2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 13´ × 19´ on the sky of NGC 3199. This nebula, in the constellation Carina, is the wind-blown partial "ring" around the Wolf-Rayet (W-R) star WR 18 (aka HD 89358), the easternmost (leftmost) of the three bright blue stars near the center of the 2MASS image. NGC 3199 and WR 18 are at a distance of about 3.6 kpc (11,736 light years) from us. W-R stars represent the final evolutionary stages of very massive stars (with ~30 solar masses or greater). The nebula shows an asymmetric appearance, i.e., only one side (the western one) of the shell is bright, both in the optical and the near-infrared. The fainter, eastern side is there, but is much fainter. Some W-R ring nebulae can be seen in 2MASS images, such as the more complete ring around M1-67. But, NGC 3199 is particularly bright in the 2MASS data. Dyson & Ghanbari (1989, A&A, 226, 270) provided an explanation for the ring's appearance through a model where a moving WR 18 is blowing a strong stellar wind into a surrounding uniform interstellar medium. Vigorous mass loss of 10-5 to 10-4 solar masses per year is characteristic of W-R stars, as the star approaches the end of its short life, although not all are surrounded by ring nebulae. Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 7´ × 7´ on the sky of the interacting galaxies NGC 4038 and NGC 4039, better known as the Antennae, or Ring Tail galaxies. The two galaxies are engaged in a tug-of-war as they collide. The mutual gravitation between them is working to distort each spiral galaxy's appearance as the two merge. The interaction is evidently impetus for an intense burst of new star formation, as can be seen from the many infrared-bright knots and bright galactic nuclei. Compare the 2MASS view of this system with that obtained by the Hubble Space Telescope in the optical. Many of the same features are seen, although 2MASS is able to peer through much of the dust seen in the galaxies' disks. The galaxy light looks smoother. Also, in the near-infrared the bright knots of star formation are likely highlighted by the light of massive red supergiant stars. The much more extended "tidal tails," which give the Antennae their name, are quite faint in the 2MASS image mosaic.


Atlas Image mosaic, covering 6´ × 6´ on the sky, of NGC 3201, a low galactic latitude globular star cluster. Although the 2MASS imaging and pipeline photometry is not optimized for such crowded regions as globular clusters, because NGC 3201 has a relatively lower concentration of stars, the near-infrared photometry resulting from the pipeline processing reasonably represents the stellar populations in the cluster, as seen in the resulting color-color and color-magnitude diagrams. In the latter diagram we clearly see the red giant, asymptotic giant, and horizontal branches for the cluster. Following the recent analysis by von Braun & Mateo (2001, AJ, 121, 1522) we show on the color-magnitude diagram an overlay of a low-metallicity isochrone, reddened by E(V-I)=0.24 at a distance of ~4.5 kpc (14670 light years), with an age of ~14 Gyr (billion years). Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 5´ × 5´ on the sky, of NGC 1501. This little-studied object is a planetary nebula, at roughly a distance of 1.3 kpc (4238 light years) from us. The pinkish-purplish nebula surrounds a very hot central star, which has a temperature of nearly 90,000 K. The morphology of the nebula appears to be affected by the vigorous wind from the star (Sabbadin et al. 2000, A&A, 361, 1112). Planetary nebulae are formed as stars, like our Sun, reach the end of their lives and lose their outer envelopes to the interstellar medium. The hot central star, originally the core of the dying star, which will eventually become a white dwarf and cool off over billions of years.


Atlas Image mosaic, covering 34´ × 34´ on the sky, of the Coma cluster, aka Abell 1656. This is a particularly rich cluster of individual galaxies (over 1000 members), most prominently the two giant ellipticals, NGC 4874 (right) and NGC 4889 (left). The remaining members are mostly smaller ellipticals, but spiral galaxies are also evident in the 2MASS image. The cluster is seen toward the constellation Coma Berenices, but is actually at a distance of about 100 Mpc (330 million light years, or a redshift of 0.023) from us. At this distance, the cluster is in what is known as the "Hubble flow," or the overall expansion of the Universe. As such, astronomers can measure the Hubble Constant, or the universal expansion rate, based on the distance to this cluster. Large, rich clusters, such as Coma, allow astronomers to measure the "missing mass," i.e., the matter in the cluster that we cannot see, since it gravitationally influences the motions of the member galaxies within the cluster. The near-infrared maps the overall luminous mass content of the member galaxies, since the light at these wavelengths is dominated by the more numerous older stellar populations. Galaxies, as seen by 2MASS, look fairly smooth and homogeneous, as can be seen from the Hubble "tuning fork" diagram of near-infrared galaxy morphology. Image mosaic by S. Van Dyk (IPAC).

































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