arXiv subjects

C. Giovanardi

Publications and source records attributed to C. Giovanardi.

At least 19 recordsLinked to original sources

arXiv2016

The Herschel Virgo Cluster Survey. XX. Dust and gas in the foreground Galactic cirrus

We study the correlation between far-infared/submm dust emission and atomic gas column density in order to derive the properties of the high Galactic latitude, low density, Milky Way cirrus in the foreground of the Virgo cluster of galaxies. Dust emission maps from 60 to 850 um are obtained from SPIRE observations carried out within the Herschel Virgo Cluster Survey, complemented by IRAS-IRIS and Planck-HFI maps. Data from the Arecibo legacy Fast ALFA Survey is used to derive atomic gas column densities for two broad velocity components, low and intermediate velocity clouds. Dust emissivities are derived for each gas component and each far-infared/submm band. For the low velocity clouds, we measure an average emissivity 0.79 +/- 0.08 times 1E-20 MJy sr^-1 cm^2 at 250 um. After fitting a modified blackbody to the available bands, we estimated a dust absorption cross-section 0.49 +/- 0.13 times 1E-25 cm^2 H^-1 at 250 um (with dust temperature T = 20.4 +/- 1.5 K and spectral index beta = 1.53 +/- 0.17). The results are in excellent agreement with those obtained by Planck over a much larger coverage of the high Galactic latitude cirrus (50% of the sky vs 0.2% in our work). For dust associated with intermediate velocity gas, we confirm earlier Planck results and find a higher temperature and lower emissivity and cross-section. After subtracting the modelled components, we find regions at scales smaller than 20' where the residuals deviate significantly from the average, cosmic-infrared-background dominated, scatter. These large residuals are most likely due to local variations in the cirrus dust properties (and/or the dust/atomic-gas correlation) or to high-latitude molecular clouds with average N_H2 <~ 1E20 cm^-2. We find no conclusive evidence for intracluster dust emission in Virgo.

astro-ph.GA↗

arXiv2016

Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

We present $^{12}$CO(1-0) and $^{12}$CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 $μ$m emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses <~ 10$^9$ M$_{\odot}$, contrary to the atomic hydrogen fraction, M$_{HI}$/M$_*$, which increases inversely with M$_*$. The flattening of the M$_{H_2}$/M$_*$ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, $τ_{dep}$, are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between HI deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity.

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arXiv2011

Young stellar clusters and associations in M33

We analyse multi-wavelength observations of 32 young star clusters and associations in M33 with known oxygen abundance (8 < 12 + log(O/H) < 8.7), using ultraviolet (UV), optical, mid-infrared (MIR), CO (1-0) and 21-cm line (HI) observations. We derive their spectral energy distribution, and we determine age, bolometric luminosities, masses and the extinction, by comparing the multi-band integrated photometry to single-age stellar population models. The stellar system ages range between 2 and 15 Myr, masses are between 3 x 10^2 and 4 x 10^4 M_sun, and the intrinsic extinction, A_V, varies from 0.3 to 1 mag. We find a correlation between age and extinction, and between the cluster mass and size. The MIR emission shows the presence of a dust component around the clusters whose fractional luminosity at 24 um, L_{24}/L_{Bol}, decreases with the galactocentric distance. However, the total IR luminosity inferred from L_{24} is smaller than what we derive from the extinction corrections. The Halpha luminosity predicted by population synthesis models is larger than the observed one, especially for low-mass systems (M < 10^4 M_sun). Such a difference is reduced, but not erased, when the incomplete sampling of the initial mass function (IMF) at the high-mass end is taken into account. Our results suggest that a non-negligible fraction of UV ionising and non-ionising radiation is leaking into the ISM outside the HII regions. This would be in agreement with the large UV and Halpha diffuse fractions observed in M33, but it implies that stellar systems younger than 3 Myr retain, on average, only 30% of their Lyman continuum photons. However, the uncertainties on cluster ages and the stochastic fluctuations of the IMF do not allow to accurately quantify this issue. We also consider the possibility that this discrepancy is the consequence of a suppressed or delayed formation of the most massive stars.

astro-ph.CO↗

arXiv2011

Stellar structures in the outer regions of M33

We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the outer regions of M33. Our aim is to search for stellar structures corresponding to extended HI clouds found in a recent 21-cm survey of the galaxy. Three fields probe a large HI complex to the southeastern (SE) side of the galaxy. An additional three fields cover the northwestern (NW) side of the galaxy along the HI warp. A final target field was chosen further north, at a projected distance of approximately 25 kpc, to study part of the large stellar plume recently discovered around M33. We analyse the stellar population at R > 10 kpc by means of V, I colour magnitude diagrams reaching the red clump. Evolved stellar populations are found in all fields out to 120' (~ 30 kpc), while a diffuse population of young stars (~ 200 Myr) is detected out to a galactocentric radius of 15 kpc. The mean metallicity in the southern fields remains approximately constant at [M/H] = -0.7 beyond the edge of the optical disc, from 40' out to 80'. Along the northern fields probing the outer \hi disc, we also find a metallicity of [M/H] = -0.7 between 35' and 70' from the centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120'. In the northernmost field, outside the disc extent, the stellar population of the large stellar feature possibly related to a M33-M31 interaction is on average more metal-poor ([M/H] = -1.3) and older (> 6 Gyr). An exponential disc with a large scale-length (~ 7 kpc) fits well the average distribution of stars detected in both the SE and NW regions from a galactocentric distance of 11 kpc out to 30 kpc. The stellar distribution at large radii is disturbed and, although there is no clear correlation between the stellar substructures and the location of the HI clouds, this gives evidence for tidal interaction or accretion events.

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arXiv2010

The far-infrared view of M87 as seen by the Herschel Space Observatory

The origin of the far-infrared emission from the nearby radio galaxy M87 remains a matter of debate. Some studies find evidence of a far-infrared excess due to thermal dust emission, whereas others propose that the far-infrared emission can be explained by synchrotron emission without the need for an additional dust emission component. We observed M87 with PACS and SPIRE as part of the Herschel Virgo Cluster Survey (HeViCS). We compare the new Herschel data with a synchrotron model based on infrared, submm and radio data to investigate the origin of the far-infrared emission. We find that both the integrated SED and the Herschel surface brightness maps are adequately explained by synchrotron emission. At odds with previous claims, we find no evidence of a diffuse dust component in M87.

astro-ph.CO↗

arXiv2010

The Herschel Virgo Cluster Survey: III. A constraint on dust grain lifetime in early-type galaxies

Passive early-type galaxies (ETGs) provide an ideal laboratory for studying the interplay between dust formation around evolved stars and its subsequent destruction in a hot gas. Using Spitzer-IRS and Herschel data we compare the dust production rate in the envelopes of evolved AGB stars with a constraint on the total dust mass. Early-type galaxies which appear to be truly passively evolving are not detected by Herschel. We thus derive a distance independent upper limit to the dust grain survival time in the hostile environment of ETGs of < 46 +/- 25 Myr for amorphous silicate grains. This implies that ETGs which are detected at far-infrared wavelengths have acquired a cool dusty medium via interaction. Given likely time-scales for ram-pressure stripping, this also implies that only galaxies with dust in a cool (atomic) medium can release dust into the intra-cluster medium.

astro-ph.CO↗

arXiv2010

The Herschel Virgo Cluster Survey: II. Truncated dust disks in HI-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.

astro-ph.CO↗

arXiv2010

The Herschel Virgo Cluster Survey: VI. The far-infrared view of M87

The origin of the far-infrared emission from the nearby radio galaxy M87 remains a matter of debate. Some studies find evidence of a far-infrared excess due to thermal dust emission, whereas others propose that the far-infrared emission can be explained by synchrotron emission without the need for an additional dust emission component. We present Herschel PACS and SPIRE observations of M87, taken as part of the science demonstration phase observations of the Herschel Virgo Cluster Survey. We compare these data with a synchrotron model based on mid-infrared, far-infrared, submm and radio data from the literature to investigate the origin of the far-infrared emission. Both the integrated SED and the Herschel surface brightness maps are adequately explained by synchrotron emission. At odds with previous claims, we find no evidence of a diffuse dust component in M87, which is not unexpected in the harsh X-ray environment of this radio galaxy sitting at the core of the Virgo Cluster.

astro-ph.CO↗

arXiv2010

The Herschel Virgo Cluster Survey: I. Luminosity functions

We describe the Herschel Virgo Cluster Survey (HeViCS) and the first data obtained as part of the Science Demonstration Phase (SDP). The data cover a central 4x4 sq deg region of the cluster. We use SPIRE and PACS photometry data to produce 100, 160, 250, 350 and 500 micron luminosity functions (LFs) for optically bright galaxies that are selected at 500 micron and detected in all bands. We compare these LFs with those previously derived using IRAS, BLAST and Herschel-ATLAS data. The Virgo Cluster LFs do not have the large numbers of faint galaxies or examples of very luminous galaxies seen previously in surveys covering less dense environments.

astro-ph.CO↗

arXiv2010

The Herschel Virgo Cluster Survey: IV. Resolved dust analysis of spiral galaxies

We present a resolved dust analysis of three of the largest angular size spiral galaxies, NGC 4501 and NGC 4567/8, in the Herschel Virgo Cluster Survey (HeViCS) Science Demonstration field. Herschel has unprecedented spatial resolution at far-infrared wavelengths and with the PACS and SPIRE instruments samples both sides of the peak in the far infrared spectral energy distribution (SED).We present maps of dust temperature, dust mass, and gas-to-dust ratio, produced by fitting modified black bodies to the SED for each pixel. We find that the distribution of dust temperature in both systems is in the range ~19 - 22 K and peaks away from the centres of the galaxies. The distribution of dust mass in both systems is symmetrical and exhibits a single peak coincident with the galaxy centres. This Letter provides a first insight into the future analysis possible with a large sample of resolved galaxies to be observed by Herschel.

astro-ph.CO↗

arXiv2010

The Herschel Virgo Cluster Survey: VII. Dust in cluster dwarf elliptical galaxies

We use the Science Demonstration Phase data of the Herschel Virgo Cluster Survey to search for dust emission of early-type dwarf galaxies in the central regions of the Virgo Cluster as an alternative way of identifying the interstellar medium.We present the first possible far-infrared detection of cluster early-type dwarf galaxies: VCC781 and VCC951 are detected at the 10 sigma level in the SPIRE 250 micron image. Both detected galaxies have dust masses of the order of 10^5 Msun and average dust temperatures ~20K. The detection rate (less than 1%) is quite high compared to the 1.7% detection rate for Hi emission, considering that dwarfs in the central regions are more Hi deficient. We conclude that the removal of interstellar dust from dwarf galaxies resulting from ram pressure stripping, harassment, or tidal effects must be as efficient as the removal of interstellar gas.

astro-ph.CO↗

arXiv2010

The Herschel Virgo cluster survey: V. Star-forming dwarf galaxies - dust in metal-poor environments

We present the dust properties of a small sample of Virgo cluster dwarf galaxies drawn from the science demonstration phase data set of the Herschel Virgo Cluster Survey. These galaxies have low metallicities (7.8 < 12 + log(O/H) < 8.3) and star-formation rates < 10^{-1} M_{sun}/yr. We measure the spectral energy distribution (SED) from 100 to 500 um and derive dust temperatures and dust masses. The SEDs are fitted by a cool component of temperature T < 20 K, implying dust masses around 10^{5} M_{sun} and dust-to-gas ratios D within the range 10^{-3}-10^{-2}. The completion of the full survey will yield a larger set of galaxies, which will provide more stringent constraints on the dust content of star-forming dwarf galaxies.

astro-ph.CO↗

arXiv2010

FIR colours and SEDs of nearby galaxies observed with Herschel

We present infrared colours (in the 25-500 mic spectral range) and UV to radio continuum spectral energy distributions of a sample of 51 nearby galaxies observed with SPIRE on Herschel. The observed sample includes all morphological classes, from quiescent ellipticals to active starbursts. Active galaxies have warmer colour temperatures than normal spirals. In ellipticals hosting a radio galaxy, the far-infrared (FIR) emission is dominated bynthe synchrotron nuclear emission. The colour temperature of the cold dust is higher in quiescent E-S0a than in star-forming systems probably because of the different nature of their dust heating sources (evolved stellar populations, X-ray, fast electrons) and dust grain properties. In contrast to the colour temperature of the warm dust, the f350/f500 index sensitive to the cold dust decreases with star formation and increases with metallicity, suggesting an overabundance of cold dust or an emissivity parameter beta<2 in low metallicity, active systems.

astro-ph.CO↗

arXiv2009

Star formation in M33: the radial and local relations with the gas

In the Local Group spiral galaxy M33, we investigate the correlation between the star formation rate (SFR) surface density, Sigma_SFR, and the gas density Sigma_gas (molecular, atomic, and total). We also explore whether there are other physical quantities, such as the hydrostatic pressure and dust optical depth, which establish a good correlation with Sigma_SFR. We use the Ha, far-ultraviolet (FUV), and bolometric emission maps to infer the SFR locally at different spatial scales, and in radial bins using azimuthally averaged values. Most of the local analysis is done using the highest spatial resolution allowed by gas surveys, 180 pc. The Kennicutt-Schmidt (KS) law, Sigma_SFR \propto (Sigma_gas)^n is analyzed by three statistical methods. At all spatial scales, with Ha emission as a SFR tracer, the KS indices n are always steeper than those derived with the FUV and bolometric emissions. We attribute this to the lack of Ha emission in low luminosity regions where most stars form in small clusters with an incomplete initial mass function at their high mass end. For azimuthally averaged values the depletion timescale for the molecular gas is constant, and the KS index is n_H2 = 1.1 +- 0.1. Locally, at a spatial resolution of 180 pc, the correlation between Sigma_SFR and Sigma_gas is generally poor, even though it is tighter with the molecular and total gas than with the atomic gas alone. Considering only positions where the CO J=1-0 line is above the 2-sigma detection threshold and taking into account uncertainties in Sigma_H2 and Sigma_SFR, we obtain a steeper KS index than obtained with radial averages: n_H2 = 2.22 +- 0.07 (for FUV and bolometric SFR tracers), flatter than that relative to the total gas (n_Htot = 2.59 +- 0.05). [abridged]

astro-ph.CO↗

arXiv2009

The HI content of early-type galaxies from the ALFALFA survey. II. The case of low density environments

We present the analysis of the HI content of a sample of early-type galaxies (ETGs) in low-density environments (LDEs) using the data set provided by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We compare their properties to the sample in the Virgo cluster that we studied in a previous paper (di Serego Alighieri et al. 2007, Paper I). We have selected a sample of 62 nearby ETGs (V< 3000 km/s) in an area of the sky where the ALFALFA data are already available (8h -17). In both cases it is 10 times higher than that of the Virgo cluster. The presence of gas can be related to a recent star formation activity: 60% of all ETGs with HI have optical emission line ratios typical of star-forming galaxies and blue colours suggesting the presence of young stellar populations, especially in the dwarf subsample. We show that the HI detection rate of ETGs depends both on the environment and mass. The fraction of early-type systems with neutral hydrogen is higher in more massive objects when compared to early-type dwarfs. The ETGs in LDEs seem to have more heterogeneous properties than their Virgo cluster counterparts, since they are able to retain a cold interstellar gas component and to support star formation activity even at recent epochs.

astro-ph.CO↗

arXiv2008

Star formation in M33: multiwavelength signatures across the disk

Aims. We use different tracers, such as Ha, ultraviolet (UV), and infrared (IR) emissions at various wavelengths, to study the dust and star-formation (SF) conditions throughout the disk of M33. Methods. We derive the radial distribution of dust, of the old and young stellar population using Spitzer and GALEX data, complemented by ground-based optical data and available surveys of atomic and molecular gas. We separate the contribution of discrete sources to the IR brightness from the diffuse emission. Results. At 8 and 24 um, discrete sources account for 40% of the IR emission in the innermost 3 kpc, and for 20% further out. We find that stochastic emission from very small grains in the diffuse interstellar medium accounts for only 10% of the diffuse 24 um emission, and that dusty circumstellar shells of unresolved, evolved AGB stars (carbon stars) are a viable alternative. The 8 um profile suggests that PAH emission declines faster with radius than the dust continuum. In annular regions, 0.24 kpc wide, we find a mean extinction value for stellar continuum AV = 0.25 mag with a weak dependence on radius, consistent with the shallow metallicity gradient observed. Dust opacity derived from the 160 um emission decreases instead by a factor 10 from the center to edge of the SF disk. Conclusions. Using extinction corrected UV and Ha maps we find the global SF rate in M33, over the last 100 Myr, to be 0.45 +- 0.10 M yr-1. FIR (far-IR) and TIR (total-IR) luminosities can trace SF even though a high conversion factor is required to recover the effective rate. If carbon stars are powering the diffuse 24 um emission in M33 this can trace star formation 1 Gyr ago and provide a more complete view of the SF history of the galaxy. [abridged]

astro-ph↗

arXiv2008

HI clouds in the proximity of M33

Neutral hydrogen clouds are found in the Milky Way and Andromeda halo both as large complexes and smaller isolated clouds. Here we present a search for Hi clouds in the halo of M33, the third spiral galaxy of the Local Group. We have used two complementary data sets: a 3^o x 3^o map of the area provided by the Arecibo Legacy Fast ALFA (ALFALFA) survey and deeper pointed observations carried out with the Arecibo telescope in two fields that permit sampling of the north eastern and south-western edges of the HI disc. The total amount of Hi around M33 detected by our survey is $\sim 10^7$ M$_{\odot}$. At least 50% of this mass is made of HI clouds that are related both in space and velocity to the galaxy. We discuss several scenarios for the origin of these clouds focusing on the two most interesting ones: $(a)$ dark-matter dominated gaseous satellites, $(b)$ debris from filaments flowing into M33 from the intergalactic medium or generated by a previous interaction with M31. Both scenarios seem to fit with the observed cloud properties. Some structures are found at anomalous velocities, particularly an extended HI complex previously detected by Thilker et al. (2002). Even though the ALFALFA observations seem to indicate that this cloud is possibly connected to M33 by a faint gas bridge, we cannot firmly establish its extragalactic nature or its relation to M33. Taking into account that the clouds associated with M33 are likely to be highly ionised by the extragalactic UV radiation, we predict that the total gas mass associated with them is > 5 x 10^7 M$_{\odot}$. If the gas is steadily falling towards the M33 disc it can provide the fuel needed to sustain a current star formation rate of 0.5 M$_{\odot}$ yr$^{-1}$.

astro-ph↗

arXiv2008

The HI distribution in the outskirts of M33 with the ALFALFA survey

Spiral galaxies appear to be dynamical systems whose disks are still forming at the current epoch and which continue to accrete mass. The presence of extraplanar gas in spirals indicates that galactic halos can contain at least part of the material needed to fuel the star formation activity in their disks. Here we present the analysis of the ALFALFA survey data in the region of M33 aimed at searching high velocity clouds around this Local Group galaxy. We find a varied population of HI clouds with masses ranging between 4 x 10^4 and few times 10^6 M_sun. We also detect an extended HI complex at anomalous velocities, whose extragalactic nature cannot be firmly established. We estimate that the total amount of neutral hydrogen mass associated to these clouds is around 10^7 M_sun.

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