We present a multiwavelength analysis of a compact group of galaxies infalling at high speed into the dynamically young cluster Abell 1367. Peculiar morphologies and unusually high Hα emission are associated with two giant galaxies and at least ten dwarfs/extragalactic HII regions, making this group the region with the highest density of star formation activity ever observed in the local clusters. Moreover Hα imaging observations reveal extraordinary complex trails of ionized gas behind the galaxies, with projected lengths exceeding 150 kpc. These unique cometary trails mark the gaseous trajectory of galaxies, witnessing their dive into the hot cluster intergalactic medium. Under the combined action of tidal forces among group members and the ram-pressure by the cluster ambient medium, the group galaxies were fragmented and the ionized gas was blown out. The properties of this group suggest that environmental effects within infalling groups may have represented a preprocessing step of the galaxy evolution during the high redshift cluster assembly phase.
We present in this paper the deepest Arecibo HI observations of Virgo cluster dwarf ellipticals (dEs) taken to date. Based on this data we argue that a significant fraction of Virgo cluster dEs recently underwent evolution. Our new observations consist of HI 21-cm line observations for 22 classified dE galaxies with optical radial velocities consistent with membership in the Virgo cluster. Cluster members VCC 390 and VCC 1713 are detected with HI masses M HI = 6 × 10 7 M ⊙ and 8 × 10 7 M ⊙ , respectively, while M HI in the remaining 20 dE galaxies have upper limits as low as ∼ 5 × 10 5 M ⊙ . We combine our results with those for 27 other Virgo cluster dEs with HI observations in the literature, 7 of which have HI detection claims. New optical images from the WIYN telescope of 5 of these HI-detected dEs, along with archival data, suggest that seven of the claimed detections are real, yielding a ≈15% detection rate. These HI-detected classified dEs are preferentially located near the periphery of the Virgo cluster. Three Virgo dEs have observed HI velocity widths > 200 km s −1 , possibly indicating the presence of a large dark matter content, or transient extended HI. We discuss the possible origins of these objects and argue that they originate from field galaxies accreted onto high angular momentum orbits by Virgo in the last few Gyr. As a result these galaxies are slowly transformed within the cluster by gradual gas stripping processes, associated truncation of star formation, and passive fading of stellar populations. Low-mass early-type cluster galaxies are therefore currently being produced as the product of cluster environmental effects. We utilize our results in a simple model to estimate the recent (past 1-3 Gyr) average mass accretion rate into the Virgo cluster, deriving a value ofṀ ∼ 50 M ⊙ year −1 . 6 The term dwarf elliptical (dE) is sometimes used in this paper to describe low-mass objects without recent star formation, and with apparently symmetric structures. Dwarf spheroidals are a sub-set of dwarf ellipticals with faint magnitudes and low surface brightnesses (Gallagher & Wyse 1994). In our view these two terms are interchangeable, and we include both in the general dE designation. To not bias the interpretation of what Virgo objects with HI detections are, we use the term low-mass cluster galaxy (LMCG) to describe them, even though these objects have been classified by others (e.g,. Binggeli et al. 1985) as dwarf ellipticals.
Abstract. High sensitivity (rms noise ∼ 0.5 mJy) 21-cm H i line observations were made of 33 galaxies in the Virgo cluster, using the refurbished Arecibo telescope, which resulted in the detection of 12 objects. These data, combined with the measurements available from the literature, provide the first set of H i data that is complete for all 355 late-type (Sa-Im-BCD) galaxies in the Virgo cluster with mp ≤ 18.0 mag. The Virgo cluster H i mass function (HIMF) that was derived for this optically selected galaxy sample is in agreement with the HIMF derived for the Virgo cluster from the blind HIJASS H i survey and is inconsistent with the Field HIMF. This indicates that both in this rich cluster and in the general field, neutral hydrogen is primarily associated with late-type galaxies, with marginal contributions from early-type galaxies and isolated H i clouds. The inconsistency between the cluster and the field HIMF derives primarily from the difference in the optical luminosity function of late-type galaxies in the two environments, combined with the HI deficiency that is known to occur in galaxies in rich clusters.
High sensitivity 21-cm H i line observations with an rms noise level of ∼0.5 mJy were made of 35 spiral galaxies in the Coma Supercluster, using the refurbished Arecibo telescope, leading to detection of 25 objects. These data, combined with the measurements available in the literature, provide the set of H i data for 94% of all late-type galaxies in the Coma Supercluster with an apparent photographic magnitude m p ≤ 15.7 mag. We confirm that the typical scale of H i deficiency around the Coma cluster is 2 Mpc, i.e. one virial radius. Comparing the H i mass function (HIMF) of cluster with non-cluster members of the Coma Supercluster we detected a shortage of high H i mass galaxies among cluster members that can be attributed to the pattern of H i deficiency found in rich clusters.
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