Gyula I. G. Jozsa

The Bluedisks project, a study of unusually H I-rich galaxies - I. H I sizes and morphology

We introduce the Bluedisk project, a large programme at the Westerbork Synthesis Radio Telescope that has mapped the H i in a sample of 23 nearby galaxies with unusually high H i mass fractions, along with a similar-sized sample of control galaxies. This paper presents the sample selection, observational set-up, data reduction strategy and a first analysis of the sizes and structural properties of the H i discs. We find that the H i-rich galaxies lie on the same H i mass versus H i size relation as normal spiral galaxies, extending it to total H i masses of 2 x 10(10) M-circle dot and radii R1 of similar to 100 kpc. The H i-rich galaxies have significantly larger values of H i-to-optical size ratio and more clumpy H i discs than those of normal spirals. There is no evidence that the discs of H i-rich galaxies are more disturbed. In fact, the centre of the H i distribution corresponds more closely with the centre of the optical light in the H i-rich galaxies than in the controls. All these results argue against a scenario in which new gas has been brought in by mergers. It is possible that they may be more consistent with cooling from a surrounding quasi-static halo of warm/hot gas.

An observational and theoretical view of the radial distribution of HI gas in galaxies

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 Λ 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.

THE SUDDEN DEATH OF THE NEAREST QUASAR

Galaxy formation is significantly modulated by energy output from supermassive black holes at the centers of galaxies which grow in highly efficient luminous quasar phases. The timescale on which black holes transition into and out of such phases is, however, unknown. We present the first measurement of the shutdown timescale for an individual quasar using X-ray observations of the nearby galaxy IC 2497, which hosted a luminous quasar no more than 70,000 years ago that is still seen as a light echo in Hannys Voorwerp, but whose present-day radiative output is lower by at least two, and more likely by over four, orders of magnitude. This extremely rapid shutdown provides new insight into the physics of accretion in supermassive black holes and may signal a transition of the accretion disk to a radiatively inefficient state.

The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems

We present new Very Large Array (VLA) H i spectral line imaging of five sources discovered by the ALFALFA extragalactic survey. These targets are drawn from a larger sample of systems that were not uniquely identified with optical counterparts during ALFALFA processing, and as such have unusually high H i mass to light ratios. The candidate “Almost Dark” objects fall into four broad categories: (1) objects with nearby H i neighbors that are likely of tidal origin; (2) objects that appear to be part of a system of multiple H i sources, but which may not be tidal in origin; (3) objects isolated from nearby ALFALFA H i detections, but located near a gas-poor early type galaxy; (4) apparently isolated sources, with no object of coincident redshift within ˜400 kpc. Roughly 75% of the 200 objects without identified counterparts in the α.40 database (Haynes et al. 2011) fall into category 1 (likely tidal), and were not considered for synthesis follow-up observations. The pilot sample presented here (AGC193953, AGC208602, AGC208399, AGC226178, and AGC233638) contains the first five sources observed as part of a larger effort to characterize H i sources with no readily identifiable optical counterpart at single dish resolution (3.‧5). These objects span a range of H i mass [7.41 <log(MHi ) <9.51] and H i mass to B-band luminosity ratios (3 <MHi /LB <9). We compare the H i total intensity and velocity fields to optical imaging drawn from the Sloan Digital Sky Survey and to ultraviolet imaging drawn from archival GALEX observations. Four of the sources with uncertain or no optical counterpart in the ALFALFA data are identified with low surface brightness optical counterparts in Sloan Digital Sky Survey imaging when compared with VLA H i intensity maps, and appear to be galaxies with clear signs of ordered rotation in the H i velocity fields. Three of these are detected in far-ultraviolet GALEX images, a likely indication of star formation within the last few hundred Myrs. One source (AGC208602) is likely tidal in nature, associated with the NGC 3370 group. Consistent with previous efforts, we find no “dark galaxies” in this limited sample. However, the present observations do reveal complex sources with suppressed star formation, highlighting both the observational difficulties and the necessity of synthesis follow-up observations to understand these extreme objects.

(Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing case of HI1232+20

We report the discovery and follow-up observations of a system of three objects identified by the ALFALFA extragalactic HI survey, cataloged as (almost) dark extragalactic sources, i.e., extragalactic HI detections with no discernible counterpart in publicly available, wide-field, imaging surveys. We have obtained deep optical imaging with WIYN pODI and HI synthesis maps with WSRT of the HI1232+20 system. The source with the highest HI flux has a newly discovered ultra-low surface brightness (LSB) optical counterpart associated with it, while the other two sources have no detected optical counterparts in our images. Our optical observations show that the detected LSB optical counterpart has a peak surface brightness of ~26.4 mag/arcsec^2 in g, which is exceptionally faint. This source (AGC 229385) has the largest accurately measured HI mass-to-light ratio of an isolated object: MHI/Lg=46 Msun/Lsun, and has an HI mass of 7.2*10^8 Msun. The other two HI sources (with HI masses 2.0*10^8 and 1.2*10^8 Msun) without optical counterparts have upper limit surface brightnesses of 27.9 and 27.8 mag/arcsec^2 in g, and lower limits on their gas mass-to-light ratio of MHI/Lg>57 and >31 Msun/Lsun. This system lies relatively close in projection to the Virgo Cluster, but velocity flow models indicate that it is located at ~25 Mpc, substantially beyond Virgo. The system appears to be quite isolated, with no known object closer than 500 kpc. These HI sources may represent both sides of the threshold between dark star-less galaxies and galaxies with stellar populations. We discuss a variety of possible formation scenarios for the HI1232+20 system.

Kinematic modeling of disk galaxies III. The warped "Spindle" NGC 2685

This is the third of a series of papers in which the structure and kinematics of disk galaxies is studied. By employing direct tilted-ring fits to the data cube as introduced in Paper I, we analyzed the Spindle galaxy NGC 2685, previously regarded as a two-ringed polar ring galaxy. Deep HI and optical (i-band) observations are presented. The HI observations strongly suggest that the gaseous structure of NGC 2685 does not consist of two separate mutually inclined regions, but forms a coherent, extremely warped disk, the appearance of two rings being due to projection effects. By comparing the HI total-intensity maps with the optical image, we demonstrate that at large radii a faint stellar disk is well aligned with the outer HI disk. The shape of the dust lanes obscuring the NE part of the inner stellar body indicates that, also at smaller radii, NGC 2685 possesses a disk containing gas, dust, and stars in which the various constituents are aligned. At smaller radii, this disk is kinematically decoupled from the central stellar body; hence, in the region of the bright, central stellar body, NGC 2685 appears to consist of two disks that share a common center, but have different orientation: a bright stellar lenticular body apparently devoid of dust and gas, and a heavily warped low-surface brightness disk containing stars, gas, and dust. The low-surface-brightness disk changes its orientation gradually and at large radii assumes the orientation of the central stellar S0 disk. Since, according to our analysis, the intrinsic orientation of the low-surface-brightness disk changes through 70 degrees, the gaseous disk is coherent, and is at no radius oriented perpendicularly with respect to the central stellar body, NGC 2685 is not likely to be a classical polar-ring galaxy.

Hanny's Voorwerp . Evidence of AGN activity and a nuclear starburst in the central regions of IC 2497

We present high- and intermediate resolution radio observations of the central region in the spiral galaxy IC 2497, performed using the European VLBI Network (EVN) at 18 cm, and the Multi-Element Radio Linked Interferometer Network (MERLIN) at 18 cm and 6 cm. We detect two compact radio sources, with brightness temperatures above 105 K, suggesting that they are related to AGN activity. We show that the total 18 cm radio emission from the galaxy is dominated neither by these compact sources nor large-scale emission, but extended emission confined within a sub-kpc central region. IC 2497 therefore appears as a typical luminous infrared galaxy that exhibits a nuclear starburst with a massive star formation rate (M > 5 M⊙) of 12.4 M⊙/yr. These results are in line with the hypothesis that the ionisation nebula “Hannys Voorwerp” at a distance of ~ 15-25 kpc from the galaxy is ionised by the radiation cone of the AGN. Table 2 is only available in electronic form at http://www.aanda.org

Revealing Hanny's Voorwerp : radio observations of IC 2497

We present multi-wavelength radio observations in the direction of the spiral galaxy IC 2497 and the neighbouring emission nebula known as Hannys Voorwerp. Our WSRT continuum observations at 1.4 GHz and 4.9 GHz reveal the presence of extended emission at the position of the nebulosity, although the bulk of the emission remains unresolved at the centre of the galaxy. e-VLBI 1.65 GHz observations show that on the milliarcsecond-scale, a faint central compact source is present in IC 2497 with a brightness temperature in excess of 4 x 105 K. With the WSRT, we detect a large reservoir of neutral hydrogen in the proximity of IC 2497. One cloud complex with a total mass of 5.6 x 10(9) M(circle dot) to the South of IC 2497 encompasses Hannys Voorwerp. Another cloud complex is located at the position of a small galaxy group similar to 100 kpc to the West of IC 2497 with a mass of 2.9 x 10(9) M(circle dot). Our data hint at a physical connection between the complexes. We also detect Hi in absorption against the central continuum source of IC 2497. Our observations strongly support the hypothesis that Hannys Voorwerp is being ionised by an AGN in the centre of IC 2497. In this scenario, a plasma jet associated with the AGN clears a path through the ISM/IGM in the direction of the nebulosity. The large-scale radio continuum emission possibly originates from the interaction between this jet and the large cloud complex in which Hannys Voorwerp is embedded. The Hi kinematics do not fit regular rotation, thus the cloud complex around IC 2497 is probably of tidal origin. From the Hi absorption against the central source, we derive a lower limit of 2.8 +/- 0.4 x 10(21) atoms cm(-2) to the H I column density. However, assuming non-standard conditions for the detected gas, we cannot exclude the possibility that the AGN in the centre of IC 2497 is Compton-thick.

Automated kinematic modelling of warped galaxy discs in large H i surveys: 3D tilted-ring fitting of H i emission cubes

Kinematical parameterisations of disc galaxies, employing emission line observations, are indispensable tools for studying the formation and evolution of galaxies. Future large-scale HI surveys will resolve the discs of many thousands of galaxies, allowing a statistical analysis of their disc and halo kinematics, mass distribution and dark matter content. Here we present an automated procedure which fits tilted-ring models to Hi data cubes of individual, well-resolved galaxies. The method builds on the 3D Tilted Ring Fitting Code (TiRiFiC) and is called FAT (Fully Automated TiRiFiC). To assess the accuracy of the code we apply it to a set of 52 artificial galaxies and 25 real galaxies from the Local Volume HI Survey (LVHIS). Using LVHIS data, we compare our 3D modelling to the 2D modelling methods DiskFit and rotcur. A conservative result is that FAT accurately models the kinematics and the morphologies of galaxies with an extent of eight beams across the major axis in the inclination range 20

(Almost) Dark Galaxies in the ALFALFA Survey: Isolated H i-bearing Ultra-diffuse Galaxies

^{circ}