Showing papers by "Fabian Walter published in 2003"

SINGS: The SIRTF Nearby Galaxies Survey

Abstract: The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multiwavelength library of ancillary and complementary observations, including radio continuum, H i, CO, submillimeter, BVRIJHK ,H a ,P aa, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

Molecular gas in the host galaxy of a quasar at redshift z = 6.42

Abstract: Observations of molecular hydrogen in quasar host galaxies at high redshifts provide fundamental constraints on galaxy evolution, because it is out of this molecular gas that stars form. Molecular hydrogen is traced by emission from the carbon monoxide molecule, CO; cold H2 itself is generally not observable. Carbon monoxide has been detected in about ten quasar host galaxies with redshifts z > 2; the record-holder is at z = 4.69 (refs 1–3). Here we report CO emission from the quasar SDSS J114816.64 + 525150.3 (refs 5, 6) at z = 6.42. At that redshift, the Universe was only 1/16 of its present age, and the era of cosmic reionization was just ending. The presence of about 2 × 1010 M⊙ of H2 in an object at this time demonstrates that molecular gas enriched with heavy elements can be generated rapidly in the youngest galaxies.

High-excitation CO in a quasar host galaxy at z = 6.42

Abstract: We report the detection of high excitation CO emission from the most distant quasar currently known, SDSS J114816.64+525150.3 (hereafter J1148+5251), at a redshift z= 6:419. The CO (J= 6! 5) and (J= 7! 6) lines were detected using the IRAM Plateau de Bure interferometer, showing a width of280 km s 1 . An upper flux limit for the CO ( J= 1! 0) line was obtained from observations with the Eelsberg 100-meter telescope. Assuming no gravitational magnification, we estimate a molecular gas mass of2 10 10 M.U sing the CO (3! 2) observations by Walter et al. (2003), a comparison of the line flux ratios with predictions from a large velocity gradient model suggests that the gas is likely of high excitation, at densities10 4:5 cm 3 and a temperature100 K. Since in this case the CO lines appear to have moderate optical depths, the gas must be extended over a few kpc. The gas mass detected in J1148+5251 can fuel star formation at the rate implied by the far-infrared luminosity for less than 10 million years, a time comparable to the dynamical time scale of the region. The gas must therefore be replenished quickly, and metal and dust enrichment must occur fast. The strong dust emission and the massive, dense gas reservoir at z 6:4 provide further evidence that vigorous star formation is co-eval with the rapid growth of massive black holes at these early epochs of the Universe.

Gas and dust in the Cloverleaf quasar at redshift 2.5

Abstract: We observed the upper fine structure line of neutral carbon, C I( 3 P2! 3 P1 )( rest = 809 GHz), the 12 CO(J= 3!2) line (rest= 345 GHz) and the 1.2 mm continuum emission from H1413+117 (Cloverleaf quasar, z= 2:5) using the IRAM inter- ferometer. Together with the detection of the lower fine structure line (Barvainis et al. 1997), the Cloverleaf quasar is now only the second extragalactic system, besides M 82, where both carbon lines have convincingly been detected. Our analysis shows that the carbon lines are optically thin and have an excitation temperature of Tex 30 K. CO is subthermally excited and the observed line luminosity ratios are consistent with n(H2) 10 3 4 cm 3 at Tkin= 30-50 K. Using three independent methods (C I, dust, CO) we derive a total molecular gas mass (corrected for magnification) of M(H2) 1.2 0.3 10 10 M. Our observations suggest that the molecular disk extends beyond the region seen in CO(7-6) to a zone of more moderately excited molecular gas that dominates the global emission in C I and the low J CO lines. described by a 3-level system. This allows us to derive its ex- citation and to constrain the physical gas conditions by obser- vations of the C I( 3 P1! 3 P0) and C I( 3 P2! 3 P1) transitions only. We compare our results from C I with estimates based on ther- mal dust emission and CO. We use H0= 75 km s 1 Mpc 1 and q0= 0:5.

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 arcsec 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.

Gas and Dust in the Cloverleaf Quasar at Redshift 2.5

Abstract: We observed the upper fine structure line of neutral carbon, CI(2-1), the CO(3-2) line and the 1.2mm continuum emission from H1413+117 (Cloverleaf quasar, z=2.5) using the IRAM interferometer. Together with the detection of the lower fine structure line (Barvainis etal. 1997), the Cloverleaf quasar is now only the second extragalactic system, besides M82, where both carbon lines have convincingly been detected. Our analysis shows that the carbon lines are optically thin and have an excitation temperature of ~30 K. CO is subthermally excited and the observed line luminosity ratios are consistent with n(H2)=10^(3-4) cm^(-3) at Tkin=30-50 K. Using three independent methods (CI, dust, CO) we derive a total molecular gas mass (corrected for magnification) of M(H2)=1.2+/-0.3*10^(10) SM. Our observations suggest that the molecular disk extends beyond the region seen in CO(7-6) to a zone of more moderately excited molecular gas that dominates the global emission in CI and the low J CO lines.

Millimeter Observations of GRB 030329: Continued Evidence for a Two-Component Jet

Abstract: We present the results of a dedicated campaign on the afterglow of GRB 030329 with the millimeter interferometers of the Owens Valley Radio Observatory (OVRO) and the Berkeley-Illinois-Maryland Association (BIMA), and with the MAMBO-2 bolometer array on the IRAM 30 m telescope. These observations allow us to trace the full evolution of the afterglow of GRB 030329 at frequencies of 100 and 250 GHz for the first time. The millimeter light curves exhibit two main features: a bright, constant flux density portion and a steep power-law decline. The absence of bright, short-lived millimeter emission is used to show that the GRB central engine was not actively injecting energy well after the burst. The millimeter data support a model, advocated by Berger et al., of a two-component jetlike outflow in which a narrow-angle jet is responsible for the high-energy emission and early optical afterglow, and a wide-angle jet carrying most of the energy is powering the radio and late optical afterglow emission.

ΛCDM and the distribution of dark matter in galaxies: A constant–density halo around DDO 47

Abstract: In this paper we present a test case for the existence of a core in the density distribution of dark halos around galaxies. DDO 47 has a rotation curve that increases linearly from the first data point, at 300 pc, up to the last one, at 5 kpc. This profile implies the presence of a (dark) halo with an (approximately) constant density over the region mapped by data. This evidences the inability of standard Cold Dark Matter scenario to account for the dark matter distribution around galaxies, and points toward the existence of an intriguing halo scale-length of homogeneity. This work adds up to the results of Blais-Ouellette et al. (2002), Trott & Webster (2002), Binney & Evans (2002), de Blok & Bosma (2002) and Bottema (2002) in suggesting that at galactic scales CDM theory should incorporate, as an intrinsic property, a "density core" feature.

SINGS: The SIRTF Nearby Galaxies Survey

Abstract: The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies, and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multi-wavelength library of ancillary and complementary observations, including radio continuum, HI, CO, submillimeter, BVRIJHK, H-alpha, Paschen-alpha, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

Chandra Observations of Expanding Shells in the Dwarf Starburst Galaxy NGC 3077

Abstract: Deep Chandra observations (53 ks, ACIS-S3) of NGC 3077, a starburst dwarf galaxy in the M81 triplet, resolve the X-ray emission from several supershells. The emission is brightest in the cavities defined by expanding shells detected previously in Hα emission. Thermal emission models fitted to the data imply temperatures ranging from ~1.3 to 4.9 × 106 K and indicate that the strongest absorption is coincident with the densest clouds traced by CO emission. The fitted emission measures give pressures of P/k ≈ (105-106)ξ-0.5f K cm-3 (where ξ is the metallicity of the hot gas in solar units and fv is the volume filling factor). Despite these high pressures, the radial density profile of the hot gas is not as steep as that expected in a freely expanding wind (e.g., as seen in the neighboring starburst galaxy M82), implying that the hot gas is still confined by the Hα shells. The chaotic dynamical state of NGC 3077 undermines reliable estimates of the escape velocity. The more relevant quantity for the ultimate fate of the outflow is probably the gas density in the rich intragroup medium. Based on the H I distribution of NGC 3077 and a connected tidal tail we argue that the wind has the potential to leave the gravitational well of NGC 3077 to the north but not to the south. The total 0.3-6.0 keV X-ray luminosity is ~(2-5) × 1039 ergs s-1 (depending on the selected thermal plasma model). Most (~85%) of the X-ray luminosity in NGC 3077 comes from the hot interstellar gas; the remainder comes from six X-ray point sources. In spite of previous claims to the contrary, we do not find X-ray emission originating from the prominent tidal tail near NGC 3077.

Young stars in the outer H i disc of NGC 6822

Abstract: 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 radiiwell beyond R 2 5 . Blue stars are also found that are associated with NGC 6822's companion H I cloud, indicating that star formation was triggered in the companion in the last 10 8 yr. In general, blue stars are present where the H I surface densities reach values > 5 x 10 2 0 cm - 2 . However, over one-third of the blue stars detected within the H I disc are found at lower surface densities. The young stars trace the distribution of the neutral hydrogen in the inner disc, but seem to be avoiding the supergiant H I 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 H I 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.

Young Stars in the Outer HI Disc of NGC 6822

Abstract: 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

HI and CO in Blue Compact Dwarf Galaxies: Haro 2 and Haro 4

Abstract: We present VLA HI imaging of two blue compact dwarf (BCD) galaxies, Haro 2 and Haro 4, and of the spiral galaxy Haro 26, which is projected some 22' SW of Haro 4. We also show a map of the CO(1-0) distribution of Haro 2 obtained with the OVRO millimeter array, as well as derive an upper limit for CO(2-1) emission from Haro 4 obtained with the CSO. The HI data of Haro 2 reveal that the kinematical major axis lies perpendicular to the photometric major axis, indicating that the atomic hydrogen rotates about the major axis of the galaxy. This confirms earlier indications based on CCD photometry that Haro 2 is a dust-lane dE rather than a dIrr. We propose that the present neutral and molecular ISM configuration is due to recent gas accretion or a merger. The HI distribution and dynamics of Haro 4 and the neighboring spiral Haro 26 suggest that they are currently undergoing a tidal interaction, reinforcing the notion that interactions play an important role in triggering the star formation witnessed in Blue Compact Galaxies.

HI and Hot Gas in the Outskirts of the M81 Group

Abstract: Results are presented from a wide area, high resolution HI synthesis survey of the outer regions of the nearby M81 group, where internal (galactic) and external (group-related) evolution processes can be studied simultaneously in great detail. The survey encompasses the star forming dwarf galaxies M81dwA, UGC4483, and HoII, where evidence of ram pressure stripping was recently discovered. The data do not reveal any intergalactic HI, but the outer parts of HoII are reminiscent of tidal tails. We argue however that those structures are equally consistent with the latest ram pressure models including cooling. The case for a hot intergalactic medium in this poor, spiral-only group is thus still open. The survey also puts tight constraints on possible counterparts to the local high velocity cloud population in an external group, reaching a 3 sigma column density of 10^19 atom/cm^2 and a 6 sigma limiting mass of 1.5x10^5 M_sun.

High-excitation CO in a quasar host galaxy at z=6.42

Abstract: We report the detection of high excitation CO emission from the most distant quasar currently known, SDSS J114816.64+525150.3 (hereafter J1148+5251), at a redshift z=6.419. The CO (J=6-5) and (J=7-6) lines were detected using the IRAM Plateau de Bure interferometer, showing a width of ~280 km/s. An upper flux limit for the CO (J=1-0) line was obtained from observations with the Effelsberg 100-meter telescope. Assuming no gravitational magnification, we estimate a molecular gas mass of ~2x10^10 M_sun. Using the CO (3-2) observations by Walter et al. (2003), a comparison of the line flux ratios with predictions from a large velocity gradient model suggests that the gas is likely of high excitation, at densities ~10^5 cm^-3 and a temperature ~100 K. Since in this case the CO lines appear to have moderate optical depths, the gas must be extended over a few kpc. The gas mass detected in J1148+5251 can fuel star formation at the rate implied by the far-infrared luminosity for less than 10 million years, a time comparable to the dynamical time of the region. The gas must therefore be replenished quickly, and metal and dust enrichment must occur fast. The strong dust emission and massive, dense gas reservoir at z~6.4 provide further evidence that vigorous star formation is co-eval with the rapid growth of massive black holes at these early epochs of the Universe.

X-ray Emission from Dwarf Galaxies: IC 2574 Revisited

Abstract: Until recently, few unequivocal detections had been reported of the hot, X-ray emitting gas thought to be associated with the large, coherent structures variously described as supershells or superbubbles in dwarf irregular (dIrr) galaxies In this contribution we report follow-up XMM-Newton and Chandra observations of our ROSAT detection of X-ray emission associated with the supergiant shell in the nearby dIrr galaxy IC 2574, a member of the M 81 group of galaxies The spectral properties of the X-ray source suggest that we are dealing with a young (age 2000 yr) supernova remnant (SNR) This SNR is most likely one of the many supernovae which have exploded in that region and which have created the impressive supergiant HI shell