New dwarfs around the curly spiral galaxy M63

Abstract

We present a deep (50 hours exposed) image of the nearby spiral galaxy M 63 (NGC 5055), taken with a 0.14-m aperture telescope. The galaxy halo exhibits the known very faint system of stellar streams extending across 110 kpc. We found 5 very low-surface-brightness dwarf galaxies around M 63. Assuming they are satellites of M 63, their median parameters are: absolute $B$-magnitude -8.8 mag, linear diameter 1.3 kpc, surface brightness $\sim$ 27.8 mag/sq. arcsec and linear projected separation 93 kpc. Based on four brighter satellites with measured radial velocities, we derived a low orbital mass estimate of M 63 to be (5.1$\pm$1.8) 10$^{11} M_{\odot}$ on a scale of $\sim$216 kpc. The specific property of M 63 is its declining rotation curve. Taking into account the declining rotation curves of the M 63 and three nearby massive galaxies NGC 2683, NGC 2903, NGC 3521, we recognize their low mean orbital mass-to-K-band luminosity ratio, (4.8$\pm$1.1) $M_{\odot}/L_{\odot}$, that is only $\sim$1/6 of the corresponding ratio for the Milky Way and M 31.