EXTRAGALACTIC ASTRONOMY WITH 2MASS
S. E. Schneider M. F. Skrutskie 1, T. J. Chester 2, T. H. Jarrett J. P. Huchra 3 1 Un iversity of Massachusetts, Amherst, MA, USA 2 Infrared Processing and Archive Center, Pasadena, CA, USA 1, 2, 3 Center fo r Astrophysics, Cambridge, MA, USA
Abstract The Two-Micron Sky Survey (2MASS) is about to begin surveying the northern sky, and will begin surveying the south in less than a year. Survey data will be released to the community approximately months after the official survey start, and will provide All 18 a wide range of opportunities for extragalactic astronomy in the infrared. The survey properties are described here, along with a few results from the 2MASS prototype camera.
1 Introduction
Almost thirty years after the Two Micron Sky Survey [4] . 2MASS is a near-infrared survey of the whole sky that will be more than 10,000 times deeper. The survey is designed to be as sensitive and uniform as possible while completing mapping in about three years. The survey was motivated by many interests, in particular: (1) searches for rare objects like brown dwarfs, extreme late-type stars, and red QSOs; (2) mapping out the structure of the entire Milky Way using red giants; and (3) cataloging galaxies in the near infrared, where the reduction in absorption allows us to find galaxies deep into the zone of avoidance and obtain total galaxy magnitudes nearly free of internal absorption. Obviously. a near-infrared-selected sample of over a million galaxies will allow a wide range of studies of galaxy properties. The survey is being carried out primarily by UMass and IPAC, with assistance from science team members at several other institutions in the United States. The northern telescope is sited at Mount Hopkins, Arizona, and the matched southern telescope will be installed at Cerro Tololo, Chile later this year. The northern telescope has just been installed, and alignments are underway. The three-channel (J, H, Ks) camera is mounted and working, and the survey will begin just as soon as a few engineering tests can be completed. Meanwhile, !PAC is completing
399 POSS 2MASS IRAS
0.43µm 2.17µm 12µm
Figure 1: Optical and near- and far-infrared images of the same field in the Galactic plane. The Palomar image is fromthe blue plate, the 2MASS image is in the K,-band, and the IRAS image is from the 12-micron channel. the "pipeline" software for data analysis. and a full end-to-end test of the system will take place shortly after this conference. We plan to provide 2MASS data to the community as soon as we can ensure its reliability, and our goals is to begin releasing the data within 18 months of the official survey start. We expect these data to be a resource analagous to the Palomar Sky Survey, with the advantages that they are digital and that we will also provide detailed source extractions. A comparison of the same fieldas seen on the POSS, by 2MASS, and by IRAS is shown in Figure L Note that while a few sources repeat, the sky at two microns looks quite different than at the shorter and longer wavelengths. In this paper, I will explain the survey methodology, describe the various data products scheduled for release, and characterize the quality of the data with some examples from the 2MASS prototype camera, which was optically almost identical to the final survey camera.
2 The Survey
In order to cover the entire sky in a reasonable amount of time. two things are necessary: sensitive detectors and a method for rapid coverage. NICMOS-:3 technology combined with a 1.3 m telescope provides the sensitivity: 2MASS reaches a Ks magnitude of 14.3 (lOu) for point sources in less than 8 seconds of total integration. The 256 x 256 chips also effectively set the resolution of the survey, since the overall timeline for the survey is budgeted for only a few years. As a result, the survey collects data with 2" pixels, which undersamples the point
400 Single Frame
5!6!hst overlap
' 10%
Figure 2: The survey technique is to slew the telescope continuously in declination while using a tip-tilt secondary to freeze the image on individualOverlap frames with a 1.3 s integration time. Frames are overlapped by 5/6ths. and neighboring scans are overlapped by 10%. spread function, but by a dithering scheme (described below) subsampling is achieved, which improves the effective resolution. Besides requiring a method for rapid coverage, a major goal of the survey is to produce highly uniform data. As a result, the 2MASS team developed a highly automated system, a camera and telescope with excellent optics, and a large amount of overlap between fields. The camera uses dichroics to split .J, H, and K,, and simply triplicates the optical path of the prototype camera which we ha,·e tested extensively. The telescope is a 1.3 m cassegrain, with accurate position acquisition and well-controlled focus behavior. The camera/telescope com bination provides excellent optical characteristics, with nearly diffraction-limited performance, an essentially uniform point spread function across the field in all three bands, and low losses. The survey technique is to collect a series of frames, while the telescope slews continuously in declination. A tip-tilt secondary freezes the image for 1.3 s integrations spaced so that each image overlaps the previous one by 5/6ths. (See Figure 2.) The array is rotated at a slight angle to the scan direction and the step size is slightly adjusted in order to sample each source at six different positions with respect to the pixel grid, providing good sampling of the point spread function. The six overlapping images of each spot on the sky are compared and combined to produce the finalimages. In addition, a brief 50 ms integration is used to measure bright sources and as part of the array reset procedure.
401 The sky is covered by almost 60,000 scans, each 8.5 arcmin wide by 6 deg long in declination. The 272 frames along each scan are used to generate the image fiat. Stars in each scan are tied into the Tycho coordinate system [2], yielding astrometry accurate to better than 0.5 arcsec. Photometric calibration sources are observed approximately every two hours to monitor the zero-point and color corrections. Ultimately, each calibration source will become a calibration field, in which all of the non-Yarying sources can be used to improve the statistical calibration errors. Furthermore, since each scan overlaps its neighbors by about 10%, comparisons of stars in the overlap regions can be used to bridge the astrometry and photometry over the whole sky. At the end of the survey, a global recalibration is planned using these features to improve the survey accuracy and uniformity.
3 The Data
The Level-1 Data Specifications for 2MASS are summarized in Table 1. All of these values have been demonstrated or exceeded in the prototype camera runs, and initial tests with the 3-channel survey camera indicate that it may be performing slightly better still.
Table - 2MASS Level 1 Specifications Point Sources: 10-a Sensitivity: J<15.8 H<15.1 K<14.3 • 1 • Photometric Accuracy: <5% • Sky Coverage: > 95% • Positional Accuracy: < 0.5" • Reliability: > 99.95% • Completeness: > 99% (lbl > 30°) • Bright source limit: K > 4.0 Galaxies: • Sensitivity: J
402 then performing a series of smoothings on the remainder image to search for extended low surface brightness objects. The second algorithm picks up some additional objects, albeit at some expense to reliability. We are currently "tuning" the parameters of this algorithm to best balance completeness and reliability. The basic data products that will be generated from the survey are described in Table 2. For extragalactic work, the primary database will be the extended source catalog. This will also include non-galaxian extended sources, but it will be dominated by galaxies. One of the main problems in defining this catalog is that there are almost as many ways to measure a galaxy as there are astronomers! We are currently recording magnitudes based on many different photometric schemes. This includes a variety of isophotal, aperture, and Petrosian magnitudes, measured with circular and elliptical fits. In addition, we cross reference the apertures between bands so that the same area is referenced in each case. This quickly mounts up to a very large number of columns we are carrying, so that compared to the Point Source Catalog, what the Extended Source Catalog lacks in length, it makes up in width! We will be paring this down for the data release to the magnitudes that prove most robust, and we will provide small images ("postage stamps") of each galaxy to allow others to apply their own favorite algorithms.
Table 2 - 2MASS Data Products •Point Source Catalog:
> 100, 000, 000 objects-positions, magnitudes, uncertainties, optical associations with POSS & ES O, etc. •Extended Source (Galaxy) Catalog:
> L 000, 000 objects-positions, magnitudes, morphology, uncertainties, circular & elliptical apertures, cross-referenced apertures, sizes & orientations, etc., etc., etc. ---+ "Postage Stamp" images • Sky Images: 17' x 8.5' images with 111 pixels-re istered band-to-band, available in fu ll-resolution and highly-compressed versions Data Archive: • g access to raw data tapes
The number of sources quoted in the table for the galaxy catalog are probably low. Since we are a magnitude or two more sensitive at H and J, we catch many of the fainter K, extended sources when we merge the bands. Based on initial results from the Mt. Hopkins facility, it appears that we will be detecting closer to 2-3 million galaxies with K, < 13.5. The total number detected down to K, < 14.5, where 2MASS becomes much less complete, is about twice this. And the overall number of sources that trigger the extended source processor may be greater than 10 million. We plan to put all of these sources in a working survey database, which will not be part of the data releases, although it will probably become available after we characterize its limitations. The total numbers of galaxies 2MASS will detect will become more firmin the next few months as we get an opportunity to survey a wider range ofenvironments and can better establish the effects of the Milky Way locally on our counts.
4 Observations with the Prototype Camera
To give an idea of the power of the 2MASS camera, we show some images of two well known Messier galaxies along with optical comparisons. The infrared images were obtained in the
403 Figure 3: Images of M104 at V, J, and K., The J and Ks images were produced by the 2MASS prototype camera operating in standard survey mode.
Figure 4: Images of M51 at V, I, and K,. The Ks image was produced by the 2MASS prototype camera operating in standard survey mode. standard 2MASS observing mode. with total integration times of about 7.8 s. The optical CCD (300 s integration) images obtained at the McDonald observatory. In Figure we show Ml04 ("the Sombrero"), which illustrates the lessening importance of dust absorption in the infrared. The dark band so evident in the visual (V) band becomes weak at J and:3 is nearly invisible at K" so that the galaxy appears essentially symmetric from north to south. This shows both why infrared surveys produce more accurate stellar fluxes of galaxies, and why from within the Milky Way we are able to observe close to the "zone of avoidance." The Ks band image of M-51 ("·the Whirlpool") in Figure 4 has a much smoother structure than in the V or I band images. It is also noteworthy that low surface brightness tidal material west of the companion and the faint outer reaches of the spiral arm to the south of the galaxy are detectable in the Ks image despite the short integration time. The smoothness of the galaxy at 2µ is due partially to the lack of dust absorption, but it is also a consequence of the lower sensitivity in the infrared to the young, blue stars in star-forming regions. The infrared light better represents the underlying mass of old and low-mass stars and more clearly delineates the two-arm structure of the galaxy. There will a lot of interesting science to do on large galaxies with 2MASS, although there will be some complications for such studies. The images shown above were specificallycentered on the galaxies. while the actual survey would more typically require careful stitching together be of neighboring scans to cover a large galaxy. We are not currently planning to address this issue as part of the survey data relases, but it should be possible to construct larger images
404 • • • Figure 5: From left to right, 2MASS J, H, K, and optical (POSS) images of representative galaxies. The top galaxy has K, = 11.4, the middle galaxy ha.S K, = 13.3, and the bottom galaxy is an object picked up by the low central surface brightness processor with relatively high confidence(a lthough it has a magnitude well below the survey limit). The images were obtained with the 2MASS prototype camera, and because the channels were obtained separately, there are some registration problems between channels. from the image products that are released. Actually, the 2MASS galaxy database will probably become somewhat confused for galaxies larger than about 1 arcmin, since the overlap between neighboring fields only guarantees that any galaxy of diameter ;:;50 arcsec will be entirely located on a single scan. The large galaxies will be a relatively small subset of the whole database, but obviously a very interesting one. Galaxies more typical of what will be cataloged by 2MASS are shown in Figure 5 along with corresponding optical images from the Palomar Sky Survey. A "bright" galaxy, one near the nominal survey limit, and a low surface brightness galaxy are shown as examples of the kinds of galaxies that will be cataloged. Note that the bright galaxy, given typical blue to infrared color differences, would be close to the limit of the Zwicky catalog [5], for example. The infrared images extend fairly clearly out to the = 20 mag arcsec-2 (inner elliptical contour), and the extent can be estimated to about a magnitude fainter using apertures with the same fixed axis ratio. The POSS images appear to µKbe perhaps a magnitude deeper in surface brightness, although the different "stretches" in the images make direct comparison difficult. On the other hand, an advantage for the infrared photometry is that the disks of spirals become bluer in the outer regions, so that a larger fraction of the total K-band light is contained within a smaller aperture. At the official survey limit of K, = 13.5 most galaxies are relatively easily discriminated from stars by using a measure ofthe diffuseness of the light. We carry out this discrimination f(r) by fitting a generalized exponential of the form: /0 c(r/a)''� to the radial distribution J(r). of pixel fluxes The parameters and f3 allow the distribution to take on a variety r114 of forms ranging from Gaussian to simplea exponential= to a de Vaucouleurs law, which covers the range from stars to spirals to ellipticals. This has proved a powerful method for star/galaxy separation, although less so for distinguishing spirals and ellipticals. The 2MASS galaxy processing system is described in more detail by Jarrett et al. [3].
405 Redshifts of 2MASS Galaxies
Finally, as a testof the effective distances that 2MASS cataloged galaxies will have, we measured 5 the redshifts of galaxies in the vicinity of one of the cluster regions mapped by the prototype camera. A region approximately � 2° x 6° was mapped near Abell 262 in order to exercise the galaxy processing algorithms, but the follow-up redshift survey indicates that most of the galaxies detected are in fact background sources. One of us ( JPH) has collected redshifts that are > 90% complete down to K, < 13 mag, and 50% complete between 13 and 13.5 mag. (Note that this fieldwas surveyed with an earlier version of the 2MASS prototype camera, and is not quite as deep as the data in the actual survey.) The resulting redshift distribution for 250 of the 337 2MASS galaxies in this region is shown in Figure 6. This essentially K-band-selected sample has several detections out to z � 0.2, and most of the galaxies lie at much higher redshifts than the targeted cluster. In fact, the mean redshift of the selected galaxies is 17000 km s-1• If this is a typical region, and if the poorer optics of the earlier incarnation of the 2MASS protocamera has not caused some sort of strange bias, then 2MASS will identify galaxies fairly densely out to z � 0.1
References
[1] de Lapparent, V., Geller, J., & Huchra, J. P. 1986, ApJ 302, Ll [2] Hog, E., et al. 1995, AA 304, 150 [3] Jarrett, T., Chester, T., Schneid?II. er, S., & Huchra, J. 1997, in "The Impact of Large Scale Near-IR Sky Surveys," eds. F. Garzon et al. (Kluwer, Netherlands) p. 213 [4] Neugebauer, G., & Leighton, R. B. 1968, Two Micron Sky Survey: A Preliminary Catalog. NASA SP-304 7 [5] Zwicky, F., 1961, Catalogue of Galaxies and Clusters of Galaxies (California Institute of Technology, Pasadena)
406 "Abell
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0.12 c .. ' ..• .. lo" r...... 1 •,• 0.08 c
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Abell 262 Figure 6: Slice diagram for a region in the direction of the Abell 262 cluster (cz � 5000 km s-1 ). For a comparison of the volumes involved, the CfA slice [l] is also shown (although note that the slices are in opposite portions of the sky). The Abell 262 cluster sits in the foreground of a much larger concentration of galaxies with a redshift close to about z = 0.08.
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