* Astronomy

NGC 6822 (also Barnard's Galaxy, IC 4895, Caldwell 57, IRAS 19421-1455, MCG -2-50-63 and PGC 63616) is a magnitude +9.3 barred irregular galaxy located 1.63 ±0.03 million light-years away in the constellation Sagittarius.

The galaxy was discovered by British astronomer Edward Emerson Barnard using a 12.7 cm (5 inch) refracting telescope at the Vanderbilt Observatory, Nashville, on the 17th August 1884. 
The galaxy was rediscovered by Maximilian Wolf in 1906 ad relisted as IC 4895. 
The galaxy was only properly catalogued in the 1920s by Edwin Hubble and Milton Humason Lasell. 

Right Ascension 19h 44m 56.6s, Declination -14° 47' 21"

Part of the Local Group of galaxies, it is one of the closer galaxies to the Milky Way. It is similar in structure and composition to the Small Magellanic Cloud. It is about 7,000 light-years in diameter
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Title: On the distance and reddening of the Local Group dwarf irregular galaxy NGC 6822
Authors: F. Fusco (Univ. "Tor Vergata" Rome, Italy), R. Buonanno (Univ. "Tor Vergata" Rome, Italy), G. Bono (Univ. "Tor Vergata" Rome, Italy), S. Cassisi (INAF, Obs. Teramo, Italy), M. Monelli (Instituto Astrofisico de Canarias, Tenerife, Spain), A. Pietrinferni (INAF, Obs. Teramo, Italy)

On the basis of a new photometric analysis of the Local Group Dwarf Irregular Galaxy NCG 6822 based on observations obtained with Advanced Camera for Surveys on board of the Hubble Space Telescope, we have obtained a new estimate of the extinction of two fields located in the southeast region of the galaxy. Due to the presence of significant differences in the distance estimates to NGC 6822 available in literature, we have decided to provide an independent determination of the distance to this galaxy based on an updated and self-consistent theoretical calibration of the tip of the Red Giant Branch (TRGB) brightness. As a result we have obtained a new determination of the distance to NGC 6822 equal to {(m-M)}_0=23.54± 0.05, and compared our measurement with the most recent determinations of this distance.

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Title: On the Origin of the Supergiant HI Shell and Putative Companion in NGC 6822
Authors: John M. Cannon, Erin M. O'Leary, Daniel R. Weisz, Evan D. Skillman, Andrew E. Dolphin, Frank Bigiel, Andrew A. Cole, W. J. G. de Blok, Fabian Walter

We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large HI hole. The resulting deep colour magnitude diagrams show that NGC 6822 has formed >50% of its stars in the last ~5 Gyr. The star formation histories of all six positions are similar over the most recent 500 Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming that the lifetime of the structure is longer than 500 Myr; such long-lived structures have now been observed in multiple systems and may be the norm in galaxies with solid-body rotation. The old stellar populations (red giants and red clump stars) of the putative companion are consistent with those of the extended halo of NGC 6822; this argues against the interpretation of this structure as a bona fide interacting companion galaxy and against its being linked to the formation of the HI hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended HI structure in NGC 6822 is a warped disk inclined to the line of sight.

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Title: Extended Star Clusters in the Remote Halo of the Intriguing Dwarf Galaxy NGC 6822
Authors: Narae Hwang, Myung Gyoon Lee, Jong Chul Lee, Won-Kee Park, Hong Soo Park, Sang Chul Kim, Jang-Hyun Park

We present a study on four new star clusters discovered in the halo of the intriguing dwarf irregular galaxy NGC 6822 from a wide field survey covering 3 deg x 3 deg area carried out with MegaCam at Canada-France-Hawaii Telescope (CFHT). The star clusters have extended structures with half-light radii R_h ~ 7.5 -- 14.0 pc, larger than typical Galactic globular clusters and other known globular clusters in NGC 6822. The integrated colours and colour magnitude diagrams (CMD) of resolved stars suggest that the new star clusters are 2 -- 10 Gyr old and relatively metal poor with Z=0.0001--0.004 based on the comparison with theoretical models. The projected distance of each star cluster from the galaxy center ranges from 10.7 arcmin (~ 1.5 kpc) to 77 arcmin (~ 11 kpc), far beyond the optical body of the galaxy. Interestingly, the new star clusters are aligned along the elongated old stellar halo of NGC 6822, which is almost perpendicular to the HI gas distribution where young stellar populations exist. We also find that the colours and half-light radii of the new clusters are correlated with the galactocentric distance: clusters farther from the galaxy center are larger and bluer than those closer to the galaxy center. We discuss the stellar structure and evolution of NGC 6822 implied by these new extended star clusters in the halo. We also discuss the current status of observational and theoretical understandings regarding the origin of extended star clusters in NGC 6822 and other galaxies.

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