Normal galaxies in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory

Abstract

We analyze the statistical properties of normal galaxies to be detected in the all-sky survey by the eROSITA X-ray telescope of the Spectrum-X-Gamma observatory. With the current configuration and parameters of the eROSITA telescope, the sensitivity of a 4-year-long all-sky survey will be ≈10−14 erg s−1 in the 0.5–2 keV band. This will allow ∼(1.5–2) × 104 normal galaxies with approximately the same contribution of star-forming and elliptical galaxies to be detected. All galaxies of the X-ray survey are expected to enter into the existing far-infrared (IRAS) or near-infrared (2MASS) catalogs; the sample of star-forming galaxies will be approximately equivalent in sensitivity to the sample of star-forming galaxies in the IRAS catalog of infrared sources. Thus, a large homogeneous sample of normal galaxies with measured X-ray, near-infrared, and far-infrared fluxes will be formed. About 90% of the galaxies in the survey are located within ∼200–400 Mpc. A typical (most probable) galaxy will have a luminosity log L X ∼ 40.5–41.0, will be located at a distance of ∼70–90 Mpc, and will be either a star-forming galaxy with a star formation rate of ∼20M ⊙ yr−1 whose X-ray emission is produced by ultraluminous X-ray sources (ULXs) or an elliptical galaxy with amass log M * ∼ 11.3 emitting through to a hot interstellar gas. The galaxies within 35 Mpc will collectively contain ∼102 ULXs with luminosities log L X > 40, ∼80% of whichwill be the only luminous source in the galaxy. Thus, although the angular resolution of the eROSITA telescope is too low for the luminosity function of compact sources in galaxies to be studied in detail, the survey data will allow one to investigate its bright end and, possibly, to impose constraints on the maximum luminosity of ULXs.