coalescence of smaller condensations as they cross the arms. The analysis of both HI and CO spectral data gives the consistent result that corotation of the two main arms in this galaxy is at a radius of 4.7+-0.3 kpc, and spiral shock waves become subsonic beyond 3.9 kpc. Perturbations are quenched beyond 6.5 kpc, where CO lines have been detected only around sporadic condensations associated with UV and MIR emission.

astro-ph.GA↗

arXiv2018

The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic HI Source Catalog

We present the catalog of ~31500 extragalactic HI line sources detected by the completed ALFALFA survey out to z < 0.06 including both high signal-to-noise ratio (> 6.5) detections and ones of lower quality which coincide in both position and recessional velocity with galaxies of known redshift. We review the observing technique, data reduction pipeline, and catalog construction process, focusing on details of particular relevance to understanding the catalog's compiled parameters. We further describe and make available the digital HI line spectra associated with the catalogued sources. In addition to the extragalactic HI line detections, we report nine confirmed OH megamasers and ten OH megamaser candidates at 0.16 < z < 0.22 whose OH line signals are redshifted into the ALFALFA frequency band. Because of complexities in data collection and processing associated with the use of a feed-horn array on a complex single-dish antenna in the terrestrial radio frequency interference environment, we also present a list of suggestions and caveats for consideration by users of the ALFALFA extragalactic catalog for future scientific investigations.

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arXiv2014

Dynamical signatures of a LCDM-halo and the distribution of the baryons in M33

We map the neutral atomic gas content of M33 using high resolution VLA and GBT observations and fit a tilted ring model to determine the orientation of the extended gaseous disk and its rotation curve. The disk of M33 warps from 8 kpc outwards without substantial change of its inclination with respect to the line of sight. Rotational velocities rise steeply with radius in the inner disk, reaching 100 km/s in 4 kpc, then the rotation curve becomes more perturbed and flatter with velocities as high as 120-130 km/s out to 23 kpc. We derive the stellar mass surface density map of M33's optical disk, via pixel -SED fitting methods based on population synthesis models, which highlights variations in the mass-to-light ratio. The stellar mass surface further out is estimated from deep images of outer disk fields. Stellar and gas maps are then used in the dynamical analysis of the rotation curve to constrain the dark matter distribution which is relevant at all radii. A dark matter halo with a Navarro-Frenk-White density profile in a LCDM cosmology, provides the best fit to the rotation curve for a dark halo concentration C=10 and a total halo mass of 4.3 10^{11}Msun. This imples a baryonic fraction of order 0.02 and the evolutionary history of this galaxy should account for loss of a large fraction of its original baryonic content.

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arXiv2013

The Herschel Virgo Cluster Survey - XIII. Dust in early-type galaxies

Aims. We study the dust content of a large optical input sample of 910 early-type galaxies (ETG) in the Virgo cluster, extending also to the dwarf ETG, and examine the results in relation with those on the other cold ISM components. Methods. We searched for far-infrared emission in all galaxies of the input sample using the 250 micron image of the Herschel Virgo Cluster Survey (HeViCS). This image covers a large fraction of the cluster. For the detected ETG we measured fluxes in 5 bands from 100 to 500 micron, and estimated the dust mass and temperature with modified black-body fits. Results. Dust is detected above the completeness limit of 25.4 mJy at 250 micron in 46 ETG, 43 of which are in the optically complete part of the input sample. In addition dust is present at fainter levels in another 6 ETG. We detect dust in the 4 ETG with synchrotron emission, including M 87. Dust appears to be much more concentrated than stars and more luminous ETG have higher dust temperatures. Dust detection rates down to the 25.4 mJy limit are 17% for ellipticals, about 40% for lenticulars (S0 + S0a) and around 3% for dwarf ETG. Dust mass does not correlate clearly with stellar mass and is often much more than that expected for a passive galaxy in a closed-box model. The dust-to-stars mass ratio anticorrelates with galaxy luminosity, and for some dwarf ETG reaches values as high as for dusty late-type galaxies. In the Virgo cluster slow rotators appear more likely to contain dust than fast ones. Comparing the dust results with those on HI from ALFALFA, there are only 8 ETG detected both in dust and in HI in the HeViCS area; 39 have dust but only an upper limit on HI, and 8 have HI but only an upper limit on dust. The locations of these galaxies in the cluster are different, with the dusty ETG concentrated in the densest regions, while the HI rich ETG are at the periphery.

astro-ph.CO↗

arXiv2012

The Herschel Virgo Cluster Survey. XI. Environmental effects on molecular gas and dust in spiral disks

We investigate the dust-to-gas mass ratio and the environmental effects on the various components of the interstellar medium for a spatially resolved sample of Virgo spirals. We have used the IRAM-30m telescope to map over their full extent NGC 4189, NGC 4298, NGC 4388, and NGC 4299 in the 12CO(1-0) and the 12CO(2-1) lines. We observed the same lines in selected regions of NGC 4351, NGC 4294, and NGC 4424. The CO observations are combined with Herschel maps in 5 bands between 100-500 μm from the HeViCS survey, and with HI data from the VIVA survey, to obtain spatially resolved dust and gas distributions. We studied the environmental dependencies by adding to our sample eight galaxies with 12CO(1-0) maps from the literature. We estimate the integrated mass of molecular hydrogen for the galaxies observed in the CO lines. We find molecular-to-total gas mass fractions between 0.04 \leq fmol \leq 0.65, with the lowest values for the dimmest galaxy in the B-band. The integrated dust-to-gas ratio ranges between 0.011 and 0.004. For the 12 mapped galaxies we derive the radial distributions of the atomic gas, molecular gas, and dust. We also study the effect of different CO-to-H2 conversion factors. Both the molecular gas and the dust distributions show steeper radial profiles for HI-deficient galaxies and the average dust-to-gas ratio for these galaxies increases or stays radially constant. On scales of \sim 3 kpc, we find a strong correlation between the molecular gas and the 250 micron surface brightness that is tighter than average for non-deficient galaxies. The correlation becomes linear if we consider the total gas surface mass density. However, the inclusion of atomic hydrogen does not improve the statistical significance of the correlation. The environment can modify the distributions of molecules and dust within a galaxy, although these components are more tightly bound than the atomic gas.

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arXiv2012

The Herschel Virgo Cluster Survey: X.The relationship between cold dust and molecular gas content in Virgo spirals

Using the far-infrared emission, as observed by the Herschel Virgo Cluster Survey (HeViCS), and the integrated HI and CO brightness, we infer the dust and total gas mass for a magnitude limited sample of 35 metal rich spiral galaxies in Virgo. The CO flux correlates tightly and linearly with far-infrared fluxes observed by Herschel. Molecules in these galaxies are more closely related to cold dust rather than to dust heated by star formation or to optical/NIR brightness. We show that dust mass establishes a stronger correlation with the total gas mass than with the atomic or molecular component alone. The dust-to-gas ratio increases as the HI deficiency increases, but in highly HI deficient galaxies it stays constant. Dust is in fact less affected than atomic gas by weak cluster interactions, which remove most of the HI gas from outer and high latitudes regions. Highly disturbed galaxies, in a dense cluster environment, can instead loose a considerable fraction of gas and dust from the inner regions of the disk keeping constant the dust-to-gas ratio. There is evidence that the molecular phase is also quenched. This quencing becomes evident by considering the molecular gas mass per unit stellar mass. Its amplitude, if confirmed by future studies, highlights that molecules are missing in Virgo HI deficient spirals, but to a somewhat lesser extent than dust.

astro-ph.CO↗

arXiv2011

The population of Young Stellar Clusters throughout the disk of M33

The properties of young stellar clusters (YSCs) in M33, identified from the center out to about twice the size of the bright star-forming disk,are investigated. We find 915 discrete MIR sources as far as the extent of the warped HI disk, i.e. 16 kpc from the galaxy center. Their surface density has a steep radial decline beyond 4.5 kpc, and flattens out beyond the optical radius at 8.5 kpc. We are able to identify YSCs out to 12 kpc. At large galactocentric radii, the paucity of very luminous clusters and the relevance of hot dust emission become evident from the analysis of the bolometric and MIR luminosity functions. The YSC mass and size are correlated with a log-log slope of 2.09, similar to that measured for giant molecular clouds in M33 and the Milky Way, which represent the protocluster environment. Most of the YSCs in our sample have low extinction and ages between 3 and 10 Myr. In the inner regions of M33 the clusters span a wide range of mass (10^2<M<3 10^5 msun) and luminosity 10^38<L{bol}<3 10^{41}erg/s, while at galactocentric radii larger than 4 kpc we find a deficiency of massive clusters. Beyond 7 kpc, where the Halpha surface brightness drops significantly, the dominant YSC population has M<10^3 msun and a slightly older age (10 Myrs). This implies the occurrence of star formation events about 10 Myr ago as far as 10-12 kpc from the center of M33. The cluster L{FUV}--L{Halpha} relation is non-linear for L{FUV}<10^{39}erg/s, in agreement with randomly sampled models of the IMF which, furthermore, shows no appreciable variation throughout the M33 disk.

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arXiv2011

The Herschel Virgo Cluster Survey. IX. Dust-to-gas mass ratio and metallicity gradients in four Virgo spiral galaxies

Using Herschel data from the Open Time Key Project the Herschel Virgo Cluster Survey (HeViCS), we investigated the relationship between the metallicity gradients expressed by metal abundances in the gas phase as traced by the chemical composition of HII regions, and in the solid phase, as traced by the dust-to-gas mass ratio. We derived the radial gradient of the dust-to-gas mass ratio for all galaxies observed by HeViCS whose metallicity gradients are available in the literature. They are all late type Sbc galaxies, namely NGC4254, NGC4303, NGC4321, and NGC4501. We examined different dependencies on metallicity of the CO-to-H$_2$ conversion factor (\xco), used to transform the $^{12}$CO observations into the amount of molecular hydrogen. We found that in these galaxies the dust-to-gas mass ratio radial profile is extremely sensitive to choice of the \xco\ value, since the molecular gas is the dominant component in the inner parts. We found that for three galaxies of our sample, namely NGC4254, NGC4321, and NGC4501, the slopes of the oxygen and of the dust-to-gas radial gradients agree up to $\sim$0.6-0.7R$_{25}$ using \xco\ values in the range 1/3-1/2 Galactic \xco. For NGC4303 a lower value of \xco$\sim0.1\times$ 10$^{20}$ is necessary. We suggest that such low \xco\ values might be due to a metallicity dependence of \xco (from close to linear for NGC4254, NGC4321, and NGC4501 to superlinear for NGC4303), especially in the radial regions R$_G<$0.6-0.7R$_{25}$ where the molecular gas dominates. On the other hand, the outer regions, where the atomic gas component is dominant, are less affected by the choice of \xco, and thus we cannot put constraints on its value.

astro-ph.CO↗

arXiv2011

On the nature of faint mid-infrared sources in M33

We investigate the nature of 24micron sources in M33 which have weak or no associated Halpha emission. Both bright evolved stars and embedded star forming regions are visible as compact infrared sources in the 8 and 24micron maps of M33 and contribute to the more diffuse and faint emission in these bands. Can we distinguish the two populations? We carry out deep CO J=2-1 and J=1-0 line searches at the location of compact mid-IR sources to unveil an ongoing star formation process. We use different assumptions to estimate cloud masses from pointed observations and analyze if SED and mid-IR colours can be used to discriminate between evolved stars and star forming regions. Molecular emission has been detected at the location of several sources at the level of 0.3 K km/s or higher in at least one of the CO rotational lines. Even though there are no giant molecular clouds beyond 4kpc in M33, our deep observations have revealed that clouds of smaller mass are very common. Sources which are known to be evolved variable stars show weaker or undetectable CO lines. Evolved stars occupy a well defined region of the IRAC color-color diagrams. Star forming regions are scattered throughout a larger area even though the bulk of the distribution has different IRAC colors than evolved variable stars. We estimate that about half of the 24 micron sources without an Halpha counterpart are genuine embedded star forming regions. Sources with faint but compact Halpha emission have an incomplete Initial Mass Function (IMF) at the high-mass end and are compatible with a population of young clusters with a stochastically sampled, universal IMF.

astro-ph.CO↗

arXiv2010

The Cluster Birthline and the formation of stellar clusters in M33

We present a new method to analyze the IMF at its high mass end in young stellar clusters, which rely on two integrated observables: the cluster bolometric and Halpha luminosity. Using several cluster samples selected in M33 we show that a stochastically sampled universal IMF is in better agreement with the data than a truncated IMF whose maximum stellar mass depends on cluster mass. We also discuss the possibility that a delayed formation of massive stars is taking place in low density star forming regions as an alternative to a strong leakage of ionizing photons from HII regions of young luminous clusters.

astro-ph.CO↗

arXiv2009

The cluster birthline in M33

We test the reliability of infrared (IR) emission to trace star formation in individual star-forming sites of M33, and outline a new method for testing the distribution function of massive stars in newly formed clusters. We select IR sources from the Spitzer survey of M33 and show that the IR and Halpha luminosities are not correlated. Complementing the infrared photometry with GALEX-UV data, we estimate the source bolometric luminosities. For a given stellar IMF we simulate a theoretical curve for the expected bolometric-to-Halpha luminosity ratio, along which stellar clusters are born. We call this the cluster birthline in the Lbol--Lbol/LHal plane. The birthline is flat for Lbol>3x10^{39}erg/s because all clusters fully sample the IMF and it increases toward lower luminosities as the upper end of the IMF becomes incompletely sampled. The observations of M33 show that young isolated clusters lie close to the theoretical birthline for a wide range of Lbol. The luminosity is not proportional to Halpha emission for low mass clusters and aging moves clusters above the birthline. The best fit to the birthline is for a randomly sampled IMF, in which the mass of most massive star in a cluster is not strictly limited by the cluster's mass. We also find that the IR luminosity of young stellar clusters in M33 is not proportional to their bolometric luminosity. This irregularity could be the result of low and patchy dust abundance in M33.

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arXiv1998

Near-infrared surface photometry of early-type spiral galaxies: II. M/L ratios and dark halos

We present mass distributions obtained from near-infrared (NIR) surface brightness decompositions and rotation curve fitting of a sample of early-type spiral galaxies. Bulge and disk mass-to-light (M/L) ratios, dark halo parameters, and the modified Newtonian dynamics (MOND) acceleration parameter are derived. We find that the mean disk NIR M/L is higher than that of the bulge, and comparison with stellar population synthesis models implies that early-type spiral bulges are, on average, younger and more metal rich than disks. NIR disk M/L is found to depend on disk luminosity, consistently with previously reported trends in ellipticals and spirals, and with cold dark matter models for disk formation. Within the optical radius, the mean ratio of stellar to dark matter is 2 and the typical dark halo mass is 10^{11} M_o. The value of the MOND acceleration parameter that best accommodates the sample as a whole is 1.3 10^{-8} cm s^{-2}.

astro-ph↗

arXiv1998

Near-infrared surface photometry of early-type spiral galaxies: I. Bulge and disk decomposition

We present near-infrared (NIR) surface photometry of a sample of 14 early-type spirals with observed rotation curves. In this first paper, we report the results of two-dimensional parametric and non-parametric decompositions to separate the bulge and disk components; the parametric bulge is modeled with a generalized exponential law of integer index n, and the disk with a simple exponential. We find that the derived bulge parameters, for a given galaxy, vary systematically with the bulge shape index n. The mean early-type bulge has a best-fit n = 2.6, and 80% of the sample has best n of 2 or 3. Bulges are rarely spherical; the median bulge intrinsic ellipticity is 0.33. The median early-type disk has (J-K)_d more than 0.1 mag bluer than the bulge, and a NIR disk surface brightness more than 1 mag arcsec^{-2} brighter than later-type disks. Our data are consistent with the well-established correlation of both bulge and disk surface brightness with physical scale length, and we note that the location of bulges within this projection of the fundamental plane depends on their shape index n. In agreement with previous work, the ratios of bulge and disk scale lengths are consistent with a constant value r_e/r_d = 0.3; however, such value again depends on the bulge index n, implying that claims for a scale-free Hubble sequence may be premature.

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arXiv1997

On the Dust Extinction in High-z Galaxies and the Case of Extremely Red Objects

We present the preliminary results of Monte Carlo simulations aimed to investigate the effects of realistic dust extinction (absorption + scattering) on the colours of high-z galaxies. In this paper, we concentrate on the case of spheroidal galaxies, and we obtain attenuation curves in the range 0.1-1.2$μ$m for different dust spatial distributions, and for a range of values of the dust optical depth, geometrical thickness and inclination of the dust disk. We find that the resultant curves are strongly dependent on the dust geometrical distribution and optical depth. A serendipitous finding is that the strength of the 2200 Åabsorption feature depends not only on the optical depth, but also on the dust geometrical distribution. As a first application, we test our results on two high-z galaxies with extremely red colours (HR10 and HR14) in order to infer clues on their ages, dust content and dust spatial distribution. We confirm that HR10 must be a very dusty galaxy, and we suggest that its stellar component should be strongly embedded in the dust in order to reproduce the observed extremely red colours.

astro-ph↗

arXiv1996

The Effect of Light Scattering by Dust in Galactic Halos on Emission Line Ratios

We present results from Monte Carlo simulations describing the radiation transfer of $Hα$ line emission, produced both by HII regions in the disk and in the diffuse ionized gas (DIG), through the dust layer of the galaxy NGC891. This allows us to calculate the amount of light originating in the HII regions of the disk and scattered by dust at high $z$, and compare it with the emission produced by recombinations in the DIG. The cuts of photometric and polarimetric maps along the $z$-axis show that scattered light from HII regions is still 10\% of that of the DIG at $z\sim 600$~pc, whereas the the degree of linear polarization is small ($<1$\%). The importance of these results for the determination of intrinsic emission line ratios is emphasized, and the significance and possible implications of dust at high $z$ are discussed.

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arXiv1995

Monte Carlo simulations of dusty spiral galaxies: extinction and polarization properties

We present Monte Carlo simulations of dusty spiral galaxies, modelled as bulge + disk systems, aimed to study their extinction and polarization properties. The extinction parameters (absorption and scattering) of dust grains are calculated from Mie's theory for a full distribution of sizes and materials; the radiation transfer is carried on for the four Stokes parameters. Photometric and polarimetric maps of galaxies of different optical depths, inclinations and bulge-to-total ratios have been produced in the B and I bandpasses. As expected, the effect of scattering is to reduce substantially the extinction for a given optical depth, in particular for what concerns the obscuration of bright bulge cores. For the same reason, scattering reduces also the reddening, as evaluated from B-I maps. On the other hand the bluing directly due to forward scattering is hardly appreciable. Radial color gradients are often found. A comparison with ``sandwich'' models shows that they fail dramatically to reproduce the extinction - optical depth relation. The degree of linear polarization produced by scattering is usually of the order of a few percent; it increases with optical depth, and with inclination (less than 80 degrees). The polarization pattern is always perpendicular to the major axis, unless the dust distribution is drastically modified. There is little local correlation between extinction and polarization degree and there is a trend of increasing polarization from the B to the I band. We discuss implications and relevance of the results for studies of the structure and morphology of spiral galaxies and of their interstellar medium.

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