arXiv subjects

W. B. Burton

Publications and source records attributed to W. B. Burton.

At least 19 recordsLinked to original sources

arXiv2025

The Flocculent Structure of the Inner Milky Way Disk

Observations of HI published in 1957 by Westerhout and Schmidt were presented as showing a global face-on view of spiral structure in the Milky Way. Since then many studies have attempted to improve on the early map, perhaps presupposing our Galaxy to be characterized by a Grand-design pattern of prominent spiral arms. We consider here two approaches to explore the nature of spiral structure of the inner Milky Way disk using the HI4PI survey. The first is to search for shallow shoulders in the high-velocity wings of HI data along the Galactic equatorial disk of the inner Milky Way that would be expected if the lines of sight swept across interarm regions of low HI density. The second is to look for broad dips in the integrated HI brightness temperature over the high-velocity wings, pertaining to gas near the subcentral region, that would be expected for the interarm region of a Grand-design. We find neither shallow shoulders nor broad dips in either the Northern quadrant I or the Southern quadrant IV indicating that the Milky Way seen interior to the Solar orbit is not characterized by a majestic spiral-structure Grand-design; this conclusion is a robust one, in that it does not depend on measures of distance. Taken together with decades of work on the bits and pieces of the quite disorganized shambles of the inner Galaxy, we suggest that the Milky Way belongs to the category of Flocculent spirals.

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arXiv2005

The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections

We present the final data release of observations of lambda 21-cm emission from Galactic neutral hydrogen over the entire sky, merging the Leiden/Dwingeloo Survey (LDS: Hartmann & Burton, 1997) of the sky north of delta = -30 deg with the Instituto Argentino de Radioastronomia Survey (IAR: Arnal et al., 2000, and Bajaja et al., 2005) of the sky south of delta = -25 deg. The angular resolution of the combined material is HPBW ~ 0.6 deg. The LSR velocity coverage spans the interval -450 km/s to +400 km/s, at a resolution of 1.3 km/s. The data were corrected for stray radiation at the Institute for Radioastronomy of the University of Bonn, refining the original correction applied to the LDS. The rms brightness-temperature noise of the merged database is 0.07 - 0.09 K. Residual errors in the profile wings due to defects in the correction for stray radiation are for most of the data below a level of 20 - 40 mK. It would be necessary to construct a telescope with a main beam efficiency of eta_{MB} > 99% to achieve the same accuracy. The merged and refined material entering the LAB Survey of Galactic HI is intended to be a general resource useful to a wide range of studies of the physical and structural characteristices of the Galactic interstellar environment. The LAB Survey is the most sensitive Milky Way HI survey to date, with the most extensive coverage both spatially and kinematically.

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arXiv2002

Compact, Isolated High-Velocity Clouds

We consider here the class of compact, isolated, high-velocity HI clouds, CHVCs, which are sharply bounded in angular extent down to a limiting column density of 1.5x10^18 cm^-2. We describe our automated search algorithm and it's application to the LDS north of dec= -28 deg. and the HIPASS data south of dec=0, resulting in an all--sky catalog numbering 246 CHVCs. We argue that these objects are more likely to represent a single phenomenon in a similar evolutionary state than would a sample which included any of the major HVC complexes. Five principal observables are defined for the CHVC population: (1) the spatial deployment of the objects on the sky, (2) the kinematic distribution, (3) the number distribution of observed HI column densities, (4) the number distribution of angular sizes, and (5) the number distribution of line widths. We show that the spatial and kinematic deployments of the ensemble of CHVCs contain various clues regarding their characteristic distance. These clues are not compatible with a location of the ensemble within the Galaxy proper. The deployments resemble in several regards those of the Local Group galaxies. We describe a model testing the hypothesis that the CHVCs are a Local Group population. The agreement of the model with the data is judged by extracting the observables from simulations, in a manner consistent with the sensitivities of the observations and explicitly taking account of Galactic obscuration. We show that models in which the CHVCs are the HI counterparts of dark-matter halos evolving in the Local Group potential provide a good match to the observables, if account is taken of tidal and ram--pressure disruption, the consequences of obscuration due to Galactic HI and of differing sensitivities and selection effects pertaining to the surveys.

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arXiv2002

High-resolution imaging of compact high-velocity clouds (II)

We have imaged five compact high-velocity clouds in HI with arcmin angular- and km/s spectral-resolution using the WSRT. Supplementary total-power data, which is fully sensitive to both the cool and warm components of HI, is available for comparison for all the sources, albeit with angular resolutions that vary from 3' to 36'. The fractional HI flux in compact CNM components varies from 4% to 16% in our sample. All objects have at least one local peak in the CNM column which exceeds about 10^19 cm^-2 when observed with arcmin resolution. It is plausible that a peak column density of 1-2x10^19 cm^-2 is a prerequisite for the long-term survival of these sources. One object in our sample, CHVC120-20-443 (Davies' cloud), lies in close projected proximity to the disk of M31. This object is characterized by exceptionally broad linewidths in its CNM concentrations (more than 5 times greater than the median value). These CNM concentrations lie in an arc on the edge of the source facing the M31 disk, while the diffuse HI component of this source has a position offset in the direction of the disk. All of these attributes suggest that CHVC120-20-443 is in a different evolutionary state than most of the other CHVCs which have been studied. Similarly broad CNM linewidths have only been detected in one other object, CHVC111-07-466, which also lies in the Local Group barycenter direction and has the most extreme radial velocity known. A distinct possibility for Davies' cloud seems to be physical interaction of some type with M31. The most likely form of this interaction might be the ram-pressure or tidal- stripping by either one of M31's visible dwarf companions, M32 or NGC205, or else by a dark companion with an associated HI condensation.

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arXiv2002

An all-sky study of compact, isolated high-velocity clouds

We combine the catalogs of compact high-velocity HI clouds extracted from the LDS and HIPASS surveys and analyze the all-sky properties of the ensemble. Five principal observables are defined for the CHVC population: (1) the spatial deployment of the objects on the sky, (2) the kinematic distribution, (3) the number distribution of observed HI column densities, (4) the number distribution of angular sizes, and (5) the number distribution of HI linewidth. Two classes of models are considered to reproduce the observed properties. The agreement of models with the data is judged by extracting these same observables from simulations, in a manner consistent with the sensitivities of the observations and explicitly taking account of Galactic obscuration. We show that models in which the CHVCs are the HI counterparts of dark-matter halos evolving in the Local Group potential provide a good match to the observables. The best-fitting populations have a maximum HI mass of 10^7 M_Sun a power-law slope of the HI mass distribution in the range -1.7 to -1.8, and a Gaussian dispersion for their spatial distributions of between 150 and 200 kpc centered on both the Milky Way and M31. Given its greater mean distance, only a small fraction of the M31 sub-population is predicted to have been detected in present surveys. An empirical model for an extended Galactic halo distribution for the CHVCs is also considered. While reproducing some aspects of the population, this class of models does not account for some key systematic features of the population.

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arXiv2002

An automated search for high-velocity clouds in the Leiden/Dwingeloo Survey

We describe an automated search through the Leiden/Dwingeloo HI Survey (LDS) for high-velocity clouds north of Dec=-28 deg. From the general catalog we extract a sample of isolated high-velocity clouds, CHVCs: anomalous-velocity HI clouds which are sharply bounded in angular extent with no kinematic or spatial connection to other HI features down to a limiting column density of 1.5*10^18cm^-2. This column density is an order of magnitude lower than the critical HI column density, about 2*10^19cm^-2, where the ionized fraction is thought to increase dramatically due to the extragalactic radiation field. As such, these objects are likely to provide their own shielding to ionizing radiation. Their small median angular size, of about 1 deg. FWHM, might then imply substantial distances, since the partially ionized HI skin in a power-law ionizing photon field has a typical exponential scale-length of 1 kpc. The automated search algorithm has been applied to the HIPASS and to the Leiden/Dwingeloo data sets. The results from the LDS are described here; Putman et al. (2002) describe application of this algorithm to the HIPASS material. We identify 67 CHVCs in the LDS which satisfy stringent requirements on isolation, and an additional 49 objects which satisfy somewhat less stringent requirements. Independent confirmation is available for all of these objects, either from earlier data in the literature or from new observations made with the Westerbork Synthesis Radio Telescope and reported here. The catalog includes 54 of the 65 CHVCs listed by Braun and Burton (1999) on the basis of a visual search of the LDS data.

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arXiv2001

HIPASS High-Velocity Clouds: Properties of the Compact and Extended Populations

A catalog of Southern anomalous-velocity HI clouds at Decl. < +2 deg is presented, based on data from the HI Parkes All-Sky Survey (HIPASS). The improved sensitivity (5sigma: T_B = 0.04 K) and resolution (15.5') of the HIPASS data results in a substantial increase in the number of individual clouds (1956, as well as 41 galaxies) compared to previous surveys. Most high-velocity emission features, HVCs, have a filamentary morphology and are loosely organized into large complexes extending over tens of degrees. In addition, 179 compact and isolated anomalous-velocity objects, CHVCs, are identified based on their size and degree of isolation. 25% of the CHVCs originally classified by Braun & Burton (1999) are reclassified. Both the entire population of high-velocity emission features and the CHVCs alone have typical HI masses of ~ 4.5 D(kpc)^2 solar masses and have similar slopes for their column density and flux distributions. On the other hand, the CHVCs appear to be clustered and the population can be broken up into three spatially distinct groups, while the entire population of clouds is more uniformly distributed with a significant percentage aligned with the Magellanic Stream. The median velocities are V_GSR = -38 km/s for the CHVCs and -30 km/s for all of the anomalous-velocity clouds. Based on the catalog sizes, high-velocity features cover 19% of the southern sky and CHVCs cover 1%. (abridged)

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arXiv2001

Arecibo imaging of compact high-velocity clouds

Ten isolated compact high-velocity clouds (CHVCs) of the type cataloged by Braun & Burton (1999) have been imaged with the Arecibo telescope and were found to have a nested core/halo morphology. We argue that a combination of high-resolution filled-aperture and synthesis data is crucial to determining the intrinsic properties of the CHVCs. We identify the halos as Warm Neutral Medium surrounding one or more cores in the Cool Neutral Medium phase. These halos are clearly detected and resolved by the Arecibo filled-aperture imaging, which reaches a limiting sensitivity (1 sigma) of N_H about 2x10^17 cm^-2 over the typical 70 km/s linewidth at zero intensity. The FWHM linewidth of the halo gas is found to be 25 km/s, consistent with a WNM thermal broadening within 10^4 K gas. Substantial asymmetries are found at high N_H (>10^18.5 cm^-2) levels in 60% of our sample. A high degree of reflection-symmetry is found at low N_H (<10^18.5 cm^-2) in all sources studied at these levels. The column-density profiles of the envelopes are described well by the sky-plane projection of a spherical exponential in atomic volume density, which allows estimating the characteristic central halo column density, N_H(0) = 4.1+/-3.2x10^19 cm^-2, and characteristic exponential scale-length, h_B=420+/-90 arcsec. For plausible values of the thermal pressure at the CNM/WNM interface, these edge profiles allow distance estimates to be made for the individual CHVCs studied here which range between 150 and 850 kpc. (abridged)

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arXiv1999

Morphological Characteristics of Compact High-Velocity Clouds Revealed by High-Resolution WSRT Imaging

A class of compact, isolated high-velocity clouds which plausibly represents a homogeneous subsample of the HVC phenomenon in a single physical state was objectively identified by Braun and Burton (1999). Six examples of the CHVCs, unresolved in single-dish data, have been imaged with the Westerbork Synthesis Radio Telescope. The high-resolution imaging reveals the morphology of these objects, including a core/halo distribution of fluxes, signatures of rotation indicating dark matter, and narrow linewidths constraining the kinetic temperature of several opaque cores. In these regards, as well as in their kinematic and spatial deployment on the sky, the CHVC objects are evidently a dynamically cold ensemble of dark-matter-dominated HI clouds accreting onto the Local Group in a continuing process of galactic evolution.

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arXiv1999

Compact High-Velocity Clouds at High Resolution

Six examples of the compact, isolated high-velocity clouds catalogued by Braun & Burton (1999) and identified with a dynamically cold ensemble of primitive objects falling towards the barycenter of the Local Group have been imaged with the Westerbork Synthesis Radio Telescope; an additional ten have been imaged with the Arecibo telescope. The imaging reveals a characteristic core/halo morphology: one or several cores of cool, relatively high-column-density material, are embedded in an extended halo of warmer, lower-density material. Several of the cores show kinematic gradients consistent with rotation; these CHVCs are evidently rotationally supported and dark-matter dominated. The imaging data allows several independent estimates of the distances to these objects, which lie in the range 0.3 to 1.0 Mpc. The CHVC properties resemble what might be expected from very dark dwarf irregular galaxies.

astro-ph↗

arXiv1999

HI Observations Towards the Sagittarius Dwarf Spheroidal Galaxy

We have measured the 21-cm line of Galactic HI over more than 50 square degrees in the direction of the Sagittarius dwarf spheroidal galaxy. The data show no evidence of HI associated with the dwarf spheroidal which might be consider analogous to the Magellanic Stream as it is associated in both position and velocity with the Large Magellanic Cloud. Nor do the HI data show evidence for any disturbance in the Milky Way disk gas that can be unambiguously assigned to interaction with the dwarf galaxy. The data shown here limit the HI mass at the velocity of the Sagittarius dwarf to <7000 solar masses over some 18 square degrees between Galactic latitudes -13 degrees and -18 degrees.

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arXiv1998

Soft X-rays from High-Velocity Clouds

The ROSAT mission gave us the unique possibility to study the galactic interstellar medium by ``shadowing'' observations on arcmin angular resolution level. The high sensitivity of the ROSAT PSPC in combination with the large field of view allows the detailed study of individual clouds, while the ROSAT all-sky survey is the main source for studies of the large-scale intensity distribution of the diffuse soft X-ray emission. We show that neutral hydrogen clouds are associated with soft X-ray emission, E(X-ray) < 3/4 keV. These neutral hydrogen clouds, known as high-velocity clouds (HVCs), are located partly within the galactic halo, and partly in intergalactic space. The ROSAT detection of soft X-ray emission from some of these HVCs is the first detection of HVCs in emission other than HI 21-cm line radiation.

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arXiv1998

The Kinematic and Spatial Deployment of Compact, Isolated High-Velocity Clouds

We have identified a class of high-velocity clouds which are compact and apparently isolated. The clouds are compact in that they have angular sizes less than 2 degrees FWHM. They are isolated in that they are separated from neighboring emission by expanses where no emission is seen to the detection limit of the available data. Candidates for inclusion in this class were extracted from the Leiden/Dwingeloo HI survey of Hartmann & Burton and from the Wakker & van Woerden catalogue of high-velocity clouds. The candidates were subject to independent confirmation using either the 25-meter telescope in Dwingeloo or the 140-foot telescope in Green Bank. We argue that the resulting list, even if incomplete, is sufficiently representative of the ensemble of compact, isolated HVCs - CHVCs - that the characteristics of their disposition on the sky, and of their kinematics, are revealing of some physical aspects of the class. The CHVCs are in fact distributed quite uniformly across the sky. A global search for the reference frame which minimizes the velocity dispersion of the ensemble returns the Local Group Standard of Rest with high confidence. The CHVCs are not stationary with respect to this reference frame but have a mean infall velocity of 100 km/s. These properties are strongly suggestive of a population which has as yet had little interaction with the more massive Local Group members. At a typical distance of about 1 Mpc these objects would have sizes of about 15 kpc and gas masses, M_HI, of a few times 10^7 M_Sun, corresponding to those of (sub-)dwarf galaxies. (abridged)

astro-ph↗

arXiv1998

Discovery of a Nearby Low-Surface-Brightness Spiral Galaxy

During the course of a search for compact, isolated gas clouds moving with anomalous velocities in or near our own Galaxy (Braun and Burton 1998 A&A, in press), we have discovered, in the data of the Leiden/Dwingeloo survey (Hartmann and Burton 1997, Atlas of Galactic Neutral Hydrogen, CUP) of Galactic hydrogen, the HI signature of a large galaxy, moving at a recession velocity of 282 km/s, with respect to our Galaxy. Deep multicolor and spectroscopic optical observations show the presence of star formation in scattered HII regions; radio HI synthesis interferometry confirms that the galaxy is rich in HI and has the rotation signature of a spiral galaxy; a submillimeter observation failed to detect the CO molecule. The radio and optical evidence combined suggest its classification as a low-surface-brightness spiral galaxy. It is located in close spatial and kinematic proximity to the galaxy NGC 6946. The newly-discovered galaxy, which we call Cepheus 1, is at a distance of about 6 Mpc. It is probably to be numbered amongst the nearest few LSB spirals.

astro-ph↗

arXiv1998

A search for soft X-ray emission associated with prominent high-velocity-cloud complexes

We correlate the ROSAT 1/4 keV all-sky survey with the Leiden/Dwingeloo HI survey, looking for soft X-ray signatures of prominent high-velocity-cloud (HVC) complexes. We study the transfer of 1/4 keV photons through the interstellar medium in order to distinguish variations in the soft X-ray background (SXRB) intensity caused by photoelectric absorption effects from those due to excess X-ray emission. The X-ray data are modelled as a combination of emission from the Local Hot Bubble (LHB) and emission from a distant plasma in the galactic halo and extragalactic sources. The X-ray radiation intensity of the galactic halo and extragalactic X-ray background is modulated by the photoelectric absorption of the intervening galactic interstellar matter. We show that large- and small-scale intensity variations of the 1/4 keV SXRB are caused by photoelectric absorption which is predominantly traced by the total N(HI) distribution. The extensive coverage of the two surveys supports evidence for a hot, X-ray emitting corona. We show that this leads to a good representation of the SXRB observations. For four large areas on the sky, we search for regions where the modelled and observed X-ray emission differ. We find that there is excess X-ray emission towards regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray emission is positionally correlated with the high-velocity clouds. Some lines of sight towards HVCs also pass through significant amounts of intermediate-velocity gas, so we cannot constrain the possible role played by IVC gas in these directions of HVC and IVC overlap, in determining the X-ray excesses.

astro-ph↗

arXiv1997

Galaxies Discovered Behind the Milky Way by the Dwingeloo Obscured Galaxies Survey

Our Galaxy blocks a significant portion of the extragalactic sky from view, hampering studies of large-scale structure. This produces an incomplete knowledge of the distribution of galaxies, and, assuming galaxies trace mass, of the gravity field. Further, just one unrecognized, nearby massive galaxy could have large influence over the Milky Way's motion with respect to the Cosmic Microwave Background. Diligent surveys in the optical and infrared wavebands can find galaxies through moderate Galactic gas and dust, but close to the Galactic Plane, only radio surveys are effective. The entire northern Zone of Avoidance is being searched at 21 cm for galaxies using the Dwingeloo 25-m telescope. A shallow search for nearby, and/or massive galaxies has been completed, yielding five objects. Two of these galaxies were previously unknown, and although they are not likely members of the Local Group, are part of the nearby Universe. A deeper search continues, which will produce a flux-limited catalog of hidden galaxies. This portion of the survey is one-third complete, and has detected about 40 objects to date. Based on present understanding of the HI mass function, the complete survey should uncover 50 - 100 galaxies.

astro-ph↗

arXiv1997

CO J=2->1 observations of the nearby galaxies Dwingeloo 1 and Dwingeloo 2

We present an exploratory survey of the central regions of the nearby spiral galaxies Dwingeloo 1 and Dwingeloo 2 made in the CO J=2->1 line at 230GHz using the James Clerk Maxwell Telescope. In Dwingeloo 1, which is probably the nearest grand-design barred spiral, the CO emission was sampled along a cross with its major axis aligned with the bar. The CO emission is an order of magnitude weaker (peak Tmb=~70mK) than in its neighbours IC 342 and Maffei 2 and appears to be confined to the nucleus, bar, and spiral arms. In Dwingeloo 2, a small system quite likely to be a companion of Dw1, a 3-sigma upper limit of 40mK for the CO J=2->1 emission from the nucleus was obtained.

astro-ph↗

arXiv1995

Neutral hydrogen in the nearby galaxies Dwingeloo 1 and Dwingeloo 2

We present observations made with the Westerbork Synthesis Radio Telescope of HI emission from Dwingeloo 1, a nearby barred spiral discovered during the Dwingeloo Obscured Galaxies Survey for galaxies hidden in the Zone of Avoidance, and of Dwingeloo 2, a small galaxy discovered in the beam of these WSRT observations. The WSRT data reveal the position of the dynamical center of Dw1, its systemic LSR velocity, its total (projected) width in velocity, its inclination on the sky, and its integrated HI flux, as well as details of the velocity field and gas distribution. Dw1 is the nearest grand-design barred spiral system, and is probably amongst the ten largest galaxies closer than about 5 Mpc. We report here also the discovery of Dwingeloo 2, a small galaxy located within the WSRT primary beam as pointed to Dw1. In view of its angular and kinematic proximity to Dw1, Dw2 may well be a companion to the larger system. The two galaxies are probably both members of the group containing Maffei 1 & 2 and IC342 and may influence the peculiar motions within that group and the morphology of its individual members.

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