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A&A 498, 407­417 (2009) DOI: 10.1051/0004-6361/200810823
c ESO 2009

Astronomy & Astrophysics

The H I content of early-type galaxies from the ALFALFA survey
II. The case of low density environments
M. Grossi1 , S. di Serego Alighieri1 ,C.Giovanardi1 , G. Gavazzi2 , R. Giovanelli3 ,M.P.Haynes3 , B. R. Kent4 , S. Pellegrini5 , S. Stierwalt3 , and G. Trinchieri6
1 2 3 4 5 6

INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Florence, Italy e-mail: grossi@arcetri.astro.it UniversitÀ di Milano-Bicocca, Piazza delle Scienze 3, 20126 Milan, Italy Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, USA Jansky Fellow of the National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA UniversitÀ di Bologna, via Ranzani 1, 40127 Bologna, Italy INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy
ABSTRACT

Received 18 August 2008 / Accepted 23 February 2009
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 < 3000 km s-1 ) inanareaof the skywhere theALFALFA data arealready available (8h < RA < 16h , 4 < Dec < 16 ), avoiding the region of the Virgo cluster. Among these, 39 have absolute B magnitudes fainter than MB = -17. Results. Fifteen out of 62 galaxies have been firmly detected with ALFALFA (25%). Five additional galaxies show a weaker Hi emission (S/N 4) and they will need deeper observations to be confirmed. Eight objects had 21-cm measurements reported in the literature. One by one comparison with the available material confirms, as expected, that ALFALFA data are, with rare exceptions, of equal or better quality than the best spectra previously obtained for these objects. All together, our analysis doubles the number of known gas-rich ETGs in this area. The Hi detection rate is 44% in luminous ETGs ( MB < -17) and 13% in dwarf ETGs ( MB > -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.
Key words. galaxies: elliptical and lenticular, cD ­ galaxies: evolution ­ galaxies: ISM

1. Introduction
In the monolithic scenario (Larson 1974; Chiosi & Carraro 2002), early-type galaxies (ETGs) form in a single, short, and highly efficient burst of star formation at high redshift (z > 3) followed by a more quiescent evolution until today as the residual gas flows into the nuclear region of the galaxy. In this model, the bulk of the stellar population was formed at early epochs and ETGs appear today as gas-poor systems dominated by an old stellar population. On the other hand, hierarchical structure formation scenarios suggest that ETGs seen today in clusters have followed different evolutionary paths compared to the galaxies in LDEs (Governato et al. 1999), which are still undergoing hierarchical assembly and therefore are expected to be on average younger with more heterogenous properties (Schweizer & Seitzer 1992; Barnes 1997). Larson et al. (1980) first pointed out that field ellipticals may have experienced a more extended star formation history with respect to their cluster counterparts. Rose et al. (1994) found an intermediate-age population in

E/S0 galaxies in LDEs, suggesting a wider spread in the epochs of major star formation activity. Spectroscopic observations of 26 ETGs located in voids (Wegner & Grogin 2008) indicate that even though there is a range of old (>9 Gyr) and young (<5Gyr) systems, there appears to be a greater proportion of young galaxies in voids than in clusters. The finding of gaseous structures, such as rings and discs, mostly in ETG field galaxies, can give further indication that merging events with gas-rich objects are taking place today in LDEs (see review by Schweizer et al. 1990). Detailed studies of nearby ETGs have shown that a fraction of them contains neutral hydrogen (Sanders 1980; Knapp et al. 1985; Huchtmeier 1994) and they can host extended Hi structures around them (Oosterloo et al. 2007), accompanied in some cases by traces of ongoing or recent star formation (Sadler et al. 2000; Serra et al. 2007; Hau et al. 2008). The presence of neutral hydrogen in ETGs may be explained as the consequence of a recent accretion of a gas-rich satellite or of a merging process between similar-size galaxies

Article published by EDP Sciences


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M. Grossi et al.: The Hi content of early-type galaxies from the ALFALFA survey. II.

(Knapp et al. 1985; van Driel & van Woerden 1991; Barnes 2002). Simulations show that spheroidal galaxies with extended gaseous discs can form in spiral mergers (Barnes 2002; Burkert & Naab 2004), as the result of the subsequent infall of the Hi from the tidal tails to the main merging remnant. This process can rebuild the gaseous disc on the timescale of few Gyr leading to an elliptical or a S0 galaxy with an extended gaseous component (Hibbard & Mihos 1995). An alternative scenario foresees that the gas is accreted from the intergalactic medium via a "cold mode", where the gas cools along filamentary structures without being shock-heated to the virial temperature of the halo (T 105 K; Keres et al. 2005; MacciÐ et al. 2006). When the Hi distribution is mapped at high resolution, gasrich ETGs show regular Hi structures appearing as low column density (<5 â 1020 cm-2 ) discs or rings in rotation (van Driel & van Woerden 1991; Oosterloo et al. 2007). The regular kinematics of these structures suggests that they are relatively old and long-lived (a few Gyr). Hi tails or clouds offset from the optical counterpart are also found in some systems (Morganti et al. 2006; Oosterloo et al. 2007) which is indicative of recent interactions or of a gas accretion process. Nonetheless, the rate of detection of neutral hydrogen in nearby ETGs varies broadly depending on the depth of observations and the choice of the sample (Sadler et al. 2002; Morganti et al. 2006; di Serego Alighieri et al. 2007). It is well known that the Hi content of late-type galaxies in dense cluster decreases when compared to systems of the same morphological type in lower density regions (Giovanardi et al. 1983a; Haynes & Giovanelli 1986; Solanes et al. 2001). But how does the Hi content of ETGs vary with the environment? Variations in the detection rate of Hi in ETGs have already been noticed in previous studies (Haynes & Giovanelli 1980; Giovanardi et al. 1983b; Huchtmeier 1994; Oosterloo et al. 2007), confirming that the probability of finding ETGs with gas is higher in lower density environments. The Hi Parkes All-Sky Survey (HIPASS; Meyer et al. 2004) in the southern hemisphere provides a large database at 21-cm which has been recently used to study the Hi content of 2500 nearby ETGs (E and S0) extracted from the RC3 catalog (de Vaucouleurs et al. 1991). HIPASS detected Hi with a rate of 6% for elliptical and 13% for S0 galaxies (Sadler et al. 2002). However this preliminary analysis of the HIPASS data is plagued by confusion, since 30%­50% of the Hi detected ellipticals have more than one neighbor with similar optical velocity within the 15 Parkes beam, and no distinction between different environments has been analysed or discussed. These results suggest the need for a systematic attempt to determine the Hi properties of ETGs with a better spatial resolution, extending this analysis to a more uniform sample of galaxies in different environments. In Paper I we have used data from the ALFALFA survey (Giovanelli et al. 2005, 2007), an ongoing blind survey at 21-cm performed with the Arecibo telescope, to start a more uniform and systematic study of the Hi content of ETGs, by analysing it in the Virgo cluster. The advantage of using ALFALFA with respect to a previous blind Hi survey such as HIPASS is twofold: its higher sensitivity and the smaller beam size which reduces the possibility of confusing the Hi detections. We have defined an optical sample of 457 ETG, extracted from the Virgo Cluster Catalog (VCC Binggeli et al. 1985), which are brighter than the VCC completeness limit at BT = 18.0. We have correlated this optical sample with the catalog of detected Hi sources from ALFALFA. Only 9 out of 457 ETGs (roughly 2%) are detected

in Hi, with the majority of ETGs with gas having peculiar morphologies and being located in the outer regions of the cluster. Here we extend our analysis to a low density sample of ETGs, to compare the Hi detection rate in different environments. The paper is organised as follows: in Sect. 2 we describe the sample selection, in Sect. 3 we give an overview of the results, presenting the 21-cm detections, and their optical properties. In Sect. 4 we discuss the role of the environment on the Hi detection rate of ETGs, and the evidence of recent star forming activity in the sample, and in Sect. 5 we present our conclusions.

2. Sample selection
The strategy we have followed in this study is the same as in Paper I for the Virgo cluster. We define a­priori an optical sample of ETGs within a definite radial velocity limit, which we then search for Hi , using the ALFALFA survey. The selection of a LDE optical sample of ETGs as similar as possible to the Virgo one, in terms of luminosity distribution, and possibly complete to a faint limiting magnitude is the crucial point of this work. For this aim we have used the Sloan Digital Sky Survey (SDSS; York et al. 2000; Stoughton et al. 2002) which provides a homogeneous data set of galaxies with both photometric and spectroscopic measurements in different environments: from voids to groups and rich clusters. Spectra are taken only for a subset of objects, the so called main spectroscopic sample, consisting of galaxies with r-band magnitudes r < 17.77 and r-band half-light surface brightnesses 50 24.5 mag arcsec-2 (Strauss et al. 2002). First we selected all the objects in the seventh data release (DR7, Abazajian & Sloan Digital Sky Survey 2008) within the sky region where ALFALFA data are currently available with the same cut in radial velocity as in Paper I, i.e. ­ V < 3000 km s-1 ; ­ 8h < RA < 16h , avoiding the region of the Virgo cluster between 12h and 13h ; ­ 4 < Dec < 16 . The criterium on the radial velocity was imposed to reach a comparable Hi mass detection limit to the one of Paper I. After this preliminary selection we obtained 307 galaxies of all morphological types. Bernardi et al. (2005) defined a set of criteria to separate early and late-type galaxies in the SDSS database from the value of two outputs by the SDSS pipeline, the spectroscopic parameter ecla s s and the photometric parameter fracDevr 1 : earlytype morphologies are defined by choosing ecla s s < 0 and fracDevr > 0.8. Applying both these cuts we obtained only 4 galaxies. However we realised that this method was not appropriate for our aims. The use of fracDevr would prevent to select early-type dwarfs, because their light profiles are generally fitted by an exponential law (Ferguson & Binggeli 1994), while the choice of ecla s s < 0 would have automatically excluded ETGs with emission line spectra, a possibility that cannot be rejected a-priori, especially for our interest in gas-rich ETGs. Moreover, because the aperture of a SDSS spectroscopic fiber (3 ) samples only the inner parts of nearby galaxies, we found that, using this criterium on its own, the bulges of some nearby late-type galaxies were mistakenly selected as early-type objects. Therefore we
ecla s s is a spectroscopic parameter giving the spectral type from a principal component analysis, while fracDevr measures the fraction of light profile that is fitted by a de Vaucoulers law.
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*

Fig. 1. The sky distribution of the selected sample of ETGs in Right Ascension and Declination. Filled dots correspond to galaxies detected at 21-cm, while empty dots show gas-poor ETGs. The ellipse indicates the area of the Leo group. The star between the two regions where we have defined our sample gives the position of M 87, while the thick dashed line traces to the super galactic plane.

examined by visual inspection the SDSS multi-colour images of the 307 galaxies and classified 50 of them as early-type (E and S0). The completeness of the main spectroscopic sample depends on the magnitude (Strauss et al. 2002). The main source of incompleteness is due to blending with saturated stars, an effect which is more significant for brighter and larger galaxies. According to Strauss et al. (2002), at bright magnitudes (r < 15) only 5% of the galaxies in the Zwicky catalog are missed, while from comparison with visual inspections of all objects brigther than r = 18 over 22 deg2 of sky, the completeness of the sample is above 99%. To remedy the incompleteness of the SDSS spectroscopic sample at bright magnitudes, we have looked for ETGs with V < 3000 km s-1 in the RC3 catalog (de Vaucouleurs et al. 1991) and in the Nasa/IPAC Extragalactic Database (NED), and we found that some galaxies had been missed by our selection from the SDSS database. From the RC3 we added 10 bright ETGs, excluded by the SDSS spectroscopic target selection algorithm. From NED we found also one faint dwarf (LeG 14) whose rband magnitude is fainter than the limit of the main sample (r < 17.77), and Leo I, which was not included in the SDSS spectroscopic sample because it is resolved. Thus we have built an apparent magnitude limited and bound in redshift sample of ETGs in LDEs which is complete down to the r-band magnitude r < 17.77 (Strauss et al. 2002). Our final list of targets is composed of 62 objects, 34 ellipticals and 28 S0s. For the galaxies in common with the RC3 catalog (de Vaucouleurs et al. 1991) we have adopted the RC3 morphological classification. 39 objects have absolute magnitudes MB > -17, and they can be considered as dwarf ETGs. Figure 1 shows the sky distribution of the galaxy sample and the regions we have selected, which also include the Leo group; in Table 1 the main observational parameters of our targets are displayed. The columns are as follows: Column (1). The name of the galaxy. Column (2). The absolute B magnitude of the galaxy. B apparent total magnitudes have been derived from the RC3 when available, otherwise they have been calculated from the model2 r, g magnitudes derived from the SDSS database and converted to the Johnson system using the following relation

Column (3). Column (4). Column (5).

Column (6).

Column (7).

Column (8). Column (9).

Column (10).
The model magnitude (modelmag) in the SDSS database is defined as the total magnitude calculated by using the de Vaucouleurs or exponential model that best fits the galaxy profile in the r band.
2 3

B = g + 0.17 + 0.47 â (g - r) (Smith et al. 2002). A correction to the g magnitudes, g = 1 - exp[-0.11(g - 17)] , has been applied to compensate for the overestimate of the local sky flux near bright large galaxies performed by the SDSS photometric pipeline (Mandelbaum et al. 2005). The optical heliocentric velocity of the galaxy extracted from NED in km s-1 . Heliocentric velocity of the Hi detections, measured as the midpoint of the 50% level of the peak flux density. Velocity width of the Hi line profile, measured at half peak value. More details on the way it has been estimated can be found in Giovanelli et al. (2007). Integrated flux density F of the source in Jy km s-1 and its estimated uncertainty. For nondetections, upper limits on the flux density have been derived by measuring the rms of the integrated spectrum, obtained over a region of 5 â 5 around the optical position. The flux value has then been calculated for a S/N = 5 and a velocity width of 200 km s-1 if MB < -17 or 80 km s-1 if MB > -17. The Hi mass (and upper limits for non-detections) in units of 107 M , obtained using the standard 2 equation MHI = 2.35 â 105 dMpc F where dMpc is the assumed distance in Mpc. Logarithm in base 10 of the Hi mass-to-stellar light ratio in solar units (and upper limits for nondetections). Distance modulus. For 12 objects (labeled with a star) direct distance measurements are available from various indicators such as the tip of the red giant branch, surface brightness fluctuations, supernovae, planetary nebulae, and they have been taken from Tully et al. (2008) and NED3 . For the other galaxies we have adopted the Hubble flow distance moduli calculated in NED, based on the local velocity field given in Mould et al. (2000), assuming H0 = 73 km s-1 Mpc-1 . The corresponding distances range between 230 kpc (Leo I) and 47 Mpc (2MASX11460404+1134529). H equivalent width in Angstrom, derived from the SDSS spectra. The H equivalent width of

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M. Grossi et al.: The Hi content of early-type galaxies from the ALFALFA survey. II.

Table 1. The sample of early-type galaxies selected in this work and their main parameters. MB Vopt VHI V50 F MHI km s-1 km s-1 Jy km s-1 107 M mag km s-1 UGC 4590 ­17.26 1901 ­ 208 <0.65 <13.3 UGC 4599 ­19.07 2072 2071 ± 1 148 35.34 ± 0.09 850.5 SDSS J093155.94+073210.3 ­14.30 2217 ­ 83 <0.42 <11.7 SDSS J093208.81+082810.8 ­14.41 2340 ­ 83 <0.46 <14.3 MRK 0706 ­17.56 2489 2501 ± 7 61 0.64 ± 0.06 22.5 SDSS J093608.59+061525.4 ­15.66 2424 2413 ± 11 127 0.60 ± 0.06 19.7 NGC 2962 ­20.08 1959 1958 ± 1 414 4.22 ± 0.12 112.7 SDSS J094218.94+044121.8 ­13.67 2036 ­ 83 <0.50 <12.1 SDSS J095036.25+124832.7 ­14.49 1335 ­ 83 <0.41 <4.8 CGCG064-021 ­18.48 2822 2676 ± 9 36 0.30 ± 0.07 13.1 CGCG064-055 ­17.92 2789 ­ 210 <0.68 <30.2 LeoI ­11.00 229 ­ 82 <0.51 <6.3 â 10- SDSS J102326.34+123542.5 ­14.49 2739 ­ 84 <0.44 <18.9 SDSS J102339.36+123725.6 ­15.16 2723 ­ 83 <0.42 <18.0 SDSS J104140.97+134929.6 ­14.34 1271 ­ 83 <0.45 <5.4 SDSS J104435.28+135622.7 ­12.20 633 ­ 82 <0.50 <0.5 LeG14a ­11.75 886 ­ 82 <0.45 <1.5 LeG17a ­14.07 1013 ­ 83 <0.33 <1.9 NGC 3377a ­19.18 684 ­ 207 <0.63 <1.9 NGC 3379a ­20.03 897 ­ 207 <0.59 <1.7 NGC 3384a ­19.57 733 ­ 82 <0.35 <1.1 CGCG066-026a ­13.10 541 ­ 82 <0.43 <0.3 SDSS J104926.70+121528.0 ­14.43 1321 1338 ± 12 47 0.31 ± 0.08 4.1 UGC 5944 ­15.15 1073 ­ 83 <0.35 <1.0 NGC 3412a ­18.93 853 ­ 207 <0.56 <1.7 SDSS J105101.51+132000.5 ­11.97 656 ­ 82 <0.37 <0.4 SDSS J105131.35+140653.2 ­11.34 832 816 ± 9 32 0.22 ± 0.05 0.5 SDSS J105204.79+150149.7 ­12.47 828 ­ 82 <0.47 <1.1 SDSS J105219.51+110235.6 ­13.45 824 ­ 82 <0.43 <0.9 UGC 6062 ­19.31 2629 ­ 209 <0.73 <30.5 NGC 3489a ­19.26 692 692 ± 2 107 0.86 ± 0.06 2.6 IC 676 ­19.42 1414 1421 ± 2 176 1.33 ± 0.07 21.9 SDSS J111445.02+123851.7a ­10.86 582 629 ± 3 44 0.62 ± 0.04 0.4 IC 2684 ­12.42 648 590 ± 2 25 0.57 ± 0.03 0.4 SDSS J111701.18+043944.2 ­13.40 1446 1441 ± 24 96 0.30 ± 0.07 4.8 SDSS J112224.02+125846.4 ­11.54 626 ­ 82 <0.40 <0.3 IC 2782 ­14.44 860 ­ 82 <0.44 <0.8 IC 2787 ­13.33 708 ­ 82 <0.37 <0.3 IC 692 ­17.31 1157 1156 ± 3 52 3.59 ± 0.06 37.2 NGC 3773 ­17.31 987 983 ± 3 90 3.06 ± 0.06 8.7 IC 719 ­17.87 1849 1848 ± 18 231 4.08 ± 0.10 53.9 UGC 6655 ­13.66 748 750 ± 3 55 1.26 ± 0.07 1.0 2MASX J11434609+1342273 ­16.58 2920 2926 ± 10 42 0.22 ± 0.05 11.2 SDSS J114516.18+135221.2 ­14.56 2957 ­ 84 <0.37 <19.1 2MASX J11460404+1134529 ­17.89 2977 2922 ± 14 168 0.72 ± 0.06 37.8 NGC 4880 ­18.36 1377 ­ 200 <0.63 <2.9 SDSS J134757.45+041850.6 ­12.01 954 ­ 83 <0.49 <2.4 SDSS J135142.91+052647.4 ­15.77 1241 ­ 83 <0.46 <5.9 MAPS-NGPO_559_1243538 ­13.83 977 ­ 83 <0.43 <2.3 UGC 8799 ­14.42 1132 ­ 83 <0.49 <2.0 NGC 5338 ­17.17 804 803 ± 3 135 0.57 ± 0.06 2.2 SDSS J135502.70+050525.2 ­15.13 1396 ­ 83 <0.49 <7.6 SDSS J135621.31+051944.2 ­14.54 1395 ­ 83 <0.45 <7.0 CGCG046-013 ­16.41 1516 ­ 83 <0.50 <8.6 SDSS J135723.57+053425.2 ­15.01 1055 ­ 83 <0.45 <3.7 UGC 8986 ­17.78 1232 ­ 207 <0.70 <8.5 SDSS J142043.54+040837.0 ­15.03 1704 ­ 83 <0.43 <8.6 SDSS J144329.18+043153.4 ­15.81 1745 1716 ± 2 74 1.36 ± 0.06 28.6 NGC 5770 ­18.41 1454 ­ 200 <0.63 <5.4 SDSS J152655.37+094657.5 ­13.96 1858 ­ 83 <0.41 <7.7 IC 1131 ­18.05 2017 ­ 200 <0.63 <18.0 NGC 6014 ­20.04 2491 ­ 200 <0.63 <25.4 Leo group member. ID m- M mag <­0.97 32.17 0.11 32.53 <0.16 32.49 <0.20 32.60 ­0.86 32.73 ­0.16 32.66 ­1.17 32.64 <0.42 32.31 <­0.31 31.54 ­1.47 32.96 <­0.88 32.99 <­2.79 21.80 <0.29 32.96 <­0.001 32.95 <­0.20 31.57 <­0.37 28.91 <0.27 30.34 <­0.54 30.95 <­2.59 30.25 <­2.98 30.21 <­2.97 30.32 <­1.01 28.54 ­0.35 31.87 <­1.25 30.22 <­2.54 30.27 <­0.40 29.12 0.01 29.87 <­0.12 29.87 <­0.60 29.75 <­1.43 32.92 ­2.48 30.41 ­1.62 32.12 0.03 28.36 ­0.52 28.79 0.13 31.90 <­0.38 28.49 <­1.08 29.69 <­1.00 28.96 ­0.55 31.61 ­1.18 30.21 ­0.61 31.87 ­0.66 28.81 ­0.77 33.14 <0.27 33.16 ­0.77 33.17 <­2.08 30.71 <0.38 30.79 <­0.73 31.85 <­0.36 30.91 <­0.65 30.61 ­1.72 30.54 <­0.36 32.05 <­0.16 32.05 <­0.82 32.17 <­0.62 31.36 <­1.37 31.78 <0.27 32.34 ­0.06 32.38 <­1.83 31.39 <0.11 32.26 <­1.16 32.71 <­1.80 33.08 log
MHI LB

H

T ­1 ­2 ­2 ­5 ­1 ­1 ­1 ­5 ­5 ­5 ­5 ­5 ­5 ­5 ­5 ­5 ­2 ­2 ­5 ­5 ­3 ­5 ­5 ­5 ­2 ­5 ­5 ­5 ­5 ­1 ­1 ­1 ­5 ­5 ­5 ­1 ­5 ­5 ­5 ­2 ­2 ­1 ­2 ­1 ­1 ­1 ­5 ­1 ­1 ­5 ­2 ­5 ­5 ­5 ­5 ­2 ­2 ­5 ­2 ­5 ­5 ­2

Code ­ a ­ ­ b b a ­ ­ c ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ c ­ ­ ­ c ­ ­ ­ a a a a c ­ ­ ­ a a a a c ­ b ­ ­ ­ ­ ­ a ­ ­ ­ ­ ­ ­ a ­ ­ ­ ­

4

å 11.3 ­0.01 ­1.7 ­3.6 123.0 21.1 ­ 0.5 9.3 ­1.0 20.7 ­ ­1.9 23.4 1.0 ­1.7 ­ ­1.1 ­ ­1.2 ­ ­1.4 102.6 ­1.1 ­ ­2.7 ­10.6 ­1.3 ­2.4 ­1.2 ­ 33.6 5.5 2.3 5.2 ­2.7 ­1.5 ­2.2 8.9 63.6 1.1 121.7 102.2 ­1.5 75.0 ­1.6 ­1.6 ­1.1 17.6 ­1.1 24.9 ­1.4 ­1.2 ­1.5 0.6 ­1.2 3.5 13.5 ­ 0.2 ­ ­

a


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NGC 3773 has been taken from Gavazzi et al. (in preparation), because it is not included in the SDSS spectroscopic sample. Column (11). Morphological type of the galaxies in the sample. T = -5 refers to ellipticals, and T = -3, ­2, ­1 correspond to S0 galaxies. Column (12). The code of the Hi detections. Given that the signal threshold in the ALFALFA catalog is S/N > 6.5for V < 400 km s-1 (Saintonge 2007), we define with code a firm detections with S/N above the adopted ALFALFA threshold, code b denotes reliable detections with a lower signal-to-noise ratio (5 < S/N < 6.5) which have an optical counterpart with a similar redshift, while code c refers to possible detections which need to be corroborated with deeper observations.

3. Results
3.1. HI proper ties

For each galaxy we have analysed the ALFALFA grids searching for 21-cm emission. We found 15 good quality detections out of 62 galaxies (25%; code a and b), while five additional candidate sources have lower S/N (4) and they will need deeper observations to be confirmed (code c). 15 objects of our sample of 62, have been previously observed at 21-cm. In particular, 6 out of our 15 detections had previous measurements, and the spectra we obtain are in good agreement with those in the literature. Upon inspection of the published spectra we find ours to be of comparable or better quality with the exception of NGC 3489 (Bregman et al. 1992), IC 719 (Giovanardi et al. 1983b), and NGC 5701 (Duprie & Schneider 1996), which are instead of sensitivity higher than ours. Two others have been previously detected but with a marked disagreement with our data; this is the case of UGC 4599 (Theureau et al. 2007; Rosenberg & Schneider 2000), and of IC 676 (Davoust & Contini 2004). In both cases our data are definitely of superior quality. As for the undetected ones, 2 objects have in the literature Hi limits worse than those here provided (NGC 3379, 5770), while for NGC 3377 (Knapp et al. 1979), NGC 3412 and NGC 4880 (Sage & Welch 2006) the published limits are lower than ours. We do not confirm the dubious detection of NGC 6014 of comparable sensitivity to ours (Lewis 1983). Finally, Sage & Welch (2006) report a 21-cm detection of NGC 3384 ( MHI = 1.8 â 106 M ) at a level well below our detection limit. In total we added 7 new 21-cm detections and, possibly, 5 more sources (code c), doubling (at least) the number of ETGs with neutral hydrogen in this region of the sky. The spectra of all the Hi detections (including the dubious ones) are shown in Fig. 2. The atomic gas is regularly rotating in UGC 4599 and NGC 2962, as indicated by the double horn profiles. Four galaxies (UGC 4599, NGC 2962, IC 719, and SDSS J111445.02+123851.7) have gas-rich neighbours to which they appear to be connected by a Hi bridge, and their contour maps are displayed in Fig. 3. The maps of UGC 4599, NGC 2962 and IC 719 show that the gas extends far beyond the optical disc in these objects. Here we discuss the Hi properties of some of the most interesting 21-cm detections. UGC 4599. It is the galaxy with the highest Hi mass of the sample ( MHI = 7.6 â 109 M ). The gaseous distribution is very extended (100 kpc) and in rotation as indicated by the double horn 21-cm profile. It belongs to the compact group No. 79

in the Updated Zwicky Catalog (UZC; Focardi & Kelm 2002). From the ALFALFA cube one can see a possible companion also detected at 21-cm (CGCG061-011) with Vhel = 2087 km s-1 (Fig. 3). In a few channels there is evidence for Hi emission in the region between the two galaxies which connects to the disc of UGC 4599, although this feature is not visible in Fig. 3. NGC 2962. The Hi emission appears to be connected to that of the galaxy SDSS J094056.3+050240.5, being located at a projected distance of about 8 arcmin (80 kpc; see Fig. 3). In the Lyon Group of Galaxies catalog (Garcia 1993) it belongs to the group No. 178 whose brightest member is NGC 2966. From the ALFALFA data, within a box of 1.5 degrees in RA and Dec and a radial velocity range of ±250 km s-1 , there are 8 systems which are detected at 21-cm. The rotating Hi structure shows the largest velocity width of the sample (V50 = 415 km s-1 ). NGC 3489. This lenticular galaxy belongs to the Leo Group, but it is in between the two subgroups defined by M 66 and M 96. It is among the faintest detection with one of the lowest Hi masses (2 â 107 M ). The Hi structure is not resolved by the Arecibo beam thus it is not shown in Fig. 3. SDSS J111445.02+123851.7 A dwarf galaxy in the M 66 subgroup of Leo. The 21-cm emission (Fig. 3) extends to the north towards the brighter companion NGC 3593 (Vhel = 628 km s-1 ; at a projected distance of 10 arcmin/30 kpc), a S0a galaxy with two counterrotating stellar discs of different scale length and surface brightness (Bertola et al. 1996). The smaller and less massive one corotates with the Hi gaseous disc, contrary to the more massive stellar disc. The accretion of a gasrich satellite is addressed as a possible mechanism to explain the double structure of NGC 3593 (Pizzella et al. 1999). IC 719. It is a S0 galaxy fairly isolated. It shows only one companion (IC 718; Vhel = 1860 km s-1 ) to which it seems to be connected by a Hi bridge (see Fig. 3).
3.2. HI content vs. B band luminosity

The SDSS database provides u,g, r, z, i photometry for all the galaxies of our sample, including those for which spectroscopic information is not available. As mentioned in Sect. 2, B apparent magnitudes have been taken from the RC3 for the galaxies included in this catalog, otherwise they have been computed from the dereddened SDSS photometry using the relation from Smith et al. (2002). Figure 4 displays the absolute magnitude distribution of all the galaxies, compared to those detected at 21-cm (filled histogram). The five code c detections are also shown. Given that our sample is complete down to r < 17.7 we can evaluate what is the completeness magnitude limit in MB . The B - r colour distibution of these galaxies ranges between 0.4 and 1.4 and the maximum distance modulus is m - M = 33.18; this implies that the sample we have selected is at least complete down to MB = -15. A large fraction of the observed luminous ( MB < -17) earlytype systems (10 out of 23, 44%) contain neutral hydrogen. At fainter magnitudes ( MB > -17) we find 5 out 39 gas-rich dwarfs (13%, not taking into account the uncertain detections), although the incompleteness of the sample becomes important for MB > -15. The detection rate per magnitude bin peaks at MB = -17 (60%), and then it drops at magnitudes fainter than MB = -16. Other studies in the field find similar high Hi detection rates in ETGs, however the difference with a dense environment like Virgo is striking. In Paper I we found that only 2 out of 55 galaxies brighter than MB = -17 have neutral hydrogen (about 4%). The fraction of luminous ETG with Hi in LDEs is 10 times higher than in Virgo. However the three brightest ellipticals of


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Fig. 2. 21-cm spectra of the ETGs with Hi emission. The last five spectra correspond to the uncertain detections (code c in Table 1).

the LDE sample (NGC 3377, NGC 3379 and NGC 3384) do not show Hi emission down to the sensitivity limit of the ALFALFA data set (see Table 1). On the fainter side of the distribution, the Hi detection rate for dwarf ETGs in LDEs is almost ten times higher than in Virgo where we found only 7 gas-rich early-type dwarfs out of 407 (1.7%, see Paper I).

3.3. The MHI /LB distribution of the sample

In Fig. 5 we compare the gas-mass-to-light ratios of the LDE and Virgo cluster samples displayed as a function of