High-Velocity Clouds and their Soft X-ray Emission

Kerp, J.; Pietz, J.; Kalberla, P. M. W.; Burton, W. B.; Egger, R.; Freyberg, M. J.; Hartmann, D.; Mebold, U.

doi:10.1017/s0252921100071438

Cited by 14 publications

(18 citation statements)

References 6 publications

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“…They possibly represent one group of local analogues of extra-planar gas features seen in the vicinity of other galaxies. IVCs and HVCs represent multi-phase structures that can contain regions of partly molecular gas (H 2 absorption, Richter et al 2001a;Wakker 2006), cold and warm neutral gas seen in 21-cm emission Ben Bekhti et al 2008), warm-ionised plasma as traced by Hα emission (Putman et al 2003;Hill et al 2009) and UV absorption (Lu et al 1994), and highly-ionised plasma seen in O vi absorption (Sembach et al 2003;Fox et al 2006) and in X-ray emission (Kerp et al 1999;de Boer 2004, and references therein).…”

Section: Introductionmentioning

confidence: 99%

An absorption-selected survey of neutral gas in the Milky Way halo

Bekhti

Winkel

Richter

et al. 2012

A&A

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Aims. We aim at analysing systematically the distribution and physical properties of neutral and mildly ionised gas in the Milky Way halo, based on a large absorption-selected data set. Methods.Multi-wavelength studies were performed combining optical absorption line data of Ca ii and Na i with follow-up H i 21-cm emission line observations along 408 sight lines towards low-and high-redshift QSOs. We made use of archival optical spectra obtained with UVES/VLT. H i data were extracted from the Effelsberg-Bonn H i survey and the Galactic All-Sky survey.For selected sight lines we obtained deeper follow-up observations using the Effelsberg 100-m telescope.Results. Ca ii (Na i) halo absorbers at intermediate and high radial velocities are present in 40-55% (20-35%) of the sightlines, depending on the column density threshold chosen. Many halo absorbers show multi-component absorption lines, indicating the presence of sub-structure. In 65% of the cases, absorption is associated with H i 21-cm emission. The Ca ii (Na i) column density distribution function follows a power-law with a slope of β ≈ −2.2 (−1.4). Conclusions. Our absorption-selected survey confirms our previous results that the Milky Way halo is filled with a large number of neutral gas structures whose high column density tail represents the population of common H i high-and intermediate-velocity clouds seen in 21-cm observations. We find that Na i/Ca ii column density ratios in the halo absorbers are typically smaller than those in the Milky Way disc, in the gas in the Magellanic Clouds, and in damped Lyman α systems. The small ratios (prominent in particular in high-velocity components) indicate a lower level of Ca depletion onto dust grains in Milky Way halo absorbers compared to gas in discs and inner regions of galaxies.

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Section: Introductionmentioning

confidence: 99%

An absorption-selected survey of neutral gas in the Milky Way halo

Bekhti

Winkel

Richter

et al. 2012

A&A

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“…In practice, the extragalactic background can be used as a "reference" to normalize the intensities of the XRB (see Sect. 4), as has been done in previous investigations using ROSAT data (e.g., Kerp et al 1999;Snowden et al 1998;Pradas et al 2003). Energy bands C1, C2 and C3 are only related to source detection and proton flare filtering.…”

Section: Selection Of the Energy Bandsmentioning

confidence: 81%

“…Read & Ponman (2003) published background maps to correct the observations for vignetting, which is of special importance for the analysis of extended sources. But the contribution of the cosmic X-ray background, originating from the Local Hot Bubble, the Galactic Halo and the extragalactic X-ray background, has not been systematically treated for XMM-Newton as it has been in case of the ROSAT mission (e.g., Kerp et al 1999;Snowden et al 1998;Pradas et al 2003). Because XMM-Newton is, in principle, able to detect very faint signals, all systematic effects must be well understood and Tables 3-6 are only available in electronic form at http://www.edpsciences.org reliable methods to eliminate their contributions have to be developed.…”

Section: Introductionmentioning

confidence: 99%

XMM-Newton data processing for faint diffuse emission

Pradas¹,

Kerp²

2005

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We present a study of the in-flight performance of the XMM-Newton EPIC MOS and pn detectors, with focus on the influence of proton flares and vignetting on the data. The very wide range in the conditions of our sample of observations, in terms of exposure length and background intensities, allows the detection of a wide range in the spectra of the proton flares, in contrast to the hard-spectrum flares proposed by Lumb et al. (2002, A&A, 389, 93) or Read & Ponman (2003, A&A, 409, 395). We also find an up to now unreported contamination in the low energy regime (E ≤ 0.5 keV) of the MOS1 observations, consisting of a significant increase in the measured intensities in two CCDs at the edges of the detector. This contamination yields in bright CCDs in the observations. Its effect must be taken into account for the study of sources detected in the affected CCDs. With respect to vignetting, we present in-flight exposure maps and we propose a method to repeat this calculation for user-definable energy bands. All the results presented here, have the goal to enable the study of very faint extended sources with XMM-Newton, like nearby galactic X-ray halos or the soft X-ray background.

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“…Hirth et al (1985) and Kerp et al (1999) showed an excess of X-ray emission that seems to be spatially correlated with Complex C, but no clear relationship with Draco itself could be established. On the other hand, based on FUV observations, Park et al (2009) reported the detection of several ionic lines in the direction of Draco, especially C IV, Si II, and O III].…”

Section: Hot Gasmentioning

confidence: 97%

Structure formation in a colliding flow: The Herschel view of the Draco nebula

Miville-Deschênes

Salomé

Martín

et al. 2017

A&A

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Context. The Draco nebula is a high Galactic latitude interstellar cloud observed at velocities corresponding to the intermediate velocity cloud regime. This nebula shows unusually strong CO emission and remarkably high-contrast small-scale structures for such a diffuse high Galactic latitude cloud. The 21 cm emission of the Draco nebula reveals that it is likely to have been formed by the collision of a cloud entering the disk of the Milky Way. Such physical conditions are ideal to study the formation of cold and dense gas in colliding flows of diffuse and warm gas. Aims. The objective of this study is to better understand the process of structure formation in a colliding flow and to describe the effects of matter entering the disk on the interstellar medium. Methods. We conducted Herschel-SPIRE observations of the Draco nebula. The clumpfind algorithm was used to identify and characterize the small-scale structures of the cloud. Results. The high-resolution SPIRE map reveals the fragmented structure of the interface between the infalling cloud and the Galactic layer. This front is characterized by a Rayleigh-Taylor (RT) instability structure. From the determination of the typical length of the periodic structure (2.2 pc) we estimated the gas kinematic viscosity. This allowed us to estimate the dissipation scale of the warm neutral medium (0.1 pc), which was found to be compatible with that expected if ambipolar diffusion were the main mechanism of turbulent energy dissipation. The statistical properties of the small-scale structures identified with clumpfind are found to be typical of that seen in molecular clouds and hydrodynamical turbulence in general. The density of the gas has a log-normal distribution with an average value of 10 3 cm −3 . The typical size of the structures is 0.1−0.2 pc, but this estimate is limited by the resolution of the observations. The mass of these structures ranges from 0.2 to 20 M and the distribution of the more massive structures follows a power-law dN/dlog(M) ∼ M −1.4 . We identify a mass-size relation with the same exponent as that found in molecular clouds (M ∼ L 2.3 ). On the other hand, we found that only 15% of the mass of the cloud is in gravitationally bound structures. Conclusions. We conclude that the collision of diffuse gas from the Galactic halo with the diffuse interstellar medium of the outer layer of the disk is an efficient mechanism for producing dense structures. The increase of pressure induced by the collision is strong enough to trigger the formation of cold neutral medium out of the warm gas. It is likely that ambipolar diffusion is the mechanism dominating the turbulent energy dissipation. In that case the cold structures are a few times larger than the energy dissipation scale. The dense structures of Draco are the result of the interplay between magnetohydrodynamical turbulence and thermal instability as self-gravity is not dominating the dynamics. Interestingly they have properties typical of those found in more classical molecular clouds.

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High-Velocity Clouds and their Soft X-ray Emission

Cited by 14 publications

References 6 publications

An absorption-selected survey of neutral gas in the Milky Way halo

An absorption-selected survey of neutral gas in the Milky Way halo

XMM-Newton data processing for faint diffuse emission

Structure formation in a colliding flow: The Herschel view of the Draco nebula

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