C. Giovanardi

THE ARECIBO LEGACY FAST ALFA SURVEY. I. SCIENCE GOALS, SURVEY DESIGN, AND STRATEGY

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map ~7000 deg2 of the high Galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range between -1600 and 18,000 km s-1 with ~5 km s-1 resolution. Exploiting Arecibos large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H I mass function in the local universe and will provide a census of H I in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H I diameter function, and enable a first wide-area blind search for local H I tidal features, H I absorbers at z < 0.06, and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H I line sources out to z ~ 0.06, including several hundred with H I masses M < 107.5 M⊙.

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.

Star formation in M 33: multiwavelength signatures across the disk

Aims. We use different tracers, such as Hα, ultraviolet (UV), and infrared (IR) emissions at various wavelengths, to study the dust and star formation (SF) conditions throughout the disk of M 33. 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 μm, 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 μm emission, and that dusty circumstellar shells of unresolved, evolved AGB stars (carbon stars) are a viable alternative. The 8 μ mp rof ile 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 the 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 μm emission decreases instead by a factor of 10 from the center to edge of the star forming disk. Conclusions. Using extinction corrected UV and Hα maps we find the global SF rate in M 33 over the last 100 Myr to be

FIR colours and SEDs of nearby galaxies observed with Herschel

We present infrared colours (in the 25-500 mu m 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 by the 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.

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

Aims. 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, A&A, 474, 851, Paper I). Our aim is both to investigate how the cool interstellar medium (ISM) of these systems depends on the galaxy mass and the environment and to relate the properties of the neutral hydrogen to the warm phases of the ISM. Methods. We have selected a sample of 62 nearby ETGs (V −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. Conclusions. 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.

The HI content of early-type galaxies from the ALFALFA survey - I. Catalogued HI sources in the Virgo cluster

Aims. We are using the Arecibo Legacy Fast ALFA survey (ALFALFA), which is covering 17% of the sky at 21 cm, to study the HI content of early-type galaxies (ETG) in an unbiased way. The aim is to get an overall picture of the hot, warm and cold ISM of ETG, as a function of galaxy mass and environment, to understand its origin and fate, and to relate it to the formation and evolution history of these objects. Methods. This paper deals with the first part of our study, which is devoted to the 8-16 deg. declination strip in the Virgo cluster. In this sky region, using the Virgo Cluster Catalogue (VCC), we have defined an optical sample of 939 ETG, 457 of which are brighter than the VCC completeness limit at B T = 18.0. We have correlated this optical sample with the catalogue of detected HI sources from ALFALFA. Results. Out of the 389 ETG from the VCC with B T < 18.0, outside the 1 deg. region of poor HI detection around M 87, and corrected for background contamination of VCC galaxies without a known radial velocity, only 9 galaxies (2.3%) are detected in HI with a completeness limit of 3.5 and 7.6 x 10 7 M ⊙ of HI for dwarf and giant ETG, respectively. In addition 4 VCC ETG with fainter magnitudes are also detected. Our HI detection rate is lower than previously claimed. The majority of the detected ETG appear to have peculiar morphology and to be located near the edges of the Virgo cluster. Conclusions. Our preliminary conclusion is that cluster ETG contain very little neutral gas, with the exceptions of a few peculiar dwarf galaxies at the edge of the ETG classification and of very few larger ETG, where the cold gas could have a recent external origin.

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

We present near-infrared (NIR) surface pho- tometry of a sample of 14 early-type spirals with observed rotation curves. In this rst paper, we report the results of two-dimensional parametric and non-parametric decom- positions to separate the bulge and disk components; the parametric bulge is modeled with a generalized exponen- tial law of integer index n, and the disk with a simple exponential. We nd 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- t n = 2.6, and 80% of the sample has best n of 2 or 3. Bulges are rarely spherical; the median bulge intrinsic el- lipticity 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 fun- damental plane depends on their shape index n. In agree- ment with previous work, the ratios of bulge and disk scale lengths are consistent with a constant value re=rd =0 .3; however, such value again depends on the bulge index n, implying that claims for a scale-free Hubble sequence may be premature.

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 10σ level in the SPIRE 250 μm image. Both detected galaxies have dust masses of the order of 10^5 M_⊙ and average dust temperatures ≈20 K. 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.

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 similar to 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.

Star formation in M 33: the radial and local relations with the gas

Aims. In the Local Group spiral galaxy M 33, we investigate the correlation between the star formation rate (SFR) surface density, ΣSFR, and the gas density Σgas (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 ΣSFR. Methods. We use the Hα, 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, ΣSFR ∝ Σ n is analyzed by three statistical methods. Results. At all spatial scales, with Hα 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 Hα 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 nH2 = 1.1 ± 0.1. Locally, at a spatial resolution of 180 pc, the correlation between ΣSFR and Σgas 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-σ detection threshold and taking into account uncertainties in ΣH2 and ΣSFR, we obtain a steeper KS index than obtained with radial averages: nH2 = 2.22 ± 0.07 (for FUV and bolometric SFR tracers), flatter than that relative to the total gas (nHtot = 2.59 ± 0.05). The gas depletion timescale is therefore larger in regions of lower ΣSFR. Lower KS indices (nH2 = 1.46 ± 0.34 and nH2 = 1.12) are found using different fitting techniques, which do not account for individual position uncertainties. At coarser spatial resolutions these indices get slightly steeper, and the correlation improves. We find an almost linear relation and a better correlation coefficient between the local ΣSFR and the ISM hydrostatic pressure or the gas volume density. This suggests that the stellar disk, gravitationally dominant with respect to the gaseous disk in M 33, has a non-marginal role in driving the SFR. However, the tight local correlation that exists between the dust optical depth and the SFR sheds light on the alternative hypothesis that the dust column density is a good tracer of the gas that is prone to star formation.