arXiv subjects

W. J. G. de Blok

Publications and source records attributed to W. J. G. de Blok.

At least 163 records · Page 9Linked to original sources

arXiv2006

Water masers in the Local Group of galaxies

We compare the number of detected 22 GHz H2O masers in the Local Group galaxies M31, M33, NGC6822, IC10, IC1613, DDO187, GR8, NGC185, and the Magellanic Clouds with the water maser population of the Milky Way. To accomplish this we searched for water maser emission in the two Local Group galaxies M33 and NGC6822 using the Very Large Array (VLA) and incorporated results from previous studies. We observed 62 HII regions in M33 and 36 regions with H-alpha emission in NGC6822. Detection limits are 0.0015 and 0.0008 Lsun for M33 and NGC6822, respectively (corresponding to 47 and 50 mJy in three channels with 0.7 km/s width). M33 hosts three water masers above our detection limit, while in NGC6822 no maser source was detected. We find that the water maser detection rates in the Local Group galaxies M31, M33, NGC6822, IC1613, DDO187, GR8, NGC185, and the Magellanic Clouds are consistent with expectations from the Galactic water masers if one considers the different star formation rates of the galaxies. However, the galaxy IC10 exhibits an overabundance of masers, which may result from a compact central starburst.

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arXiv2006

High Resolution Optical Velocity Fields of 11 Low Surface Brightness Galaxies

We present high resolution two-dimensional velocity fields from integral field spectroscopy along with derived rotation curves for eleven low surface brightness galaxies. We fit NFW and pseudo-isothermal halo models to the new data combined with previous long-slit and HI data. In most cases we find the pseudo-isothermal halo to better represent the data than the NFW halo, as the NFW concentrations are often lower than expected for a LambdaCDM cosmology. We also compare our results to previous studies and find that including the new two-dimensional optical data does not significantly alter the halo parameters, but does decrease the uncertainties by roughly a factor of 2.

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arXiv2005

Two Dimensional Velocity Fields of Low Surface Brightness Galaxies

We present high resolution two dimensional velocity fields from integral field spectroscopy along with derived rotation curves for nine low surface brightness galaxies. This is a positive step forward in terms of both data quality and number of objects studied. We fit NFW and pseudo-isothermal halo models to the observations. We find that the pseudo-isothermal halo better represents the data in most cases than the NFW halo, as the resulting concentrations are lower than would be expected for LCDM.

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arXiv2005

The stellar population and interstellar medium in NGC 6822

We present a comprehensive study of the stellar population and the interstellar medium in NGC 6822 using high-quality \HI data (obtained with the Australia Telescope Compact Array) and optical broad/narrow-band data (obtained with Subaru and the INT). Our H$α$ observations are an order of magnitude deeper than previous studies and reveal a complex filamentary network covering almost the entire central disk of NGC 6822. We find hitherto unknown HII regions in the outskirts of NGC 6822 and the companion galaxy. The old and intermediate age stellar population can be traced out to radii of over 0.6 deg (> 5 kpc), significantly more extended than the HI disk. In sharp contrast, the distribution of the young, blue stars, closely follows the distribution of the HI disk and displays a highly structured morphology. We find evidence for an older stellar population in the companion galaxy -- the current star formation activity, although likely to have been triggered by the interaction with NGC 6822, is not the first star formation episode in this object. We show that the properties of the giant kpc-sized hole in the outer HI disk of NGC 6822 are consistent with it being formed by the effects of stellar evolution.

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arXiv2005

Halo Mass Profiles and Low Surface Brightness Galaxies Rotation Curves

A recent study has claimed that the rotation curve shapes and mass densities of Low Surface Brightness (LSB) galaxies are largely consistent with $Λ$CDM predictions, in contrast to a large body of observational work. I demonstrate that the method used to derive this conclusion is incapable of distinguishing the characteristic steep CDM mass-density distribution from the core-dominated mass-density distributions found observationally: even core-dominated pseudo-isothermal haloes would be inferred to be consistent with CDM. This method can therefore make no definitive statements on the (dis)agreement between the data and CDM simulations. After introducing an additional criterion that does take the slope of the mass-distribution into account I find that only about a quarter of the LSB galaxies investigated are possibly consistent with CDM. However, for most of these the fit parameters are so weakly constrained that this is not a strong conclusion. Only 3 out of 52 galaxies have tightly constrained solutions consistent with $Λ$CDM. Two of these galaxies are likely dominated by stars, leaving only one possible dark matter dominated, CDM-consistent candidate, forming a mere 2 per cent of the total sample. These conclusions are based on comparison of data and simulations at identical radii and fits to the entire rotation curves. LSB galaxies that are consistent with CDM simulations, if they exist, seem to be rare indeed.

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arXiv2005

The HIPASS Catalogue: III - Optical Counterparts & Isolated Dark Galaxies

We present the largest catalogue to date of optical counterparts for HI radio-selected galaxies, Hopcat. Of the 4315 HI radio-detected sources from the HI Parkes All Sky Survey (Hipass) catalogue, we find optical counterparts for 3618 (84%) galaxies. Of these, 1798 (42%) have confirmed optical velocities and 848 (20%) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16%) have confirmed optical velocities and a further 258 (6%) galaxies are without confirmed velocities. For 481 (11%), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5%) have no obvious optical galaxy present. Most of these 'blank fields' are in crowded fields along the Galactic plane or have high extinctions. Isolated 'Dark galaxy' candidates are investigated using an extinction cut of ABj < 1 mag and the blank fields category. Of the 3692 galaxies with an ABj extinction < 1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow-up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the Hipass survey.

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arXiv2004

The cosmological significance of Low Surface Brightness galaxies found in a deep blind neutral-hydrogen survey

We have 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-hour R-band Tech Pan films from the UK Schmidt Telescope covering 36 square degrees of the survey area to reach a very deep isophotal limit of 26.5 R mag/sq. arcsec. 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 HI sources to estimate the significance of LSB galaxies (defined to be those at least 1.5 magnitudes dimmer in effective surface-brightness than the peak in the observed distribution seen in optical surveys). We calculate the contribution of LSB galaxies to the total number, neutral hydrogen density, luminosity density, baryonic mass density, dynamical mass density and cross-sectional area of gas-rich galaxies. 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 either be rare, gas-poor or outside the survey detection limits.

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arXiv2004

Oxygen Abundances and Chemical Evolution in Low Surface Brightness Galaxies

We report the oxygen abundances of the HII regions of a sample of low surface brightness (LSB) galaxies. We provide analytic functions describing the McGaugh (1991) calibration of the R_23 method. We use this and the equivalent width (EW) method to determine oxygen abundances, and also make direct estimates in a few cases where the temperature sensitive [OIII] 4363 line is available. We find LSB galaxies to be metal poor, consistent with the L-Z relation of other galaxies. The large gas mass fractions of these objects provide an interesting test of chemical evolution models. We find no obvious deviation from the closed-box model of galactic chemical evolution. Based on our abundance and gas mass fraction measurements, we infer that LSB galaxies are not fundamentally different than other galaxy types but are perhaps at an early stage of evolution.

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arXiv2004

First Results from THINGS: The HI Nearby Galaxy Survey

We describe The HI Nearby Galaxy Survey (THINGS), the largest program ever undertaken at the VLA to perform 21-cm HI observations of the highest quality (~7'', <= 5 km/s resolution) of nearby galaxies. The goal of THINGS is to investigate key characteristics related to galaxy morphology, star formation and mass distribution across the Hubble sequence. A sample of 34 objects with distances between 3 and 10 Mpc will be observed, covering a wide range of evolutionary stages and properties. Data from THINGS will complement SINGS, the Spitzer Infrared Nearby Galaxy Survey. For the THINGS sample, high-quality observations at comparable resolution will thus be available from the X-ray regime through to the radio part of the spectrum. THINGS data can be used to investigate issues such as the small-scale structure of the ISM, its three-dimensional structure, the (dark) matter distribution and processes leading to star formation. To demonstrate the quality of the THINGS data products, we present some prelimary HI maps here of four galaxies from the THINGS sample.

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arXiv2004

Star Formation in HI Selected Galaxies I: Sample Characteristics

A sample of 69 galaxies with radial velocities less than 2500 km/s was selected from the HI Parkes All Sky Survey (HIPASS) and imaged in broad band B and R and narrow band H-alpha to deduce details about star formation in nearby disk galaxies while avoiding surface brightness selection effects. The sample is dominated by late-type, dwarf disks (mostly Sc and Sm galaxies) with exponential disk scale lengths of about 1 to 5 kpc. The HIPASS galaxies on average have lower star formation rates (SFRs) and are bluer and lower surface brightness than an optically selected sample. HII regions were detected in all but one of the galaxies. Many galaxies had as few as two to five HII regions. The galaxies' H-alpha equivalent widths, colors, and SFRs per unit HI mass are best explained by young mean ages (about 3 to 5 Gyr according to Schmidt Law models) with star formation histories where the SFRs were higher in the past. Comparison of the surface brightness coverage of the HIPASS galaxies with that of an optically selected sample shows that such a sample may miss about 10% of the local galaxy number density and could possibly miss as much as 3 to 4% of the SFR density. The amount lower surface brightness galaxies contribute to the total luminosity density may be insignificant, but this conclusion is somewhat dependent on how the fluxes of these objects are determined.

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arXiv2004

Structure and kinematics of edge-on galaxy discs -- II. Observations of the neutral hydrogen

We present Australia Telescope Compact Array (ATCA) and Westerbork Synthesis Radio Telescope (WSRT) HI observations of 15 edge-on spiral galaxies of intermediate to late morphological type. The global properties and the distribution and kinematics of the HI gas are analysed and discussed. We determine the rotation curves using the envelope-tracing method. For 10 spiral galaxies with a stellar disc truncation we find an average ratio of the HI radius to the truncation radius of the stellar disc of 1.1 +/- 0.2 (1 sigma). This paper has been accepted by MNRAS and is available in pdf-format at the following URL: http://www.astro.rug.nl/$\sim $vdkruit/jea3/homepage/paperII.pdf

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arXiv2004

The nuclear ring in the unbarred galaxy NGC 278: result of a minor merger?

We present fully sampled high angular resolution two-dimensional kinematics in the H alpha spectral line, optical and near-infrared imaging, as well as 21 cm atomic hydrogen data of the spiral galaxy NGC 278. This is a small non-barred galaxy, which has a bright star forming inner region of about 2 kpc in diameter, reminiscent of nuclear rings seen mainly in barred galaxies. The H alpha kinematics show a disturbed velocity field, which may be partly the result of spiral density wave streaming motions. The 21 cm data trace the atomic hydrogen well outside the optical disk. The HI is not abundant but clearly shows disturbed morphology and kinematics. We postulate that the current structure of NGC 278 is a result of a recent interaction with a small gas-rich galaxy, which is now dispersed into the outer disk of NGC 278. Non-axisymmetries set up in the disk by this minor merger may well be the cause of the intense star formation in the inner region, which can be interpreted as a rare example of a nuclear ring in a non-barred galaxy. Rather than being induced by a bar, this nuclear ring would then be the direct result of an interaction event in the recent history of the galaxy.

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arXiv2003

A multi-beam HI survey of the Virgo Cluster - two isolated HI clouds ?

We have carried out a fully sampled large area ($4^{\circ} \times 8^{\circ}$) 21cm \HI line survey of part of the Virgo cluster using the Jodrell Bank multi-beam instrument. The survey has a sensitivity some 3 times better than the standard HIJASS and HIPASS surveys. We detect 31 galaxies, 27 of which are well known cluster members. The four new detections have been confirmed in the HIPASS data and by follow up Jodrell Bank pointed observations. One object lies behind M86, but the other 3 have no obvious optical counter parts upon inspection of the digital sky survey fields. These 3 objects were mapped at Arecibo with a smaller \am{3}{6} HPBW and a 4 times better sensitivity than the Jodrell Bank data, which allow an improved determination of the dimensions and location of two of the objects, but surprisingly failed to detect the third. The two objects are resolved by the Arecibo beam giving them a size far larger than any optical images in the nearby field. To our mass limit of $5 \times 10^{7}$ $\frac{Δv}{50 km s^{-1}}$ $M_{\odot}$ and column density limit of $3 \times 10^{18}$ $\frac{Δv}{50 km s^{-1}}$ atoms cm$^{-2}$ these new detections represent only about 2% of the cluster atomic hydrogen mass. Our observations indicate that the \HI mass function of the cluster turns down at the low mass end making it very different to the field galaxy \HI mass function. This is quite different to the Virgo cluster optical luminosity function which is much steeper than that in the general field. Many of the sample galaxies are relatively gas poor compared to \HI selected samples of field galaxies, confirming the 'anaemic spirals' view of Virgo cluster late type galaxies.

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arXiv2003

CDM in LSB Galaxies: Toward the Optimal Halo Profile

Low Surface Brightness (LSB) galaxies are dominated by dark matter. High-resolution rotation curves suggest that their total mass-density distributions are dominated by constant density cores rather than the steep and cuspy distributions found in Cold Dark Matter (CDM) simulations. The data are best described by a model with a soft core with an inner power-law mass-density slope alpha = 0.2 +/- 0.2. However no single universal halo profile provides an adequate description of the data. The observed mass profiles appear to be inconsistent with LambdaCDM.

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arXiv2003

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

We have carried out an extremely long integration-time (9000 s/beam) 21-cm blind survey of 60 square degrees in Centaurus using the Parkes multibeam system. We find that the noise continues to fall as sqrt(t) throughout, enabling us to reach an HI column-density limit of 4.2 x 10^18 cm^-2 for galaxies with a velocity width of 200 km/s in the central 32 square degree region, making this the deepest survey to date in terms of column density sensitivity. The HI 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_HI = 10^(20.65 +/- 0.38). This is at least an order of magnitude above our sensitivity limit, with a scatter only marginally larger than the errors on N_HI. This needs explaining. If confirmed it means that HI surveys will only find low surface brightness (LSB) galaxies with high M_HI/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 HI surveys being carried out at Parkes and Jodrell Bank.

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arXiv2003

Radiative transfer in disc galaxies -- III. The observed kinematics of dusty disc galaxies

We present SKIRT (Stellar Kinematics Including Radiative Transfer), a new Monte Carlo radiative transfer code that allows the calculation of the observed stellar kinematics of a dusty galaxy. The code incorporates the effects of both absorption and scattering by interstellar dust grains, and calculates the Doppler shift of the emerging radiation exactly by taking into account the velocities of the emitting stars and the individual scattering dust grains. The code supports arbitrary distributions of dust through a cellular approach, whereby the integration through the dust is optimized by means of a novel efficient trilinear interpolation technique. We apply our modelling technique to calculate the observed kinematics of realistic models for dusty disc galaxies. We find that the effects of dust on the mean projected velocity and projected velocity dispersion are severe for edge-on galaxies. For galaxies which deviate more than a few degrees from exactly edge-on, the effects are already strongly reduced. As a consequence, dust attenuation cannot serve as a possible way to reconcile the discrepancy between the observed shallow slopes of the inner rotation curves of LSB galaxies and the predictions of CDM cosmological models. For face-on galaxies, the velocity dispersion increases with increasing dust mass due to scattering, but the effects are limited, even for extended dust distributions. Finally, we show that serious errors can be made when the individual velocities of the dust grains are neglected in the calculations.

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arXiv2003

Young Stars in the Outer HI Disc of NGC 6822

We present wide-field optical imaging covering the entire neutral hydrogen disc of the Local Group dwarf galaxy NGC 6822. These observations reveal the presence of numerous blue, young stars at large galactocentric radii well beyond R_25. Blue stars are also found that are associated with NGC 6822's companion HI cloud, indicating that star formation was triggered in the companion in the last 10^8 yr. In general, blue stars are present where the HI surface densities reach values > 5 x 10^20 cm^(-2). However, over one-third of the blue stars detected are found at lower surface densities. The young stars trace the distribution of the neutral hydrogen surprisingly well, but seem to be avoiding the supergiant HI shell in NGC 6822, setting a lower limit for its age of 10^8 yr. The extended distribution of young stars implies that stars can form at large galactocentric radii in dwarf galaxies; the HI is therefore not necessarily much more extended than the stellar population. This finding has important consequences for the chemical enrichment of the interstellar medium throughout (dwarf) galaxies

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arXiv2003

The 1000 Brightest HIPASS Galaxies: HI Mass Function and Omega_HI

We present a new accurate measurement of the HI mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest HI peak flux densities in the southern hemisphere (Koribalski et al. 2003). This sample spans nearly four orders of magnitude in HI mass (from log M_HI/M_sun=6.8 to 10.6, H0=75) and is the largest sample of HI selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies, and show that this is a robust method, insensitive to the effects of large scale structure. The resulting HI mass function can be fitted satisfactorily with a Schechter function with faint-end slope alpha=-1.30. This slope is found to be dependent on morphological type, with later type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the HI mass function, including peculiar motions of galaxies, large scale structure, selection bias, and inclination effects, and quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: Omega_HI=(3.8 +/- 0.6) x 10^{-4}. Low surface brightness galaxies contribute only 15% to this value, consistent with previous findings.

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