Multiphase High-Velocity Clouds toward HE 0226–4110 and PG 0953+414

By A. J. Fox, B. Wakker, B. Savage, T. Tripp, K. Sembach, Joss Bland-Hawthorn. 2005

165 citations recorded by the source corpus.

We study the physical conditions, elemental abundances, and kinematics of the high-velocity clouds (HVCs) along the sight lines toward active galaxies HE 0226-4110 and PG 0953+414 using Hubble Space Telescope Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer data. No 21 cm H I emission is detected in these clouds, but our observations reveal multiple components of HVC absorption in lines of H I, C II, C III, C IV, O VI, Si II, Si III, and Si IV in both directions. We investigate whether photoionization by the extragalactic background radiation or by escaping Milky Way radiation can explain the observed ionization pattern. We find that photoionization is a good explanation for the C II, C III, Si II, and Si III features but not for the O VI or C IV associated with the HVCs, suggesting that two principal phases exist: a warm (T ≈ 104 K), photoionized phase and a hotter (T = 1-3 × 105 K), collisionally ionized phase; the broader line widths of the high ions are consistent with this multiphase hypothesis. The warm HVCs toward HE 0226-4110 have high levels of ionization (97%-99%) and metallicities ([Z/H] between -0.9 and -0.4) close to those in the Magellanic Stream, which lies 11° away on the sky at similar velocities. These HVCs may well be stripped fragments of the Stream that have been ionized by the pervading radiation field; they have thermal pressures that would place them close to equilibrium in a fully ionized 106 K Galactic corona with nH = 4-9 × 10-5 cm-3 at 50 kpc. The warm HVCs seen at -146 and 125 km s-1 toward PG 0953+414 have [Z/H] = -0.6 ± 0.2 and -0.8 ± 0.2, respectively, suggesting they are not formed from purely Galactic material. A minisurvey of the hot, collisionally ionized HVC components seen here and in five other sight lines finds that in 11/12 cases, the high ions have kinematics and ionic ratios that are consistent with an origin in conductive interfaces, where energy flows into the HVCs from a hot surrounding medium and produces O VI- and C IV-bearing boundary layers. However, the broad absorption wing on the O VI profile toward PG 0953+414 is not completely explained by the interface scenario. This feature may be tracing the outflow of hot gas into the Milky Way halo as part of a Galactic fountain or wind.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/ae3aaff4f2b5190475c79c74471c1e150a616439

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Citations in the corpus

Time-dependent Ionization in Radiatively Cooling Gas (2006, methodology): “…spectroscopy of hot gas in the Galactic halo (e.g., Sembach&Savage 1992; Spitzer 1996), including,more recently, in discrete ionized ‘‘high-velocity clouds’’ (Savage et al. 2002; Fox et al. 2004, 2005, 2006; see also Sternberg et al. 2002; Gnat & Sternberg 2004; Maller & Bullock 2004).”
THE COS-HALOS SURVEY: ORIGINS OF THE HIGHLY IONIZED CIRCUMGALACTIC MEDIUM OF STAR-FORMING GALAXIES (2016, background): “A similar challenge arises from the diffuse gas known as high velocity clouds (HVCs) in the halo of the Milky Way (Sembach et al. 2003; Fox et al. 2004, 2005, 2006; Lehner et al. 2009; Wakker et al. 2012).”
A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers (2007, background): “62 line is partially blended with Galactic C IV (see Fox et al. 2005), but the distinctive component structure seen in the O VI λ1031.”
THE BIMODAL METALLICITY DISTRIBUTION OF THE COOL CIRCUMGALACTIC MEDIUM AT z ≲ 1 (2013, background): “The fact that we do not rely on the high ions to constrain our models also has the benefit of reducing the uncertainties from leaking photons from galaxies near the absorbers: for the Milky Way, Fox et al. (2005) showed that C II/C III (Si II/Si III) for gas at ∼ 50 kpc from the Milky Way is almost ”
The Vertical Structure of Warm Ionised Gas in the Milky Way (2008, background): “This high pressure is consistent with the expectation that any small warm clouds at these heights should be enveloped in a hot halo (e.g., Fox et al. 2005).”
THE STRUCTURE OF THE MILKY WAY'S HOT GAS HALO (2013, methodology): “In particular, we compare our model pressure to pressures associated with HVCs measured by Fox et al. (2005), which are at temperatures ranging from 104 to 105 K.”
The Circumgalactic Medium (2017, background): “It soon became impossible even to address these big mysteries of galaxies without appealing to gas flows between the ISM, the IGM, and by implication, the CGM.”
VERTICAL STRUCTURE OF A SUPERNOVA-DRIVEN TURBULENT, MAGNETIZED INTERSTELLAR MEDIUM (2012, background): “…1 of JM06), and 2 × 105 K, which is thermally unstable and near the maximum cooling efficiency due to O vi. Gas in this transition-temperature regime is normally assumed to lie on interfaces between hot and warm clouds (Fox et al. 2005; Savage & Wakker 2009) because it is highly thermally unstable.”
DO HOT HALOS AROUND GALAXIES CONTAIN THE MISSING BARYONS? (2010, background): “We also consider high-velocity clouds, using UV measurements of the temperature and density of numerous partially ionized metals and inferred gas pressures (Fox et al. 2005).”
THREE-DIMENSIONAL INTEGRAL FIELD OBSERVATIONS OF 10 GALACTIC WINDS. I. EXTENDED PHASE (≳10 Myr) OF MASS/ENERGY INJECTION BEFORE THE WIND BLOWS (2010, background): “Secondly, there is now strong evidence for large-scale outflows in the Galaxy across the electromagnetic spectrum (Bland-Hawthorn & Cohen 2003; Fox et al. 2005; Keeney et al. 2006).”
Probing Galactic Halos with Fast Radio Bursts (2019, background): “We note that the xHI value may vary significantly from sightline to sightline (e.g., Fox et al. 2005; Howk et al. 2006), the mean value adopted here provides a bulk estimate of the contribution of the Galactic ionized hydrogen to DM.”