ROSAT Detection of an X-ray Shadow in the 1/4-keV Diffuse Background in the Draco Nebula

Snowden, S. L.; Mebold, U.; Hirth, W.; Herbstmeier, U.; Schmitt, J. H. M. M.

doi:10.1126/science.252.5012.1529

Cited by 113 publications

(64 citation statements)

References 16 publications

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“…The previously mentioned HVCs as well as other gas clouds cast shadows against a diffuse background of soft (kT-03 keV) X-ray emission coming from beyond (Snowden et al 1991;Herbstmeier et al 1995). Distance measurements have shown that shadowing structures like, e.g., the Draco Nebula, are located high-several hundred pc-above the Galactic plane (Lilienthal et al 1991), which places the shadowed X-ray emitting gas even farther away from the disk.…”

Section: Warm and Hot Ionized Gas In The Halomentioning

confidence: 99%

Gaseous Halos of Late-Type Spiral Galaxies

Dahlem¹

1997

PASP

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ABSTRACT. This article reviews the most recent observational results on disk-halo interactions in nearby galaxies. The implications of these results on our understanding of the structure of the interstellar medium (ISM) in our Galaxy and external spiral galaxies, with particular emphasis on the halo ISM, are discussed. Disk-halo interactions occur only above the brightest HII regions, which are found almost exclusively in late-type spirals and dwarf and irregular galaxies. Circumnuclear starbursts are the most energetic flavor of this phenomenon. The existence, the shapes, and the properties of gaseous halos depend on the level of energy input into the disk ISM per unit surface area, i.e., on the feedback of mass, momentum, and energy produced by vigorous star formation. Thus, current theoretical models of the ISM take into account that gaseous halos are natural extensions of disk ISMs and are heated by the winds and ionizing radiation of massive stars, and by the shock waves of Supernovae and their remnants. Observationally, this is reflected by the fact that all phases of the ISM known to exist in galaxy disks have also been detected in halos. Disk-halo interactions are a very effective way of redistributing energy, metals, and magnetic fields in galaxies and of expelling them into intergalactic space when outflows reach escape velocity. MOTIVATION AND OUTLINESpiral galaxies consist of three spatially and dynamically dinstinct components, namely bulge, disk, and halo. Of these, the central bulge consists primarily of old stars and has a low gas fraction. The disk and halo consist of stars plus gas, dust, and in many cases "dark matter." Here I want to concentrate on the interstellar medium (ISM) in the disks and halos of galaxies, with emphasis on late-type spirals. Massive dark halos will not be described (see, e.g., Ashman 1992; Holt and Bennett 1995). In the current context "latetype" stands for disk-dominated spirals with rather inconspicuous bulges (Hubble types Sb -Sdm). The ISM of elliptical galaxies and early-type spirals (S0-Sab) will also not be presented here (cf. Walsh et al. 1989;and Fabbiano 1989).Studies of the ISM contribute significantly to our understanding of the structure, kinematics, and evolution of galaxies. The level of star formation (SF), its spatial distribution, and its temporal changes depend strongly on the properties of the interstellar gas from which stars can form. Stars feed energy and metal-enriched matter back into the ambient medium via stellar winds and supernovae (SNe), thereby distributing processed material throughout the ISM. Hence, through this gas-star-gas cycle, the ISM becomes a mediator of SF.In Sec. 2, the different phases of the ISM in galaxies will be introduced, and Sec. 3 presents observations of the ISM in the Milky Way. Section 4 will show that the Galactic ISM is l A\\ correspondence should be directed to this address. not confined to the thin disk, but extends to high distances (z) above the plane, forming a gaseous halo. However, being located within the...

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Section: Warm and Hot Ionized Gas In The Halomentioning

confidence: 99%

Gaseous Halos of Late-Type Spiral Galaxies

Dahlem¹

1997

PASP

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“…One of the first unique discoveries of ROSAT was the observation of deep shadows in the general SXRB cast by the Draco Nebula (l,b ~ 90°,40°, Burrows & Mendenhall 1991;Snowden et al 1991). These observations unequivocally proved that there is significant | keV Xray emission in at least the lower Galactic halo: roughly 50% of the observed flux in that direction originates <; 300 pc from the Sun, <; 200 pc above the Galactic plane.…”

Section: Introductionmentioning

confidence: 66%

“…The Draco region, however, shows a strong intensity enhancement over a relatively limited solid angle (~ 0.03 sr). This enhancement, the first unambiguous Galactic halo emission discovered (Burrows & Mendenhall 1991;Snowden et al 1991), has led to a biased view of the dumpiness of the halo emission as well as to an overestimate of the mean halo surface brightness. The halo emission is brighter in the region of the north Galactic pole than the south while halo emission in the south increases more strongly toward lower latitudes.…”

Section: \ Kev Emission In the Galactic Halomentioning

confidence: 99%

A New View of the LHB and ¼ keV X-ray Halo

Snowden

1997

International Astronomical Union Colloquium

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A b s t r a c t . The X-ray sky at \ keV is completely dominated by diffuse emission. It has become clear that it originates as at least three separate components: local emission within the nearest ~ 100 pc from the Sun, halo emission from beyond most of the neutral material of the Galactic disk, and the superposition of unresolved extragalactic sources. The only way to determine the temperatures and relative emission measures of the hot plasma responsible for the Galactic components is to use the X-ray intensity variations due to column density variations in the intervening Hi to separate the components. "Shadowing" studies have been pursued for individual objects using ROSAT data, from both pointed observations and the all-sky survey with considerable success.This paper presents the results of an all-sky analysis of the | keV background from the ROSAT survey. A Local Hot Bubble is found consistent with, although somewhat smaller than, previous models. It has a temperature of 10 6 ' 1 K and an emission measure which varies by a factor of ~ 3.3 over large angles. The halo emission has a temperature near 10 6 0 K with an emission measure which varies from near zero to more than five times that of the local emission.

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“…Without doubt the most spectacular was the observation of soft X-ray shadows (Snowden et al, 1991;Burrows & Mendenhall, 1991) which showed that the "local bubble" was not, in fact, local, but that much of the 1/4 keV emission in the general direction of the Galactic poles was coming from distant regions. In principle, one can still invoke a wall boundary in the Galactic plane, (see, for example, Snowden, this volume) but the supporting data are quite thin and may be more easily explained in other ways.…”

Section: Historical Commentsmentioning

confidence: 99%