High Ion Column Densities Research Articles

EXPLORING THE ORIGIN AND FATE OF THE MAGELLANIC STREAM WITH ULTRAVIOLET AND OPTICAL ABSORPTION

(Abridged) We present an analysis of ionization and metal enrichment in the Magellanic Stream (MS), the nearest gaseous tidal stream, using HST/STIS and FUSE ultraviolet spectroscopy of two background AGN, NGC 7469 and Mrk 335. For NGC 7469, we include optical spectroscopy from VLT/UVES. In both sightlines the MS is detected in low-ion and high-ion absorption. Toward NGC 7469, we measure a MS oxygen abundance [O/H]_MS=[OI/HI]=-1.00+/-0.05(stat)+/-0.08(syst), supporting the view that the Stream originates in the SMC rather than the LMC. We use CLOUDY to model the low-ion phase of the Stream as a photoionized plasma using the observed Si III/Si II and C III/C II ratios. Toward Mrk 335 this yields an ionization parameter log U between -3.45 and -3.15 and a gas density log (n_H/cm^-3) between -2.51 and -2.21. Toward NGC 7469 we derive sub-solar abundance ratios for [Si/O], [Fe/O], and [Al/O], indicating the presence of dust in the MS. The high-ion column densities are too large to be explained by photoionization, but also cannot be explained by a single-temperature collisional-ionization model (equilibrium or non-equilibrium). This suggests the high-ion plasma is multi-phase. Summing over the low-ion and high-ion phases, we derive conservative lower limits on the ratio N(total H II)/N(H I) of >19 toward NGC 7469 and >330 toward Mrk 335, showing that along these two directions the vast majority of the Stream has been ionized. The presence of warm-hot plasma together with the small-scale structure observed at 21 cm provides evidence for an evaporative interaction with the hot Galactic corona. This scenario, predicted by hydrodynamical simulations, suggests that the fate of the MS will be to replenish the Galactic corona with new plasma, rather than to bring neutral fuel to the disk.

Metal-enriched plasma in protogalactic halos

We continue our recent work to characterize the plasma content of high-redshift damped and sub-damped Lyman-alpha systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey N V absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67<z<4.28 with unblended coverage of the N V doublet, using data from VLT/UVES, Keck/HIRES, and Keck/ESI. In DLAs, we find eight secure N V detections, four marginal detections, and 79 non-detections. The detection rate of N V in DLAs is therefore 13^{+5}_{-4}%. Two sub-DLA N V detections are found among a sample of 18, at a similar detection rate of 11^{+15}_{-7}%. We show that the N V detection rate is a strong function of metallicity, increasing by a factor of ~4 at [N/H]=[NI/HI]>-2.3. The N V and CIV component b-value distributions in DLAs are statistically similar, but the median b(N V) of 18 km/s is lower than the median b(O VI) of 25 km/s. Some ~20% of the N V components have b<10 km/s and thus arise in warm photoionized plasma at log (T/K)<4.92; local sources of ionizing radiation (as opposed to the extragalactic background) are required to keep the cloud sizes physically reasonable. The nature of the remaining ~80% of (broad) N V components is unclear; models of radiatively-cooling collisionally-ionized plasma at log(T/K)=5.2-5.4 are fairly successful in reproducing the observed integrated high-ion column density ratios and the component line widths, but we cannot rule out photoionization by local sources. Finally, we identify several unusual DLAs with extremely low metallicity (<0.01 solar) but strong high-ion absorption [log N(N V)>14 or log N(O VI)>14.2] that present challenges to either galactic inflow or outflow models.

Metallicity and Ionization in High‐Velocity Cloud Complex C

We analyze HST and FUSE ultraviolet spectroscopic data for 11 sight lines passing through the infalling high-velocity cloud (HVC) Complex C. These sight lines pass through regions with H I column densities in the range N = 1018.1-1020.1 cm-2. From [O I/H I] abundances, we find that Complex C metallicities range from 0.09 to 0.29 Z☉, with a column density weighted mean of 0.13 Z☉. Nitrogen (N I) is underabundant by factors of (0.01-0.07)(N/H)☉, significantly less than oxygen relative to solar abundances. This pattern suggests nucleosynthetic enrichment by Type II SNe, consistent with an origin in the Galactic fountain or infalling gas produced in winds from Local Group galaxies. The range of metallicity and its possible (2 σ) dependence on N could indicate some mixing of primordial material with enriched gas from the Milky Way, but the mixing mechanism is unclear. We also investigate the significant highly ionized component of Complex C, detected in C IV, Si IV, and O VI, but not in N V. High-ion column density ratios show little variance and are consistent with shock ionization or ionization at interfaces between Complex C and a hotter surrounding medium. Evidence for the former mechanism is seen in the Mrk 876 line profiles, where the offset in line centroids between low and high ions suggests a decelerating bow shock.

Highly Ionized Gas Surrounding High‐Velocity Cloud Complex C

We present Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope observations of high-, intermediate-, and low-ion absorption in high-velocity cloud (HVC) Complex C along the lines of sight toward five active galaxies. Our purpose is to investigate the idea that Complex C is surrounded by an envelope of highly ionized material, arising from the interaction between the cloud and a hot surrounding medium. We measure column densities of high-velocity high-ion absorption and compare the kinematics of low-, intermediate-, and high-ionization gas along the five sight lines. We find that in all five cases, the H I and O VI high-velocity components are centered within 20 km s-1 of one another, with an average displacement of = 3 ± 12 km s-1. In those directions where the H I emission extends to more negative velocities (the so-called high-velocity ridge), so does the O VI absorption. The kinematics of Si II is also similar to that of O VI, with = 0 ± 15 km s-1. We compare our high-ion column density ratios to the predictions of various models, adjusted to account for both recent updates to the solar elemental abundances and relative elemental abundance ratios in Complex C. Along the PG 1259+593 sight line, we measure N(Si )/N(O ) = 0.10 ± 0.02, N(C )/N(O ) = 0.35, and N(N )/N(O ) < 0.07 (3 σ). These ratios are inconsistent with collisional ionization equilibrium at one kinetic temperature. Photoionization by the extragalactic background is ruled out as the source of the high ions since the path lengths required would make HVCs unreasonably large; photoionization by radiation from the disk of the Galaxy also appears unlikely since the emerging photons are not energetic enough to produce O VI. By themselves, ionic ratios are insufficient to discriminate between various ionization models, but by considering the absorption kinematics as well, we consider the most likely origin for the highly ionized high-velocity gas to be at the conductive or turbulent interfaces between the neutral/warm ionized components of Complex C and a surrounding hot medium. The presence of interfaces on the surface of HVCs provides indirect evidence for the existence of a hot medium in which the HVCs are immersed. This medium could be a hot (T ≳ 106 K) extended Galactic corona or hot gas in the Local Group.

On the Origin of the High‐Ionization Intermediate‐Velocity Gas toward HD 14434

We present Far-Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph observations of high-ionization interstellar absorption toward HD 14434 (l ¼ 135=1, b ¼� 3=8; d � 2:3 kpc), an O5.5 V star in the Perseus OB1 association. Intermediate-velocity interstellar Si iv and C iv absorption is present at VLSR ¼� 67 km s � 1 , while low-ionization gas associated with the Perseus arm is detected at about � 50 km s � 1 . Neither N v nor O vi is detected at VLSR ¼� 67 km s � 1 , although Al iii and Fe iii, tracers of warm ionized gas, are seen. The high-ion column densities in the � 67 km s � 1 component are log½NðC ivÞ� ¼ 13:92 � 0:02 cm � 2 ,l og½NðSi ivÞ� ¼ 13:34 � 0:02 cm � 2 ,l og½NðN vÞ� � 12:65 cm � 2 ,a nd log½NðO viÞ� � 13:73 cm � 2 (3 � limits). The observed Civ/Siiv ratio of 3:8 � 0:3 in this intermediate-velocity cloud (IVC) is similar to the Galactic average (4:3 � 1:9). Our analysis of the Si iv and C iv line widths yields a temperature of T � 10; 450 � 3400 K for this component. At this low temperature, neither Si iv nor C iv can be produced via collisions. We investigate several photoionization models to explain the intermediatevelocity Si iv and C iv absorption toward HD 14434. Photoionization models employing cooling of a hot (T � 10 6 K) diffuse plasma as the source of ionizing radiation reproduce the observed properties of the IVC toward HD 14434 quite well. The hot plasma responsible for the ionizing radiation in these models may be attributed to hot gas contained in a supershell in or near the Perseus arm or from a more generally distributed hot ionized medium. Subject headings: ISM: abundances — ISM: atoms — ISM: structure — open clusters and associations: individual (Perseus OB1) — stars: individual (HD 14434) — ultraviolet: ISM

Observational Evidence of Supershell Blowout in GS 018−04+44: The Scutum Supershell

Emission in the H I 21 cm line has been mapped for a region of the Galaxy that includes two known supershells, GS 018(04)44 and GS 034(06)65. We focus on the GS 018(04)44, hereafter referred to as the Scutum Supershell, which is an elongated shell about 5i in diameter extending to (7i below the Galactic plane. The Scutum shell lies at a kinematic distance of D3300 pc, implying a shell diameter of D290 pc with a vertical extension of D400 pc away from the Galactic plane. The Scutum shell con- tains 6.2 ) 105 swept into the walls. We observe that the top of the shell is missing, and a substan- M _ tial column of H I rises from the shell walls to b \( 11i, culminating in a large cloud of neutral hydrogen, 3.74 ) 104 located D630 pc from the plane. ROSAT data show X-ray emission that M _ , closely anticorrelates with the 21 cm emission. This emission probably originates from hot gas within the Scutum Supershell. After approximately correcting for the foreground absorption, we —nd that the 1.5 keV X-rays peak at the base of the shell, the 0.75 keV emission peaks in the interior and at the top of the shell, and the 0.25 keV emission extends to high latitudes above the shell. The X-ray luminosity is roughly D5 ) 1036 ergs s~1. The Wisconsin H-Alpha Mapper (WHAM) survey shows the presence of Ha emission that exhibits a morphology similar to that of the H I. Spectra indicate the presence of ionized hydrogen at velocities similar to the H I, placing ionized material at the same kinematic distance as the neutral material. IRAS images in the 60 and 100 km wavebands reveal the presence of dust corre- lated with the neutral hydrogen. Infrared surface brightness indicates an excess in the 100 km emission, which could indicate a molecular hydrogen component with a column density of 2.4 ) 1021 cm~2 in the densest regions of the high-latitude cloud of neutral hydrogen. IUE ultraviolet high dispersion spectra of HD 177989 and HD 175754 reveal the presence of very (l 17i.89, b \( 11i.88) (l 16i.40, b \( 9i.92) strong absorption by highly ionized gas at a velocity that associates the absorption with the ejecta of the Scutum Supershell. In the case of HD 177989, the high ion column density ratios suggest an origin in a turbulent mixing layer where hot and cool gases mix in the presence of shear —ows. The Ha and X-ray emission suggest that a multitude of energetic phenomena exist in this region, providing the necessary ionizing radiation. Indeed, there are multiple supernova remnants, H II regions, and hot stars, which could all contribute sizeable amounts of energy and ionizing radiation. The combination of these data sets indicates observational evidence of a ii blowout ˇˇ phenomena whereby hot material produced within the Scutum Supershell has blown through the top of the shell and been pushed to high latitude. Subject headings: Galaxy: structureISM: bubblesISM: individual (Scutum Supershell) ¨ ISM: structureradio lines: ISMsupernova remnants

GHRS andORFEUS IIObservations of the Highly Ionized Interstellar Medium toward ESO 141‐055

We present Goddard High Resolution Spectrograph (GHRS) and ORFEUS II measurements of Si IV, C IV, N V, and O VI absorption in the interstellar medium of the Galactic disk and halo toward the nucleus of the Seyfert galaxy ESO 141-055. The high-ionization absorption is strong, with line strengths consistent with the spectral signature expected for hot (T ≳ 105 K) collisionally ionized gas in either a "Galactic fountain" or an inhomogeneous medium containing a mixture of conductive interfaces and turbulent mixing layers. The total O VI column density of ≈1015 cm-2 suggests that the scale height of O VI is large (≳3 kpc) in this direction. Models of the C IV velocity distribution along the sight line are consistent with a large scale height for the highly ionized gas. Comparison of the high ion column densities with measurements for other sight lines indicates that the highly ionized gas distribution is patchy. The amount of O VI perpendicular to the Galactic plane varies by at least a factor of ~4 among the complete halo sight lines thus far studied. In addition to the high ion absorption, lines of low-ionization species are also present in the spectra. With the possible exception of Ar I, which may have a lower than expected abundance resulting from partial photoionization of gas along the sight line, the absorption strengths are typical of those expected for the warm, neutral interstellar medium. The sight line intercepts a cold molecular cloud with N(H2) ≈ 1019 cm-2. The cloud has an identifiable counterpart in IRAS 100 μm emission maps of this region of the sky. We detect a Lyα absorber associated with ESO 141-055 at z = 0.03492. This study presents an enticing glimpse into the interstellar and intergalactic absorption patterns that will be observed at high spectral resolution by the Far Ultraviolet Spectroscopic Explorer.

High‐Resolution Ultraviolet Observations of the Highly Ionized Interstellar Gas toward Radio Loops I and IV

We present new Goddard High-Resolution Spectrograph (GHRS) echelle observations of the high ionization lines of Si IV, C IV, and N V toward HD 119608, a halo star at d = 4.1 kpc behind the Loop I and IV supernova remnants. Absorption along the path to HD 119608 makes it possible to study energetic processes that may result in the flow of gas into the Galactic halo. The data have a resolution (FWHM) of ≈ 3.5 km s-1 and S/N ratios of 30:1-50:1. The integrated high ion column densities log N = 13.57 ± 0.02, 14.48 ± 0.06, and 13.45 ± 0.07 for Si IV, C IV, and N V, respectively, imply a factor of 2-4 enhancement in the amount of highly ionized gas compared to average sight lines through the Galactic disk and halo. The integrated high ion column density ratios, N(C IV)/N(Si IV) = 8.1 ± 1.1 and N(C IV)/N(N V) = 10.7 ± 2.1, are also several times larger than normal. These high ion results suggest the absorption is influenced by passage of the sight line through the center of Loop IV. The HD 119608 C IV absorption profile has a bimodal velocity structure indicative of an expanding shell; we tentatively derive an expansion velocity of 16 km s-1 for Radio Loop IV. A detailed analysis of the high ion profile structure indicates that multiple types of highly ionized gas with a range of properties exist along this sight line. We also reexamine the high ionization properties of the QSO 3C 273 sight line using new intermediate-resolution (FWHM ≈ 20 km s-1) GHRS data. We obtain log N = 14.49 ± 0.03 and 13.87 ± 0.06 for C IV and N V, respectively. The C IV column density, which is 0.12 dex smaller than earlier estimates, leads to somewhat smaller ionic ratios than previously determined. We find N(C IV)/N(Si IV) = 5.1 ± 0.6 and N(C IV)/N(N V) = 4.2 ± 0.6. However, as for HD 119608, the high ion column densities toward 3C 273 are larger than normal by factors of 2-4. The 3C 273 high ion absorption profiles are much broader than those seen toward HD 119608 and other sight lines near the center of Loop IV. The larger line widths may result because the sight line passes through the turbulent edge of Loop IV as well as the X-ray and radio continuum emission regions of the North Polar Spur. We have compiled a list of the highest quality IUE and GHRS high ion measurements available for interstellar sight lines through the disk and halo and find the following median averaged results: N(C IV)/N(Si IV) = 3.8 ± 1.9 and N(C IV)/N(N V) = 4.0 ± 2.4. These ratios are lower than those found for four Loop IV sight lines. We suggest a model for the production of highly ionized gas in Loop IV in which the contributions from turbulent mixing layers and conductive interfaces/SNR bubbles to the total high ion column densities are approximately equal. Much of the high ion absorption toward HD 119608 and 3C 273 may occur within a highly fragmented medium within the remnant or the outer cavity walls of the remnant.

High Signal-to-Noise Echelle Spectroscopy of Quasar Absorption-Line Systems in the Direction of HS 1946+7658

view Abstract Citations (74) References (79) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High Signal-to-Noise Echelle Spectroscopy of Quasar Absorption-Line Systems in the Direction of HS 1946+7658 Tripp, Todd M. ; Lu, Limin ; Savage, Blair D. Abstract We have obtained a high signal-to-noise (40 <= S/N <= 80) high- resolution (FWHM = 20 km s^-1^) spectrum of the radio-quiet QSO HS 1946+7658 (z_em_ = 3.051) with the echelle spectrograph on the KPNO 4 m telescope. We detect 11 metal systems in the direction of this QSO, including two Mg II systems, five C IV systems, two damped Lyα systems, and two associated systems. We use the apparent column density technique and profile fitting to measure the heavy element column densities and to assess the effects of absorption-line saturation. Profile fitting indicates that three of the C IV systems are narrow with b < 8 km s^-1^. This implies that T < 50,000 K, and therefore these systems are probably photoionized. The abundance patterns in the damped Lyα systems are strikingly similar to those observed in low- metallicity Milky Way stars and suggest that these absorption lines are due to galaxies in early stages of chemical enrichment (see Lu et al.). The prominent associated system at z_abs_ = 3.0496,3.0504 is detected in H I, C II, CIV, SiII, Si III, Si IV, A1 II, Al III, and N V. The high ion column density ratios in this associated system imply that the gas is more highly ionized than the Galactic halo. The high degree of ionization is not surprising given the extraordinary luminosity of the QSO (Hagen et al.). To study ionization and abundances in this associated system, we compare the observed column densities to a series of CLOUDY models in which photoionization by the QSO is the dominant ionization mechanism. For the input radiation field, we have used the various spectral energy distributions of HS 1946+7658 observed by Kuhn et al. The model that best fits the observed column densities of singly and doubly ionized species has solar relative and absolute abundances, but models with absolute metallicities a few times greater than solar fit comparably well. None of the models produce enough Si IV and C IV, but the QSO flux near the ionization potentials of these ions is uncertain. This photoionization modeling raises a dilemma: the high metallicity implied by the best models suggests that the associated absorption occurs near the nucleus of the QSO where the star formation rate is likely to be enhanced, but the ionization parameter from the best model combined with a density upper limit from C II^*^ implies that the distance between the QSO and the associated absorber is greater than 300 kpc. We briefly discuss possible explanations of these discordant conclusions, including the possibility that the gas was enriched near the QSO nucleus and then ejected. By combining our HS 1946+7658 data with similar quality observations from the literature, we find that the number density of C IV systems with rest equivalent width W_r_ > 0.03 A is dN/dz = 71 + 1.7. This is ~3 times larger than the number density implied by a sample of C IV systems with W_r_ > 0.15 A, which indicates that the number of C IV systems per unit redshift is dominated by very weak lines. An attempt to detect a turndown in the C IV equivalent width distribution, which could indicate that some Lyα clouds contain no metals, resulted in ambiguous conclusions due to the small sample size. We briefly discuss recent oscillator strength revisions which are likely to have a significant impact on QSO absorption studies. Publication: The Astrophysical Journal Supplement Series Pub Date: February 1996 DOI: 10.1086/192258 Bibcode: 1996ApJS..102..239T Keywords: GALAXIES: QUASARS: ABSORPTION LINES; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: HS 1946+7658 full text sources ADS | data products NED (22) SIMBAD (1)

Ultraviolet gas absorption and dust extinction toward M8

view Abstract Citations (4) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ultraviolet Gas Absorption and Dust Extinction toward M8 Boggs, Don ; Bohm-Vitense, Erika Abstract Interstellar absorption lines are analyzed using high-resolution IUE spectra of 11 stars in the young cluster NGC 6530 located in the M8 region. High-velocity clouds at -35 km/s and -60 km/s are seen toward all cluster stars. The components arise in gases that are part of large interstellar bubbles centered on the cluster and driven by stellar winds of the most luminous members. Absorption lines of species of different ionization states are separated in velocity. The velocity stratification is best explained as a 'champagne' flow of ionized gas away from the cluster. The C IV/Si IV ratios toward the hotter cluster members are consistent with simple photoionization models if the gas-phase C/Si ratio is increased by preferential accretion onto dust grains. High ion column densities in the central cluster decline with distance from W93, suggesting that radiation from a hot source near W93 has photoionized gas in the central cluster. Publication: The Astrophysical Journal Pub Date: August 1990 DOI: 10.1086/168998 Bibcode: 1990ApJ...358..441B Keywords: Absorption Spectra; Galactic Clusters; Gas Ionization; Interstellar Extinction; Interstellar Gas; Iue; B Stars; Cosmic Dust; H Ii Regions; Interstellar Matter; Photoionization; Astrophysics; CLUSTERS: OPEN; INTERSTELLAR: GRAINS; NEBULAE: INDIVIDUAL MESSIER NUMBER: M8; NEBULAE: H II REGIONS full text sources ADS | data products SIMBAD (25) MAST (1) INES (1)