2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 7´ × 7´ on the sky of NGC 6440. This globular star cluster is near the Galactic center, at a distance of 8.5 kpc (27710 light years) and reddened by about three visual magnitudes (Ortolani et al. 1994, A&AS 108, 653). NGC 6440 is also interesting because it has long shown to contain bright X-ray sources; a bright transient source was detected by the BeppoSAX satellite in 1998 August. This source has been studied by Verbunt et al. (2000, A&A, 359, 960). The X-ray point sources are similar in property to low-mass X-ray binaries seen in the Galactic plane. These interacting binary systems likely contain ultracompact primary stars, possibly black holes (Sidoli et al. 2001, A&A, 368, 451). Image mosaic by S. Van Dyk (IPAC).


Atlas Image mosaic, covering 5´ × 5´ on the sky of NGC 346. This is the brightest open stellar cluster and HII (or ionized hydrogen) region in the Small Magellanic Cloud (SMC), one of the dwarf satellite galaxies to our own Milky Way Galaxy. The SMC is at a distance from us of ~63 kpc (205,400 light years). In the optical this cluster is highlighted by many hot, young, massive stars (Massey, Parker, & Garmany 1989, AJ, 98, 1305). In the near-infrared the most prominent stars are a much older, underlying population of red giant stars. Also revealed are several dust-obscured stars. These populations can be seen in the 2MASS color-color and color-magnitude diagrams. On the latter diagram in green are shown isochrones (for the appropriate low metallicity, or heavy element abundance, of the SMC) for 4 Myr, 15 Myr, and 10 Gyr. The bluer, hot, young population is faintly detected by 2MASS, but a large ensemble of red giants with ages of about 10 billion years (not a rigorous fit) is prevalent. (The three brightest stars seen in the diagrams are likely Galactic foreground stars.) The bright red star to the southeast in the 2MASS image has a Ks magnitude of 12.15 and color J-Ks=2.95, and is likely a dust-obscured asymptotic giant branch star. Reflected starlight and likely molecular hydrogen emission from the nebula can also be seen in the 2MASS image. Image mosaic by S. Van Dyk (IPAC).


Atlas Image, covering 8´ × 8´ on the sky of SS 433 and its environs. The 433rd entry in the Stephenson-Sanduleak (1977, ApJS, 33, 459) catalog of H emission stars in the Milky Way, aka V1343 Aquilae, is one of the most peculiar and possibly unique objects in the Galaxy (it is seen as the relatively bright star at the center of the 2MASS image). The strong emission lines seen in the spectrum of SS 433 appear as two sets which alternately redshift and blueshift in a smooth and regular fashion. The line sets, when redshifted, reach an impressive maximum of 50,000 km s-1, while the sets, when blueshifted, reach a maximum of 30,000 km s-1. These velocities both are a significant fraction of the speed of light! The basic model hypothesizes that the emission arises from two opposing collimated jets of matter, which rotate once every 164 days. The jets emanate from an accretion disk around a neutron star in an interacting binary system, possibly with a B-type companion star. That the compact object is likely a neutron star is consistent with the likely association of SS 433 with the supernova remnant W50; the jets interact with the nebula (Dubner et al. 1998, AJ, 116, 1842). SS 433, on the date that the routine 2MASS southern operations observed it (1999 Aug 11 UT), had magnitudes J=9.40, H=8.75, Ks=8.18. Kodaira, Nakada, & Backman (1985, ApJ, 296, 232) found the near-infrared emission to be, not unexpectedly, variable and likely mostly arising from the disk. The 2MASS color-color and color-magnitude diagrams (SS 433 is the red star on the latter diagram) indicate that the recent radio distance (Dubner et al.) of ~3.0 kpc (9780 light years) and a visual extinction of 8 to 9 magnitudes are consistent with the brightnesses and colors of many of the stars in SS 433's environment.


Atlas Image, covering 5.5´ × 5.5´ on the sky of NGC 5253. This is a dwarf galaxy of Hubble type S0/I0 and is a member of the Centaurus group of galaxies, along with Messier 83 and Centaurus A (NGC 5128). Although the extended light of the galaxy is essentially characterized by a dwarf spheroidal (or S0), NGC 5253 is highlighted by many recently-formed star clusters within the central 100 pc (Calzetti et al. 1997, AJ, 114, 1834). These clusters contain a large number of massive stars, including Wolf-Rayet stars; the integrated light of these very massive, evolved stars leads to the designation for NGC 5253 as a "Wolf-Rayet galaxy" (Conti 1991, ApJ, 377, 115). In the 2MASS near-infrared image the galaxy looks significantly different from optical images. The infrared light, from near- to far-, is dominated by the emission from a "supernebula" (seen in the 2MASS image as the brightest knot just northeast of the galaxy center). This object is also a bright thermal radio source (Turner, Beck, & Ho 2000, ApJ, 532, L109). Gorjian, Turner, & Beck (2001, ApJ, 554, L29) recently studied the supernebula in the mid-infrared with the Keck 10-m telescope, and determined that it is excited by 105-106 stars, with the size and mass of a typical globular cluster, such as 47 Tucanae in the Milky Way. However, this cluster is very dust-obscured and likely very young, possibly the youngest known globular cluster in any galaxy.


Atlas Image mosaic, covering 11´ × 11´ on the sky of W43, which is one of the giant HII, or ionized hydrogen, regions in the Milky Way Galaxy. The gas is ionized by a cluster of dozens of hot, young, massive stars, similar to the conditions in NGC 3603. However, unlike NGC 3603, W43 is completely obscured visually. 2MASS reveals the nebula and stars in this image. 2MASS color-color and color-magnitude diagrams, generated for an area with radius 2.5´ centered on W43, show considerable variable reddening in the field. The distance to W43 derived from the radio is ~7 kpc (22820 light years). Foreground stars appear appreciably less reddened, with the cluster population having J-Ks~2.5 and redward. This corresponds to visual extinction to the cluster stars of at least 15 magnitudes. Blum, Damineli, & Conti (1999, AJ, 117, 1392) studied the central region of W43 in the near-infrared and find that the central dense clustering contains several very massive stars behind ~34 visual magnitudes of extinction. These are the red stars between Ks~12 and 9 mag. In the 2MASS image a number of objects can be seen at primarily Ks, with weak H and J detections, which may be young stars embedded in their natal cloud on the outskirts of W43. A scenario where the energy input from the central cluster leads to the propagation of star formation elsewhere in the molecular cloud is possible. The bright star toward the southwest is the late-type pulsating variable giant star AB Aql (it lies right at the edges of two 2MASS tiles and is not associated with W43). 2MASS is ideal for revealing recent star formation throughout the Galaxy. Image mosaic by S. Van Dyk (IPAC).

































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