2MASS Picture of the Week Archive Captions

Atlas Image mosaic, covering 5´ × 5´ on the sky of the galaxy system IC 2554, which is comprised of three late-type galaxies, components A, B, and C. The disturbed morphology of this system implies that the galaxies are gravitationally interacting or merging with one another. The system, aka IRAS 10075-6647, is also infrared-luminous, indicating possible vigorous ongoing star formation as a result of the interaction. The system is seen at only about 9° below the Galactic plane and is at a distance of about 23 Mpc (75 million light years) from us, for a distance scale of 65 km/s/Mpc (based on the system's recession velocity from NED).


Atlas Image mosaic, covering 16´ × 16´ on the sky of a previously-unknown old open star cluster near the Galactic Plane, uncovered by J.D. Kirkpatrick during the quality assurance (QA) phase of the 2MASS data processing pipeline. The cluster is centered at about RA=22h10m17.74s, Dec=+58d47m57.7s. The view in the optical of this field, from the Digitized Sky Survey, does not obviously reveal the cluster. This is because it is obscured by the intervening dust in the Plane of the Milky Way. A preliminary and approximate analysis of the 2MASS near-infrared color-color and color-magnitude diagrams show that the cluster is old, like Messier 67, with a pronounced "red clump" (see, e.g., Grocholski & Sarajedini 2002, AJ, 123, 1603) and ascending branch of red giant stars. The position of the red clump indicates a distance of 7.2 kpc (23500 light years) and about 5 visual magnitudes of extinction. It is not straightforward to adequately constrain the age: the Girardi et al. (2000, A&AS, 141, 371) solar-metallicity isochrones are 1 Gyr (red), 2 Gyr (blue), 4 Gyr (green), and 8 Gyr (magenta). Several such uncatalogued clusters were found during QA. Special thanks to A. Cole (UMass). Image mosaic by S. Van Dyk (IPAC). Digitized Sky Survey image © 1995 by AURA, Inc., under Contract NAS5-26555 with NASA.


Atlas Image mosaic, covering 6.5´ × 6.5´ on the sky of a previously-unknown globular star cluster near the Galactic Plane, uncovered by Hurt et al. (2000, AJ, 120, 1876) in the 2MASS image data. The cluster likely formed billions of years ago, but is "new" to our knowledge of the contents of the Milky Way Galaxy. It was not known before, due to about 17 visual magnitudes of extinction from nearly 4 kpc (13000 light years) of dust in the Plane (Ivanov et al. 2000, A&A, 362, L1); see an optical image here from the Digitized Sky Survey for comparison. This is the second of two clusters discovered by Hurt et al.; the first has already been featured elsewhere in our Image Gallery; the stars in both clusters are likely to be relatively rich in heavy elements (Ivanov et al.). Image mosaic by S. Van Dyk (IPAC). Digitized Sky Survey image © 1995 by AURA, Inc., under Contract NAS5-26555 with NASA.


Atlas Image mosaic, covering 9´ × 9´ on the sky of the ionized hydrogen, or "HII", region NGC 1491, aka Sharpless 206 and RAFGL 5111. NGC 1491 is an example of a "blister" type of H II region, where, in this case, the adjacent dense molecular cloud is being excited by the hot, young O6-type star, BD +50°886 (aka Sh 2-206 IRS 1), seen toward the center of the 2MASS image (Mookerjea et al. 1999, ApJ, 522, 285). The excited molecular gas emits primarily in the 2MASS Ks band, which results in the pinkish/purplish color of the filamentary nebulosity, seen primarily to the west of the exciting star. The star and the region are at a distance of 3.3 kpc (10760 light years; Snell et al. 1990, ApJ, 352, 139) from us. Previous near-infrared mapping was done by Picmis & Mampaso (1991, MNRAS, 249, 385), but at much lower resolution: they detect the central star and the two stars, immediately northeast and northwest of BD+50°886, which they identify as early B-type stars. They also estimate that the stars are seen through about four visual magnitudes of extinction.


Atlas Image mosaic, covering 5´ × 5´ on the sky of the variable star V628 Cassiopeiae, aka MWC 1080. This star is an example of a Herbig Be star, which are the massive pre-stellar equivalents of the lower-mass T Tauri stars. V628 Cas has a strong outflow of matter in the form of a wind. Its optical spectrum is characterized by intense emission lines, indicating copious circumstellar matter, possibly in a flat, dense accretion disk (Hillenbrand et al. 1992, ApJ, 397, 613). The star appears variable, since it is an eclipsing binary system, with a nearly 3-day period, with periodic 0.16 visual magnitude variations and 0.3-mag irregular ones (Manset & Bastien 2001, AJ, 122, 3453). The obvious extended emission seen in the 2MASS image is associated with a known molecular outflow from the star. The pinkish "star" due south of V628 Cas is actually a known persistence artifact.
































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