Marco Grossi

A Blind HI Survey of the M81 Group

Results are presented of the first blind H I survey of the M81 Group of galaxies. The data were taken as part of the H I Jodrell All-Sky Survey (HIJASS). The survey reveals several new aspects to the complex morphology of the H I distribution in the group. All four of the known dwarf irregular (dIrr) galaxies close to M81 can be unambiguously seen in the HIJASS data. Each forms part of the complex tidal structure in the area. We suggest that at least three of these galaxies may have formed recently from the tidal debris in which they are embedded. The structure connecting M81 to NGC 2976 is revealed as a single tidal bridge of mass 2.1 × 108 M☉ and projected spatial extent 80 kpc. Two spurs of H I projecting from the M81 complex to lower declinations are traced over a considerably larger spatial and velocity extent than by previous surveys. The dwarf elliptical (dE) galaxies BK5N and Kar 64 lie at the spatial extremity of one of these features and appear to be associated with it. We suggest that these may be the remnants of dIrrs that have been stripped of gas and transmuted into dEs by close gravitational encounters with NGC 3077. The nucleated dE galaxy Kar 61 is unambiguously detected in H I for the first time and has an H I mass of ~108 M☉, further confirming it as a dE/dIrr transitional object. HIJASS has revealed one new possible group member, HIJASS J1021+6842. This object contains 2 × 107 M☉ of H I and lies 105 from IC 2574. It has no optical counterpart on the Digital Sky Survey.

First Results from the HI Jodrell All Sky Survey: inclination-dependent selection effects in a 21-cm blind survey

Details are presented of the H i Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (H I ) survey of the northern sky (δ > 22°), being conducted using the multibeam receiver on the Lovell Telescope (full width at half-maximum beamwidth 12 arcmin) at Jodrell Bank. HIJASS covers the velocity range -3500 to 10 000 km s - 1 , with a velocity resolution of 18.1 km s - 1 and spatial positional accuracy of ∼2.5 arcmin. Thus far about 1115 deg 2 of sky have been surveyed. The average rms noise during the early part of the survey was around 16 mJy beam - 1 . Following the first phase of the Lovell Telescope upgrade (in 2001), the rms noise is now around 13 mJy beam - 1 . We describe the methods of detecting galaxies within the HIJASS data and of measuring their H I parameters. The properties of the resulting H i-selected sample of galaxies are described. Of the 222 sources so far confirmed, 170 (77 per cent) are clearly associated with a previously catalogued galaxy. A further 23 sources (10 per cent) lie close (within 6 arcmin) to a previously catalogued galaxy for which no previous redshift exists. A further 29 sources (13 per cent) do not appear to be associated with any previously catalogued galaxy. The distributions of peak flux, integrated flux, H I mass and cz are discussed. We show, using the HIJASS data, that H I self-absorption is a significant, but often overlooked, effect in galaxies with large inclination angles to the line of sight. Properly accounting for it could increase the derived H I mass density of the local Universe by at least 25 per cent. The effect that this will have on the shape of the H I mass function will depend on how self-absorption affects galaxies of different morphological types and H i masses. We also show that galaxies with small inclinations to the line of sight may also be excluded from H I-selected samples, since many such galaxies will have observed velocity widths that are too narrow for them to be distinguished from narrow-band radio-frequency interference. This effect will become progressively more serious for galaxies with smaller intrinsic velocity widths. If, as we might expect, galaxies with smaller intrinsic velocity widths have smaller HI masses, then compensating for this effect could significantly steepen the faint-end slope of the derived HI mass function.

HIDEEP: an extragalactic blind survey for very low column-density neutral hydrogen

We have earned out an extremely long integration time (9000 s beam - 1 ) 21-cm blind survey of 60 deg 2 in Centaurus using the Parkes multibeam system. We find that the noise continues to fall as √t o b s throughout, enabling us to reach an HI column-density limit of 4.2 x 10 1 8 cm - 2 for galaxies with a velocity width of 200 km s - 1 in the central 32 deg 2 region, making this the deepest survey to date in terms of column density sensitivity. The H i data are complemented by very deep optical observations from digital stacking of multi-exposure UK Schmidt Telescope R-band films, which reach an isophotal level of 26.5 R mag arcsec - 2 (≃27.5 B mag arcsec - 2 ). 173 Hi sources have been found, 96 of which have been uniquely identified with optical counterparts in the overlap area. There is not a single source without an optical counterpart. Although we have not measured the column densities directly, we have inferred them from the optical sizes of their counterparts. All appear to have a column density of N H 1 = 10 2 0 . 6 5 ′ 0 . 3 8 . This is at least an order of magnitude above our sensitivity limit, with a scatter only marginally larger than the errors on N H 1 . This needs explaining. If confirmed it means that H 1 surveys will only find low surface brightness (LSB) galaxies with high M H 1 /L B . Gas-rich LSB galaxies with lower HI mass to light ratios do not exist. The paucity of low column-density galaxies also implies that no significant population will be missed by the all-sky H 1 surveys being carried out at Parkes and Jodrell Bank.

21 cm Synthesis Observations of VIRGOHI 21?A Possible Dark Galaxy in the Virgo Cluster

Many observations indicate that dark matter dominates the extragalactic universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously, we have suggested that VIRGOHI 21, a 21 cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line width (~200 km s(-1)) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10(8) yr ago with a perturber of ~10(11) M☉. This we take as additional evidence for the massive nature of VIRGOHI 21, as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the H I and find none down to an I-band surface brightness limit of 31.1 ± 0.2 mag arcsec(-2).

The cosmological significance of low surface brightness galaxies found in a deep blind neutral hydrogen survey

Minchin et al. have recently placed limits on the cosmological significance of gas-rich low surface brightness (LSB) galaxies as a proportion of the total population of gas-rich galaxies by carrying out a very deep survey (HIDEEP) for neutral hydrogen (HI) with the Parkes multibeam system. Such a survey avoids the surface brightness selection effects that limit the usefulness of optical surveys for finding LSB galaxies. To complement the HIDEEP survey, we have digitally stacked eight 1-h R-band Tech Pan films from the UK Schmidt Telescope covering 36 deg 2 of the survey area to reach a very deep isophotal limit of 26.5 R mag arcsec -2 , At this level, we find that all of the 129 HI sources within this area have optical counterparts and that 107 of them can be identified with individual galaxies. We have used the properties of the galaxies identified as the optical counterparts of the H I sources to estimate the significance of LSB galaxies (defined to be those at least 1.5 mag dimmer in effective surface brightness than the peak in the observed distribution seen in optical surveys). Two different methods of correcting for ease of detection do not yield significantly different results: LSB galaxies make up 62 ± 37 per cent of gas-rich galaxies by number according to our first method (weighting by H I mass function), which includes a correction for large-scale structure, or 51 ± 20 per cent when calculated by our second method (1/V max correction). We also find that LSB galaxies provide 30 ± 10 per cent of the contribution of gas-rich galaxies to the neutral hydrogen density of the Universe, 7 ± 3 per cent of their contribution to the luminosity density of the Universe, 9 ± 4 of their contribution to the baryonic mass density of the Universe, 20 ± 10 per cent of their contribution to the dynamical mass density of the Universe, and 40 ± 20 per cent of their cross-sectional area. We do not find any crouching giant LSB galaxies such as Malin 1, nor do we find a population of extremely low surface brightness galaxies not previously found by optical surveys. Such objects must be either rare, gas-poor or outside the survey detection limits.

Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group

We analyse the properties of three unusual dwarf galaxies in the Centaurus A group discovered with the HIPASS survey. From their optical morphology, they appear to be low surface brightness dwarf spheroidals, yet they are gas rich (M H I /L B > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. Therefore these systems appear different from any dwarfs of the Local Group. They should be favoured hosts for starburst, whereas we find a faint star formation region in only one object. We have obtained 21-cm data and Hubble Space Telescope photometry in V and I bands, and have constructed colour magnitude diagrams (CMDs) to investigate their stellar populations and to set a constraint on their age. From the comparison of the observed and model CMDs, we infer that all three galaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr) and remain gas rich because their star formation rates have been very low (≤10 -3 M ⊙ yr -1 ) throughout. In such systems, star formation appears to have been sporadic and local, though one object (HIPASS J1321-31) has a peculiar red plume in its CMD suggesting that many of its stars were formed in a miniburst 300-500 Myr ago. The question of why there are no similar dwarf galaxies in the Local Group remains open.

Anomalous Evolution of the Dwarf Galaxy HIPASS J1321–31*

We present Hubble Space Telescope/WFPC2 observations of the dwarf galaxy HIPASS J1321-31. This unusual galaxy lies in the direction of the Centaurus A group of galaxies and has a color-magnitude diagram with a distinctive red plume of luminous stars. This feature could arise from (1) a red giant branch if the galaxy were much nearer than previously recognized, (2) a peculiar asymptotic giant branch, or (3) an ~1 Gyr old population of intermediate-mass red supergiants, which we find to be the most likely explanation. However, the lack of equally luminous blue stars requires that the star formation has dropped substantially since these stars were formed. Evidently HIPASS J1321-31 experienced an episode of enhanced star formation rather recently in its star formation history followed by a period of relative quiescence that has led to the evolution of the main-sequence stars into the red supergiant branch. The stellar populations in HIPASS J1321-31 reflect a star formation history that is uncommon in star-forming dwarf galaxies. This is the first time such a star formation history has been noted, although the literature contains a small number of other dwarf galaxies with similar color-magnitude diagrams. Therefore, HIPASS J1321-31 and these other galaxies represent a different path of dwarf galaxy evolution that has not been well explored and an important probe into how dwarf galaxies evolve.

A dark galaxy in the Virgo Cluster imaged at 21-cm

Dark Matter supposedly dominates the extragalactic, yet no totally dark structure of galactic proportions has ever been convincingly identified. Earlier (Minchin et al. 2005) we suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster at Jodrell Bank using single-dish observations (Davies et al. 2004), was probably such a dark galaxy because of its broad line-width (~ 200 km/s) unaccompanied by any visible gravitational source to account for it. Now we have managed to image VIRGOHI 21 in the neutral-hydrogen line, and indeed we find what appears to be a dark, edge-on, spinning disc with the mass and diameter of a typical spiral galaxy. Moreover the disc has unquestionably interacted with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor invariably responsible for such a feature. Published numerical models (Vollmer, Huchtmeier & van Driel 2005) of NGC 4254 call for a close interaction ~ 10^8 years ago with a perturber of 10^11 solar masses. This we take as completely independent evidence for the massive nature of VIRGOHI 21.

The Herschel Virgo Cluster Survey. I. Luminosity function [Letter]

We describe the Herschel Virgo Cluster Survey (HeViCS) and the first data obtained as part of the science demonstration phase (SDP). The data cover a central 4×4 squ2009deg region of the cluster. We use SPIRE and PACS photometry data to produce 100, 160, 250, 350 and 500 μm luminosity functions (LFs) for optically bright galaxies that are selected at 500 μm and detected in all bands. We compare these LFs with those previously derived using IRAS, BLAST and Herschel-ATLAS data. The Virgo cluster LFs do not have the large numbers of faint galaxies or examples of very luminous galaxies seen previously in surveys covering less dense environments.