M 101 group galaxies

Abstract

Based on archival Hubble Space Telescope images, we have performed stellar photometry for the galaxy M 101 and other neighboring galaxies located at a small angular distance from M 101 and having radial velocities similar to that of M 101: M 51, M 63, NGC 5474, NGC 5477, UGC 9405, Ho IV, KUG1413+573, and others. Based on the TRGB method, we have determined the distances to these galaxies. We have found that the M 101 group lies at a distance of 6.8 Mpc and is a small compact galaxy group consisting of four galaxies: NGC 5474, NGC 5477, UGC 9405, and Ho IV. The bright massive galaxies M 51 and M 63 are considerably farther (D = 9.0 and 9.3 Mpc, respectively) than the M 101 group and do not belong to it. Applying the virial theorem to 27 objects (H II regions and galaxies), M 101 satellites located at different distances from the galaxy, has revealed an increase in the dynamical mass of M 101 with increasing sizes of the system of satellites used in calculating the mass. The maximum calculated mass of M 101 is 7.5 × 10 11 M . The dynamical mass of M 101 calculated on the basis of the four galaxies constituting the group is 6.2 × 10 11 M . The mass-to-light ratio for this mass is M/L = 18 (at the adopted luminosity of M 101, M B = −20.8).

The M 101 group complex: new dwarf galaxy candidates and spatial structure

“…It has been debated whether M 51 and M 63 plus satellites should be counted as members of the M 101 group. Tikhonov et al (2015) argue against this view. In the present work, based on our analysis of the galaxy distribution in the region, we will use the term M 101 group complex for all three galaxies and their satellites and the term subgroup for an individual host and its satellite population (M 101 subgroup, M 51 subgroup, and M 63 subgroup).…”

The Distance to M51*

“…Our TRGB measurement overlaps in uncertainties with the TRGB distances reported recently by Tikhonov et al (2015) for three different fields in M51. Our measurement is an improvement over these previous values for a number of reasons.…”

“…Some of the studies measured multiple distances based on the same data set (i.e., Tully 1988; Tutui & Sofue 1997;Takáts & Vinkó 2006Dessart et al 2008;Bose & Kumar 2014;Rodríguez et al 2014;Tikhonov et al 2015). Typically, each study reported a final number based on combining the individual measurements.…”

“…Finally, we use a TRGB calibration specific to the HST filters. Tikhonov et al (2015) do not explicitly state the calibration they used, but refer to the Lee et al (1993) paper when referencing their final TRGB distance modulus. This early paper calibrated the TRGB luminosity in the I-band using RRLyrae distances with metallicity corrections based on a V−I color.…”

“…First, the uncertainties on our photometry for the upper RGB are lower as the photometric depth is ∼1 mag deeper, reaching ∼2 mag below the TRGB. Second, we use a ML technique for our final distance, instead of the Sobel filter used in Tikhonov et al (2015), which reduces the uncertainties on our measurement. Finally, we use a TRGB calibration specific to the HST filters.…”