The Two Micron All Sky Survey at IPAC

The Crab Nebula, or Messier 1, is one of the most spectacular and intensively studied objects in the sky. It is the remnant of a supernova in AD 1054, observed as a "guest star" by the Chinese in today's constellation Taurus. It is among the brightest remnants across a broad wavelength spectrum. The Crab Nebula is probably the best-known synchrotron emission nebula. The synchrotron is what is primarily seen in the 2MASS image. In addition,the central power-law source, the Crab pulsar, is photo-exciting line emission. The gas in the nebula has not yet mixed with the interstellar medium, and so study of the line-emitting gas gives us insight on the progenitor star. The blue-green [Fe II] 1.644 µm emission is excited in the optically thick filaments in the Crab Nebula by the power-law photoionization source within; H₂ seen as K-band emission in the filaments could not normally survive in these nebular conditions and must have formed early in the remnant's expansion when densities were higher (Graham, Wright, & Longmore 1990, ApJ, 352, 172). (Field size 8.2´ × 10.2´. Image size 298 kb.)


Atlas Image Mosaic of the intermediate-age Galactic supernova remnant IC 443, which is at a distance of about 1.5 kpc. What can be seen over this large area are two regions of near-infrared emission from the remnant. The bright bluish arc to the northeast appears to be line emission from excited iron in these remnant filaments, bright in the J band (Rho et al., in preparation). Along the south, from east to west, is the interaction of the remnant with the nearby molecular cloud. IC 443 offers a unique laboratory for studying such interactions. The supernova shock is exciting 2.12-micron H_2 molecular line emission, very bright in the K_s band (Richter, Graham, & Wright 1995, ApJ, 454, 277). Some H_2 emission is also seen weakly to the north. The emission from the partially dissociative "J-type" shock-cloud interaction is complex and clumpy. The overall structure of the remnant, as seen by 2MASS, follows both the radio and X-ray emission contours, showing that the radiation in all these wavelength regimes seems to arise from the same regions in the remnant. Analysis of this large-area 2MASS mosaic will provide important insight into the interaction of supernova remnants with their immediate environment, and therefore into the probable nature of the supernova remnant's stellar progenitor. These data are part of the 2MASS Spring 1999 Incremental data release. Image mosaic by E. Kopan (IPAC). (Field size 0.83° × 0.97°. Image size 3.4 Mb!)


Atlas Image mosaic of the source Cassiopeia A. Cas A, for short, has a faint, ghostly shell-like appearance in the 2MASS image, with several brighter filaments. Cas A was first discovered in the late '40s by radio astronomers as the brightest radio object in the sky at 1 GHz. It is now understood to be possibly the youngest supernova remnant in the Milky Way Galaxy! No clear records of the supernova's appearance in the optical sky exist historically, as is the case for the Crab Nebula (the Chinese 'guest star' of 1054 AD), but it has been linked to a 6th magnitude star, no longer visible near the constellation Cassiopeia, in a catalogue by Flamsteed from 1680 AD. The inference is that Cas A may have been a subluminous supernova, resulting from the explosion of a Wolf-Rayet star; this picture is motivated by the chemical composition of the various fast and slow moving knots in the remnant, which, along with analyses of the radio and X-ray emission, are consistent with the explosion of a very massive star with very little hydrogen in its envelope. Cas A is at a distance of ~3.4 kpc (11,000 ly); at only ~2° from the Galactic Plane, it is seen behind ~5-6 mag of visual extinction (Hurford & Fesen 1996, ApJ, 469, 246). Most of the energy from Cas A is, in fact, radiated in the infrared. The mid-infrared emission, as seen by the recent ISO satellite, is mostly thermal dust emission, with some line emission, and is associated primarily with the fast-moving knots, suggesting that dust is condensing out in the expanding supernova ejecta (Lagage et al. 1996, A&A, 315, L273). Image mosaic by S. Van Dyk (IPAC). (Field size 6.0´ × 6.0´. Image size 192 kb.)


Atlas Image mosaic of RCW 103, a young supernova remnant (SNR) in the Milky Way, less than 1° from the Plane of the Galaxy. As a result, the interstellar extinction along the line of sight to the SNR is about 4.5 visual magnitudes (Oliva, Moorwood, & Danziger 1990, A&A, 240, 453). In the near-infrared 2MASS image, one sees filamentary emission of a bluish-green color, forming what appears to be an incomplete shell, with some fainter emission interior to this partial shell, surrounded by reddish emission, particularly in the southeast. The red color is from molecular hydrogen (H₂) line emission at 2.12 µm. The blue-green color is [FeII] (forbidden singly-ionized iron) line emission at 1.64 µm. Oliva et al. (1999, A&A, 341, L75) find that the morphology of the [Fe II] emission identically traces out the optical line emission, and that the H₂ emission arises from a region outside the SNR, as seen at optical, radio, and X-ray wavelengths. The H₂ emission is from molecular gas around the SNR which has not yet been reached by the fast-moving (~1200 km/s) shock wave, but is most likely a dense molecular cloud heated by the X-rays being emitted by the shock. The remnant likely has an age of only about 1000 years, arising from the explosion of a massive star. The nature of the progenitor star can be inferred from the presence of a 69-millisecond-period radio and X-ray pulsar near RCW 103 (Kaspi et al. 1998, ApJ, 503, L161; Torii et al. 1998, ApJ, 496, L207); pulsars are rapidly rotating neutron stars, and neutron stars are the remnant collapsed cores of stars more massive than about 8 times the mass of our Sun. The picture that emerges is of a massive star not unexpectedly ending its life explosively within its natal gas cloud. Image mosaic by S. Van Dyk (IPAC). (Field size 11.0´ × 11.0´. Image size 499 kb.)


Atlas Image of the supernova remnant (SNR) N49 in the Large Magellanic Cloud (LMC). This appears to be the first near-infrared image of this SNR. The optical, X-ray, near-infrared and radio morphology for N49 all are in general agreement, with an incomplete shell covering nearly 1´ in diameter. Emission due to the hydrogen Paschen beta line and lines of forbidden singly-ionized iron ([Fe II]) in the J and H bands likely dominate what we see. Strong K_s-band emission, as in the case of the SNR IC 443 in our Galaxy, due to molecular hydrogen (H₂) line emission excited by shock-molecular cloud interaction, is surprising lacking, given the apparent interaction with an extended dense cloud to the southeast, as deduced by the optical (Vancura et al. 1992, ApJ, 394, 158) and X-ray (Hughes et al. 1998, ApJ, 505, 732) observations. N49 is also intriguing, with its possible association with the soft gamma-ray repeater SGR 0525-66 near the SNR's northern edge, however, this association has not yet been firmly established. SGRs are linked to isolated neutron stars, so if the association holds, this is further possible evidence of a massive stellar progenitor for this SNR. These data are included in the Second Incremental Release! (Field size 5.0´ × 5.0´. Image size 144 kb.)


Atlas Image of the spiral galaxy NGC 3877, taken as part of routine operations on 1998 Dec 24 UT at the 2MASS Northern Facility on Mt. Hopkins, AZ. What is most interesting about this image is that the supernova SN 1998S was caught in the act in the near-infrared, about 49´´ southwest of the galaxy's nucleus, along the spiral arm! SN 1998S (Li, Filippenko, & Moran 1998, IAUC 6829) is of type II-"narrow" (Schlegel 1990, MNRAS, 244, 269). Supernovae of this subtype tend to show broad emission lines of hydrogen (and other elements) in their optical spectra (thus, the Type II classification), due to the rapid expansion of the hydrogen-rich supernova ejecta. But, atop the broad lines are narrow lines, likely due to emission from the interaction region of the SN shock with slower-moving very dense circumstellar matter lost by the progenitor star in the late stages of its evolution prior to explosion. SN 1998S emits strongly in the near-infrared, primarily due to the broad hydrogen recombination lines, but also due to the first and second overtone bands of CO (the carbon monoxide molecule) in the H and K_s bands (Gerardy et al. 1998, BAAS, 30, 1324). Gerardy et al. find from their near-IR spectra a CO overabundance of ~2 and a temperature in the CO-forming region of 4000 to 4500 K. Their spectra may indicate that molecule formation is common in Type II supernovae; this is important, since molecular emission can be a strong coolant and a first step toward dust formation. The SN had K_s magnitude 13.16, indicating a possible brightening in this and the other near-IR bands since observations by others on 1998 Nov 10 UT (Garnavich et al. 1998, IAUC 7058), which already indicated a large infrared excess had developed then. (Field size 5.8´ × 5.8´. Image size 167 kb.)


Atlas Image of the environment of Supernova 1987A in the Large Magellanic Cloud (LMC). SN 1987A was the closest supernova, or exploding star, to us, at ~50 kpc (163,000 light years), since Kepler's in 1604 (distance < 20,000 light years), which occurred in our own Milky Way Galaxy. SN 1987A is not actually visible in the 2MASS image, since it is quite faint and also difficult to resolve from its two blue neigboring stars, but its position is indicated by the top of the white line on the image. At very high resolution in the optical, the Hubble Space Telescope (HST) first revealed the amazing ring structure around the supernova. From the 2MASS color-color and color-magnitude diagrams, we see that the environment of SN 1987A consists of several different populations of stars. The colored lines on the color-magnitude diagram roughly trace out the LMC populations: green, the many, old (~14 Gyr) red giants; blue, the top of the hot main sequence; and red, the young (~20 Myr), primarily red, supergiant stars. The latter group are predominately from the large stellar association, Lucke-Hodge 90, near the northeast corner of the image. Noticeably lacking are older and dust-obscured asymptotic giant branch stars. The group of stars at J-K_s~0.4 are stars in the Galactic foreground. SN 1987A is beginning to interact with the rings, as seen in recent HST images, and continues to be a wondrous object. (Field size 8´ × 8´. Image size 291 kb.)

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