Distance, structure and bright stellar content of the dwarf irregular galaxy UGC 685

Universitätssternwarte München, Scheiner Str. 1, D-81679 München, Germany

Send offprint request to: U. Hopp

Received: 16 June 1998
Accepted: 27 August 1998

Abstract

B and R CCD images and J NICMOS3 frames taken with the Calar Alto 3.5 m telescope of the dwarf irregular galaxy UGC 685 are presented. The brightest part of the stellar population is resolved in B and R, very few also in J. The stellar color-magnitude diagram is discussed. An estimate of the distance to UGC 685 of 5.5 Mpc is derived based on the brightest blue supergiant stars. Most of the resolved bright stars show colors in the range . The continuum light of the unresolved stars is distributed rather regular in B, R, and J, showing only minor irregularities. This surface brightness distribution follows an exponential law in all three colors with a scale length kpc. The central surface brightnesses are , , and 20.11 ± 0.11 mag, in B, R, and J, respectively. The surface brightness can be traced out to 5 r_c in B and R. Thus, UGC 685 belongs to the class of dwarfs where the HI distribution is much more extended (here 2.6 times) than the optical (stellar) light distribution, but contrary to many objects of this type, it does not belong to the class of low-surface brightness objects. No color gradient was detected in UGC 685 except that the very center is slightly bluer. The overall colors are , and the magnitude is (). The classification as an irregular dwarf from survey plates results from the few HII regions of UGC 685 which are all concentrated to the South-East of the center of the galaxy. On an Hα Calar Alto 2.2 m telescope CCD image, I identified only 5 HII regions, one of them being rather bright. The total Hα flux corresponds to a mildly on-going star formation with a rate of 0.003  yr^-1, a low rate even in comparison to other dwarfs. The available (and limited) data do not indicate any major deviation from this rather low star formation rate within the last 10⁹ yr.