A 500 kpc HI Extension of the Virgo Pair NGC4532/DDO137 Detected by the Arecibo Legacy Fast ALFA (ALFALFA) Survey

We report the discovery of a ~500 kpc H I extension southwest of the Virgo Cluster H I-rich pair NGC 4532/DDO 137, detected as part of the Arecibo Legacy Fast ALFA (ALFALFA) Survey. The feature is the longest and most massive H I tail structure so far found in the Virgo Cluster and, at 1.8 Mpc from M87, the most distant from the main concentration of the intracluster medium. The structure is spatially and spectrally separated into two ridges and is defined by diffuse emission and discrete clumps of mass (2.5–6.8) × 107 M☉. All emission is blueshifted with respect to the NGC 4532/DDO 137 pair emission. Including diffuse emission, the structure has a total mass of up to 7 × 108 M☉, equivalent to ~10% of the system\u27s H I mass. Optical R-band imaging finds no counterparts to a level of 26.5 mag arcsec−2. The characteristics of the structure appear most consistent with a tidal origin

Wide HI profile galaxies

We investigate the nature of objects in a complete sample of 28 galaxies selected from the first sky area fully covered by ALFALFA, being well-detected and having HI profiles wider than 550 km/s. The selection does not use brightness, morphology, or any other property derived from optical or other spectral bands. We investigate the degree of isolation, the morphology, and other properties gathered or derived from open data bases and show that some objects have wide HI profiles probably because they are disturbed or are interacting, or might be confused in the ALFALFA beam. We identify a sub-sample of 14 galaxies lacking immediate interacting neighbours and showing regular, symmetric, two-horned HI profiles that we propose as candidate high-mass disk systems (CHMDs). We measure the net-Halpha emission from the CHMDs and combine this with public multispectral data to model the global star formation (SF) properties of each galaxy. The Halpha observations show SFRs not higher than a few solar masses per year. Simple SF models indicate that the CHMDs formed most of their stars almost a Hubble time ago, but most also underwent an SF event in the last 1-10 Myrs; the young stars now produce 10 to 30% of the visible light. The spatial distribution of the SF-regions is compatible with recycled stellar ejecta. We calculate representative dynamical masses from 1 to $\sim7 10^{11}$ M_sun, larger by factors of 2.5 to 7.5 than the baryonic masses of the luminous stars and gas. We test the Tully-Fisher relation for the CHMDs and show that these lie below the relation defined for lower mass galaxies, i.e., that their M_dyn is lower than expected when extrapolating the relation from lower mass galaxies to higher HI line widths.Comment: MNRAS in pres