AHSTStudy of the Stellar Populations in the Cometary Dwarf Irregular Galaxy NGC 2366

Abstract

We present Vand I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram reaches down to I $ 26:0 mag. It reveals not only a young population of blue main-sequence stars (age P30 Myr) but also an intermediate-age population of blue and red supergiants (20 Myr P age P100 Myr) and older evolved populations of asymptotic giant branch (AGB) stars (age k100 Myr) and red giant branch (RGB) stars (age k1 Gyr). The measured magnitude I ¼ 23:65 AE 0:10 mag of the RGB tip results in a distance modulus m À M ¼ 27:67 AE 0:10, which corresponds to a distance of 3:42 AE 0:15 Mpc, in agreement with previous distance determinations. The youngest stars are associated with the bright complex of H ii regions NGC 2363 (=Mrk 71) in the southwest extremity of the galaxy. As a consequence of the diffusion and relaxation processes of stellar ensembles, the older the stellar population is, the smoother and more extended is its spatial distribution. An underlying population of older stars is found throughout the body of NGC 2366. The most notable feature of this older population is the presence of numerous relatively bright AGB stars. The number ratio of AGB to RGB stars and the average absolute brightness of AGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403, indicating a younger age of the AGB stars in NGC 2366. In addition to the present burst of age P100 Myr, there has been strong star formation activity in the past of NGC 2366, from $100 Myr to P3 Gyr ago.

Stellar subsystems of the galaxy NGC 1313

“…Our new distance estimate, D = 3.13 ± 0.25 Mpc, agrees with that of Karachentsev et al [27], but differs significantly from estimate of Thuan and Izotov [26].…”

“…Abrupt changes in the slopes of the luminosity functions of fields SI, S2, S3, and S4 in result is increased by our use of more than a dozen stars at the red-giant boundary to calculate the tip of the red-giant branch. Our distance estimate for NGC 2366 agrees with that of Karachentsev et al [27], but not with the value obtained by Thuan and Izotov [26]. This discrepancy must be due to the use of different calibration methods.…”

“…The populated red-giant branch, which is conspicuous in the CM-diagrams (Fig. 2), can be used to find the distance via the Tip of the Red-Giant Branch (TRGB) method [28], which was used to by Thuan et al [26] and Karachentsev et al [27]. Both teams based their estimates on only a single field, whereas we have used images of four fields.…”

Blue Compact Dwarf Galaxies: Laboratories for probing the Primordial Universe

“…Photometric studies combined with spectral population synthesis models of a few BCDs give ages of a few Gyr for the older stellar population, considerably smaller than the age of the universe. For example, the study of the two "cometary" BCDs Mrk 59 and Mrk 71 by Noeske et al (2000) gives an age of ∼ 4 Gyr for the older stellar population, consistent with the age found by Thuan & Izotov (2005b) from a HST/WFPC2 color-magnitude diagram study of Mrk 71 (NGC 2366).…”

HubbleSpace Telescope study of resolved red giant stars in the outer halos of nearby dwarf starburst galaxies

“…They are usually gas-rich systems with a chemical composition comparable to that of distant primeval galaxies (e.g. Thuan & Izotov 1999). Investigating the properties of underlying older populations in present-day starbursting systems is the key to understanding their evolution over the cosmological timescale.…”