Galactic absorption measured by X-ray observations of clusters of galaxies at the low Galactic latitude

Abstract

The amount of the interstellar gas in the Galaxy has been conventionally estimated through observations at various wavelengths. The estimation of the total hydrogen column density (N_H) depends on assumptions such as temperature. The X-ray absorption process is the photoelectric absorption, which depends on the number of atoms to encounter X-ray photons, and hence X-ray observations would be able to derive the N_H values independently on the condition of the interstellar matter. We measured the Galactic absorption using clusters of galaxies at the low Galactic latitude. Comparing the observed N_H with the calculated N_H} values from HI and CO intensities indicates that the observed values are systematically larger than the calculated values. The observed $N_{\rm H}$ values at high Galactic latitude (N_H<10^{22} cm^{-2}) are comparable to those estimated from N_HI} and optical reddening values using the method by Willingale et al. (2013, MNRAS, 431, 394), but the values near to the Galactic plane (N_H >10^{22} cm^{-2}) are larger than the estimated ones. The dust optical depth at 353 GHz, tau_{353}, and the observed N_H values are expressed by a linear function of N_H=(1.01-1.59)x10^{26} tau_{353} cm^{-2} even at N_H >10^{23} cm^{-2}. We also confirmed a linear correlation between the optical reddening, E(B-V), and the N_H values expressed by N_H=(6.3-9.5)x10^{21} E(B-V) cm^{-2}. This work is an additional and independent test of the relation among the amount of interstellar gas, the optical depth, and the optical reddening.