Very Red Objects Detectable by 2MASS

The list below, which is not meant to be comprehensive, outlines some of the objects having very red J-K, J-H, or H-K colors. (For now, the focus will be on fields at high galactic latitude where, for example, contamination by M giants is negligible.) The following discussion describes each class of object and how it might be distinguished from the others on the color-color diagrams of Figures 2, 3, and 4.

 
Figure 2: Infrared color-color diagram showing the locations of various kinds of objects discussed in the text. Actual 2MASS protocamera photometry has been used for the NLTT proper motion stars, the late 2MASS object, and objects in the random 2MASS field. Photometry for the known late-type dwarfs, the PG quasars, and the IR-bright quasars and HLIRGs comes from the literature.

• Extremely red dwarf stars: Figure 2 shows the 2MASS JHK diagram for the 247 NLTT objects (discussed above, plotted as filled circles) together with known dwarfs of type M7 and cooler (filled squares) whose JHK magnitudes have been taken from the literature. Note that the stars form a diffuse cloud on this diagram, and although the ultra-cool dwarfs have redder J-H and H-K colors on average, they are not easily distinguishable from the other stars on this diagram. However, if the 2MASS photometry on these objects is merged with B and R magnitudes from the POSS-I survey, a better discrmination in temperature is possible. This is shown in Figure 3, where the later dwarfs separate out well from stars of warmer temperature.
• PG quasars: Figure 2 also shows the location of quasars taken from the Palomar-Green survey, where photometry is taken from the literature. While the J-H colors of these objects are similar to the colors of stars, their H-K colors are on average much redder. Figure 3 also shows that the quasar locus separates out well from the stellar locus, where the R magnitudes of the quasars have been taken from the APM POSS-I data. Here, the quasars have J-K colors as red as 2.5, although R-K colors are never redder than 4.0. Stars, on the other hand, can have R-K colors up to 8.0 while the J-K colors only approach 1.5.
• IR-bright quasars and hyper-luminous IR galaxies: Because the PG survey used UV excess as a selection criterion, the quasars discussed above may not be representative of the quasar population that 2MASS will detect. The kinds of very red extragalactic objects 2MASS is likely to uncover are the IR-bright quasars and hyper-luminous IR galaxies first detected by IRAS. Examples of these objects are also plotted in Figure 2 and 3, where IR photometry is again from the literature. These objects overlap the locus of PG quasars, although the most extreme cases are even further separated from the stellar locus.

 
Figure 3: Infrared/optical color-color diagram showing the same objects plotted in Figure 1. The sources for the IR photometry are the same as before, but the R photometry in all cases has been taken from the APM POSS-I scans.

Because information in multiple bandpasses is available, pseudo-colors can be invented which serve to separate objects out even more cleanly. One such pseudo-color diagram is shown in Figure 4, where the pseudo-colors have been chosen to pull the quasar locus even further from the area occupied by stars. Plots such as those shown in Figures 2, 3, and 4 can be used in concert to discriminate between various classes of objects imaged by 2MASS.