The Stellar Content and Distance of the Galaxy NGC 2403 IN the M81 Group

Abstract

view Abstract Citations (322) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Stellar Content and Distance of the Galaxy NGC 2403 IN the M81 Group Tammann, G. A. ; Sandage, Allan Abstract Fifty-nine variables have been found in the nearby Sc galaxy NGC 2403, which is a member of the M81 group. Of these, seventeen are Cepheids ranging in period from 87448 to 2O~23O; eight are very bright irregular blue variables of the type previously found in M31 and M33; seventeen are bright, red irregular supergiant variables similar to those in h and x Per; there is one eclipsing binary; and sixteen are unclassified variables of undetermined nature. NGC 2403 is the first galaxy outside the Local Group in which Cepheids have been detected. Light-curves have been determined relative to a photoelectric sequence of seventy-three stars which covers the interval from B = 9.15 to B = 22.70, V = 8.39 to V = 21.80. Three-color photoelectric observations of the sequence stars brighter than V = 19.5 show that the local reddening due to the galactic system is E(B - V) = 0.06 mag. The period-luminosity relations for Cepheids in B and V at maximum light provide two well-determined upper envelope fits to the standard P-L function, giving apparent distance moduli of NGC 2403 of (m - M)AB = 27.80 ± 0.1 and (m - M)AV = 27.75 ± 0.1. These reduce to true moduli of (m - M)o,B = 27.56, and (m - M)oy = 27.57 from the Cepheids alone. Four other distance indicators give closely comparable values. (1) Star counts across the face of NGC 2403, corrected for foreground contamination, give B = 18.25 for the brightest resolved stars. Previous calibration of MB = -9.3 in galaxies of the Local Group gives (m - M)AB = 27.55. (2) The brightest blue irregular variable occurs at B(max) = 18.2, giving (m - M)AB =27.5 ± 0.2 if MB(max) = -9.3 is adopted from M31 and M33. (3) The brightest irregular red variable occurs at V(max) = 19.98, B(max) = 22.02. Calibration via the LMC, NGC 6822, and the SMC gives a preliminary value of Mv(max) = -8.00, MB(max) = -6.00 for these stars, giving apparent moduli of (m - M)AB = 28.02, and (m - M)AV = 27.98. (4) The angular sizes of the largest, and mean of the first five largest Hii regions, calibrated via the LMC and M33, give (m - M)O,L = 27.54 ± 0.18 and (m - M)0,5 = 27.52 ± 0.40. The average of all methods is given in Table 12 as (m - = 27.55 ± 0.13 (AD). The agreement between the methods is good and provides evidence that no systematic difference exists from galaxy to galaxy among the various distance indicators. One unexpected result is that the red supergiants, both constant and variable, may be a new and pre- cise distance indicator. There appears to be an upper envelope to the absolute luminosity of these stars at about M~ = -8.0. Such red stars can be located with ease in all Sc and Irr galaxies with (m - M)AV <29 by comparing red and blue plates and, therefore, they can be important in an intermediate step for the redetermination of the Hubble constant. The ratio of mean redshift of the M81 group-each galaxy corrected individually for solar motion- to the distance to NGC 2403 of r = (3.25 ± 0.20) X 106 pc iS v/r = (65 ± 15) km sec' Mpc'. This is not claimed to have any relation to the Hubble constant because the sample contains only one group, but similar analysis of many additional groups may eventually lead to an adequate value of H Publication: The Astrophysical Journal Pub Date: March 1968 DOI: 10.1086/149487 Bibcode: 1968ApJ...151..825T full text sources ADS | data products NED (74) SIMBAD (10)