Total HI Research Articles

ANALYTIC H i-to-H2 PHOTODISSOCIATION TRANSITION PROFILES

ABSTRACT We present a simple analytic procedure for generating atomic (H i) to molecular ( ) density profiles for optically thick hydrogen gas clouds illuminated by far-ultraviolet radiation fields. Our procedure is based on the analytic theory for the structure of one-dimensional H i/ photon-dominated regions, presented by Sternberg et al. Depth-dependent atomic and molecular density fractions may be computed for arbitrary gas density, far-ultraviolet field intensity, and the metallicity-dependent H2 formation rate coefficient, and dust absorption cross section in the Lyman–Werner photodissociation band. We use our procedure to generate a set of transition profiles for a wide range of conditions, from the weak- to strong-field limits, and from super-solar down to low metallicities. We show that if presented as functions of dust optical depth, the and density profiles depend primarily on the Sternberg “ parameter” (dimensionless) that determines the dust optical depth associated with the total photodissociated column. We derive a universal analytic formula for the transition points as a function of just . Our formula will be useful for interpreting emission-line observations of H i/ interfaces, for estimating star formation thresholds, and for sub-grid components in hydrodynamics simulations.

The void galaxy survey: Star formation properties

We study the star formation properties of 59 void galaxies as part of the Void Galaxy Survey (VGS). Current star formation rates are derived from $\rm{H\alpha}$ and recent star formation rates from near-UV imaging. In addition, infrared 3.4 $\rm{\mu m}$, 4.6 $\rm{\mu m}$, 12 $\rm{\mu m}$ and 22 $\rm{\mu m}$ WISE emission is used as star formation and mass indicator. Infrared and optical colours show that the VGS sample displays a wide range of dust and metallicity properties. We combine these measurements with stellar and HI masses to measure the specific SFRs ($\rm{SFR/M_{*}}$) and star formation efficiencies ($\rm{SFR/M_{HI}}$). We compare the star formation properties of our sample with galaxies in the more moderate density regions of the cosmic web, 'the field'. We find that specific SFRs of the VGS galaxies as a function of stellar and HI mass are similar to those of the galaxies in these field regions. Their $\rm{SFR\alpha}$ is slightly elevated than the galaxies in the field for a given total HI mass. In the global star formation picture presented by Kennicutt-Schmidt, VGS galaxies fall into the regime of low average star formation and correspondingly low HI surface density. Their mean $\rm{SFR\alpha/M_{HI}}$ and $\rm{SFR\alpha/M_{*}}$ are of the order of $\rm{10^{-9.9}}$ $\rm{yr^{-1}}$. We conclude that while the large scale underdense environment must play some role in galaxy formation and growth through accretion, we find that even with respect to other galaxies in the more mildly underdense regions, the increase in star formation rate is only marginal.

The DiskMass Survey

We present results from 21 cm radio synthesis imaging of 28 spiral galaxies from the DiskMass Survey obtained with the VLA, WSRT, and GMRT facilities. We detail the observations and data reduction procedures and present a brief analysis of the radio data. We construct 21 cm continuum images, global HI emission-line profiles, column-density maps, velocity fields, and position-velocity diagrams. From these we determine star formation rates (SFRs), HI line widths, total HI masses, rotation curves, and azimuthally-averaged radial HI column-density profiles. All galaxies have an HI disk that extends beyond the readily observable stellar disk, with an average ratio and scatter of R_{HI}/R_{25}=1.35+/-0.22, and a majority of the galaxies appear to have a warped HI disk. A tight correlation exists between total HI mass and HI diameter, with the largest disks having a slightly lower average column density. Galaxies with relatively large HI disks tend to exhibit an enhanced stellar velocity dispersion at larger radii, suggesting the influence of the gas disk on the stellar dynamics in the outer regions of disk galaxies. We find a striking similarity among the radial HI surface density profiles, where the average, normalized radial profile of the late-type spirals is described surprisingly well with a Gaussian profile. These results can be used to estimate HI surface density profiles in galaxies that only have a total HI flux measurement. We compare our 21 cm radio continuum luminosities with 60 micron luminosities from IRAS observations for a subsample of 15 galaxies and find that these follow a tight radio-infrared relation, with a hint of a deviation from this relation at low luminosities. We also find a strong correlation between the average SFR surface density and the K-band surface brightness of the stellar disk.

The Arecibo Galaxy Environment Survey – X. The structure of halo gas around M33

As part of the HI Arecibo Galaxy Environments Survey (AGES) we have observed 5$\times$4 degrees of sky centred on M33, reaching a limiting column density of $\sim 1.5 \times 10^{17}$ cm$^{-2}$ (line width of 10 km s$^{-1}$ and resolution 3.5\arcmin). We particularly investigate the absence of optically detected dwarf galaxies around M33, something that is contrary to galaxy formation models. We identify 22 discrete HI clouds, 11 of which are new detections. The number of objects detected and their internal velocity dispersion distribution is consistent with expectations from standard galaxy formation models. However, the issue remains open as to whether the observed velocity dispersions can be used as a measure of the HI clouds total mass i.e. are the velocities indicative of virialised structures or have they been influenced by tidal interactions with other structures in the Local Group? We identify one particularly interesting HI cloud, AGESM33-31, that has many of the characteristics of HI distributed in the disc of a galaxy, yet there is no known optical counterpart associated with it. This object has a total HI mass of $1.22 \times 10^{7}$ M$_{\odot}$ and a diameter of 18 kpc if at the distance of M33 ($D_{M33}=840$ kpc). However, we also find that there are numerous other HI clouds in this region of sky that have very similar velocities and so it is plausible that all these clouds are actually associated with debris from the Magellanic stream.

Towards the statistical detection of the warm–hot intergalactic medium in intercluster filaments of the cosmic web

Modern analyses of structure formation predict a universe tangled in a 'cosmic web' of dark matter and diffuse baryons. These theories further predict that at low-z, a significant fraction of the baryons will be shock-heated to $T \sim 10^{5}-10^{7}$K yielding a warm-hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation. We present a novel experiment to detect the WHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable QSO sightline that passes within $\Delta d = 3$ Mpc from the 7 inter-cluster axes connecting 7 independent cluster-pairs at redshifts $0.1 \le z \le 0.5$. We find tentative excesses of total HI, narrow HI (NLA; Doppler parameters $b<50$ km/s), broad HI (BLA; $b \ge 50$ km/s) and OVI absorption lines within rest-frame velocities of $\Delta v \lesssim 1000$ km/s from the cluster-pairs redshifts, corresponding to $\sim 2$, $\sim 1.7$, $\sim 6$ and $\sim 4$ times their field expectations, respectively. Although the excess of OVI likely comes from gas close to individual galaxies, we conclude that most of the excesses of NLAs and BLAs are truly intergalactic. We find that the covering fractions, $f_c$, of BLAs close to cluster-pairs are $\sim 4-7$ times higher than the random expectation (at the $\sim 2 \sigma$ c.l.), whereas the $f_c$ of NLAs and OVI are not significantly enhanced. We argue that a larger relative excess of BLAs compared to those of NLAs close to cluster-pairs may be a signature of the WHIM in inter-cluster filaments. By extending the present analysis to tens of sightlines our experiment offers a promising route to detect the WHIM.

COLD AND WARM ATOMIC GAS AROUND THE PERSEUS MOLECULAR CLOUD. II. THE IMPACT OF HIGH OPTICAL DEPTH ON THE HI COLUMN DENSITY DISTRIBUTION AND ITS IMPLICATION FOR THE HI-TO-H2TRANSITION

We investigate the impact of high optical depth on the HI saturation observed in the Perseus molecular cloud by using Arecibo HI emission and absorption measurements toward 26 radio continuum sources. The spin temperature and optical depth of individual HI components are derived along each line-of-sight, enabling us to estimate the correction for high optical depth. We examine two different methods for the correction, Gaussian decomposition and isothermal methods, and find that they are consistent (maximum correction factor ~ 1.2) likely due to the relatively low optical depth and insignificant contribution from the diffuse radio continuum emission for Perseus. We apply the correction to the optically thin HI column density on a pixel-by-pixel basis, and find that the total HI mass increases by ~10%. Using the corrected HI column density image and far-infrared data from the IRIS Survey, we then derive the H2 column density on ~0.4 pc scales. For five dark and star-forming sub-regions, the HI surface density is uniform with Sigma_HI ~ 7-9 solar mass/pc2, in agreement with the minimum HI surface density required for shielding H2 against photodissociation. As a result, Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2 show a tight relation. Our results are consistent with predictions for H2 formation in steady state and chemical equilibrium, and suggest that H2 formation is mainly responsible for the Sigma_HI saturation in Perseus. We also compare the optically thick HI with the observed "CO-dark" gas, and find that the optically thick HI only accounts for ~20% of the "CO-dark" gas in Perseus.

VLT/UVES observations of extremely strong intervening damped Lyman-αsystems

We present a detailed analysis of three extremely strong intervening DLAs (log N(HI)>=21.7) observed towards quasars with VLT/UVES. We measure overall metallicities of [Zn/H]~-1.2, -1.3 and -0.7 at respectively zabs=2.34 towards SDSS J2140-0321 (log N(HI) = 22.4+/-0.1), zabs=3.35 towards SDSS J1456+1609 (log N(HI) = 21.7+/-0.1) and zabs=2.25 towards SDSS J0154+1935 (log N(HI) = 21.75+/-0.15). We detect H2 towards J2140-0321 (log N(H2) = 20.13+/-0.07) and J1456+1609 (log N(H2) = 17.10+/-0.09) and argue for a tentative detection towards J0154+1935. Absorption from the excited fine-structure levels of OI, CI and SiII are detected in the system towards J2140-0321, that has the largest HI column density detected so far in an intervening DLA. This is the first detection of OI fine-structure lines in a QSO-DLA, that also provides us a rare possibility to study the chemical abundances of less abundant atoms like Co and Ge. Simple single phase photo-ionisation models fail to reproduce all the observed quantities. Instead, we suggest that the cloud has a stratified structure: H2 and CI likely originate from both a dense (log nH~2.5-3) cold (80K) and warm (250K) phase containing a fraction of the total HI while a warmer (T>1000 K) phase probably contributes significantly to the high excitation of OI fine-structure levels. The observed CI/H2 column density ratio is surprisingly low compared to model predictions and we do not detect CO molecules: this suggests a possible underabundance of C by 0.7 dex compared to other alpha elements. The absorber could be a photo-dissociation region close to a bright star (or a star cluster) where higher temperature occurs in the illuminated region. Direct detection of on-going star formation through e.g. NIR emission lines in the surrounding of the gas would enable a detailed physical modelling of the system.

COLD AND WARM ATOMIC GAS AROUND THE PERSEUS MOLECULAR CLOUD. I. BASIC PROPERTIES

(Abridged) Using the Arecibo Observatory we have obtained neutral hydrogen (HI) absorption and emission spectral pairs in the direction of 26 background radio continuum sources in the vicinity of the Perseus molecular cloud. Strong absorption lines were detected in all cases allowing us to estimate spin temperature (T_s) and optical depth for 107 individual Gaussian components along these lines of sight. Basic properties of individual HI clouds (spin temperature, optical depth, and the column density of the cold and warm neutral medium, CNM and WNM) in and around Perseus are very similar to those found for random interstellar lines of sight sampled by the Millennium HI survey. This suggests that the neutral gas found in and around molecular clouds is not atypical. However, lines of sight in the vicinity of Perseus have on average a higher total HI column density and the CNM fraction, suggesting an enhanced amount of cold HI relative to an average interstellar field. Our estimated optical depth and spin temperature are in stark contrast with the recent attempt at using Planck data to estimate properties of the optically thick HI. Only ~15% of lines of sight in our study have a column density weighted average spin temperature lower than 50 K, in comparison with >85% of Planck's sky coverage. The observed CNM fraction is inversely proportional to the optical-depth weighted average spin temperature, in excellent agreement with the recent numerical simulations by Kim et al. While the CNM fraction is on average higher around Perseus relative to a random interstellar field, it is generally low, 10-50%. This suggests that extended WNM envelopes around molecular clouds, and/or significant mixing of CNM and WNM throughout molecular clouds, are present and should be considered in the models of molecule and star formation.

A low H I column density filament in NGC 2403: signature of interaction or accretion

Observed HI accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion happens in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To contrain the presence and properties of such flows, we present deep HI observations obtained with the NRAO Green Bank Telescope of an area measuring 4 by 4 degrees around NGC 2403. These observations, with a 5 sigma detection limit of 2.4 x 10^18 cm^-2 over a 20 km/s linewidth, reveal the presence of a low-column density, extended cloud outside the main HI disk, about 17' (~16 kpc or ~2R25) to the NW of the center of the galaxy. The total HI mass of the cloud is 6.3 x 10^6 Msun, or 0.15 percent of the total HI mass of NGC 2403. The cloud is associated with an 8-kpc anomalous-velocity HI filament in the inner disk, previously observed in deep VLA observations by Fraternali et al. (2001, 2002). We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigbouring dwarf galaxy.

SOME LIKE IT HOT: LINKING DIFFUSE X-RAY LUMINOSITY, BARYONIC MASS, AND STAR FORMATION RATE IN COMPACT GROUPS OF GALAXIES

We present an analysis of the diffuse X-ray emission in 19 compact groups of galaxies (CGs) observed with Chandra. The hottest, most X-ray luminous CGs agree well with the galaxy cluster X-ray scaling relations in $L_X-T$ and $L_X-\sigma$, even in CGs where the hot gas is associated with only the brightest galaxy. Using Spitzer photometry, we compute stellar masses and classify HCGs 19, 22, 40, and 42 and RSCGs 32, 44, and 86 as fossil groups using a new definition for fossil systems that includes a broader range of masses. We find that CGs with total stellar and HI masses $\gtrsim10^{11.3}$ M$_\odot$ are often X-ray luminous, while lower-mass CGs only sometimes exhibit faint, localized X-ray emission. Additionally, we compare the diffuse X-ray luminosity against both the total UV and 24 $\mu$m star formation rates of each CG and optical colors of the most massive galaxy in each of the CGs. The most X-ray luminous CGs have the lowest star formation rates, likely because there is no cold gas available for star formation, either because the majority of the baryons in these CGs are in stars or the X-ray halo, or due to gas stripping from the galaxies in CGs with hot halos. Finally, the optical colors that trace recent star formation histories of the most massive group galaxies do not correlate with the X-ray luminosities of the CGs, indicating that perhaps the current state of the X-ray halos is independent of the recent history of stellar mass assembly in the most massive galaxies.

H I-TO-H2TRANSITIONS AND H I COLUMN DENSITIES IN GALAXY STAR-FORMING REGIONS

We present new analytic theory and radiative transfer computations for the atomic to molecular (HI-to-H2) transitions, and the build-up of atomic-hydrogen (HI) gas columns, in optically thick interstellar clouds, irradiated by far-ultraviolet photodissociating radiation fields. We derive analytic expressions for the total HI column densities for (1D) planar slabs, for beamed or isotropic radiation fields, from the weak- to strong-field limits, for gradual or sharp atomic to molecular transitions, and for arbitrary metallicity. Our expressions may be used to evaluate the HI column densities as functions of the radiation field intensity and the H2-dust-limited dissociation flux, the hydrogen gas density, and the metallicity-dependent H2 formation rate-coefficient and far-UV dust-grain absorption cross-section. We make the distinction between "HI-dust" and "H2-dust" opacity, and we present computations for the "universal H2-dust-limited effective dissociation bandwidth". We validate our analytic formulae with Meudon PDR code computations for the HI-to-H2 density profiles, and total HI column densities. We show that our general 1D formulae predict HI columns and H2 mass fractions that are essentially identical to those found in more complicated (and approximate) spherical (shell/core) models. We apply our theory to compute H2 mass fractions and star-formation thresholds for individual clouds in self-regulated galaxy disks, for a wide range of metallicities. Our formulae for the HI columns and H2 mass fractions may be incorporated into hydrodynamics simulations for galaxy evolution.

Discovery of a transparent sightline at ρ ≲ 20 kpc from an interacting pair of galaxies

We report the discovery of a transparent sightline at projected distances of {\rho} < 20 kpc to an interacting pair of mature galaxies at z = 0.12. The sightline of the UV-bright quasar PG1522+101 at z = 1.328 passes at {\rho} = 11.5 kpc from the higher-mass galaxy (M_* = 10^10.6 M_Sun) and {\rho} = 20.4 kpc from the lower-mass one (M_* = 10^10.0 M_Sun). The two galaxies are separated by 9 kpc in projected distance and 30 km/s in line-of-sight velocity. Deep optical images reveal tidal features indicative of close interactions. Despite the small projected distances, the quasar sightline shows little absorption associated with the galaxy pair with a total HI column density it no greater than log N(HI) = 13.65. This limiting HI column density is already two orders-of-magnitude less than what is expected from previous halo gas studies. In addition, we detect no heavy-element absorption features associated with the galaxy pair with 3-{\sigma} limits of log N(MgII) < 12.2 and log N(OVI) < 13.7. The probability of seeing such little absorption in a sightline passing at a small projected distance from two non-interacting galaxies is 0.2%. The absence of strong absorbers near the close galaxy pair suggests that the cool gas reservoirs of the galaxies have been significantly depleted by the galaxy interaction. These observations therefore underscore the potential impact of galaxy interactions on the gaseous halos around galaxies.

An H i study of NGC 3521 – a galaxy with a slow-rotating halo

A study is presented of HI line observations of the nearby spiral galaxy NGC 3521 observed with the VLA as part of The HI Nearby Galaxy Survey. Clearly evident in the HI data cube is the presence of an anomalous HI component that is both diffuse and slow-rotating. The data cube is dynamically decomposed into regular and anomalous HI components. A mass of M_HI=1.5 x 10^9 Msun is estimated for the anomalous HI - 20 per cent of the total HI mass. Standard HI data products and rotation curves are produced for each dynamical component. In terms of circular rotation speed, the anomalous HI is found to lag the regular HI by ~25 - 125 km/s. Three-dimensional models are generated and used to determine the possible location of the anomalous HI. The results strongly suggest it to be distributed in a thick disc with a scale-height of a few kpc (~3.5 kpc). It is concluded that the anomalous HI in NGC 3521 constitutes a slow-rotating halo gas component, consistent with similar findings for other nearby galaxies. A study of the radial distribution of the anomalous HI shows it to be spatially coincident with the inner regions of the stellar disc where the star formation rate is highest. It is most likely a galactic fountain that has deposited gas from the disc of the galaxy into the halo.

The temperature of the diffuse H i in the Milky Way - II. Gaussian decomposition of the H i-21 cm absorption spectra

We discuss physical conditions in Galactic neutral hydrogen based on deep, high velocity resolution interferometric HI 21 cm absorption spectroscopy towards 33 compact extra-galactic radio sources. The HI 21 cm optical depth spectra have root-mean-square noise values \lesssim 10^{-3} per 1 km/s velocity channel, i.e., sufficiently sensitive to detect HI 21 cm absorption by the warm neutral medium (WNM). Comparing these spectra with HI 21 cm emission spectra from the Leiden-Argentine-Bonn (LAB) survey, we show that some of the absorption detected on most sightlines must arise in gas with temperatures higher than that in the stable cold neutral medium (CNM). A multi-Gaussian decomposition of 30 of the HI 21 cm absorption spectra yielded very few components with line widths in the temperature range of stable WNM, with no such WNM components detected for sixteen of the thirty sightlines. We find that some of the detected HI 21 cm absorption along thirteen of these sightlines must arise in gas with spin temperatures larger than the CNM range. For these sightlines, we use very conservative estimates of the CNM spin temperature and the non-thermal broadening to derive strict upper limits to the gas column densities in the CNM and WNM phases. Comparing these upper limits to the total HI column density, we find that typically at least 28% of the gas must have temperatures in the thermally unstable range (200-5000 K). Our observations hence robustly indicate that a significant fraction of the gas in the Galactic interstellar medium has temperatures outside the ranges expected for thermally stable gas in two-phase models.

EVOLUTION IN THE H I GAS CONTENT OF GALAXY GROUPS: PRE-PROCESSING AND MASS ASSEMBLY IN THE CURRENT EPOCH

We present an analysis of the neutral hydrogen (HI) content and distribution of galaxies in groups as a function of their parent dark matter halo mass. The Arecibo Legacy Fast ALFA survey alpha.40 data release allows us, for the first time, to study the HI properties of over 740 galaxy groups in the volume of sky common to the SDSS and ALFALFA surveys. We assigned ALFALFA HI detections a group membership based on an existing magnitude/volume-limited SDSS DR7 group/cluster catalog. Additionally, we assigned group "proximity" membership to HI detected objects whose optical counterpart falls below the limiting optical magnitude--thereby not contributing substantially to the estimate of the group stellar mass, but significantly to the total group HI mass. We find that only 25% of the HI detected galaxies reside in groups or clusters, in contrast to approximately half of all optically detected galaxies. Further, we plot the relative positions of optical and HI detections in groups as a function of parent dark matter halo mass to reveal strong evidence that HI is being processed in galaxies as a result of the group environment: as optical membership increases, groups become increasingly deficient of HI rich galaxies at their center and the HI distribution of galaxies in the most massive groups starts to resemble the distribution observed in comparatively more extreme cluster environments. We find that the lowest HI mass objects lose their gas first as they are processed in the group environment, and it is evident that the infall of gas rich objects is important to the continuing growth of large scale structure at the present epoch, replenishing the neutral gas supply of groups. Finally, we compare our results to those of cosmological simulations and find that current models cannot simultaneously predict the HI selected halo occupation distribution for both low and high mass halos.

KAT-7 SCIENCE VERIFICATION: USING H I OBSERVATIONS OF NGC 3109 TO UNDERSTAND ITS KINEMATICS AND MASS DISTRIBUTION

HI observations of the Magellanic-type spiral NGC 3109, obtained with the seven dish Karoo Array Telescope (KAT-7), are used to analyze its mass distribution. Our results are compared to what is obtained using VLA data. KAT-7 is the precursor of the SKA pathfinder MeerKAT, which is under construction. The short baselines and low system temperature of the telescope make it sensitive to large scale low surface brightness emission. The new observations with KAT-7 allow the measurement of the rotation curve of NGC 3109 out to 32', doubling the angular extent of existing measurements. A total HI mass of 4.6 x 10^8 Msol is derived, 40% more than what was detected by the VLA observations. The observationally motivated pseudo-isothermal dark matter (DM) halo model can reproduce very well the observed rotation curve but the cosmologically motivated NFW DM model gives a much poorer fit to the data. While having a more accurate gas distribution has reduced the discrepancy between the observed RC and the MOdified Newtonian Dynamics (MOND) models, this is done at the expense of having to use unrealistic mass-to-light ratios for the stellar disk and/or very large values for the MOND universal constant a0. Different distances or HI contents cannot reconcile MOND with the observed kinematics, in view of the small errors on those two quantities. As for many slowly rotating gas-rich galaxies studied recently, the present result for NGC 3109 continues to pose a serious challenge to the MOND theory.

Is GBT 1355+5439 a dark galaxy?

We present HI imaging of GBT 1355+5439 performed with the Westerbork Synthesis Radio Telescope. This is a dark HI object recently discovered close to the nearby galaxy M101. We find GBT 1355+5439 to be an HI cloud 5x3 arcmin in size. The total HI image and the kinematics show that the cloud consists of condensations that have small (~10 km/s) motions with respect to each other. The column densities of the HI are low; the observed peak value is 7.1x10^{19} cm^{-2}. The velocity field shows a mild velocity gradient over the body of GBT 1355+5439, possibly due to rotation, but it may also indicate large-scale radial motions. Although our data are limited in sensitivity, at all positions the HI velocity dispersion is higher than 5 km/s and no narrow, cold, HI component is seen. Because its distance is not known, we considered various possibilities for the nature of GBT 1355+5439. Both the scenarios that it is a tidal remnant near M101 and that it is a dark dwarf companion of M101 meet difficulties. Neither do the data fit the properties of known compact High-Velocity Clouds in the Galactic halo exactly, but we cannot entirely exclude this option and deeper observations are required. We also considered the possibility that GBT 1355+5439 is a gas-rich dark minihalo in the outer regions of the Local Group. Interestingly, it would then have similar properties as the clouds of a proposed Local Group population recently found in the ALFALFA survey. In this case, the HI mass of GBT 1355+5439 would be about a few times 10^5 Msol, its size about 1 kpc, and the dynamical mass M_dyn > 5x10^7 Msol. However, if GBT 1355+5439 is a dark Local Group object, the internal kinematics of the HI appears to be different from that of gas-dominated, almost dark galaxies of similar size.

HALOGAS: Extraplanar gas in NGC 3198

We present the analysis of new, deep HI observations of the spiral galaxy NGC 3198, as part of the HALOGAS (Westerbork Hydrogen Accretion in LOcal GAlaxieS) survey, with the main aim of investigating the presence, amount, morphology and kinematics of extraplanar gas. We present models of the HI observations of NGC 3198: the model that matches best the observed data cube features a thick disk with a scale height of ~3 kpc and an HI mass of about 15% of the total HI mass; this thick disk also has a decrease in rotation velocity as a function of height (lag) of 7-15 km/s/kpc (though with large uncertainties). This extraplanar gas is detected for the first time in NGC 3198. Radially, this gas appears to extend slightly beyond the actively star-forming body of the galaxy (as traced by the Halpha emission), but it is not more radially extended than the outer, fainter parts of the stellar disk. Compared to previous studies, thanks to the improved sensitivity we trace the rotation curve out to larger radii. We model the rotation curve in the framework of MOND (Modified Newtonian Dynamics) and we confirm that, with the allowed distance range we assumed, fit quality is modest in this galaxy, but the new outer parts are explained in a satisfactory way.

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

We introduce the "Bluedisk" project, a large program at the Westerbork Synthesis Radio Telescope (WSRT) that has mapped the HI in a sample of 23 nearby galaxies with unusually high HI mass fractions, along with a similar-sized sample of control galaxies matched in stellar mass, size, inclination and redshift. This paper presents the sample selection, observational set-up, data reduction strategy, and a first analysis of the sizes and structural properties of the HI disks. We find that the HI-rich galaxies lie on the same HI mass versus HI size relation as normal spiral galaxies, extending it to total HI masses of $2 \times 10^{10} M_{\odot}$ and radii R1 of $\sim 100$ kpc (where R1 is defined as the radius where the HI column density reaches 1 $M_{\odot}$ pc$^{-2}$). HI-rich galaxies have significantly larger values of HI-to-optical size ratio at fixed stellar mass, concentration index, stellar and star formation rate surface density compared to the control sample. The disks of HI-rich galaxies are also significantly more clumpy (i.e. have higher HI Gini and $\Delta$Area coefficient) than those of normal spirals. There is no evidence that the disks of HI-rich galaxies are more disturbed: HI-rich galaxies exhibit no difference with respect to control samples in their distributions of HI asymmetry indices or optical/HI disk position angle differences. In fact, the center of the HI distribution corresponds more closely with the center of the optical light in the HI-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.

Choirs, H i galaxy groups: catalogue and detection of star-forming dwarf group members

H{\alpha} observations centred on galaxies selected from the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001) typically show one and sometimes two star-forming galaxies within the approximately 15-arcminute beam of the Parkes 64-m HI detections. In our Survey of Ionization in Neutral Gas Galaxies (SINGG, Meurer et al. 2006) we found fifteen cases of HIPASS sources containing four or more emission line galaxies (ELGs). We name these fields Choir groups. In the most extreme case we found a field with at least nine ELGs. In this paper we present a catalogue of Choir group members in the context of the wider SINGG sample. The dwarf galaxies in the Choir groups would not be individually detectable in HIPASS at the observed distances if they were isolated, but are detected in SINGG narrow-band imaging due to their membership of groups with sufficiently large total HI mass. The ELGs in these groups are similar to the wider SINGG sample in terms of size, H{\alpha} equivalent width, and surface brightness. Eight of these groups have two large spiral galaxies with several dwarf galaxies and may be thought of as morphological analogues of the Local Group. However, on average our groups are not significantly HI-deficient, suggesting that they are at an early stage of assembly, and more like the M81 group. The Choir groups are very compact at typically only 190 kpc in projected distance between the two brightest members. They are very similar to SINGG fields in terms of star formation efficiency (the ratio of star formation rate to HI mass; SFE), showing an increasing trend in SFE with stellar mass.