Search for Hot Gas in the Local Group with ASCA

Abstract

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 M31 and M33, 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 $2.8 \times 10^{-9} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1} \,\mathrm{sr}^{-1}$ 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 $\beta$-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 $1.3 \times 10^{-4} \,\mathrm{cm}^{-3}$. The plasma column density is too low to contribute significantly to the observed quadrupole anisotropy in the cosmic microwave background.