2MASS Picture of the Week Archive Captions
Atlas Image mosaic, covering 25´ × 25´ on the sky of the Local Group dwarf irregular galaxy NGC 6822, also known as Barnard's Galaxy. Compare the 2MASS near-infrared image with that in the optical. The optical light is dominated by the young, hot, blue stars and HII, or ionized hydrogen, regions, whereas the view from 2MASS is of the older, more evolved giant, supergiant, and asymptotic giant branch stars (the nebulae are far less obvious). These cooler stars better represent the overall mass distribution in galaxies, even irregular ones. The distance of NGC 6822 from us is about 500 kpc (1.63 million light years).


Atlas Image mosaic, covering 10´ × 10´ on the sky of the globular star cluster Messier 68 (M68). Salaris & Weiss (2002, A&A, 388, 492) recently determined the age of the cluster to be 11.2 Gyr, which is slightly older than the ~10 Gyr found by Brocato, Castellani, & Piersimoni (1997, ApJ, 491, 789). This cluster is relatively metal-poor. The light from globular clusters in the near-infrared is dominated by old red giant stars, which have approximately the same near-infrared colors. Thus, all the stars appear to be about the same color, although not necessarily the same brightness.


Atlas Image mosaic, covering 28´ × 28´ on the sky of the open cluster IC 1590. The cluster centers around the Trapezium-like system, HD 5005 (the bright blue object north of image center). The cluster is associated with, and generally embedded in, the nebula NGC 281 (Sharpless 184). Guetter & Turner (1997, AJ, 113, 2116) performed detailed optical photometry and spectroscopy, and limited K-band photometry, of stars in the cluster and found the distance of 2.9 kpc (9450 light years) to the optically-visible cluster, behind nearly 2 visual magnitudes of extinction. They find the cluster to be quite young, ~3.5 Myr. What is most remarkable is that, comparing an optical image of NGC 281 with the 2MASS image, what are dark regions of dust in the optical nebula are bright regions of dust and molecular gas emission in the near-infrared. More importantly, deeply embedded newly-forming stars are evident, to the southwest of the optical cluster (additional star formation appears to be occurring to the east as well). The 2MASS color-color and color-magnitude diagrams for both clusters show that the age, distance, and reddening are consistent for the stars around HD 5005 (green tracks on the diagrams), but the more embedded, probably very young objects are behind up to ~15 visual magnitudes of extinction.


Atlas Image mosaic, covering 25´ × 31´ on the sky of the filamentary, or snake-like, infrared dark cloud G11.11-0.12. This is an example of various regions in the Milky Way Galaxy with very high visual extinction, in excess of 25 magnitudes. They were discovered by routine survey observations of the Galactic plane by the Infrared Space Observatory and the Midcourse Space Experiment. From the lack of emission between 8 and 100 μm and high mid-infrared opacities, Egan et al. (1998, ApJ, 494, L199) concluded that the infrared dark clouds contain cold (T<13 K) dust and suggested that they are dense molecular cores. Carey et al. (2000, ApJ, 543, L157) find strong submillimeter bright, compact sources in eight dark clouds, using SCUBA at the JCMT, and these authors suggest that these highly embedded sources are potentially the sites for very early star formation in the Galaxy. These clouds, of course, look very dark on the near-infrared 2MASS images. Image mosaic by S. Van Dyk (IPAC). These data are included in the Second Incremental Data Release!


Atlas Image mosaic, covering 15´ × 15´ on the sky of the prototypical reflection nebula NGC 7023, aka the Iris Nebula. The filamentary structures to the north and south are the edges of molecular hydrogen gas clouds emitting in the near infrared. A bipolar cavity is seen to to the east (and north) in this molecular emission (Gerin et al. 1998, ApJ, 500, 329). Rogers, Heyer, & Dewdney (1995, ApJ, 442, 694) indicate that a bipolar outflow from the young, central Herbig Be star, HD 200775 (the bright star toward the center of the 2MASS image), is responsible for the formation of the cavity. Compare the Iris seen here with that in the optical.

































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