Abstract.We report the discovery of a large H  cloud in the central regions of the Virgo cluster. It is 110 × 25 kpc in size and contains 3.4 × 10 8 M of H . The morphology and kinematics of this cloud strongly suggest that it consists of H  removed from the galaxy NGC 4388 by ram-pressure stripping. It is more likely the result of an interaction of the ISM of NGC 4388 with the hot halo of the M 86 group and not with the ICM centred on M 87. The large extent of the plume suggests that gas stripped from cluster galaxies can remain neutral for at least 10 8 yr. Locally, the column density is well above 10 20 cm −2 , suggesting that the intra-cluster H  regions known to exist in Virgo may have formed from gas stripped from cluster galaxies. The existence of the H  plume suggests that stripping of infalling spirals contributes to the enrichment of the ICM. The H  object in the Virgo cluster recently reported by Minchin et al. (2005, ApJ, 622, L21) may have a similar origin and may therefore not be a "dark galaxy".

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We present optical imaging and spectroscopy and HI imaging of the Virgo Cluster galaxy IC 3418, which is likely a "smoking gun" example of the transformation of a dwarf irregular into a dwarf elliptical galaxy by ram pressure stripping. IC 3418 has a spectacular 17 kpc length UV-bright tail comprised of knots, head-tail, and linear stellar features. The only Hα emission arises from a few HII regions in the tail, the brightest of which are at the heads of head-tail UV sources whose tails point toward the galaxy ("fireballs"). Several of the elongated tail sources have Hα peaks outwardly offset by ∼80-150 pc from the UV peaks, suggesting that gas clumps continue to accelerate through ram pressure, leaving behind streams of newly formed stars which have decoupled from the gas. Absorption line strengths, measured from Keck DEIMOS spectra, together with UV colors, show star formation stopped 300±100 Myr ago in the main body, and a strong starburst occurred prior to quenching. While neither Hα nor HI emission are detected in the main body of the galaxy, we have detected 4×10 7 M ⊙ of HI from the tail with the VLA. The gas consumption timescale in the tail is relatively long, implying that most of the stripped gas does not form stars but joins the ICM. The velocities of tail HII regions, measured from Keck LRIS spectra, extend only a small fraction of the way to the cluster velocity, suggesting that star formation does not happen in more distant parts of the tail. Stars in the outer tail have velocities exceeding the escape speed, but some in the inner tail should fall back into the galaxy, forming halo streams. One likely fallback stream is identified. 7 In this paper we will follow the majority of recent authors and refer to the more massive early type dwarf galaxies as dEs, and lower mass ones as dSphs. However some authors (Kormendy & Bender 2012) prefer the name Spheroidal (Sph) for more massive early type dwarfs, since they are distinct systems from giant Ellipticals, and dEs (or Sphs) and dSphs seem to be part of the same family. dE can be interpreted as "early type dwarf".

We present neutral hydrogen, ultraviolet, optical, and near-infrared imaging, and optical spectroscopy, of Minkowski's Object ( MO), a star-forming peculiar galaxy near NGC 541. The observations strengthen evidence that star formation in MO was triggered by the radio jet from NGC 541. Key new results are the discovery of a 4:9 ; 10 8 M double H i cloud straddling the radio jet downstream from MO, where the jet changes direction and decollimates; strong detections of MO, also showing double structure, in UVand H; and numerous H ii regions and associated clusters in MO. In UV, MO resembles the radio-aligned, rest-frame UV morphologies in many high-redshift radio galaxies (HzRGs), also thought to be caused by jet-induced star formation. MO's stellar population is dominated by a 7.5 Myr old, 1:9 ; 10 7 M instantaneous burst, with a current star formation rate of 0.52 M yr À1 (concentrated upstream from where the H i column density is high). This is unlike the jet-induced star formation in Centaurus A, where the jet interacts with preexisting cold gas; in MO, the H i may have cooled out of a warmer, clumpy intergalactic or interstellar medium as a result of jet interaction, followed by the collapse of the cooling clouds and subsequent star formation (consistent with numerical simulations). Since the radio source that triggered star formation in MO is much less luminous, and therefore more common than powerful HzRGs, and because the environment around MO is not particularly special in terms of abundant dense, cold gas, jet-induced star formation in the early universe might be even more prevalent than previously thought.