2MASS Picture of the Week Archive Captions

The galactic HII regions Sharpless 254 through 258, covering 21.5´ × 14.5´ on the sky. The molecular cloud associated with Sharpless 254 through 258 is an active, ongoing star formation location. This cloud is part of the larger Gem OB1 complex. These regions of ionized gas are at a distance of 2.5 kpc (8150 light years) from us and are located near the Galactic anticenter. These regions were previously imaged at K-band by Hodapp (1994, ApJS, 94, 615). A complex of nebulosity is seen in the 2MASS image. The bright blue, hot ionizing stars in the HII regions are surrounded by bluish reflection nebulae (dust scattering the stars' blue light). Also seen throughout the field and, particularly, in the central cluster of Sharpless 255 are a large number of infrared bright, reddish, presumably high-mass young stars and stellar objects, still embedded in the cloud. A number of OH and H20 masers are also associated with these regions, indicating recent and ongoing star formation. These data are part of the Spring 1999 data release. Image mosaic by S. Van Dyk (IPAC).


The star OH 345.0+15.7 (AFGL source GL 1822; IRAS 16029-3041) is an OH (hydroxyl) line emitter and has no optical counterpart. It is likely a late M-type star with a very dense dusty circumstellar envelope at a distance of about 6 kpc (19600 light years) from us. It is losing its mass at a rate of 1.4 × 10-4 solar masses per year, implying that it is in a superwind phase at the tip of the asymptotic giant branch (Persi et al. 1990, A&A, 237, 153). The wind is moving out from the star at about 13 km/sec. Persi et al. in 1985 June found a K magnitude of 6.82 for the star (the 2MASS Ks magnitude is 6.87 ± 0.02; the 2MASS color is J-Ks=8.72). The OH/IR star is likely nearing the end of its life. The red "stars" trailing to the south in decreasing brightness are ghost artifacts of the bright OH/IR star.


The relatively nearby (redshift z=0.016), very rich galaxy cluster Abell 3627. The image covers 21.7´ × 41.7´ on the sky. The cluster, which lies at the core of the Great Attractor, is at galactic longitude l=325°, but, more importantly, is at the low galactic latitude b=-7.2°, i.e., near the Galactic Plane, where extinction from Galactic dust is significant. The cluster has an internal velocity dispersion which implies a gravitational mass comparable to that of the Coma cluster, another rich nearby cluster. But, most intriguingly, Abell 3627's distance and direction puts it near the predicted location of the center of the Great Attractor, implying that the cluster may sit at the bottom of the Attractor's gravitational potential well. The Great Attractor, so dubbed, is a position in the sky toward which a bulk flow of galaxies appears to be moving (Lynden-Bell et al. 1988, ApJ, 326, 19). In effect, the Attractor represents the center of a large mass concentration of galaxies and galaxy clusters, of which our own Local Group is included, known as the Local Supercluster. 2MASS is less susceptible to the extinction toward Abell 3627 and the Great Attractor than observations at optical wavelengths, and therefore will provide valuable clues as to its true nature. Image mosaic by S. Van Dyk (IPAC).
(The full JPG image above is 2.5 Mbytes. For a smaller version [449 kbytes], click


The barred Seyfert galaxy NGC 1097. This three-color JHKs composite image mosaic of NGC 1097 shows the prominent bar in this LINER (now Seyfert 1) galaxy. The presence of the bar may perhaps be due to tidal interaction with the fainter small elliptical companion galaxy, NGC 1097A, also seen in the image. In the inset one can see the very bright well-known circumnuclear ring of gas, dust, and stars, where intense star formation is occurring around the active nucleus of NGC 1097. The ring, both in the optical and in the near-infrared, resolves into two-armed spirals, which appear to be common in barred galaxies and is in accord with simulations of bar-driven inflow of gas (Barth et al. 1995, AJ, 110, 1009). Extremely compact young star clusters were found by Barth et al. along the ring in Hubble Space Telescope imaging. One of the important questions is the connection between circumnuclear rings and the active nucleus. Are the gas and stars in the ring feeding the putative supermassive black hole at the nucleus? The broad optical emission line profiles seen in 1996 by Storchi-Bergmann et al. (1997, ApJ, 489, 87) from the galaxy's nucleus is consistent with attributing their origin to an accretion disk that has formed abruptly from the tidal disruption of a star by the black hole.
Image mosaic by S. Van Dyk (IPAC).


The globular cluster 47 Tucanae. This mosaic of one of the brightest Milky Way globular star clusters, 47 Tuc (NGC 104), covers 19.2´ × 23.3´ on the sky. This cluster, seen near the Small Magellanic Cloud in the sky, and at a distance of 4.6 kpc (15000 light years) from us and 7.3 kpc (23800 light years) from the Galactic Center, likely contains about 1 million stars. Its optical half-light radius is 2.79´, or only 3.7 pc; the stars in globular clusters, such as 47 Tuc, are clearly densely packed. This cluster is known to be typically metal-rich, relative to many other globular clusters. Globular clusters formed early in the Galaxy's history and, therefore, must have been chemically enriched by massive short-lived stars. The luminosity functions for globular clusters vary, with metal-rich clusters having flatter function slopes than metal-poor clusters; recent indications are that a cluster's interaction with the Galactic disk could strip lower-mass stars from the cluster, leading to a relative overabundance of higher-mass stars, which would lead to greater enrichment of elements, such as oxygen. In the near-IR, globular cluster stars look very homogeneous, as can be seen in this image, with very little in the way of color or population gradients, particularly in the central regions (see also Montegriffo et al. 1995, MNRAS, 276, 739). The near-IR light is dominated by the old red giants and asymptotic giant branch stars in the cluster.
Image mosaic by E. Kopan (IPAC).
































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