The Two Micron All Sky Survey: 2MASS

2MASS is a NASA funded ground-based imaging survey of the entire sky at the three near infrared wavebands of J (1.25 µm ), H (1.65 µm ) and K_s (2.16 µm ) to be made by two dedicated 1.3 m telescopes in the northern and southern hemispheres. The project will map the entire sky to K_s = 14.5^m (point source, 5 sigma detection), 12^m fainter than the limit of the previous full-sky 2 micron survey (Neugebauer and Leighton 1969), and is to be completed by the year 2000. The 2MASS project will produce and initiate a very wide range of exciting science; among its major goals are:

• Mapping luminous red giant stars throughout the galactic plane and measuring the structure of the Galaxy.

• A census of galaxies in the nearby Universe. The spectral energy distribution of almost all galaxies, whatever their morphological type, peaks in the near-infrared because of the high luminosity of the cool evolved stellar population and because the internal extinction is lower at infrared than at optical wavelengths. Further, the much lower Galactic extinction at infrared wavelengths means that galaxies can be found to much lower Galactic latitudes than in the optical. These factors will make the 2MASS galaxy survey by far the most complete full-sky survey of galaxies to date.

• Searches for exotic objects, especially brown dwarfs, high redshift quasars and dust enshrouded quasars, as well as objects of previously unknown types.

In addition, the 2MASS catalogues and images provide major support for future space missions, particularly for SIRTF. Since the data are of very widespread general interest, 2MASS, like the SDSS, will release its data in a timely manner and with no proprietary period.

2MASS will observe with three arrays with a pixel size of 2" at K_s . The positional accuracy is reproducible to ~ 0.2" for strong sources. The limiting magnitudes (for a 5 sigma detection limit for a point source at high galactic latitude) of the 2MASS and SDSS projects are compared in Table 2.1.

The scientific and technical payoff which will result from a combination of the 2MASS and SDSS projects is likely to be immense. Some examples of the scientific payoffs are:

• A definitive search for very cool stars and for brown dwarfs, and a measurement of the luminosity function at the bottom of the main sequence and below. We expand on this below.
• Wide wavelength coverage for galaxies and stars, to enable studies of the stellar populations in several hundred thousand galaxies of all spectral types.
• Redshifts and spectra for all but the very reddest galaxies found by 2MASS within the SDSS footprint.
• Spectra and redshifts for essentially all but the very reddest active galaxies and quasars detected by 2MASS within the SDSS footprint.
• Identification of very red quasars. Two types of quasar with very red colors are those at high redshift (z>5) or those with large amounts of local or intervening extinction (see section 3.3).
• Greatly enhanced possibilities for serendipitous discovery

• help 2MASS with image analysis, particularly in the areas of star/galaxy separation and galaxy morphological separation.
• provide more accurate positions, and (in the South) a large amount of proper motion data, for 2MASS stars and galaxies.
• provide spectroscopic data for a very large number of the high latitude 2MASS objects, which will allow their nature to be determined.

The Lower Main Sequence and Brown Dwarfs

Chapter 3.6 will discuss the wealth of stellar astronomy which the SDSS will make possible. In this section we discuss an example of how the combination of SDSS and 2MASS will enable a whole range of interesting work on low luminosity stars, in particular stars close to, and below, the hydrogen burning limit. This fascinating subject is a perennial `hot topic' for many reasons: (1) identifying the low mass cutoff for a true hydrogen burning main sequence star is an important test of our understanding of stellar structure and evolution; (2) deuterium-burning stars do not last long and are rare, as well as very faint; (3) self-gravitating bodies of substellar mass are prime candidates for dark matter; and (4) the luminosity function at the bottom of the main sequence and below contains vital information about star and planet formation.

As discussed in Chapter 3.6, the lowest-luminosity main sequence stars have M_i' ~ 15^m (Schneider et al. 1991; Bessel et al. 1991; Leggett 1992; Tinney et al. 1993; Tinney 1993; Kirkpatrick et al. 1993) and can be detected by the SDSS to distances of ~ 200 pc, roughly one scale height of the galactic disk. The high galactic latitude data will then enable a reliable luminosity function at the faint end to be constructed because we are basically dealing with a distance limited sample. Incidentally, contamination by red giants or AGB stars is either negligible - or very interesting! - because these stars are 10⁹ times as luminous as their main sequence relatives and so will not be seen in the faint star counts unless there is a significant intergalactic (Local Group) component of these short-lived stars. Recent deep HST images by Bahcall et al. (1994) show that this is very unlikely.

Table 2.2 lists the I - K color (which correlates well with spectral type, Bessell 1991) for known lower main sequence stars and possible brown dwarfs, including GL 229 B, which almost certainly is such an object (Nakajima et al. 1995; Geballe et al. 1996; Matthews et al. 1996; Tinney 1996). All of these stars are cool enough that they will be detected by the SDSS most readily in the z' band data. The data in Table 2.2 are from Bessell (1991), Kirkpatrick et al. (1991, 1993), Tinney et al. (1993), Tinney (1993), Davidge and Boeshaar (1992) and Nakajima et al. (1995).

Figure 2.1

Broad band spectra of very cool stars. The figure shows two lower main sequence stars and the brown dwarf GL 229B (from Nakajima et al. 1995).

2MASS will detect many brown dwarf candidates, but identifying them as such for spectroscopic follow up requires optical data, as illustrated by the spectra shown in Figure 2.1 from Nakajima et al. (1995). Comparison of Tables 2.1 and 2.2 shows that all of the objects in Table 2.2 if detected at the K limit of 2MASS, would also be detected by the SDSS in at least its two longest wavelength filters, while objects cooler than this will not. In the Southern survey of the SDSS (Section 5.5), the data will be about 2 magnitudes deeper, allowing even cooler stars to be detected by both surveys. The Southern survey will also provide proper motion data for the brighter stars, enabling the kinematics of the brown dwarf population to be studied.

Stars below the main sequence hydrogen burning limit will only be visible in either survey if they are fairly young, and can be expected to show strong H alpha emission such as that found in PC0025+044 (Schneider et al. 1991; cf., Fig. 3.6.2). This gives a characteristic color signature in the SDSS filter set - very red i' - z' colors but not particularly red r' - i' colors. A final class of low-luminosity objects with peculiar colors is worth mentioning. Zuckerman and Becklin (1992) searched for low-mass companions of white dwarfs by imaging them in the K band. Such white dwarf - red dwarf pairs will also have an unmistakable color signature in the combined SDSS/2MASS data.

Relations Between the SDSS and 2MASS Projects

Since the 2MASS and SDSS projects have much in common, both formal and informal contact between the two groups has already been made. Robert Lupton (Princeton) has served as advisor for the galaxy classification software under development at IPAC. Both projects have recently obtained test imaging data in the same part of the sky and are comparing results. David Monet (USNO) is a member of the 2MASS science team, Michael Strauss (Princeton) is a member of the external review board for 2MASS and Jill Knapp (Princeton) is on the IPAC Users' Committee.

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