Search for hot gas in the local group with ASCA

Abstract

An X-ray study was made to examine whether some part of the soft X-ray background is coming from hot gas in the Local Group. For this purpose, four consecutive pointings were made with ASCA toward a sky region between M 31 and M 33, which is close to the direction of the center of the Local Group. By comparing the X-ray surface brightness in this sky direction with that in another blank sky region near the north equatorial pole, an upper limit on any soft excess X-ray background was determined to be 2.8×10 −9 erg cm −2 s −1 sr −1 with a 90% confidence level statistical error. Assuming an optically thin thermal bremsstrahlung energy spectrum (Raymond-Smith model) for a temperature of 1 keV and a β-model electron density distribution for a core radius of 100 kpc for the X-ray halo, the upper limit of the central plasma density was obtained to be 1.3×10 −4 cm −3 . The plasma column density is too low to contribute significantly to the observed quadrupole anisotropy in the cosmic microwave background.

Passive Spiral Formation from Halo Gas Starvation: Gradual Transformation into S0s

“…where the halo gas temperature is assumed to be proportional to M T /R h (i.e., the temperature of the gas is equivalent to the virial temperature). Recent X-ray observations of the Galaxy halo by ASCA have demonstrated that the halo density within 100 kpc of the Galaxy is less than 7.62 × 10 −5 cm −5 (Osone et al 2000), which is very close to the above critical value. Therefore, the above equation implies that even if a galaxy passes through an ICM with a density two orders of magnitude lower than the central density of Coma (∼ 5.64 × 10 −3 cm −3 ), the galaxy halo gas can still be greatly affected by the ICM.…”

Constraints on a Local Group X‐Ray Halo

“…Regarding the other density structure parameter, r c , Suto et al (1996) use r c = 150 kpc as does Bland-Hawthorn (1999), while Maloney & Bland-Hawthorn (1999) consider the range r c = [0, 350 kpc]. Banday & Gorski (1996) and Osone et al (2000) only restrict themselves to core radii smaller than the distance from the observer to the LGH center. For a relatively poor group like the Local Group, a value of r c = 150 kpc (again typical of clusters) seems rather large when viewed in light of observational results on other groups (Mulchaey et al 1996b;Pedersen, Yoshii, & Sommer-Larsen 1997;Dahlem & Thiering 2000).…”

“…Regarding the metallicity Z of halo gas, Z = 0.3Z ⊙ is used in all previous models when modelling the LGH X-ray emission (Suto et al 1996;Osone et al 2000), although Maloney & Bland-Hawthorn (1999) also consider Z = 0.01Z ⊙ and Z = 0.1Z ⊙ . The value Z = 0.3Z ⊙ , however, is typical of clusters and rich groups of galaxies (e.g., Hwang et al 1999) whereas poor low-temperature (T 1 − 1.5 keV) groups tend to have somewhat lower metallicities, possibly because their gravitational potentials are too shallow to retain all the enriched ejecta of the constituent galaxies (Tawara et al 1998;Davis et al 1999;Hwang et al 1999).…”

“…These all-sky maps have been removed of point sources to a uniform flux level for which the original survey source catalog (RASS-I) was complete over 90% of the sky. To obtain a qualitative picture of the amount of any excess 1 keV emission towards M31, we resampled the original "point source-removed" 102 • by 102 A recent, much more detailed investigation by Osone et al (2000), to our knowledge the first one to be carried out with the distinct purpose of constraining LGH parameters from direct observations of associated X-ray emission, concentrated on 4 neighboring ASCA GIS pointings centered approximately 6 • south of M31 (i.e. a few degrees off the line connecting M31 and M33 as seen from the Sun).…”

“…Previous models of intergalactic gas distributed in a Local Group halo and/or its possible diffuse X-ray emission include those of Kahn & Woltjer (1959) and Oort (1970); Hunt & Sciama (1972); Suto et al (1996), and related works by Banday & Gorski (1996) and Pildis & McGaugh (1996); Bland-Hawthorn (1999) and Maloney & Bland-Hawthorn (1999); Sidher, Sumner, & Quenby (1999); and a recent observational study by Osone et al (2000).…”