Stellar content of the metal-poor galaxy DDO 68

STELLAR POPULATIONS AND STAR FORMATION HISTORY OF THE METAL-POOR DWARF GALAXY DDO 68∗

“…The presence of stars of all ages in the arc-shaped structure suggests that this is a possible dwarf galaxy in interaction with the central body of DDO 68, since stars older than ∼ 1 Gyr wouldn't be able to remain in the form of a gravitationally unstable arc. Indeed, in a recent work, Tikhonov et al (2014) suggested that DDO 68 consists of two distinct objects: a more massive central galaxy, DDO 68 A, traced by the spatial distribution of the bulk of the old stars, and a dwarf galaxy in interaction with DDO 68 A, namely DDO 68 B, corresponding to the arc-shaped structure, an hypothesis that we find very interesting and worth of further investigation.…”

“…In the RGB spatial map we also recognize a small concentration of stars toward the upper right edge of the ACS chip, noticed also by Tikhonov et al (2014); new results on this feature will be presented in a forthcoming paper based on new LBT data with a much larger field of view (Annibali et al 2016). …”

“…Their distance, although larger than the ∼ 9.9 Mpc predicted by its recession velocity (v = 502 ± 4 km/s), 6.5 Mpc from the apparent magnitudes of the brightest stars (Pustilnik et al 2005), and 12.0 Mpc derived from the Tip of the Red Giant Branch (TRGB) (Tikhonov et al 2014), places DDO 68 a factor 1.5 closer than I Zw 18, close enough to resolve its old stars with HST.…”

“…So, the absence of an easily identifiable companion to DDO 68 leaves the question of how its recent SF was triggered open. From the analysis of the spatial distribution of stars resolved in HST /ACS imaging, (Tikhonov et al 2014) argued that DDO 68 consists in fact of two different systems: a central most massive body and an elongated arc-shaped body (DDO 68 B). In a recent work based on Very Large Array (VLA) observations, Cannon et al (2014) claimed to have found an object with the same systemic velocity of DDO 68.…”

Study of the Lynx-Cancer void galaxies. – V. The extremely isolated galaxy UGC 4722

“…Like UGC 4722, many void pair and triplet members, show elevated values of M (Hi)/LB (∼2 to 25), blue colours of their outer regions and low O/H. Examples include: the merger remnant/triplet DDO 68 (Ekta, Chengalur, Pustilnik 2008;Tikhonov et al 2014;Cannon et al 2014), the triplet J0723+36 (Chengalur & Pustilnik 2013), the pair HS 0822+3542/SAO 0822+3545 (Chengalur et al 2006), pair J0852+13 and the triplet MRK 407 (Chengalur et al, in prep.). A more detailed comparison of these unusual systems in voids is postponed to the separate paper.…”