2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 11.0´ × 11.0´ on the sky, of RCW 103, a young supernova remnant (SNR) in the Milky Way, less than 1° from the Plane of the Galaxy. As a result, the interstellar extinction along the line of sight to the SNR is about 4.5 visual magnitudes (Oliva, Moorwood, & Danziger 1990, A&A, 240, 453). In the near-infrared 2MASS image, one sees filamentary emission of a bluish-green color, forming what appears to be an incomplete shell, with some fainter emission interior to this partial shell, surrounded by reddish emission, particularly in the southeast. The red color is from molecular hydrogen (H2) line emission at 2.12 µm. The blue-green color is [FeII] (forbidden singly-ionized iron) line emission at 1.64 µm. Oliva et al. (1999, A&A, 341, L75) find that the morphology of the [Fe II] emission identically traces out the optical line emission, and that the H2 emission arises from a region outside the SNR, as seen at optical, radio, and X-ray wavelengths. The H2 emission is from molecular gas around the SNR which has not yet been reached by the fast-moving (~1200 km/s) shock wave, but is most likely a dense molecular cloud heated by the X-rays being emitted by the shock. The remnant likely has an age of only about 1000 years, arising from the explosion of a massive star. The nature of the progenitor star can be inferred from the presence of a 69-millisecond-period radio and X-ray pulsar near RCW 103 (Kaspi et al. 1998, ApJ, 503, L161; Torii et al. 1998, ApJ, 496, L207); pulsars are rapidly rotating neutron stars, and neutron stars are the remnant collapsed cores of stars more massive than about 8 times the mass of our Sun. The picture that emerges is of a massive star not unexpectedly ending its life explosively within its natal gas cloud. (See also Burton & Spyromilio, PASA, 10, 327.) Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 7.0´ × 7.0´ on the sky, of Tonantzintla 2 (Pismis 26), which is one of the least-known Galactic globular clusters. At l=350.8° and b=-3.4°, it is quite close to the Galactic Center. From V and I photometry using the ESO 3.6-m NTT, Bica, Ortolani, & Barbuy (1996, A&AS, 120, 153) found that the positions of red giant branch and horizontal branch stars on the optical color-magnitude diagram indicate that the chemical composition of the cluster stars are similar to those in 47 Tucanae. They also found a reddening to the cluster which implies an extinction of AV~4 magnitudes, and a distance from us of 6.4 kpc (20,900 light years). Bica et al. conclude that Ton 2 belongs to the bulge population of the Milky Way, but is only moderately "metal-rich". Image mosaic by S. Van Dyk. Look also at preliminary 2MASS near-infrared color-color and color-magnitude diagrams. (These include all stars in the Atlas Image Mosaic detected by the 2MASS pipeline processing; crowding affects and limits the detections and photometry of stars in the densest regions of the cluster.)


Atlas Image mosaic, covering 1° × 1° on the sky, of Messier 17 (M17), aka the Omega Nebula, the Swan Nebula, the Horseshoe Nebula, and the Lobster Nebula, located in the constellation Sagittarius. It is roughly 5,000 light years from the Sun. The 2MASS image penetrates the obscuring veil of dust and reveal the otherwise invisible contents of a dark molecular cloud. The giant molecular cloud associated with M17 is one of the most luminous and active star-forming complexes in the Milky Way, and contains one of the youngest and richest stellar clusters in our galaxy. Although the gas in the molecular cloud is too cold to emit visible or near-infrared light, traces of the densest portions of the cloud can still be seen as dark, obscuring, cloudy structures in the 2MASS mosaic, by E. Kopan (IPAC). (N.B.: The full JPG image is 12.7 Mb in size! A smaller version [1.4 Mb] can be obtained here.)


Atlas Image mosaic, covering 6.0´ × 6.0´ on the sky, of the planetary nebula NGC 6781. Planetary nebulae (PNe) are formed as low-mass stars, like the Sun, reach the end of their lives and lose their outer envelopes to the interstellar medium. NGC 6781 has a very similar "ring-like" morphology to the famous PN, the Ring Nebula (M57). The red glow of NGC 6781's ring in the near-infrared is due to 2.12 µm emission from molecular hydrogen (H2), which is strong in the 2MASS Ks band. Although the main ring is quite bright, the fainter H2 filaments within and halo emission outside of the bright ring, as studied by Kastner et al. (1994, ApJ, 421, 600), can just barely be seen in the 2MASS image. Kastner et al. postulate that if all PNe showing axial symmetry, and therefore, possessing bipolar structure, are H2-bright, as first found by Zuckerman & Gatley (1988, ApJ, 324, 501), then ring-like PNe, like NGC 6781, with strong shocked H2 emission may be bipolar nebulae viewed with an inclined polar axis with respect to the plane of the sky. The bright ring is the equatorial torus, from a high-density slow wind from the evolving star, whereas the fainter halo structures are the polar lobes and the remnants of a faster, low-density wind. For NGC 6781 the lobe geometry may be a bipolar cylinder with half the radius of and within the bright torus. Near-infrared observations, such as those by 2MASS, provide very important information about PNe and the evolution of stars like our Sun.


Atlas Image mosaic, covering 10.0´ × 10.0´ on the sky, of the barred spiral galaxy Messier 83 (NGC 5236). This bright southern galaxy is seen nearly face-on and is one of the nearest large barred galaxies to the Milky Way. The bulge region of this galaxy is experiencing an intense starburst; the structure of this burst of circumnuclear star formation at high spatial resolution in the near-infrared, and other wavelengths, is complex (Gallais et al. 1992, A&A, 243, 309). A prominent dust lane emanates in both directions along the bar and out along the spiral arms. Although the overall appearance of the galaxy is quite smooth in the near-infrared (e.g, Adamson, Adams, & Warwick 1987, MNRAS, 224, 367), compared to its appearance in the optical, a large number of bright knots are seen along the arms, corresponding to clusters of red (supergiant) stars in and near the many OB stellar associations and H II regions. Clearly, star formation is happening today not only in the nucleus, but all over M83. This galaxy has been host to six historical supernovae, four of which are thought to arise from young, massive stellar progenitors, both near the nucleus and in the spiral arms, further evidence for the vigorous galactic star formation. Image mosaic by S. Van Dyk (IPAC).
































Return to the 2MASS Home Page