ALBERT

Description: Using data taken as part of the Bluedisk project, we study the connection between neutral hydrogen (H i ) in the environment of spiral galaxies and that in the galaxies themselves. We measure the total H i mass present in the environment in a statistical way by studying the distribution of noise peaks in the H i data cubes obtained for 40 galaxies observed with Westerbork Synthesis Radio Telescope. We find that galaxies whose H i mass fraction is high relative to standard scaling relations have an excess H i mass in the surrounding environment as well. Gas in the environment consists of gas clumps which are individually below the detection limit of our H i data. These clumps may be hosted by small satellite galaxies and/or be the high-density peaks of a more diffuse gas distribution in the intergalactic medium. We interpret this result as an indication for a picture in which the ${\rm H}\,{\small {I}}$ -rich central galaxies accrete gas from an extended gas reservoir present in their environment.

Description: We present results of a blind 21cm H i -line imaging survey of a galaxy overdensity located behind the Milky Way at , b 160°, 0 $_{.}^{\circ}$ 5. The overdensity corresponds to a zone-of-avoidance crossing of the Perseus-Pisces Supercluster filament. Although it is known that this filament contains an X-ray galaxy cluster (3C 129) hosting two strong radio galaxies, little is known about galaxies associated with this potentially rich cluster because of the high Galactic dust extinction. We mapped a sky area of ~9.6 deg 2 using the Westerbork Synthesis Radio Telescope in a hexagonal mosaic of 35 pointings observed for 12 h each, in the radial velocity range cz = 2400–16 600 km s –1 . The survey has a sensitivity of 0.36 mJy beam –1 rms at a velocity resolution of 16.5 km s –1 . We detected 211 galaxies, 62 per cent of which have a near-infrared counterpart in the UKIDSS Galactic Plane Survey. We present a catalogue of the H i properties and an H i atlas containing total intensity maps, position–velocity diagrams, global H i profiles and UKIDSS counterpart images. For the resolved galaxies we also present H i velocity fields and radial H i surface density profiles. A brief analysis of the structures outlined by these galaxies finds that 87 of them lie at the distance of the Perseus-Pisces Supercluster ( cz ~ 4000–8000 km s –1 ) and seem to form part of the 3C 129 cluster. Further 72 detections trace an overdensity at a velocity of cz 10 000 km s –1 and seem to coincide with a structure predicted from mass density reconstructions in the first 2MASS Redshift Survey.

Description: We report a tight linear relation between the H i circular velocity measured at 6 R e and the stellar velocity dispersion measured within 1 R e for a sample of 16 early-type galaxies with stellar mass between 10 10 and 10 11 M . The key difference from previous studies is that we only use spatially resolved v circ (H i ) measurements obtained at large radius for a sizeable sample of objects. We can therefore link a kinematical tracer of the gravitational potential in the dark-matter dominated outer regions of galaxies with one in the inner regions, where baryons control the distribution of mass. We find that v circ (H i )= 1.33 e with an observed scatter of just 12 per cent. This indicates a strong coupling between luminous and dark matter from the inner- to the outer regions of early-type galaxies, analogous to the situation in spirals and dwarf irregulars. The v circ (H i )– e relation is shallower than those based on v circ measurements obtained from stellar kinematics and modelling at smaller radius, implying that v circ declines with radius – as in bulge-dominated spirals. Indeed, the value of v circ (H i ) is typically 25 per cent lower than the maximum v circ derived at ~0.2 R e from dynamical models. Under the assumption of power-law total density profiles   r – , our data imply an average logarithmic slope 〈〉 = 2.18 ± 0.03 across the sample, with a scatter of 0.11 around this value. The average slope and scatter agree with recent results obtained from stellar kinematics alone for a different sample of early-type galaxies.

Description: One of the key goals of the Bluedisk survey is to characterize the impact of gas accretion in disc galaxies in the context of galaxy evolution. It contains 50 disc galaxies in the stellar mass range 10 10 –10 11 M , of which half are bluer and more H i -rich galaxies than their H i -normal (control) counterparts. In this paper, we investigate how ongoing disc growth affects the molecular gas distribution and the star formation efficiency in these galaxies. We present 12 CO observations from the IRAM 30-m telescope in 26 galaxies of the Bluedisk survey. We compare the amount and spatial distribution of the molecular gas to key quantities such as atomic gas, stellar mass and surface density, star formation rate (SFR) and metallicity. We analyse the SFR per unit gas (SFR/H i and SFR/H 2 ) and relate all those parameters to general galaxy properties (H i -rich/control disc, morphology, etc.). We find that the H i -rich galaxies have similar H 2 masses as the control galaxies. In their centres, H i -rich galaxies have lower H 2 /H i ratios and marginally shorter molecular gas depletion times. However, the main differences between the two samples occur in the outer parts of the discs, with the H i -rich galaxies having slightly smaller CO discs (relative to the optical radius R 25 ) and steeper CO and metallicity gradients than the control galaxies. The ongoing accretion of H i at large radii has thus not led to an appreciable growth of the CO discs in our sample. Based on depletion times, we estimate that this gas will contribute to star formation on time-scales of at least 5 Gyr.

Description: As part of the Bluedisk survey, we analyse the radial gas-phase metallicity profiles of 50 late-type galaxies. We compare the metallicity profiles of a sample of H i -rich galaxies against a control sample of H i -‘normal’ galaxies. We find the metallicity gradient of a galaxy to be strongly correlated with its H i mass fraction ( $\textrm {M(H\,\small {I})} / \textrm {M}_{\ast }$ ). We note that some galaxies exhibit a steeper metallicity profile in the outer disc than in the inner disc. These galaxies are found in both the H i -rich and control samples. This contradicts a previous indication that these outer drops are exclusive to H i -rich galaxies. These effects are not driven by bars, although we do find some indication that barred galaxies have flatter metallicity profiles. By applying a simple analytical model, we are able to account for the variety of metallicity profiles that the two samples present. The success of this model implies that the metallicity in these isolated galaxies may be in a local equilibrium, regulated by star formation. This insight could provide an explanation of the observed local mass–metallicity relation.

Description: Recent work suggests blue ellipticals form in mergers and migrate quickly from the blue cloud of star-forming galaxies to the red sequence of passively evolving galaxies, perhaps as a result of black hole feedback. Such rapid reddening of stellar populations implies that large gas reservoirs in the pre-merger star-forming pair must be depleted on short time-scales. Here we present pilot observations of atomic hydrogen gas in four blue early-type galaxies that reveal increasing spatial offsets between the gas reservoirs and the stellar components of the galaxies, with advancing post-starburst age. Emission line spectra show associated nuclear activity in two of the merged galaxies, and in one case radio lobes aligned with the displaced gas reservoir. These early results suggest that a kinetic process (possibly feedback from black hole activity) is driving the quick truncation of star formation in these systems, rather than a simple exhaustion of gas supply.

Description: As part of the Bluedisk survey, we analyse the radial gas-phase metallicity profiles of 50 late-type galaxies. We compare the metallicity profiles of a sample of H i -rich galaxies against a control sample of H i -‘normal’ galaxies. We find the metallicity gradient of a galaxy to be strongly correlated with its H i mass fraction ( $\textrm {M(H\,\small {I})} / \textrm {M}_{\ast }$ ). We note that some galaxies exhibit a steeper metallicity profile in the outer disc than in the inner disc. These galaxies are found in both the H i -rich and control samples. This contradicts a previous indication that these outer drops are exclusive to H i -rich galaxies. These effects are not driven by bars, although we do find some indication that barred galaxies have flatter metallicity profiles. By applying a simple analytical model, we are able to account for the variety of metallicity profiles that the two samples present. The success of this model implies that the metallicity in these isolated galaxies may be in a local equilibrium, regulated by star formation. This insight could provide an explanation of the observed local mass–metallicity relation.

Description: Our work is based on the ‘Bluedisk’ project, a programme to map the neutral gas in a sample of 25 H i -rich spirals and a similar number of control galaxies with the Westerbork Synthesis Radio Telescope (WSRT). In this paper, we focus on the H i properties of the galaxies in the environment of our targeted galaxies. In total, we extract 65 galaxies from the WSRT cubes with stellar masses between 10 8 and 10 11 M . Most of these galaxies are located on the same H i mass–size relation and ‘H i -plane’ as normal spiral galaxies. We find that companions around H i -rich galaxies tend to be H i -rich as well and to have larger $R_{\rm 90,H\,\small {I}}/R_{\rm 50,H\,\small {I}}$ . This suggests a scenario of ‘H i conformity’, similar to the colour conformity found by Weinmann et al.: galaxies tend to adopt the H i properties of their neighbours. We visually inspect the outliers from the H i mass–size relation and galaxies which are offset from the H i plane and find that they show morphological and kinematical signatures of recent interactions with their environment. We speculate that these outliers have been disturbed by tidal or ram-pressure stripping processes, or in a few cases, by accretion events.

Description: We analyse the radial distribution of H i gas for 23 disc galaxies with unusually high H i content from the Bluedisk sample, along with a similar-sized sample of ‘normal’ galaxies. We propose an empirical model to fit the radial profile of the H i surface density, an exponential function with a depression near the centre. The radial H i surface density profiles are very homogeneous in the outer regions of the galaxy; the exponentially declining part of the profile has a scalelength of ~0.18 R1, where R1 is the radius where the column density of the H i is 1 M  pc –2 . This holds for all galaxies, independent of their stellar or H i mass. The homogenous outer profiles, combined with the limited range in H i surface density in the non-exponential inner disc, results in the well-known tight relation between H i size and H i mass. By comparing the radial profiles of the H i -rich galaxies with those of the control systems, we deduce that in about half the galaxies, most of the excess gas lies outside the stellar disc, in the exponentially declining outer regions of the H i disc. In the other half, the excess is more centrally peaked. We compare our results with existing smoothed particle hydrodynamical simulations and semi-analytic models of disc galaxy formation in a cold dark matter universe. Both the hydro simulations and the semi-analytic models reproduce the H i surface density profiles and the H i size–mass relation without further tuning of the simulation and model inputs. In the semi-analytic models, the universal shape of the outer H i radial profiles is a consequence of the assumption that infalling gas is always distributed exponentially.