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

Abstract

B,R, and J frames taken with the Calar Alto 3.5m 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 $-0.1 \le B-R \le +0.7$. 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 $r_c = 0.33$ kpc. The central surface brightnesses are $21.57 \pm 0.09 mag/\Box''$, $20.65 \pm 0.06mag/\Box''$, and $20.11 \pm 0.11 mag/\Box''$, 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. The overall colors are $B-R = + 0.97$, $B-J = +1.55$ and the magnitude is B = 14.55 ($M_B^o$ = -14.5). The classification as an irregular dwarf results from the few HII regions of UGC 685. On an H${\alpha}$ Calar Alto 2.2m telescope CCD image, I identified only 5 HII regions. The total H${\alpha}$ flux corresponds to a mildly on-going star formation with a rate of 0.003 \Msolar yr$^{-1}$, a low rate even in comparison to other dwarfs. The available data do not indicate any major deviation from this rather low star formation rate within the last $10^9$ yr.