Showing papers by "Fabian Walter published in 2002"

Molecular Gas in M82: Resolving the Outflow and Streamers

Abstract: We present a high-resolution (36, 70 pc) 12CO (J = 1 → 0) mosaic of the molecular gas in M82 covering an area of 25 × 35 (2.8 × 3.9 kpc) obtained with the Owens Valley Radio Observatory millimeter interferometer. The observations reveal the presence of huge amounts of molecular gas (>70% of the total molecular mass, Mtot ≈ 1.3 × 109 M☉) outside the central 1 kpc disk. Molecular streamers are detected in and below M82's disk out to distances from the center of ~1.7 kpc. Some of these streamers are well correlated with optical absorption features; they form the basis of some of the prominent tidal H I features around M82. This provides evidence that the molecular gas within M82's optical disk is disrupted by the interaction with M81. Molecular gas is found in M82's outflow/halo, reaching distances up to 1.2 kpc below the plane; CO line splitting has been detected for the first time in the outflow. The maximum outflow velocity is ~230 km s-1; we derive an opening angle of ~55° for the molecular outflow cone. The total amount of gas in the outflow is greater than 3 × 108 M☉, and its kinetic energy is of order 1055 ergs, about 1% of the estimated total mechanical energy input of M82's starburst. Our study implies that extreme starburst environments can move significant amounts of molecular gas into a galaxy's halo (and even to the intergalactic medium).

A High-Resolution Rotation Curve of NGC 6822: A Test-case for Cold Dark Matter

Abstract: We present high resolution rotation curves of the local group dwarf irregular galaxy NGC 6822 obtained with the Australia Telescope Compact Array. Our best curves have an angular resolution of 8'' or 20 pc and contain some 250 independent points. The stellar and gas components of NGC 6822 cannot explain the shape of the curve, except for the very inner regions, and NGC 6822 is consequently very dark matter dominated. There is no evidence for the presence of a steep density cusp down to scales of ~20 pc, contrary to the predictions of Cold Dark Matter.

GRB 010222: A Burst within a Starburst

Abstract: We present millimeter- and submillimeter-wavelength observations and near-infrared K-band imaging toward the bright gamma-ray burst GRB 010222. Over seven different epochs, a constant source was detected with an average flux density of 3.74 ± 0.53 mJy at 350 GHz and 1.05 ± 0.22 mJy at 250 GHz, giving a spectral index α = 3.78 ± 0.25 (where F ∝ να). We rule out the possibility that this emission originated from the burst or its afterglow, and we conclude that it is due to a dusty, high-redshift starburst galaxy (SMM J14522+4301). We argue that the host galaxy of GRB 010222 is the most plausible counterpart of SMM J14522+4301, based in part on the centimeter detection of the host at the expected level. The optical/near-IR properties of the host galaxy of GRB 010222 suggest that it is a blue sub-L* galaxy, similar to other GRB host galaxies. This contrasts with the enormous far-infrared luminosity of this galaxy based on our submillimeter detection (LBol ≈ 4 × 1012 L☉). We suggest that this GRB host galaxy has a very high star formation rate, SFR ≈ 600 M☉ yr-1, most of which is unseen at optical wavelengths.

The Interacting Dwarf Galaxy NGC 3077: The Interplay of Atomic and Molecular Gas with Violent Star Formation

Abstract: We present a comprehensive multiwavelength study of the nearby interacting dwarf galaxy NGC 3077 (member of the M81 triplet). High-resolution VLA H I observations show that most of the atomic gas (~90%) around NGC 3077 is situated in a prominent tidal arm with a complex velocity structure. Little H I (~5 × 107 M⊙) is associated with NGC 3077 itself. High-resolution Owens Valley Radio Observatory (OVRO) observations of the molecular component (CO) reveal the presence of 16 molecular complexes near the center of NGC 3077 (total mass: ~1.6 × 106 M⊙). A virial mass analysis of the individual complexes yields a lower CO-to-H2 conversion factor in NGC 3077 than the Galactic value—a surprising result for a dwarf galaxy. The lower conversion factor can be explained by extreme excitation conditions and the metallicity of the molecular gas. The total (atomic and molecular) gas content in the center of NGC 3077 is displaced from the stellar component of NGC 3077—this implies that not only the gas at large galactocentric radii is affected by the interaction within the triplet but also the center. We speculate that the starburst activity of NGC 3077 was triggered by this redistribution of gas in the center: Hα, as well as Paα, images show the presence of violent central star formation, as well as dramatic ionized supershells reaching galactocentric distances of ~1 kpc. Some of these supershells are surrounded by neutral hydrogen. In a few cases, the rims of the ionized supershells are associated with dust absorption. The most prominent star-forming region in NGC 3077 as probed by Paα observations is hidden behind a dust cloud that is traced by the molecular complexes. Correcting for extinction we derive a star-forming rate of 0.05 M⊙ yr-1; i.e., given the reservoir in atomic and molecular gas in NGC 3077, star formation may proceed at a similar rate for a few times 108 yr. The efficiency of forming stars out of molecular gas in NGC 3077 is similar to that in M82.

The Broadband Afterglow of GRB 980329

Abstract: We present radio observations of the afterglow of the bright γ-ray burst GRB 980329 made between 1 month and several years after the burst, a reanalysis of previously published submillimeter data, and late-time optical and near-infrared (NIR) observations of the host galaxy. From the absence of a spectral break in the optical/NIR colors of the host galaxy, we exclude the earlier suggestion that GRB 980329 lies at a redshift of z 5. We combine our data with the numerous multiwavelength observations of the early afterglow, fit a comprehensive afterglow model to the entire broadband data set, and derive fundamental physical parameters of the blast wave and its host environment. Models for which the ejecta expand isotropically require both a high circumburst density and extreme radiative losses from the shock. No low-density model (n 10 cm-3) fits the data. A burst with a total energy of ~1051 ergs, with the ejecta narrowly collimated to an opening angle of a few degrees, driven into a surrounding medium with density of ~20 cm-3, provides a satisfactory fit to the light curves over a range of redshifts.

The Distribution of Dark Matter in Galaxies: a Constant-Density Halo around DDO 47

Abstract: In this paper we present strong evidence for a core in the density distribution of the dark halo around a (dwarf) galaxy. DDO 47 has a rotation curve that increases linearly from the first data point, at 300 pc, up to to the last one, at 5 kpc. This profile cannot be reproduced unless we consider a dark halo with a flat density core. The optical region of this galaxy is dominated by a homogeneous halo of costant density of ~ 10^{-24} g/cm^3 and a size of ~3 kpc, about twice that of the stellar disk. This result is a formidable case of the inability of standard Lambda Cold Dark Matter scenario to account for the dark mass distribution around galaxies, and points toward the existence of an intriguing halo scale-length. With respect to the recent claim that the "status of CDM on galactic scales" is satisfactorily in agreement with observations (Primack, 2002), this paper adds up to the results of Blais-Ouellette et al. (2002), Trott & Webster (2002), Binney & Evans (2002), de Blok & Bosma (2002), Bottema (2002) in providing evidence that, "succesfull CDM" is instead obliged to implement the "soft core" feature as a general property of the distribution of galactic dark matter.

Rosat x-ray observations of the dwarf galaxy holmberg ii

Abstract: We present a study of the irregular dwarf galaxy Holmberg II based on ROSAT Position Sensitive Proportional Counter observations (total exposure time: 22 ks). Holmberg II is a nearby (3.2 Mpc), well-studied dwarf irregular galaxy. It is famous for its interstellar medium (ISM), which is dominated by expanding structures such as H I holes and shells. We search for X-ray emission from point sources as well as for diffuse emission, down to the detection limit of the ROSAT data. Using X-ray hardness ratio diagrams we differentiate between thermal plasma and power-law X-ray spectra, which helps to determine the nature of the individual sources. Correlating the X-ray data with complementary observations ranging from the far-ultraviolet to the radio regime, we increase the probability of correctly identifying sources belonging to Holmberg II. We did not detect soft X-ray emission originating from hot gas within supergiant H I shells above our luminosity sensitivity limit of Llimit(0.1-2.1 keV) ≥ 1037ergs s-1. This finding can probably be attributed to blowout in the case of the largest holes and insufficient sensitivity (owing to strong photoelectric absorption) in the case of the smaller H I holes. However, we find faint X-ray sources well beyond the stellar body but within the H I distribution of Holmberg II, which suggests the presence of X-ray binaries. This indicates that star formation has taken place across the entire gaseous disk of Holmberg II in the past, some of which may have created the structures seen in the ISM at large galactocentric radii.

Molecular Gas in M82: Resolving the Outflow and Streamers

Abstract: We present a high-resolution (3.6'', 70pc) CO(1-0) mosaic of the molecular gas in M 82 covering an area of 2.5' x 3.5' (2.8kpc x 3.9kpc) obtained with the OVRO millimeter interferometer. The observations reveal the presence of huge amounts of molecular gas (> 70% of the total molecular mass, M_tot=1.3 x 10^9 M_sun) outside the central 1 kpc disk. Molecular streamers are detected in and below M82's disk out to distances from the center of 1.7 kpc. Some of these streamers are well correlated with optical absorption features; they form the basis of some of the prominent tidal HI features around M 82. This provides evidence that the molecular gas within M 82's optical disk is disrupted by the interaction with M 81. Molecular gas is found in M 82's outflow/halo, reaching distances up to 1.2 kpc below the plane; CO line-splitting has been detected for the first time in the outflow. The maximum outflow velocity is 230 km/s; we derive an opening angle of 55 deg for the molecular outflow cone. The total amount of gas in the outflow is >3 x 10^8 M_sun and its kinetic energy is of order 10^55 erg, about one percent of the estimated total mechanical energy input of M 82's starburst. Our study implies that extreme starburst environments can move significant amounts of molecular gas in to a galaxy's halo (and even to the intergalactic medium).

ROSAT X-ray observations of the dwarf galaxy Holmberg II

Abstract: We present a study of the irregular dwarf galaxy Holmberg II based on ROSAT PSPC observations (total exposure time: 22 ksec). Holmberg II is a nearby (3.2 Mpc), well-studied dwarf irregular galaxy. It is famous for its interstellar medium which is dominated by expanding structures such as HI holes and shells. We search for X-ray emission from point sources as well as for diffuse emission, down to the detection limit of the ROSAT data. Using X-ray hardness ratio diagrams we differentiate between thermal plasma and power-law X-ray spectra which helps to determine the nature of the individual sources. Correlating the X-ray data with complementary observations ranging from the far-ultraviolet to the radio regime we increase the probability of correctly identifying sources belonging to Holmberg II. We did not detect soft X-ray emission originating from hot gas within supergiant HI shells above our luminosity sensitivity limit of (L_limit(0.1 - 2.1 keV) ~ 10^{37}erg/s). This finding can probably be attributed to blow-out in the case of the largest holes and insufficient sensitivity (due to strong photoelectric absorption) in case of the smaller HI holes. However we find faint X-ray sources well beyond the stellar body but within the HI distribution of Holmberg II, which suggests the presence of X-ray binaries. This indicates that star formation has taken place across the entire gaseous disk of Holmberg II in the past, some of which may have created the structures seen in the ISM at large galactocentric radii.

Bloated Dwarfs: The Thickness of the HI Disks in Irregular Galaxies

Abstract: Original paper can be found at: http://www.astrosociety.org/pubs/cs/253.html--Copyright Astronomical Society of the Pacific