Extragalactic Background Sources Research Articles

The relation between Lyαabsorbers and local galaxy filaments

Context.The intergalactic medium (IGM) is believed to contain the majority of baryons in the universe and to trace the same dark matter structure as galaxies, forming filaments and sheets. Lyαabsorbers, which sample the neutral component of the IGM, have been extensively studied at low and high redshift, but the exact relation between Lyαabsorption, galaxies, and the large-scale structure is observationally not well constrained.Aims.In this study, we aim at characterising the relation between Lyαabsorbers and nearby over-dense cosmological structures (galaxy filaments) at recession velocities Δv ≤ 6700 km s−1by using archival observational data from various instruments.Methods.We analyse 587 intervening Lyαabsorbers in the spectra of 302 extragalactic background sources obtained with the Cosmic Origins Spectrograph (COS) installed on theHubbleSpace Telescope (HST). We combine the absorption line information with galaxy data of five local galaxy filaments from the V8k catalogue.Results.Along the 91 sightlines that pass close to a filament, we identify 215 (227) Lyαabsorption systems (components). Among these, 74 Lyαsystems are aligned in position and velocity with the galaxy filaments, indicating that these absorbers and the galaxies trace the same large-scale structure. The filament-aligned Lyαabsorbers have a ∼90% higher rate of incidence (d𝒩/dz = 189 for logN(H I) ≥ 13.2) and a slightly shallower column density distribution function slope (−β = −1.47) relative to the general Lyαpopulation atz = 0, reflecting the filaments’ matter over-density. The strongest Lyαabsorbers are preferentially found near galaxies or close to the axis of a filament, although there is substantial scatter in this relation. Our sample of absorbers clusters more strongly around filament axes than a randomly distributed sample would do (as confirmed by a Kolmogorov–Smirnov test), but the clustering signal is less pronounced than for the galaxies in the filaments.

Heightened Faraday complexity in the inner 1 kpc of the galactic centre

ABSTRACT We have measured the Faraday rotation of 62 extra-galactic background sources in 58 fields using the CSIRO Australia Telescope Compact Array (ATCA) with a frequency range of 1.1–3.1 GHz with 2048 channels. Our sources cover a region $\sim 12\, \times 12\, \mathrm{deg^{ 2}}$ (∼1 kpc) around the Galactic Centre region. We show that the Galactic Plane for |l| < 10° exhibits large Rotation Measures (RMs) with a maximum |RM| of $1691.2 \pm 4.9\, \mathrm{rad}\, \mathrm{m}^{-2}$ and a mean $|\mathrm{RM}| = 219 \pm 42\, \mathrm{rad}\, \mathrm{m}^{-2}$. The RMs decrease in magnitude with increasing projected distance from the Galactic Plane, broadly consistent with previous findings. We find an unusually high fraction (95 per cent) of the sources show Faraday complexity consistent with multiple Faraday components. We attribute the presences of multiple Faraday rotating screens with widely separated Faraday depths to small-scale turbulent RM structure in the Galactic Centre region. The second-order structure function of the RM in the Galactic Centre displays a line with a gradient of zero for angular separations spanning 0.83°–11° (∼120–1500 pc), which is expected for scales larger than the outer scale (or driving scale) of magneto-ionic turbulence. We place an upper limit on any break in the SF gradient of 66 arcsec, corresponding to an inferred upper limit to the outer scale of turbulence in the inner 1 kpc of the Galactic Centre of 3 pc. We propose stellar feedback as the probable driver of this small-scale turbulence.

A VLBA Survey of Radio Stars in the Orion Nebula Cluster. I. The Nonthermal Radio Population

Abstract We present first results of a four-epoch VLBA survey for nonthermal emission toward all 556 compact radio sources previously identified in a deep VLA survey of the Orion Nebula Cluster (ONC). We identify VLBA counterparts toward an unprecedented 123 sources. Of these, 41 do not have X-ray counterparts, of which 34 also do not display near-infrared counterparts. Since these cannot be explained by extragalactic background sources, this suggests a component of the ONC population of young stellar objects that are too deeply embedded for even X-rays to be detectable. We find pervasive variability and detect even most of the highest-S/N sources in only one out of four epochs. Neither a negative spectral index nor extreme variability in the VLA data is a good predictor of a VLBA detection.

A Population of Compact Radio Variables and Transients in the Radio-bright Zone at the Galactic Center Observed with the Jansky Very Large Array

Abstract Using Jansky Very Large Array data obtained from high-resolution observations at 5.5 GHz at multiple epochs in 2014 and 2019, we have detected a population of radio variables and transients in the radio-bright zone at the Galactic center. With observations covering a sky area of 180 arcmin2 at an angular resolution of 0.″4, we report new detections of 110 Galactic center compact radio (GCCR) sources with a size of <1″. The flux densities of GCCR sources exceed 70 μJy, with at least 10σ significance. Among these sources, 82 are variable or transient and 28 are nonvariable. About 10% of them are expected to be extragalactic background sources. We discuss the possible astrophysical nature of the detected sources. As compared to the Galactic disk (GD) population of normal pulsars (NPs) and millisecond pulsars (MSPs), a majority (80%) of the GCCR sources appear to fall within the high flux density tail of the pulsar distribution, as extrapolated from a sample of NPs in the GD. However, MSPs extrapolated from the GD population are too weak to have contributed significantly to the GCCR population that has been detected. We also cross-correlated the GCCR sources with X-ray sources in Chandra X-ray catalogs and found that 42 GCCR sources have candidate X-ray counterparts. Most of the GCCR sources having X-ray counterparts are likely to be associated with unresolved or slightly resolved radio jets launched from X-ray binaries with a compact object, either a black hole or a neutron star.

A population of high-velocity absorption-line systems residing in the Local Group

Aims. We investigated the ionisation conditions and distances of Galactic high-velocity clouds (HVCs) in the Galactic halo and beyond in the direction of the Local Group (LG) barycentre and anti-barycentre, by studying spectral data of 29 extragalactic background sources obtained with the Cosmic Origins Spectropgraph (COS) installed on the Hubble Space Telescope (HST). Methods. We model column-densities of low, intermediate, and high ions such as Si II, C II, Si III, Si VI, and C IV, and use these data to construct a set of Cloudy ionisation models. Results. In total, we found 69 high-velocity absorption components along the 29 lines of sight. The components in the direction of the LG barycentre span the entire range of studied velocities, 100 ≲ |vLSR| ≲ 400 km s−1, while those in the anti-barycentre sample have velocities up to about 300 km s−1. For 49 components, we infer the gas densities. In the direction of the LG barycentre, the gas densities exhibit a wide range from log nH = −3.96 to −2.55, while in the anti-barycentre direction the densities are systematically higher, log nH > −3.25. The barycentre absorbers can be split into two groups based on their density: a high-density group with log nH > −3.54, which can be affected by the Milky Way radiation field, and a low-density group (log nH ≤ −3.54). The latter has very low thermal pressures of P/k < 7.3 K cm−3. Conclusions. Our study shows that part of the absorbers in the LG barycentre direction trace gas at very low gas densities and thermal pressures. These properties indicate that the absorbers are located beyond the virial radius of the Milky Way. Our study also confirms results from earlier, single-sightline studies, suggesting the presence of a metal-enriched intragroup medium filling the LG near its barycentre.

Radio continuum emission in the northern Galactic plane: Sources and spectral indices from the THOR survey

Context Radio continuum surveys of the Galactic plane can find and characterize H II regions, supernova remnants (SNRs), planetary nebulae (PNe), and extragalactic sources. A number of surveys at high angular resolution (≤25″) at different wavelengths exist to study the interstellar medium (ISM), but no comparable high-resolution and high-sensitivity survey exists at long radio wavelengths around 21 cm. Aims. Our goal is to investigate the 21 cm radio continuum emission in the northern Galactic plane at < 25″ resolution. Methods We observed a large percentage of the Galactic plane in the first quadrant of the Milky Way (l = 14.0−67.4° and |b| ≤ 1.25°) with the Karl G. Jansky Very Large Array (VLA) in the C-configuration covering six continuum spectral windows (SPW). These data provide a detailed view on the compact as well as extended radio emission of our Galaxy and thousands of extragalactic background sources. Results We used the BLOBCAT software and extracted 10 916 sources. After removing spurious source detections caused by the side lobes of the synthesized beam, we classified 10 387 sources as reliable detections. We smoothed the images to a common resolution of 25″ and extracted the peak flux density of each source in each SPW to determine the spectral indices α (assuming I(ν) ∝ να). By cross-matching with catalogs of H II regions, SNRs, PNe, and pulsars, we found radio counterparts for 840 H II regions, 52 SNRs, 164 PNe, and 38 pulsars. We found 79 continuum sources that are associated with X-ray sources. We identified 699 ultra-steep spectral sources (α < −1.3) that could be high-redshift galaxies. About 9000 of the sources we extracted are not classified specifically, but based on their spatial and spectral distribution, a large percentage of these are likely to be extragalactic background sources. More than 7750 sources do not have counterparts in the SIMBAD database and more than 3760 sources do not have counterparts in the NED database. Conclusions Studying the long wavelengths centimeter continuum emission and the associated spectral indices allows us to characaterize a large percentage of Galactic and extragalactic radio sources in the area of the northern inner Milky Way. This database will be extremely useful for future studies of a diverse set of astrophysical objects.

An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way’s circumgalactic medium and the Local Group

To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen. Along 270 sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and CIV and associated HI 21 cm emission in HVCs in the velocity range |v_LSR|=100-500 km s^-1. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the absorption characteristics with that of Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG. Our study demonstrates that the Milky Way CGM contains sufficient gaseous material to maintain the Galactic star-formation rate at its current level. We show that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The Magellanic Stream clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Way's CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way (abridged).

An HST/COS legacy survey of intervening Si III absorption in the extended gaseous halos of low-redshift galaxies

Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening SiIII-selected absorbers and their relation to the circumgalactic medium (CGM) of galaxies at low redshift (z<=0.1), based on the analysis of UV absorption spectra along 303 extragalactic lines of sight obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). Along a total redshift path of Dz=24 we identify 69 intervening SiIII systems that all show associated absorption from other low and high ions. We derive a bias-corrected number density of dN/dz(SiIII)=2.5 for absorbers with column densities log N(SiIII)>12.2. We develop a geometrical model for the absorption-cross section of the CGM around the local galaxy population and find excellent agreement between the model predictions and the observations. We further compare redshifts and positions of the absorbers with that of ~64,000 galaxies using archival galaxy-survey data. For the majority of the absorbers we identify possible host galaxies within 300 km/s of the absorbers and derive impact parameters rho<200 kpc, demonstrating that the spatial distributions of SiIII absorbers and galaxies are highly correlated. Our study indicates that the majority of SiIII-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ~70 per cent. From a detailed ionization model we estimate that diffuse gas in the CGM around galaxies, as traced by SiIII, contains substantially more baryonic mass than their neutral interstellar medium.

HCO, c-C3H and CF+: three new molecules in diffuse, translucent and “spiral-arm” clouds

%methods {We used the EMIR receiver and FTS spectrometer at the IRAM 30m to construct absorption spectra toward bright extra-galactic background sources at 195 kHz spectral resolution ($\approx$ 0.6 \kms). We used the IRAM Plateau de Bure interferometer to synthesize absorption spectra of \hthcop\ and HCO toward the galactic HII region W49.} %results {HCO, \cc3h\ and CF\p\ were detected toward the blazars \bll\ and 3C111 having \EBV\ = 0.32 and 1.65 mag. HCO was observed in absorption from ``spiral-arm'' clouds in the galactic plane occulting W49. The complement of detectable molecular species in the 85 - 110 GHz absorption spectrum of diffuse/translucent gas is now fully determined at rms noise level $\delta_\tau \approx 0.002$ at \EBV\ = 0.32 mag (\AV\ = 1 mag) and $\delta_\tau$/\EBV\ $\approx\ 0.003$ mag$^{-1}$ overall.} %conclusions {As with OH, \hcop\ and \cch, the relative abundance of \cc3h\ varies little between diffuse and dense molecular gas, with N(\cc3h)/N({\it o-c}-\c3h2) $\approx$ 0.1. We find N(CF\p)/N(H$^{13}$CO\p) $\approx 5$, N(CF\p)/N(\cch) $\approx$ 0.005-0.01 and because N(CF\p) increases with \EBV\ and with the column densities of other molecules we infer that fluorine remains in the gas phase as HF well beyond \AV\ = 1 mag. We find N(HCO)/N(H$^{13}$CO\p) = 16 toward \bll, 3C111 and the 40 km/s spiral arm cloud toward W49, implying X(HCO) $\approx 10^{-9}$, about 10 times higher than in dark clouds. The behaviour of HCO is consistent with previous suggestions that it forms from C\p\ and \HH, even when \AV\ is well above 1 mag. The survey can be used to place useful upper limits on some species, for instance N(\hhco)/N(\HH CS) $>$ 32 toward 3C111, compared to 7 toward TMC-1, confirming the possibility of a gas phase formation route to \hhco.}

AN X-RAY AND INFRARED SURVEY OF THE LYNDS 1228 CLOUD CORE

The nearby Lynds 1228 (L1228) dark cloud at a distance of ~200 pc is known to harbor several young stars including the driving sources of the giant HH 199 and HH 200 Herbig-Haro outflows. L1228 has been previously studied at optical, infrared, and radio wavelengths but not in X-rays. We present results of a sensitive 37 ks Chandra ACIS-I X-ray observation of the L1228 core region. Chandra detected 60 X-ray sources, most of which are faint (<40 counts) and non-variable. Infrared counterparts were identified for 53 of the 60 X-ray sources using archival data from 2MASS, Spitzer, and WISE. Object classes were assigned using mid-IR colors for those objects with complete photometry, most of which were found to have colors consistent with extragalactic background sources. Seven young stellar object (YSO) candidates were identified including the class I protostar HH 200-IRS which was detected as a faint hard X-ray source. No X-ray emission was detected from the luminous protostar HH 199-IRS. We summarize the X-ray and infrared properties of the detected sources and provide IR spectral energy distribution modeling of high-interest objects including the protostars driving the HH outflows.

Polarized synchrotron radiation from the Andromeda galaxy M 31 and background sources at 350 MHz

Polarization measurements at low radio frequencies allow detection of small Faraday rotation measures caused by regular magnetic fields in galaxies and in the foreground of the Milky Way. The galaxy M31 was observed in two overlapping pointings with the Westerbork Synthesis Radio Telescope (WSRT) resulting in ~4' resolution in total intensity and linearly polarized emission. The frequency range 310-376 MHz was covered by 1024 channels which allowed the application of RM synthesis. We derived a data cube in Faraday depth and compared two symmetric ranges of negative and positive Faraday depths. This new method avoids the range of high instrumental polarization and allows the detection of very low degrees of polarization. For the first time, diffuse polarized emission from a nearby galaxy is detected below 1 GHz. The degree of polarization is only 0.21 +/- 0.05 %, consistent with extrapolation of internal depolarization from data at higher radio frequency. A catalogue of 33 polarized sources and their Faraday rotation in the M31 field is presented. Their average depolarization is DP(90,20) = 0.14 +/- 0.02, 7 times stronger depolarized than at 1.4 GHz. We argue that this strong depolarization originates within the sources, e.g. in their radio lobes, or in intervening galaxies on the line of sight. On the other hand, the Faraday rotation of the sources is mostly produced in the foreground of the Milky Way and varies significantly across the ~9 square degree M31 field. As expected, polarized emission from M31 and extragalactic background sources is much weaker at low frequencies compared to the GHz range. Future observations with LOFAR, with high sensitivity and high angular resolution to reduce depolarization, may reveal diffuse polarization from the outer disks and halos of galaxies.

The Milky Way halo as a QSO absorption-line system

We use archival UV absorption-line data from HST/STIS to statistically analyse the absorption characteristics of the high-velocity clouds (HVCs) in the Galactic halo towards more than 40 extragalactic background sources. We determine absorption covering fractions of low- and intermediate ions (OI, CII, SiIII, MgII, FeII, SiIII, CIV, and SiIV) in the range fc = 0.20 - 0.70. For detailed analysis we concentrate on SiII absorption components in HVCs, for which we investigate the distribution of column densities, b-values, and radial velocities. Combining information for SiII and MgII, and using a geometrical HVC model we investigate the contribution of HVCs to the absorption cross section of strong MgII absorbers in the local Universe. We estimate that the Galactic HVCs would contribute on average ~52 % to the total strong MgII cross section of the Milky Way, if our Galaxy were to be observed from an exterior vantage point. We further estimate that the mean projected covering fraction of strong MgII absorption in the Milky Way halo and disc from an exterior vantage point is fc(sMgII) = 0.31 for a halo radius of R = 61 kpc. These numbers, together with the observed number density of strong MgII absorbers at low redshift, indicate that the contribution of infalling gas clouds (i.e., HVC analogues) in the halos of Milky Way-type galaxies to the cross section of strong MgII absorbers is <34 %. These findings are in line with the idea that outflowing gas (e.g., produced by galactic winds) in the halos of more actively star-forming galaxies dominate the absorption-cross section of strong MgII absorbers in the local Universe.

THE ABUNDANCE OF C 3 H 2 AND OTHER SMALL HYDROCARBONS IN THE DIFFUSE INTERSTELLAR MEDIUM

Hydrocarbons are ubiquitous in the interstellar medium, observed in diverse environments ranging from diffuse to molecular dark clouds and strong photon-dominated regions near HII regions. Recently, two broad diffuse interstellar bands (DIBs) at 4881{\AA} and 5450{\AA} were attributed to the linear version of propynylidene l-C3H2, a species whose more stable cyclic conformer c-C3H2 has been widely observed in the diffuse interstellar medium at radio wavelengths. This attribution has already been criticized on the basis of indirect plausibility arguments because the required column densities are quite large, N(l-C3H2)/EB-V = 4 \times 1014 cm-2 mag-1. Here we present new measurements of N(l-C3H2) based on simultaneous 18-21 GHz VLA absorption profiles of cyclic and linear C3H2 taken along sightlines toward extragalactic radiocontinuum background sources with foreground Galactic reddening EB-V = 0.1 - 1.6 mag. We find that N(l-C3H2)/N(c-C3H2) ? 1/15 - 1/40 and N(l-C3H2)/EB-V ? 2 \pm 1 \times 1011 cm-2 mag-1, so that the column densities of l-C3H2 needed to explain the diffuse interstellar bands are some three orders of magnitude higher than what is observed. We also find N(C4H)/EB-V < 1.3 \times 1013 cm-2 mag-1 and N(C4H-)/EB-V < 1 \times 1011 cm-2 mag-1 (3?). Using available data for CH and C2H we compare the abundances of small hydrocarbons in diffuse and dark clouds as a guide to their ability to contribute as DIB carriers over a wide range of conditions in the interstellar medium.

Imaging diffuse clouds: bright and dark gas mapped in CO

We wish to relate the degree scale structure of galactic diffuse clouds to sub-arcsecond atomic and molecular absorption spectra obtained against extragalactic continuum background sources. To do this, we used the ARO 12m telescope to map J=1-0 CO emission at 1' resolution over 30' fields around the positions of 11 background sources occulted by 20 molecular absorption line components, of which 11 had CO emission counterparts. We compare maps of CO emission to sub-arcsec atomic and molecular absorption spectra and to the large-scale distribution of interstellar reddening. The main results are: 1) Typical covering factors of individual features at the 1 K.km/s level were 20%. 2) CO-H2 conversion factors as much as 4-5 times below the mean value N(H2)/Wco = 2e20 H2 cm^-2 /(K.km/s) are required to explain the luminosity of CO emission at/above the level of 1 K.km/s. Small conversion factors and sharp variability of the conversion factor on arcminute scales are due primarily to CO chemistry and need not represent unresolved variations in reddening or total column density. Hence, like FERMI and PLANCK we see some gas that is dark in CO and other gas in which CO is overluminous per H2. A standard CO-H2 conversion factor applies overall owing to balance between the luminosities per H2 and surface covering factors of bright and dark CO., but with wide variations.

COMMISSION 21: LIGHT OF THE NIGHT SKY

Commission 21 consists of IAU members and consultants with expertise and interest in the study of the light of the night sky and its various diffuse components, at all accessible electromagnetic frequencies. In cosmic distance scales, the subjects of Commission 21 range from airglow and tropospheric scattering in Earth's atmosphere, through zodiacal light in the solar system, including thermal emission from interplanetary dust, integrated starlight in the Milky Way galaxy, diffuse galactic light due to dust scattering in the galactic diffuse interstellar medium, thermal emissions from interstellar dust and free free emission from ionized interstellar gas, to various diffuse extragalactic background sources, including the cosmologically important cosmic microwave background (CMB). Observations of the diffuse night sky brightness at any frequency typically include signals from several of these sources, and it has been the historic mandate of Commission 21 to foster the necessary collaboration of experts from the different astronomical sub-disciplines involved.

AFar Ultraviolet Spectroscopic ExplorerSurvey of Molecular Hydrogen in Intermediate‐Velocity Clouds in the Milky Way Halo

Far Ultraviolet Spectroscopic Explorer (FUSE) data are used to investigate the molecular hydrogen (H2) content of intermediate-velocity clouds (IVCs) in the lower halo of the Milky Way. We analyze interstellar absorption toward 56 (mostly extragalactic) background sources to study H2 absorption in the Lyman and Werner bands in 61 IVC components at H I column densities ≥1019 cm-2. For data with good signal-to-noise ratio (S/N) (~9 per resolution element and higher), H2 in IVC gas is convincingly detected in 14 cases at column densities varying between ~1014 and ~1017 cm-2. We find an additional 17 possible H2 detections in IVCs in FUSE spectra with lower S/N. The molecular hydrogen fractions, f, vary between 10-6 and 10-3, implying a dense, mostly neutral gas phase that is probably related to the cold neutral medium (CNM) in these clouds. If the H2 stays in formation-dissociation equilibrium, the CNM in these clouds can be characterized by compact (D ~ 0.1 pc) filaments with volume densities on the order of nH ~ 30 cm-3. The relatively high detection rate of H2 in IVC gas implies that the CNM in these clouds is ubiquitous. More dense regions with much higher molecular fractions may exist, but it would be difficult to detect them in absorption because of their small size.

The Distribution of Metal-absorbing High-Velocity Clouds in the Galaxy

view Abstract Citations (38) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Distribution of Metal-absorbing High-Velocity Clouds in the Galaxy Bowen, David V. ; Blades, J. Chris ; Pettini, Max Abstract We present Hubble Space Telescope (HST) GHRS spectra of Mg II absorption from our own Galaxy toward seven extragalactic background sources. Along four sight lines, the bulk of the absorption occurs at velocities consistent with those expected for halo gas corotating with the disk of the Galaxy. In the other three directions the line profiles extend to peculiar velocities, differing from 0 to 50 km s-1 from corotating gas. Only in one direction toward Q1219+755, originally discussed by Bowen & Blades (1993)do we find Mg II absorption at velocities greater than 100 km s-1, characteristic of high-velocity clouds (HVCs). The remaining six lines of sight show no absorption to 2 σ equivalent width limits of 51-73 mÅ, corresponding to column densities N(Mg II) ≃ 2 × 1012 cm-2. For plausible ionization conditions these limits correspond to H I column densities N(H I) ∼1016-17 cm-2, ≈1-2 dex less than those detectable from 21 cm emission. We conclude that the covering factor of HVCs at these low levels is ∼14%±14%, not as high as previously speculated. We find no HVCs along any sight line in our sample with a column density high enough to produce Mg II lines detectable with both GHRS and FOS. This is in marked contrast with the results of Savage et al. (1993) who found absorbing HVCs toward seven of the 15 QSO sight lines they observed with the FOS. However, we show that by adopting similar selection procedures, the two data sets yield covering factors which differ at only the ∼2 σ level, a result which is not unexpected given the known clumping of HVCs. By combining the two sets of data we deduce that HVCs capable of producing Mg II absorption with W(λ2796) ≳ 0.4 Å cover ∼14%±8% of the sky. A literature search for 21 cm emission in directions close to our sight lines show that there are HVCs within ≈1°-4° of five of the background probes. We interpret the fact that these HVCs do not produce Mg II absorption in our spectra as evidence that HVCs are sharply bounded, probably by ionization; this would lead to a natural discontinuity in the covering factor of HVCs at N(H I) 1017 cm-2. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/175994 Bibcode: 1995ApJ...448..662B Keywords: GALAXY: HALO; ISM: ABUNDANCES; ISM: CLOUDS; ULTRAVIOLET: ISM full text sources ADS | data products NED (8) SIMBAD (7) MAST (1) ESA (1)

VLA observations of candidate supernova remnants from the Clark Lake 30.9 MHz Galactic plane survey

view Abstract Citations (3) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of Candidate Supernova Remnants From the Clark Lake 30.9 MHz Galactic Plane Survey Gorham, P. W. ; Kulkarni, S. R. ; Prince, T. A. Abstract We report the results of 1464 MHz continuum VLA observations of eight fields containing unidentified small-diameter objects associated with candidate supernova remnants from the Clark Lake 30.9 MHz galactic plane survey. The observations were made in the C configuration giving a resolution of ~12-20 arcsec, and a sensitivity of typically < 0.5 mJy per beam. Polarization measurements were made as well. One of the 30.9 MHz candidates, G41.4+ 1.2, appears to be confirmed as a supernova remnant by our observations. Of the remaining seven fields observed, three were found to contain small-diameter objects which met some of the criteria for nonthermal origin, but will require further study to evaluate whether they are associated with the candidate supernova remnants. Two of the fields were found to contain groups of unresolved objects consistent with expectations for extragalactic background sources. In these cases the 30.9 MHz observations, which could not resolve the individual sources but would view them as a single extended source, may have mistakenly identified them as remnants. Finally, two fields contained bright H II regions. Publication: The Astronomical Journal Pub Date: January 1993 DOI: 10.1086/116428 Bibcode: 1993AJ....105..314G Keywords: Extragalactic Radio Sources; H Ii Regions; Sky Surveys (Astronomy); Supernova Remnants; Very Large Array (Vla); Galactic Structure; Linear Polarization; Massive Stars; Stellar Evolution; Astrophysics; SUPERNOVA REMNANTS full text sources ADS | data products SIMBAD (20) NED (12)

Observations of the galactic radio structure CTB 63 (G47.5 + 6.0)

view Abstract Citations (2) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of the Galactic Radio Structure CTB 63 (G47.5+6.0) Caswell, J. L. ; Landecker, T. L. ; Roger, R. S. Abstract The radio source CTB 63 is extended and diffuse and has been regarded as a possible galactic supernova remnant. We present new observations at 408 and 1420 MHz made with the DRAO synthesis telescope that delineate the boundary and suggest that it is a galactic spur rather than a discrete supernova remnant. The spur has a length of ~6^deg^ and at one extremity merges with the diffuse galactic disk emission; it is clearly nonthermal and we explore some possibilities for its origin. We also catalog small-diameter sources in the region, most of which are extragalactic background sources. Publication: The Astronomical Journal Pub Date: June 1989 DOI: 10.1086/115111 Bibcode: 1989AJ.....97.1721C Keywords: Extragalactic Radio Sources; Galactic Radio Waves; Galactic Structure; Supernova Remnants; Astronomical Spectroscopy; Brightness Temperature; High Resolution; Radio Galaxies; Astrophysics; NEBULAE: SUPERNOVA REMNANTS; GALAXIES: THE GALAXY full text sources ADS | data products SIMBAD (150) NED (149)

Distance and Chemical Composition of High-Velocity Clouds

AbstractThis contribution reports results of recent absorption-line studies on La Palma and reviews earlier work done elsewhere. CaII has been found in absorption in five HVCs, mostly against extragalactic background sources. The CaII/HI ratios differ strongly, from 0.002 to 0.1 times solar, suggesting different origins of HVCs. Nondetections of CaII absorption in stellar spectra may provide lower limits to HVC distances, or be caused by small-scale structure or a low CaII/HI ratio in the HVC. Proper interpretation requires comparison of CaII/HI ratios in stellar and extragalactic probes, and use of high-resolution HI maps. No solid direct distance determinations of HVCs are available yet. The distance of Complex C is controversial.