2MASS Picture of the Week Archive Captions

Aitoff projection of the three-color composite JHKs source count map of the entire sky, based on 95,851,173 stars with Ks 13.5. What appears most prominently are the Galactic plane and the Galactic bulge. The plane is cut by dark dust lanes and clouds, even in the near-infrared, and several dusty regions, including Orion, are conspicuous. One can also see the two Magellanic Clouds, Large and Small. (Note the prominent bar and incipient spiral structure of the Large Cloud.) Near the Small Magellanic Cloud is the globular cluster 47 Tucanae. Cutting almost north-south through one side of the bulge is the dwarf Sagittarius galaxy, which, along with the Clouds, is a satellite of the Milky Way. 2MASS has completed the observational part of the survey and is now preparing to reprocess all of the data for a Final public Release, in late 2002. The source generation was performed by M.F. Skrutskie (UMass; Principal Investigator, 2MASS), the flux maps were compiled by J.M. Carpenter (Caltech), and the color composite was assembled by R. Hurt (IPAC/Caltech). It is this composite flux map that comprises the new 2MASS logo, seen above.


Atlas Image mosaic, covering 14.8´ × 20.0´ on the sky, of the Trifid Nebula, aka Messier 20 and NGC 6514. The Trifid is only about 1.5 degrees northwest on the sky of the larger Lagoon Nebula (Messier 8) in the constellation Sagittarius, and is at a distance from us of 1.68 kpc (or 5477 light years), near the plane of our Milky Way Galaxy. It gets its name from its optical appearance, from three dark dust lanes that divide it. Like the Lagoon, much of the optical emission is dominated by the red light from hydrogen, forming an "H II region" of ionized gas around the bright small cluster of hot stars just to the southeast of the image center; the rest of the emission is reflected blue light from these hot stars, primarily from the brightest one, HD 164492A. In the near-infrared we can see through much of the obscuring dust in the Trifid, including the name-giving dust lanes, but still see much of the bluish light reflected by the dust. In the 2MASS image, much of the dark dust is still seen, but also many more stars than are seen optically. The Trifid is less than 1 million years old, and young, massive still-forming stellar objects can be seen as well. Visit the Trifid and other Messier objects in the 2MASSier Object Gallery. Image mosaic by E. Kopan (IPAC).


Atlas Image mosaic, covering 12´ × 12´ on the sky, of the reflection nebula IC 2087. The bluish nebula has a arc-like appearance, fanning off to the east from the low-mass young stellar object (YSO) Elias 18 (aka Elias 3-18 and IRAS 04369+2539), within the Taurus molecular cloud, specifically the Heiles Cloud 2. Elias (1978, ApJ, 224, 857) found that this YSO is behind about 17 visual magnitudes of extinction. Shuping et al. (2001, ApJ, 547, L161) argue, based on near-infrared spectroscopy, that, since Elias 18 is associated with two Herbig-Haro objects (HH 395A and B) to the northeast (not seen in this image) and IC 2087, the YSO is near the front of Heiles Cloud 2 and that most of this extinction is in the form of a highly-inclined circumstellar disk around the YSO. Elias 18 has Ks=6.25, J-H=2.63, H-Ks=1.78 mag. Known diffraction spike artifacts are evident around, and persistence artifacts trail (in decreasing brightness) both due north and due south of, Elias 18. The infrared-bright source to the southwest in the 2MASS mosaic is the YSO TMC 1A (IRAS 04365+2535). Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 16´ × 16´ on the sky, of the HII region Gum 29, aka RCW 49. This southern nebula of ionized hydrogen and other gases is lit up by the radiation from the hot, young stars in the compact cluster Westerlund 2, toward the center of the region. In the near-infrared we also see the nebula as reflected starlight and excited molecular hydrogen in the natal cloud from which the stars have very recently formed. Various gas filaments and dusty tendrils can be seen in Gum 29. Also visible are various holes in the nebula, the main one around the cluster and the Wolf-Rayet (W-R) star WR 20a (the bright star to the northwest of the cluster), and another prominent, yet smaller hole around the W-R star WR 20b (the bright star to the southeast). These same holes, or shells, result from the vigorous winds being blown by the stars, and can be seen also in the radio; the main hole expands to the west to form an extended blister (Whiteoak & Uchida 1997, A&A, 317, 563). W-R stars represent the evolved phase for the most massive stars in the Milky Way. Piatti, Bica, & Clariá (1997, A&AS, 127, 423), from optical photometry, find that Westerlund 2 is behind about five magnitudes of visual extinction, at a distance of 5.7 kpc (18600 light years), with an age of 2 to 3 million years, which is very young (a number of infrared-bright knots in Gum 29 indicate that star formation may still be ongoing!). The 2MASS color-color and color-magnitude diagrams are consistent with this distance and age (although the larger distance estimate of 7.9 kpc is also reasonable; Moffat, Shara, & Potter 1991, AJ, 102, 642), but with about 6.5 magnitudes of extinction (note: the accuracy of 2MASS photometry for such dense star clusters is compromised by crowding and blending effects). Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 8´ × 8´ on the sky, of the dwarf elliptical galaxy Messier 32 (M32). M32 is a satellite of the Andromeda Galaxy (Messier 31) and therefore is also a member of the Local Group of galaxies, which includes the Milky Way. The proximity of this small early-type galaxy to us has allowed astronomers to probe its nuclear regions, where there is now a preponderance of evidence for the presence of a supermassive black hole (with a mass of a few million solar masses; e.g., Joseph et al. 2001, ApJ, 550, 668), demonstrating that such black holes appear to reside even in dwarf galaxies. The mottled appearance of the galaxy in the 2MASS image is due to faint individual infrared-bright stars. The purplish spot due south of the nucleus is a known persistence artifact, created by the very bright image of the galactic nucleus. Image mosaic by S. Van Dyk and R. Hurt (IPAC).

































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