The Distance to the Giant Elliptical Galaxy M87 and the Size of Its Stellar Subsystem

Abstract

Stellar photometry in nine fields around the giant elliptical galaxy M87 in the Virgo cluster is obtained from archival images of the Hubble Space Telescope. The resulting Hertzsprung-Russell diagrams show populated red-giant and AGB branches. The position of the tip the red-giant branch (the TRGB discontinuity) is found to vary with galactocentric distance. This variation can be interpreted as the effect of metal-rich red giants on the procedure of the measurement of the TRGB discontinuity or as a consequence of the existence of a weak gas-and-dust cloud around M87 extending out to 10’ along the galactocentric radius and causing I-band absorption of up to \(0\mathop .\limits^{\rm{m}} 2\) near the center of the galaxy. The TRGB stars located far fromtheM87 center yield an average distancemodulus of (m-M) = 30.91 ± 0.08, which corresponds to the distance of D = 15.4± 0.6Mpc. It is shown that stars in the field located between M86 and M87 galaxies at angular separations of 37’ and 40’ are not intergalactic stars, but belong to the M87 galaxy, i.e., that the stellar halo of this galaxy can be clearly seen at a galactocentric distance of 190 kpc. The distances are measured to four dwarf galaxies P4anon, NGC4486A, VCCA039, and dSph-D07, whose images can be seen in the fields studied. The first three galaxies are M87 satellites, whereas dSph-D07 is located at a greater distance and is a member of the M86 group.