2MASS Picture of the Week Archive Captions

Atlas Image Mosaic, covering 5.0´ × 5.0´ on the sky, of the Hickson Compact Group (HCG) 79 of galaxies, also known as Seyfert's Sextet. The dominant early-type (S0) galaxy north of the image center is NGC 6027. The other galaxies, of various morphological types, are NGC 6027a (E0), b (S0), c (S0), d (Sdm), and e (Scd). The redshifts for NGC 6027 and NGC6027a-c all agree, at z=0.0145; however, the redshift for NGC6027d (the small galaxy just southeast of NGC 6027) is discordant, at z=0.067 (Sulentic & Lorre 1983, A&A, 120, 36), but this is likely due to chance projection (e.g., Mendes de Oliveira 1985, MNRAS, 273, 139). This compact group shows evidence for tidal interactions between the galaxies (Rubin, Hunter, & Ford 1991, ApJS, 76, 153). Click here to obtain a diagram of the default colors obtained by the galaxy photometry package, GALWORKS, in the 2MASS processing pipeline (shown for comparison are the default colors for galaxies in the Second Incremental Data Release, K corrections, in steps of z=0.1, and stellar tracks). GALWORKS measures accurate colors for NGC 6027 and 6027a, but in compact groups, where galaxy confusion is a problem, the package "masks" out the smaller, fainter galaxies, resulting in only estimates for their colors. When the uncompressed Atlas Images for the Second Release are soon online, users can perform more accurate photometry "by hand" on all compact group galaxies, such as those in HCG 79. Image mosaic by S. Van Dyk (IPAC).


Atlas Image Mosaic, covering 11.0´ × 11.0´ on the sky, of the HII region NGC 7538, also known as Sharpless 158. The large emission region to the northwest corresponds to what is seen optically; this region is ionized by the bright, hot stars toward the center of the nebula. The region can be seen in the 2MASS image to be quite dusty; much of the starlight is being reflected by dust, giving the filamentary main nebula a purplish-pink hue. What is unseen optically, but most visible in the 2MASS image is the embedded star formation in the molecular cloud to the southeast. NGC 7538, at a distance of 2.8 kpc (9128 light years), is the site of massive stellar objects in various early stages of development. Bloomer et al. (1998, ApJ, 506, 727) recently studied in detail the infrared-bright knot of very young O-type stars (IRS 1, 2, and 3) just southeast of the image center, behind at least 16 visual magnitudes of extinction. The very Ks-bright source in the small southeast cloud is IRS 9, discovered by Werner et al. (1979, MNRAS, 188, 463), who identified it as a massive protostar. A progression of massive star formation appears to be spreading spatially across the molecular cloud. Image mosaic by S. Van Dyk (IPAC).


Atlas Image Mosaic, covering 11.0´ × 11.0´ on the sky, of the globular star cluster NGC 6541. This cluster has been poorly studied. Alcaino et al. (1997, AJ, 114, 2638) conducted a multi-band optical CCD photometric study of NGC 6541, and concluded that its properties were quite similar to the well-studied globular clusters M13 and M79. At a distance from us of 7.4 kpc (24124 light years), NGC 6541 is 1.4 kpc below the Galactic plane (Kennedy, Bates, & Kemp 1998, A&A, 336, 315). For a 2MASS color-magnitude diagram of NGC 6541, with a 14 Gyr low-metallicity isochrone from Bertelli et al. (1994, A&AS, 106, 275), click here; confusion and blending affect the 2MASS pipeline profile-fitting photometry. Image mosaic by S. Van Dyk (IPAC).


Atlas Image Mosaic, covering 7.0´ × 7.0´ on the sky, of the nearly edge-on Seyfert 2 (NED) galaxy NGC 2683. This Sb galaxy has a small, very bright nucleus in a peanut-shaped bulge (see also Luetticke, Dettmar, & Pohlen 2000, A&AS, 145, 405; Merrifield & Kuijken 1999, A&A, 345, L47), with filamentary arms and dark lanes on one side (as described in the original "Reference Catalog of Bright Galaxies," de Vaucouleurs & de Vaucouleurs 1964). Barbon & Capaccioli (1975, A&A, 42, 221) found the dusty spiral arms of this galaxy to be trailing. Harris et al. (1985, AJ, 90, 2495) found NGC 2683 to have a population of globular clusters at least as numerous as that of our Galaxy. Image mosaic by S. Van Dyk (IPAC).


Atlas Image Mosaic, covering 27.0´ × 41.0´ on the sky, of the H II region W40. This region is part of a larger molecular cloud complex at a distance of 600-700 pc (1960-2300 light years). A bright cluster of stars is evident in the near-infrared 2MASS image which is not seen in the optical. The previous near-IR imaging of W40 revealed only 7 infrared-bright sources, all but one of which was associated with an optical star (Smith et al. 1985, ApJ, 291, 571). In the 2MASS image, scores of stars are seen in the cluster, among the dust and nebulosity of the molecular cloud in which the stars are embedded. To see a 2MASS color-color diagram of the star cluster, click here; the tracks are for dwarf and giant stars from Bessell and Brett (1988, PASP, 100, 1134). The cluster of young stars is behind at least 15 magnitudes of visual extinction. The full 2MASS mosaic shows the cluster's overall environment. (N.B.: Image is 3.8 Mb!) Image mosaic by S. Van Dyk (IPAC).































Return to the 2MASS Home Page