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

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 (NH) 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 NH 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 NH with the calculated NH values from H i and CO intensities indicates that the observed values are systematically larger than the calculated values. The observed NH values at high Galactic latitude (NH < 10²² cm⁻²) are comparable to those estimated from N_ H\, I and optical reddening values using the method by Willingale et al. (2013, MNRAS, 431, 394), but the values near to the Galactic plane (NH > 10²² cm⁻²) are larger than the estimated ones. The dust optical depth at 353 GHz, τ ₃₅₃, and the observed NH values are expressed by a linear function of NH=(1.01\!-\!1.59)× 10²⁶\ τ ₃₅₃ cm⁻² even at NH > 10²³ cm⁻². We also confirmed a linear correlation between the optical reddening, $E(B-V)$, and the NH values expressed by NH=(6.3\!-\!9.5)× 10²¹\ 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.