The Astronomical Journal, 126:1346 1361, 2003 September # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE ASYMMETRIC THICK DISK: A STAR-COUNT AND KINEMATIC ANALYSIS. I. THE STAR COUNTS Jennifer E. Parker and Roberta M. Humphreys Department of Astronomy, University of Minnesota, 116 Church Street, SE, Minneapolis, MN 55455; parker@astro.umn.edu, roberta@aps.umn.edu and Jeffrey A. Larsen Lunar and Planetary Laboratory, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85719; jlarsen@lpl.arizona.edu Received 2003 April 10; accepted 2003 May 21 ABSTRACT We report a statistically significant asymmetry in the distribution of thick-disk and possibly inner halo stars interior to the solar orbit 1 2.5 kpc from the Sun. We have compared the star counts in the 120 POSS I fields, 40 each above and below the Galactic plane in quadrant I with the 40 complementary fields above the plane in quadrant IV. We find a spatially extended region in quadrant I with a significant excess, 20% to 25%, in the numbers of blue- and intermediate-colored stars. While the region of the asymmetric distribution is somewhat irregular in shape, it is also fairly uniform, stretching over several hundred square degrees on the sky. It is therefore a major substructure in the Galaxy due to more than small-scale clumpiness in the thick disk or inner halo. In this first paper, we describe the observations and our star-count and statistical analysis. We also discuss three possible explanations for the asymmetry: the fossil remnant of a merger, a triaxial thick disk or halo, and interaction of the thick-disk/inner halo stars with the bar in the disk. Key words: galaxies: halos Galaxy: structure 1. INTRODUCTION It is now apparent from several independent avenues of research that our Milky Way is a much more complex system than we thought only 10 years ago. Studies of both stars and gas are revealing significant structure and asymmetries in their motions and spatial distributions including the bar of stars and gas in the Galactic bulge (Stanek et al. 1994) and the evidence from infrared surveys for a larger stellar bar in the inner disk (Weinberg 1992; López-Correidoira et al. 1999), the flattening of the bulge (Blanco & Terndrup 1989) and of the inner halo (Hartwick 1987; Wyse & Gilmore 1989; Larsen & Humphreys 1994). Each of these observations provides a significant clue to the history of the Milky Way. When combined with the growing evidence for Galactic mergers, i.e., the Sagittarius dwarf (Ibata, Gilmore, & Irwin 1994, 1995), the presence of star streams in the halo plus smaller moving groups (e.g., at the north Galactic pole [NGP]; Majewski, Munn, & Hawley 1994, 1996), and the debris trail from the Sagittarius dwarf (Ibata et al. 2001; Majewski et al. 1999; Yanny et al. 2000), we now realize that the structure and evolution of our Galaxy have been significantly altered by mergers with other systems. Several surveys are now in progress to probe the Galactic halo with deep CCD imaging and spectroscopy. These pencil-beam surveys have the advantage of going very faint with good photometric accuracy but are restricted to small areas on the sky (Morrison et al. 2000). Consequently, all-sky surveys like the digitized photographic surveys and the new digital surveys, the Sloan Digital Sky Survey and the Two Micron All-Sky Survey, are required to obtain a global picture of the halo and thick disk and to search for asymmetries on large spatial scales. Larsen & Humphreys (1996) reported evidence for a large-scale asymmetry in the distribution of faint blue stars 1346 in the inner part of the Galaxy. They used star counts from the Automated Plate Scanner (APS) Catalog of the POSS I for four paired fields, 16 deg 2 each, near b 30 40. They found a 30% excess in the number of faint blue stars in all four fields in the first quadrant (l =20 45 ) compared with their complementary longitudes on the other side of the Sun-center line. No evidence for a similar asymmetric distribution was found in the anticenter direction. They concluded that the excess was due to an asymmetry in the distribution of the inner halo and/or thick-disk stars but were unable to determine whether the nature of the excess was due to a triaxial halo/thick disk or an interaction between the thick disk and the stellar bar in the thin disk. To better understand the possible origin(s) of this feature, we first need to map the extent and shape of the asymmetric distribution and further identify the contributing population of stars. In this paper, we report our investigation of the star counts extending our search to include data from 40 (2) contiguous POSS I fields from Galactic latitude +15 to +55 and with longitudes from 25 to 80. We confirm the Larsen & Humphreys result and find evidence for a asymmetric excess dominated by blue- and intermediatecolored stars but extending nearly 700 deg 2 from approximately l 20 to 55 at b 25 to 45. Extending our survey below the Galactic plane with 40 additional fields in the longitude range l 25 80, we find evidence for the same asymmetry. Although the star-count excess is relatively uniform across large spatial areas, there is some variation in its size and significance from field to field and also above and below the plane. The large spatial extent of the asymmetry feature parallel to the plane has important implications for its origins. In the next section, we briefly describe the observational data, and in x 3 we present our star-count analysis and the ratios between the paired fields and their uncertainties. In x 4, we discuss the evidence for an
ASYMMETRIC THICK DISK. I. (a) (b) P802 [340, + 35 ] 16 22 22 20 18 18 16 O Magnitude 14 14 12 12 P566 [21, + 34 ] 20 O Magnitude 1347-2 0 2 4 6-2 0 2 4 6 Fig. 1. Typical CMDs for (a) P566 (21, +34 ) in quadrant I and (b) P802 (340, +35 ) in quadrant IV excess of star counts and an asymmetry in the distribution of the halo/thick-disk stars in lines of sight toward the inner Galaxy at estimated distances of 1 2.5 kpc from the Sun. In the final section, we discuss the possible origins of the observed asymmetry. 2. OBSERVATIONAL DATA AND FIELD SELECTION The Automated Plate Scanner (APS) Catalog of the POSS I is derived from our digitized scans of glass duplicates of the original blue (O) and red (E) Palomar Observatory Sky Survey (POSS I). The scanning procedures and parameters are described in Pennington et al. (1993). The resulting database includes positions, magnitudes, colors, object classification, and other image parameters for all matched images on the 632 plate pairs with b 20 at the plate center.1 The stellar and nonstellar images are separated with a neural network classifier (Odewahn et al. 1992, 1993). Each of the plates is independently calibrated. The stellar magnitude calibration uses a magnitude-diameter relation derived from CCD BVRC and photoelectric BVRJ sequences and the diameters of the best-fit ellipses to the stellar images (Humphreys et al. 1991). When supplemented with the B and V photoelectric data in the Guide Star Photometry Catalog (Lasker et al. 1988), we have direct calibrations from 10th to 21st magnitude for the blue plates and 13th to 20th magnitude for the red plates. The magnitudes are transformed to the instrumental system, the O (blue) and E (red) passbands, of the sky survey plates, and 1 See http://aps.umn.edu. the observed relation is then fitted by a smooth function based on the stellar brightness profile (see King 1971; Kormendy 1973). Figures in Larsen & Humphreys (2003) and Cabanela et al. (2003) show examples of magnitudediameter fits. The calibrations have a typical rms of 0.15 mag over the relevant range, O = 15 20 mag, discussed in this paper. More information about the photometric calibration is given in these papers and at the APS Web site. The calibrating sequences are also available at the Web site.2 Typical color-magnitude diagrams for two of our fields are shown in Figure 1. Our O E color is approximately 1.8 times the B V color with the exception of the reddest stars. To map the shape and full extent of the asymmetry in the star counts first reported by Larsen & Humphreys (1996), we have selected the POSS I fields with plate centers between b +20 and +55, and from l 20 to 80 and their complementary fields at l 340 280 for a total of 40 matched plate pairs. We later added the 40 corresponding POSS I fields below the Galactic plane (l 20 80 ). All of the fields are listed in Table 1 with their central coordinates, Galactic longitude and latitude, and plate limits in O and E magnitudes. Their distribution on the sky is shown in Figure 2. 3. STAR COUNTS For our star-count study, we have selected the stars fainter than O = 15 mag from the central 16 deg2 region on each plate and counted nearly 6 million stars. Because interstellar extinction can be significant, especially for our fields 2 See http://aps.umn.edu/catalog/getphotom.html.
TABLE 1 Midlatitude Galactic Structure Fields Plate Centers (B1950.0) Plate Limits Completeness Limits APS Field R.A. Decl. l b O E E(B V ) E(B V ) phot Blue Inter. Red P568... 17 16 39 +05 53 49 27.5 23.2 22.0 21.0 0.09 0.01 0.25 18 18.5 19 P815... 20 05 26 17 43 39 25.0 24.6 21.9 20.3 0.10 0.02 0.21 18 18 18.5 P915... 14 48 30 30 18 35 331.5 25.6 20.6 19.8 0.10 0.04 0.26 18 18 19.5 P567... 16 52 39 +05 50 45 24.4 28.4 21.7 20.3 0.10 0.01 0.18 18 18 18.5 P816... 20 29 24 17 40 52 27.5 29.9 21.9 20.0 0.04 0.01 0.19 18 18 18.5 P859... 14 48 20 24 18 44 334.9 30.8 21.2 20.0 0.14 0.04 0.21 18 18 18.5 P508... 17 16 25 +11 53 56 33.4 25.9 21.9 20.0 0.16 0.02 0.32 18 18.5 19.5 P756... 20 29 13 11 40 45 33.7 27.5 22.0 21.3 0.04 0.03 0.22 18 18 18.5 P914... 14 22 25 30 20 28 325.8 28.1 21.0 20.0 0.09 0.02 0.26 18 18 18.5 P448... 17 16 11 +17 53 52 39.5 28.3 21.8 20.1 0.08 0.02 0.25 18 18.5 19 P697... 20 53 01 05 38 20 42.9 30.0 21.4 21.0 0.04 0.01 0.22 18 18 18.5 P913... 13 56 19 30 21 59 319.8 30.0 21.2 20.2 0.05 0.02 0.21 18 18 19 P389... 17 23 58 +23 54 51 46.7 28.7 21.8 19.5 0.07 0.01 0.18 18 18 18.5 P637... 20 52 52 +00 21 42 48.8 27.0 22.2 20.7 0.09 0.02 0.16 18 18 18.5 P912... 13 30 13 30 23 13 313.4 31.4 20.9 19.6 0.03 0.01 0.25 18 18 18.5 P333... 17 23 41 +29 54 56 53.2 30.6 22.0 20.6 0.03 0.01 0.23 18 18 18.5 P638... 21 16 51 +00 24 55 52.5 32.0 21.9 20.3 0.05 0.02 0.36 18 18 18.5 P911... 13 04 07 30 24 05 306.9 32.1 21.5 20.3 0.09 0.01 0.21 18 18 18.5 P278... 17 19 22 +35 54 21 59.8 33.0 21.8 19.8 0.03 0.01 0.17 18 18 18.5 P579... 21 40 44 +06 25 57 62.5 33.3 22.5 20.8 0.05 0.01 0.19 18 18 18.5 P910... 12 37 59 30 24 46 300.2 32.1 21.5 20.8 0.08 0.01 0.21 18 18 18.5 P228... 17 32 59 +41 56 13 67.4 31.5 22.0 20.2 0.01 0.01 0.22 18 18 18.5 P519... 21 40 36 +12 25 58 67.8 29.4 22.0 20.2 0.07 0.01 0.19 18 18 19 P909... 12 11 53 30 25 02 293.6 31.5 21.2 20.1 0.07 0.02 0.18 18 18 18.5 P181... 17 36 31 +47 56 23 74.5 31.8 22.0 20.2 0.01 0.00 0.21 18 18 18.5 P460... 22 04 32 +18 27 43 77.4 29.3 21.8 20.6 0.04 0.01 0.21 18 18 18.5 P908... 11 45 47 30 25 01 287.2 30.2 20.5 19.8 0.05 0.01 0.19 18 18 18.5 P566... 16 28 40 +05 47 42 21.0 33.7 22.0 20.7 0.01 0.01 0.20 18 18 18.5 P872... 20 25 27 23 45 10 20.6 31.4 21.5 20.3 0.05 0.01 0.16 18 18 18.5 P802... 14 53 22 18 23 06 339.8 35.2 22.0 20.5 0.06 0.02 0.21 18 18 18.5 P506... 16 28 22 +11 47 37 27.4 36.5 22.0 21.0 0.07 0.02 0.24 18 18 18.5 P873... 20 52 22 23 42 54 22.8 37.0 21.6 20.6 0.05 0.01 0.17 18 18 18.5 P801... 14 29 18 18 25 14 333.7 38.1 21.4 20.0 0.10 0.01 0.21 18 18 18.5 P507... 16 52 29 +11 50 22 30.5 31.2 21.5 20.7 0.06 0.01 0.19 18 18 18.5 P817... 20 53 21 17 38 16 30.0 35.2 21.6 20.4 0.05 0.01 0.11 18 18 18.5 P858... 14 22 17 24 20 27 328.7 33.6 21.7 20.0 0.08 0.01 0.21 18 18 18.5 P447... 16 52 12 +17 50 43 37.0 33.6 21.7 20.5 0.08 0.01 0.24 18 18 18.5 P757... 20 53 11 11 38 17 36.6 32.8 21.6 20.9 0.05 0.02 0.25 18 18 18.5 P857... 13 56 13 24 21 59 321.9 35.8 21.4 20.5 0.06 0.02 0.19 18 18 18.5 P388... 16 57 58 +23 51 11 44.4 34.4 21.5 20.4 0.05 0.01 0.16 18 18 18.5 P758... 21 17 09 11 35 52 39.8 38.1 21.8 20.4 0.04 0.01 0.21 18 18 18.5 P856... 13 30 08 24 23 10 314.8 37.3 21.5 20.3 0.06 0.01 0.23 18 18 18.5 P332... 16 57 42 +29 51 19 51.4 36.1 21.8 20.9 0.03 0.02 0.22 18 18 18.5 P698... 21 17 00 05 35 57 46.4 35.2 22.2 20.7 0.06 0.02 0.25 18 18 18.5 P855... 13 04 03 24 24 07 307.4 38.1 21.4 20.0 0.10 0.02 0.21 18 18 18.5 P277... 16 51 25 +35 50 23 58.5 38.5 22.3 20.3 0.00 0.00 0.15 18 18 18.5 P639... 21 40 52 +00 26 04 56.7 37 21.7 20.4 0.05 0.02 0.11 18 18 18.5 P854... 12 37 58 24 24 43 299.8 38.1 22.2 20.2 0.09 0.03 0.17 18 18 18.5 P227... 17 03 02 +41 52 02 66.4 37.0 22.0 20.2 0.12 0.01 0.18 18 18 18.5 P580... 22 04 45 +06 27 38 67.3 37.8 22.0 20.8 0.07 0.01 0.21 18 18 18.5 P853... 12 11 53 24 25 01 292.4 37.4 21.8 20.5 0.15 0.03 0.17 18 18 18.5 P180... 17 02 35 +47 52 05 73.9 37.5 21.5 20.2 0.01 0.01 0.19 18 18 18.5 P520... 22 04 39 +12 27 41 72.6 33.7 21.8 20.6 0.05 0.01 0.16 18 18 18.5 P852... 11 45 48 24 25 01 285.2 36.0 21.4 19.8 0.08 0.02 0.22 18 18 18.5 P565... 16 04 41 +05 44 26 17.2 38.8 22.0 20.2 0.01 0.00 0.23 18 18 18.5 P929... 20 52 32 29 42 58 15.5 38.6 21.1 20.1 0.10 0.01 0.20 18 18 18.5 P742... 14 53 11 12 23 08 344.2 40.1 21.5 20.0 0.01 0.00 0.21 18 18.5 18.5 P505... 16 04 29 +11 44 27 24.1 41.7 22.0 20.4 0.04 0.01 0.18 18 19 19 P874... 21 18 20 23 41 06 25.1 42.7 21.8 20.6 0.01 0.01 0.21 18 18 18.5 P741... 14 29 09 12 25 16 337.6 43.4 21.5 20.0 0.09 0.01 0.19 18 18 18.5 P446... 16 28 15 +17 47 35 34.4 38.9 21.9 20.3 0.05 0.01 0.25 18 18 18.5 P818... 21 17 18 17 35 55 32.7 40.5 21.6 20.3 0.04 0.02 0.14 18 18 18.5 P800... 14 05 14 18 27 04 327.1 40.6 22.0 20.0 0.09 0.01 0.21 18 18 19
ASYMMETRIC THICK DISK. I. 1349 TABLE 1 Continued Plate Centers (B1950.0) Plate Limits Completeness Limits APS Field R.A. Decl. l b O E E(B V ) E(B V ) phot Blue Inter. Red P387... 16 32 00 +23 47 45 42.1 40.0 21.5 20.4 0.05 0.01 0.17 18 18 18.5 P759... 21 41 06 11 33 56 43.3 43.3 21.9 20.7 0.02 0.00 0.17 18 18 19 P799... 13 41 11 18 28 39 319.9 42.4 21.6 20.5 0.07 0.01 0.21 18 18 18.5 P331... 16 31 43 +29 47 56 49.8 41.5 22.0 20.7 0.02 0.01 0.19 18 18 19 P699... 21 40 59 05 34 01 50.3 40.3 21.2 20.6 0.01 0.01 0.28 18 18 18.5 P798... 13 17 06 18 29 56 312.2 43.6 21.5 20.8 0.08 0.01 0.27 18 18 18.5 P276... 16 23 27 +35 46 48 57.5 44.1 22.0 20.0 0.01 0.01 0.15 18 18 18.5 P700... 22 04 58 05 32 17 54.7 45.3 21.7 20.7 0.04 0.01 0.21 18 18 18.5 P797... 12 53 02 18 30 53 304.3 44.1 21.4 20.5 0.05 0.01 0.21 18 18 18.5 P226... 16 33 06 +41 48 08 65.9 42.6 22.0 20.6 0.00 0.00 0.48 18 18 18.5 P640... 22 04 52 +00 27 47 61.4 41.8 21.8 20.4 0.02 0.01 0.14 18 18 18.5 P796... 12 28 58 18 31 28 296.4 43.8 21.5 20.2 0.02 0.02 0.21 18 18 18.5 P179... 16 28 42 +47 47 36 74.1 43.1 22.0 20.8 0.00 0.00 0.21 18 18 18.5 P581... 22 28 47 +06 29 17 72.8 42.1 21.2 20.6 0.06 0.01 0.12 18 18 18.5 P795... 12 04 53 18 31 44 288.7 42.8 21.7 20.1 0.00 0.01 0.14 18 18 18.5 P564... 15 40 42 +05 41 40 13.0 43.8 21.6 20.0 0.00 0.01 0.20 18 18.5 19 P930... 21 18 27 29 40 57 17.8 44.1 21.9 21.0 0.06 0.03 0.17 18 18.5 19 P682... 14 53 01 06 23 05 349.2 44.8 22.0 20.8 0.02 0.01 0.36 18 18.5 18.5 P504... 15 40 30 +11 41 45 20.3 46.9 21.5 20.7 0.02 0.01 0.16 18 18.5 19 P931... 21 44 22 29 39 10 18.5 49.7 21.9 20.8 0.00 0.01 0.20 18 18 18.5 P681... 14 29 00 06 25 22 342.3 48.4 22.0 20.0 0.06 0.01 0.59 18 18 18.5 P445... 16 04 17 +17 44 28 31.6 44.2 21.9 20.4 0.03 0.01 0.19 18 18 18.5 P819... 21 41 15 17 33 57 35.6 45.8 22.0 20.3 0.02 0.01 0.17 18 18 18.5 P740... 14 05 06 12 27 08 330.3 46.1 21.5 20.1 0.08 0.01 0.22 18 18 18.5 P386... 16 06 03 +23 44 43 39.8 45.7 22.0 20.8 0.00 0.01 0.24 18 18 18.5 P759... 21 41 06 11 33 56 43.3 43.3 21.2 20.6 0.02 0.00 0.17 18 18 19 P739... 13 41 04 12 28 41 322.3 48.2 21.2 20.2 0.04 0.01 0.15 18 18 18.5 P330... 16 05 48 +29 44 32 48.3 47.1 22.0 20.7 0.03 0.01 0.20 18 18.5 18.5 P760... 22 05 48 11 32 07 47.4 48.6 22.1 21.2 0.01 0.01 0.08 18 18 18.5 P738... 13 17 01 12 30 00 313.6 49.5 21.3 19.8 0.03 0.01 0.19 18 18 18.5 P275... 15 55 34 +35 43 24 56.9 49.8 21.5 20.0 0.01 0.01 0.21 18 18.5 18.5 P701... 22 28 56 05 30 40 59.9 50.2 21.8 20.5 0.03 0.01 0.21 18 18.5 18.5 P737... 12 52 58 12 30 56 304.5 50.1 22.0 20.5 0.00 0.01 0.26 18 18.5 18.5 P225... 16 03 12 +41 44 31 66.0 48.2 21.6 20.0 0.00 0.00 0.19 18 18.5 19 P641... 22 28 51 +00 29 20 66.8 46.3 21.9 20.9 0.04 0.01 0.19 18 18.5 19 P736... 12 28 55 +12 31 29 295.4 49.8 22.0 20.1 0.00 0.01 0.17 18 18 19 P178... 15 54 52 +47 43 46 75.3 48.8 22.0 20.3 0.00 0.00 0.24 18 18 18.5 P642... 22 52 51 +00 30 22 73.2 50.5 22.2 20.9 0.04 0.01 0.21 18 18 18.5 P735... 12 04 53 12 31 44 286.6 48.7 21.8 20.5 0.01 0.01 0.12 18 18 18.5 P444... 15 40 19 +17 41 43 28.4 49.5 22.2 20.5 0.01 0.01 0.22 18 18 18.5 P875... 21 44 15 23 39 21 27.4 48.4 22.6 20.8 0.01 0.01 0.21 18 18 18.5 P680... 14 04 59 06 27 08 321.1 51.5 21.5 20.2 0.00 0.00 0.21 18 18 18.5 P385... 15 40 07 +23 41 39 37.4 51.4 21.6 19.9 0.03 0.00 0.17 18 18 18.5 P820... 22 05 11 17 32 10 38.8 51.1 21.4 21.3 0.00 0.01 0.19 18 18 18.5 P679... 13 40 58 06 28 49 325.3 53.9 21.9 20.9 0.00 0.01 0.18 18 18 18.5 P329... 15 39 52 +29 41 41 46.9 52.6 21.9 20.5 0.03 0.01 0.27 18 18 18.5 P821... 22 29 08 17 30 40 42.6 56.3 21.5 20.5 0.01 0.01 0.17 18 18 18.5 P678... 13 16 57 06 29 58 315.3 55.4 21.5 20.0 0.03 0.01 0.24 18 18 18.5 P274... 15 27 41 +35 40 19 56.9 55.4 21.7 20.7 0.01 0.01 0.20 18 18 18.5 P761... 22 29 02 11 30 44 51.9 53.6 21.9 20.7 0.04 0.01 0.11 18 18 18.5 P677... 12 52 55 06 30 57 304.7 56.1 21.5 20.4 0.00 0.00 0.22 18 18 18.5 P224... 15 33 21 +41 40 44 67.0 53.7 21.6 19.8 0.03 0.00 0.16 18 18 18.5 P702... 22 52 55 05 29 37 66.2 54.8 22.1 20.7 0.03 0.01 0.21 18 18 18.5 P676... 12 28 54 06 31 31 294.1 55.7 21.9 20.7 0.00 0.01 0.16 18 18 18.5 P177... 15 21 05 +47 39 52 77.8 54.2 21.5 20.1 0.00 0.00 0.21 18 18 18.5 P643... 23 16 52 +00 31 09 80.8 54.4 21.9 20.4 0.02 0.01 0.21 18 18 18.5 P675... 12 04 52 06 31 44 283.9 54.4 21.4 20.0 0.01 0.01 0.21 18.5 18.5 19 Note. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. below b =30, we have corrected all of our data for interstellar reddening using the H i infrared dust emission maps and extinction tables from Schlegel, Finkbeiner, & Davis (1998). The reddening in E(B V ) for each star was then transformed to our O E color system using the universal extinction law. The magnitude and color of each star is then individually corrected. The mean reddenings in B V for each plate and the dispersion are also given in Table 1 with
1350 PARKER, HUMPHREYS, & LARSEN Vol. 126 Galactic Latitude [ ] -50 0 50 P274 P177 P224 P329 P385 P275 P178 P444 P225 P330 P504 P386 P276 P445 P179 P564 P226 P331 P505 P387 P446 P565 P180 P277 P227 P332 P506 P388 P447 P566 P181 P278 P228 P333 P507 P389 P448 P567 P508 P568 P815 P637 P756 P460 P519 P697 P816 P872 P520 P579 P638 P757 P698 P817 P580 P639 P758 P873 P929 P699 P818 P581 P640 P759 P874 P700 P930 P641 P819 P760 P875 P642 P701 P931 P820 P643 P761 P702 P821 P678 P677 P676 P679 P675 P680 P681 P738 P737 P736 P739 P735 P682 P740 P741 P799 P798 P797 P796 P795 P742 P800 P801 P856 P855 P854 P853 P852 P802 P857 P858 P912 P911 P910 P909 P859 P913 P908 P914 P915 POSS I Boundary 150 100 50 0 310 260 210 Galactic Longitude [ ] Fig. 2. Map of the 120 POSS I fields used in this study plotted in Galactic coordinates. The dotted line is the celestial equator, while the dotted oval-like shape is the area in which there is no POSS I data. phot, the combined photometric error from the O and E photometric calibrations and the uncertainty in the reddening correction, E(B V ). It is obvious that the effects of interstellar reddening on our star-count data will be small for most fields, but Figure 3 illustrates the shift in apparent O magnitude and color when the data is corrected for reddening for a field close to the plane. To identify the stellar population(s) responsible for the asymmetry, we must discriminate among the major components of the Galaxy: the thin disk, thick disk, and halo. We use the three component Galactic model GALMOD described in Larsen & Humphreys (2003) to predict their expected contributions to the observed star counts as a function of magnitude, color, and direction. Figure 4 shows the observed color distribution of the stars, binned by magnitude, for fields at different latitudes compared with the expected distributions of the thin disk, thick disk, and halo stars. While it is clear that we cannot completely isolate a specific Galactic component with our O E color alone, the model allows us to identify the color range over which a particular Galactic component or population makes its greatest contribution to the star counts. Although each POSS I plate is independently calibrated, a strict color cutoff to separate the different components will be vulnerable to small zero-point shifts from plate to plate, affecting the star-count statistics. Fortunately, there O Magnitude 22 20 18 16 14 12 10 (a) P568 [23, +27.5 ] Uncorrected for Interstellar Extinction O Magnitude 22 20 18 16 14 12 10 (b) P568 [23, +27.5 ] Corrected for Interstellar Extinction -2 0 2 4 6-2 0 2 4 6 Fig. 3. Illustration of the effects of the correction for interstellar extinction. (a) The observed CMD O magnitude vs. O E color for field P568 (23, +27=5) as compared with (b) the extinction-corrected CMD.
No. 3, 2003 ASYMMETRIC THICK DISK. I. 1351 0 500 1000 (a) O= 15-16 Data Total Thick Disk Disk Halo O= 16-17 0 200 400 600 800 (b) O= 15-16 O= 16-17 0 100 200 300 (c) O= 15-16 O= 16-17 0 500 1000 1500 0 500 1000 1500 2000 O= 17-18 -2-1 0 1 2 3 4 0 500 1000 0 500 1000 1500 O= 17-18 -2-1 0 1 2 3 4 0 200 400 0 200 400 600 O= 17-18 -2-1 0 1 2 3 4 Fig. 4. Observed and model-predicted distributions of the stars as a function of magnitude and color for three different latitudes: (a) P506 (27, +37=5), (b) P801 (334, +38 ), and (c) P274 (57, +55 ). is a clearly identifiable feature in the color distributions that can be used as a point of reference. This feature, which we call the blue ridge, occurs at O E 1.0 1.1 mag or a B V 0.6 mag in most fields after the data is corrected for extinction. Its B V color corresponds to disk main-sequence G stars and to the main-sequence turnoff for subdwarfs. Instead of adopting a specific color for the peak of the blue ridge, we determine the (O E) peak in the color distribution independently for every plate and each whole magnitude bin. The peak is easily pinpointed using a parabola fit to the color histogram with an uncertainty better than 0.1 mag. Figure 5 illustrates this procedure for field P505 for O-magnitude bins 15 19 mag. 0 200 400 600 0 200 600 1000 (a) (b) O=15-16 O=16-17 -1 0 1 2 3 4 0 500 1000 1500 0 500 1000 1500 (c) (d) O=17-18 O=18-19 -1 0 1 2 3 4 Fig. 5. Color histograms for plate P505 (24, +47 ) for O-magnitude bins (a) 15 16, (b) 16 17, (c) 17 18, and (d ) 18 19. The blue ridge, or (O E) peak,is illustrated by the black vertical line. The peak fitting error, peak,is0.1 mag.
1352 PARKER, HUMPHREYS, & LARSEN Vol. 126 With the peak of the blue ridge as a reference point, we then define three population or color groups; blue, intermediate, and red, based on their colors with respect to the (O E) peak : Blue. (O E) peak to (O E) peak 1.0 mag. Intermediate. (O E) peak 0.5 mag to (O E) peak +0.5 mag. Red. (O E) peak + 0.5 mag to (O E) peak + 1.5 mag. Halo and thick-disk stars make a major contribution to the blue color bin and will dominate the blue star counts fainter than O = 17th magnitude. The halo and thick-disk stars make a significant contribution to the intermediate color counts as well, but the disk stars will dominate at the brighter magnitudes. Fainter than 17th magnitude, however, the number of halo and thick-disk stars are comparable to the number of disk stars in the intermediate color range. In addition, of course the star counts in the red color range are almost exclusively composed of the old disk stars at all magnitudes. Defining the color bins in this way and following this procedure, we are in effect putting all of the star-count data on the same photometric system to within 0.1 mag. 3.1. Correcting for Blended Images For the fields closer to the Galactic plane, the high density of objects makes it increasingly difficult for the neural network to resolve and differentiate between a faint galaxy and close or overlapping stars. Optical examination of many blended images at low latitude confirm that at the faint limits many objects classified as galaxies are primarily two stars of about the same diameter or magnitude. This effect can be seen in Figure 6, which shows the number of nonstellar objects or galaxies from 217 plates in the range +90 b +20. There is an increasing number of nonstellar objects at lower latitudes, and the blended image problem is clearly most serious for fields with b 45. We can statistically correct our star counts for the expected number of merged stellar images if we know the expected number of galaxies as a function of apparent magnitude. To determine this, we combined the number counts of nonstellar images from our 120 fields with the 97 POSS I fields within 30 of the NGP, from the MAPS-NGP (Cabanela 1999). 3 We used all of the fields with b >+45 to determine the mean number of galaxies; to account for galaxy clustering, we adopted this number plus 3 as our baseline for the expected number of galaxies. All nonstellar images above this number are assumed to be blended stellar images. Treating these blended images statistically as two stars of comparable magnitude, we add them to our star counts according to each field s color distribution at that magnitude. The effect on the star counts naturally depends on the Galactic latitude of the field. The correction for blended images was 10% or less of the total counts for 80% of our fields. Only nine low-latitude fields had a statistical correction for blends of 15% 25%. 3.2. Completeness Tests The limiting magnitudes for our plates are typically 21 22 mag in O and 20th mag for the E plate; however, the completeness limit for our star counts will be significantly brighter, especially for our restricted color ranges. We determine the completeness limit from the observed luminosity function, the log N versus apparent magnitude plot, where log N is the logarithm of the differential star counts. The sample is considered to be incomplete where the plot begins to deviate from a linear relation or begins to turn over. In Figure 7, we show the O-band luminosity function for the combined star counts in all three color ranges for P323, at the NGP, from 12 to 22 mag. The luminosity function shows a gradual curvature or bending at the bright end but there is a well-defined linear section from 15 to 20 mag, where the luminosity function begins to turn over. For comparison, we also show the predicted luminosity function from GALMOD which parallels the observed star counts very well including the bend at the bright end. We estimate the completeness limit for our color groups on each plate from a least-squares fit to the linear or nearly linear portion of each luminosity function. This fit is illustrated in Figure 7 for P323, which is complete to O 20 3 See ftp://aps.umn.edu/pub/juan/mapsngp/. N gal 0 2000 4000 6000 8000 O = 15-18 <N > gal <N > + 3σ gal 20 30 40 50 60 70 80 90 Galactic Latitude [ ] Fig. 6. Number of objects classified as galaxy or nonstellar as a function of Galactic latitude. Only plates from b +45 were used to calculate the mean. 10 + log(a m ) 11 12 13 14 15 P323 [-1 2.5] GALMOD 12 13 14 15 16 17 18 19 20 21 22 O Magnitude Fig. 7. Luminosity function for P323 (65, +86 ) illustrates the leastsquares fit used to determine the completeness limit. In this case, at O 20 mag, the function turns over and is the completness limit. The asterisks are the predicted luminosity function from GALMOD. Note that it parallels the observed function.
No. 3, 2003 ASYMMETRIC THICK DISK. I. 1353-1 -0.5 0 0.5 1-1 -0.5 0 0.5 1-1 -0.5 0 0.5 1 (a) Blue Stars (b) Intermediate Stars (c) Red Stars 15 16 17 18 19 20 21 22 O Magnitude Fig. 8. Residuals from a least-squares fit to the luminosity function are used to identify the completeness limit for each plate in each color range. The asterisks are the residuals to a least-squares fit to the predicted luminosity function from GALMOD. It is important to note the GALMOD residuals follow the same trends and predict the same completeness limits. For field P323 (65, +86 ) at the NGP with (O E) peak =1.1,(a) blue= 18.5, (b) intermediate = 19, and (c) red = 19.5. mag for the combined color range. 4 We plot the residuals of the fit, so that the faintest magnitudes at which the counts begin to deviate is obvious. Figure 8 shows the residuals for P323 for the three color ranges. The completeness limit may be different for each color range. A comparison in each color range with the GALMOD predicted counts suggests the completeness limits may actually be somewhat fainter than we have determined. However, we are confident that 4 Our completeness limits are somewhat brighter than those listed in Larsen & Humphreys (2003), because we are determining it for subsets of the data selected by color, while they used all of the available star-count data. we have not overestimated the completeness limits of our star counts. 3.3. Star-Count Ratios and Their Uncertainties After all of the star-count data in each field have been corrected for interstellar reddening, we then count the number of stars in 1 mag bins from an O = 15 mag to the completeness limit determined for each of the three color ranges (Table 1). 5 A total of 5.6 million stars on the 120 fields are used in this study. The star-count ratio is then simply the ratio of these numbers for the matched plate pairs. The adopted completeness limit for a matched pair is set by the field with the brighter limit, which is almost always the field at the more southern declination. We find, in general, that the POSS I fields at the most negative declinations do not go as faint as the other POSS I fields due to a combination of shorter exposure times and higher air mass. In Table 2, we summarize the measured star-count ratios (R obs ) for the 40 matched pairs in quadrants I and IV above the Galactic plane for the three color ranges. We also include the error or uncertainty in the ratio obs and the expected ratio from GALMOD (R mod ). If the galaxy is symmetric, the expected ratio between matched fields on either side of the Sun-center line should be near one. However, many of the paired fields have plate centers that are not exactly matched in l and b, which can lead to ratios that are slightly different from one, but the expected ratio from the model accounts for these small differences, or offsets in the plate centers, for comparison with the observed counts. The errors in the counts are determined for each magnitude interval for each field by adding in quadrature the Poisson counting error and the zero-point error due to the uncertainty in the (O E) peak of 0.1 mag. The photometric error is random and compared with the Poisson and zeropoint errors will not make a substantial contribution to the total error. The total error also includes the additional uncertainty due to the number of stars added from the blended image correction. The error in the ratio for each plate pair is then sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Robs ¼ N 1 2 1 þ 2 2 ; ð1þ N 2 N 1 N 2 where N 1 and N 2 are the star counts for each plate in the relevant magnitude and color range, and 1 and 2 their respective errors in the counts. In a given color range, each plate may contribute differently to each error term. Table 2 also includes the super-ratio (R s ) of the observed to model ratios, its uncertainty Rs, and its significance (s) defined as ðr s 1Þ= Rs. This s-parameter is a measure of the significance of how much R s deviates from one. R s and s provide a statistical way for us to easily compare our many different measurements and evaluate the strength and importance of an excess or a deficiency at a specific l and b compared with the other fields. Even a cursory glance at the data in Table 2 shows that many fields in quadrant I have a significant excess of star counts, especially in the intermediate and blue color bins as 5 The actual star counts can be found in Tables B1 B4 at http://aps.umn.edu/parker/tables/.
TABLE 2 Star-Count Ratios +QI versus +QIV: Blue, Intermediate, and Red Plate Pair Blue Intermediate Red O Mag. Bin R obs R mod R s Rs s R obs R mod R s Rs s R obs R mod R s Rs s P568/ P915... 15 16 1.23 1.25 0.98 0.18 0.09 1.32 1.19 1.11 0.03 3.78 1.29 1.30 0.99 0.27 0.03 16 17 1.18 1.28 0.92 0.18 0.42 1.23 1.23 1.00 0.02 0.05 1.26 1.19 1.06 0.30 0.20 17 18 1.54 1.26 1.22 0.21 1.04 1.55 1.26 1.23 0.03 8.54 1.47 1.14 1.30 0.31 0.96 16 18 1.39 1.27 1.09 0.14 0.66 1.41 1.25 1.13 0.02 7.24 1.39 1.15 1.20 0.22 0.93 P567/ P859... 15 16 1.17 1.21 0.96 0.20 0.18 1.02 1.16 0.88 0.03 4.69 1.47 1.21 1.22 0.46 0.48 16 17 1.47 1.22 1.20 0.25 0.80 1.17 1.19 0.99 0.03 0.44 1.14 1.13 1.01 0.35 0.02 17 18 0.96 1.20 0.80 0.15 1.35 0.93 1.22 0.76 0.02 14.97 0.92 1.12 0.82 0.21 0.85 16 18 1.11 1.20 0.93 0.13 0.59 1.01 1.21 0.84 0.01 11.40 0.97 1.12 0.87 0.18 0.74 P508/ P914... 15 16 1.41 1.21 1.16 0.23 0.71 1.35 1.16 1.17 0.03 4.89 1.27 1.22 1.04 0.29 0.14 16 17 1.23 1.22 1.01 0.18 0.06 1.34 1.18 1.13 0.03 4.16 1.43 1.13 1.26 0.36 0.74 17 18 1.80 1.20 1.50 0.27 1.87 1.61 1.21 1.33 0.04 8.91 1.21 1.11 1.08 0.29 0.29 16 18 1.55 1.21 1.29 0.16 1.73 1.50 1.20 1.25 0.03 9.77 1.28 1.12 1.15 0.23 0.65 P448/ P913... 15 16 1.60 1.15 1.40 0.28 1.42 1.42 1.11 1.28 0.04 6.32 0.74 1.13 0.66 0.21 1.65 16 17 1.58 1.15 1.37 0.26 1.46 1.58 1.13 1.40 0.04 10.27 0.87 1.08 0.81 0.26 0.76 17 18 1.79 1.14 1.58 0.29 1.97 1.93 1.15 1.68 0.04 15.16 1.20 1.08 1.11 0.29 0.37 16 18 1.70 1.14 1.49 0.20 2.46 1.77 1.14 1.55 0.03 18.49 1.07 1.08 0.99 0.20 0.07 P389/ P912... 15 16 1.05 1.22 0.86 0.17 0.83 1.12 1.17 0.96 0.03 1.25 0.97 1.18 0.82 0.26 0.68 16 17 1.23 1.22 1.01 0.20 0.03 1.06 1.19 0.89 0.03 4.31 0.63 1.11 0.57 0.18 2.37 17 18 1.08 1.19 0.90 0.19 0.52 1.17 1.22 0.96 0.02 1.90 0.94 1.12 0.84 0.23 0.69 16 18 1.14 1.20 0.94 0.14 0.41 1.12 1.21 0.93 0.02 4.18 0.82 1.12 0.74 0.16 1.68 P333/ P911... 15 16 1.74 1.11 1.57 0.31 1.85 1.61 1.09 1.48 0.05 9.49 1.20 1.09 1.11 0.40 0.27 16 17 1.48 1.11 1.33 0.27 1.21 1.65 1.10 1.51 0.05 10.44 1.80 1.06 1.70 0.55 1.27 17 18 2.33 1.09 2.13 0.42 2.65 2.03 1.11 1.83 0.07 11.51 1.60 1.06 1.51 0.38 1.35 16 18 1.92 1.10 1.74 0.25 2.97 1.86 1.11 1.68 0.04 15.59 1.67 1.06 1.57 0.31 1.84 P278/ P910... 15 16 1.08 0.94 1.14 0.29 0.49 0.95 0.95 1.00 0.03 0.02 0.55 0.96 0.57 0.20 2.11 16 17 1.13 0.94 1.20 0.27 0.73 0.90 0.95 0.95 0.03 1.49 0.59 0.97 0.61 0.19 2.00 17 18 1.05 0.95 1.11 0.21 0.51 0.99 0.94 1.05 0.03 1.62 0.82 0.97 0.85 0.21 0.72 16 18 1.08 0.95 1.14 0.17 0.85 0.95 0.94 1.01 0.02 0.46 0.74 0.97 0.76 0.15 1.63 P228/ P909... 15 16 0.92 0.99 0.93 0.19 0.36 0.99 0.99 0.99 0.03 0.18 1.17 0.99 1.18 0.42 0.43 16 17 0.80 0.99 0.81 0.17 1.16 0.82 0.99 0.83 0.02 7.51 1.02 1.00 1.03 0.31 0.08 17 18 0.92 0.99 0.93 0.19 0.38 0.88 0.99 0.89 0.03 4.45 0.77 1.00 0.77 0.17 1.30 16 18 0.87 0.99 0.88 0.13 0.97 0.85 0.99 0.86 0.02 7.92 0.85 1.00 0.85 0.15 0.98 P181/ P908... 15 16 0.92 0.89 1.04 0.22 0.18 0.93 0.91 1.02 0.04 0.61 0.63 0.92 0.68 0.25 1.29 16 17 1.02 0.88 1.16 0.21 0.77 1.00 0.90 1.12 0.04 2.81 0.63 0.94 0.67 0.20 1.65 17 18 0.89 0.89 1.00 0.18 0.01 1.04 0.88 1.18 0.03 5.89 0.66 0.93 0.71 0.16 1.86 16 18 0.95 0.89 1.06 0.13 0.47 1.03 0.89 1.15 0.03 6.16 0.65 0.94 0.69 0.13 2.44 P566/ P802... 15 16 1.58 1.10 1.43 0.33 1.33 1.33 1.08 1.23 0.04 5.51 1.29 1.09 1.19 0.42 0.45 16 17 1.40 1.10 1.27 0.27 1.02 1.30 1.09 1.19 0.03 6.28 1.40 1.06 1.32 0.41 0.78 17 18 1.53 1.09 1.40 0.30 1.34 1.34 1.11 1.21 0.03 6.69 1.26 1.06 1.19 0.29 0.66 16 18 1.47 1.09 1.35 0.21 1.69 1.33 1.10 1.20 0.02 9.04 1.30 1.06 1.23 0.24 0.96 P506/ P801... 15 16 1.20 1.10 1.09 0.29 0.31 1.45 1.08 1.34 0.08 4.39 1.42 1.07 1.33 0.52 0.63 16 17 1.70 1.09 1.56 0.36 1.55 1.49 1.09 1.36 0.04 8.69 1.29 1.05 1.22 0.41 0.55 17 18 1.52 1.08 1.41 0.27 1.48 1.40 1.10 1.27 0.03 8.15 1.27 1.06 1.19 0.26 0.75 16 18 1.59 1.08 1.47 0.22 2.13 1.43 1.10 1.31 0.03 11.87 1.27 1.06 1.20 0.22 0.93 P507/ P858... 15 16 1.19 1.19 1.00 0.21 0.00 1.12 1.15 0.98 0.03 0.70 1.04 1.16 0.89 0.31 0.35 16 17 1.31 1.19 1.10 0.22 0.46 1.19 1.17 1.02 0.03 0.63 1.12 1.11 1.01 0.32 0.04 17 18 1.26 1.17 1.08 0.21 0.37 1.30 1.20 1.09 0.03 3.56 1.42 1.11 1.28 0.32 0.88 16 18 1.28 1.18 1.09 0.15 0.57 1.26 1.19 1.06 0.02 3.19 1.32 1.11 1.19 0.24 0.81 P447/ P857... 15 16 1.06 1.16 0.92 0.21 0.39 1.14 1.12 1.02 0.04 0.49 0.90 1.13 0.80 0.27 0.73 16 17 1.16 1.16 1.00 0.23 0.01 1.16 1.14 1.01 0.03 0.39 0.81 1.09 0.75 0.22 1.16 17 18 1.35 1.14 1.18 0.23 0.79 1.31 1.17 1.12 0.03 4.16 0.87 1.09 0.80 0.18 1.10 16 18 1.28 1.15 1.11 0.17 0.67 1.25 1.16 1.08 0.02 3.67 0.85 1.09 0.78 0.14 1.56 P388/ P856... 15 16 1.61 1.20 1.34 0.31 1.08 1.25 1.16 1.08 0.04 1.82 0.90 1.14 0.78 0.28 0.78 16 17 1.60 1.19 1.34 0.32 1.07 1.33 1.18 1.13 0.04 3.44 0.90 1.10 0.81 0.26 0.71 17 18 1.59 1.17 1.36 0.31 1.14 1.43 1.21 1.18 0.03 5.20 1.05 1.12 0.94 0.24 0.24 16 18 1.59 1.18 1.35 0.22 1.55 1.39 1.20 1.16 0.03 6.23 1.00 1.11 0.90 0.18 0.56 P332/ P855... 15 16 0.98 1.13 0.86 0.21 0.66 1.07 1.10 0.97 0.03 0.80 2.15 1.09 1.97 0.71 1.36 16 17 1.20 1.13 1.06 0.25 0.25 1.22 1.12 1.09 0.04 2.16 1.89 1.07 1.78 0.50 1.55 17 18 1.28 1.12 1.15 0.24 0.63 1.18 1.14 1.03 0.08 0.42 1.23 1.08 1.14 0.25 0.57 16 18 1.25 1.12 1.12 0.18 0.66 1.19 1.13 1.05 0.05 1.03 1.39 1.07 1.30 0.22 1.32 P277/ P854... 15 16 1.12 1.01 1.10 0.24 0.43 0.98 1.01 0.97 0.04 0.84 1.10 1.01 1.09 0.36 0.25 16 17 0.93 1.01 0.92 0.18 0.44 0.91 1.01 0.89 0.03 4.05 1.21 1.01 1.21 0.31 0.67 17 18 0.80 1.02 0.79 0.12 1.77 0.83 1.02 0.82 0.02 8.63 1.09 1.00 1.09 0.18 0.48 1354
TABLE 2 Continued Plate Pair Blue Intermediate Red O Mag. Bin R obs R mod R s Rs s R obs R mod R s Rs s R obs R mod R s Rs s 16 18 0.85 1.01 0.84 0.10 1.64 0.86 1.02 0.85 0.02 9.28 1.13 1.00 1.12 0.16 0.78 P227/ P853... 15 16 1.15 1.03 1.12 0.27 0.42 1.26 1.02 1.23 0.05 4.47 0.57 1.02 0.55 0.22 2.01 16 17 1.03 1.03 1.00 0.24 0.00 1.36 1.03 1.32 0.05 6.18 0.87 1.02 0.86 0.25 0.55 17 18 0.93 1.03 0.90 0.20 0.48 1.28 1.03 1.24 0.04 5.49 1.03 1.02 1.02 0.22 0.07 16 18 0.97 1.03 0.94 0.15 0.38 1.31 1.03 1.27 0.03 8.18 0.98 1.02 0.96 0.17 0.22 P180/ P852... 15 16 1.02 0.92 1.10 0.28 0.37 1.02 0.94 1.09 0.04 2.17 0.77 0.96 0.80 0.32 0.61 16 17 0.87 0.93 0.94 0.24 0.26 0.92 0.93 0.99 0.03 0.41 0.83 0.96 0.86 0.24 0.56 17 18 0.81 0.94 0.86 0.22 0.63 0.91 0.93 0.98 0.07 0.22 0.80 0.95 0.84 0.20 0.80 16 18 0.84 0.94 0.89 0.16 0.65 0.91 0.93 0.99 0.04 0.35 0.81 0.96 0.85 0.15 0.97 P565/ P742... 15 16 1.21 1.08 1.13 0.28 0.46 1.19 1.06 1.12 0.04 3.03 1.11 1.05 1.06 0.42 0.13 16 17 1.25 1.07 1.17 0.25 0.66 1.30 1.07 1.22 0.04 5.16 1.00 1.04 0.96 0.31 0.13 17 18 1.03 1.06 0.97 0.20 0.15 1.15 1.08 1.06 0.03 2.12 1.16 1.05 1.11 0.26 0.42 16 18 1.11 1.06 1.05 0.15 0.30 1.21 1.07 1.12 0.02 5.04 1.11 1.04 1.06 0.20 0.29 P505/ P741... 15 16 1.27 1.09 1.17 0.26 0.66 1.18 1.07 1.11 0.05 2.21 1.45 1.06 1.37 0.58 0.63 16 17 1.17 1.08 1.09 0.23 0.37 1.08 1.08 1.00 0.03 0.07 1.31 1.05 1.25 0.38 0.64 17 18 1.03 1.07 0.96 0.20 0.20 1.04 1.09 0.96 0.03 1.53 1.12 1.06 1.06 0.24 0.24 16 18 1.08 1.07 1.01 0.15 0.06 1.06 1.09 0.97 0.02 1.28 1.17 1.05 1.11 0.20 0.56 P446/ P800... 15 16 1.47 1.09 1.35 0.30 1.17 1.39 1.07 1.29 0.05 5.39 1.68 1.06 1.58 0.57 1.03 16 17 1.58 1.08 1.46 0.35 1.29 1.39 1.08 1.28 0.08 3.31 1.56 1.05 1.49 0.46 1.06 17 18 1.25 1.07 1.17 0.30 0.56 1.45 1.09 1.33 0.15 2.19 1.77 1.06 1.67 0.43 1.56 16 18 1.38 1.08 1.28 0.23 1.22 1.42 1.09 1.31 0.09 3.30 1.70 1.05 1.61 0.33 1.88 P387/ P799... 15 16 1.17 1.12 1.04 0.25 0.18 1.19 1.10 1.09 0.04 2.03 1.02 1.07 0.95 0.35 0.15 16 17 1.17 1.11 1.05 0.22 0.24 1.14 1.11 1.03 0.03 0.85 1.06 1.06 0.99 0.29 0.02 17 18 1.16 1.09 1.06 0.21 0.28 1.36 1.12 1.21 0.04 5.52 1.40 1.08 1.30 0.28 1.06 16 18 1.16 1.10 1.06 0.16 0.37 1.27 1.12 1.13 0.03 5.19 1.29 1.07 1.20 0.21 0.95 P331/ P798... 15 16 1.48 1.10 1.35 0.30 1.16 1.77 1.08 1.64 0.07 8.66 0.69 1.06 0.66 0.22 1.55 16 17 1.22 1.09 1.12 0.25 0.49 1.61 1.09 1.48 0.06 8.38 1.20 1.05 1.14 0.33 0.43 17 18 1.03 1.07 0.96 0.21 0.19 1.30 1.10 1.19 0.08 2.41 1.28 1.06 1.21 0.26 0.80 16 18 1.10 1.08 1.02 0.16 0.15 1.42 1.09 1.30 0.05 5.67 1.25 1.06 1.18 0.21 0.90 P276/ P797... 15 16 0.98 0.99 0.98 0.28 0.06 0.96 0.99 0.97 0.05 0.67 0.57 0.99 0.57 0.23 1.88 16 17 1.21 0.99 1.23 0.31 0.75 1.03 0.99 1.04 0.04 0.94 0.57 1.00 0.57 0.17 2.47 17 18 1.15 0.98 1.17 0.26 0.67 1.16 0.99 1.18 0.04 4.62 0.76 1.00 0.76 0.15 1.55 16 18 1.18 0.99 1.20 0.20 0.99 1.11 0.99 1.12 0.03 4.07 0.69 1.00 0.70 0.12 2.58 P226/ P796... 15 16 0.86 1.04 0.82 0.20 0.88 0.94 1.04 0.90 0.06 1.72 1.43 1.02 1.40 0.53 0.75 16 17 0.99 1.04 0.96 0.22 0.19 0.84 1.04 0.80 0.03 5.64 0.95 1.02 0.93 0.26 0.27 17 18 0.94 1.02 0.92 0.21 0.39 0.89 1.04 0.86 0.03 5.12 0.99 1.03 0.96 0.20 0.21 16 18 0.96 1.03 0.94 0.15 0.42 0.87 1.04 0.83 0.02 7.58 0.98 1.03 0.95 0.16 0.32 P179/ P795... 15 16 1.04 0.97 1.07 0.30 0.23 0.95 0.98 0.97 0.05 0.72 0.95 0.98 0.97 0.39 0.09 16 17 0.94 0.97 0.97 0.22 0.16 0.93 0.97 0.95 0.04 1.31 1.02 0.99 1.03 0.33 0.09 17 18 0.91 0.96 0.95 0.20 0.28 0.89 0.96 0.92 0.03 2.64 1.06 0.98 1.08 0.24 0.31 16 18 0.92 0.96 0.96 0.15 0.31 0.90 0.97 0.93 0.02 2.85 1.05 0.98 1.06 0.20 0.32 P564/ P682... 15 16 0.96 1.05 0.91 0.22 0.39 1.10 1.04 1.06 0.04 1.38 1.04 1.03 1.01 0.37 0.01 16 17 1.12 1.04 1.07 0.26 0.29 1.11 1.05 1.06 0.03 1.69 0.75 1.03 0.73 0.22 1.26 17 18 1.01 1.04 0.97 0.19 0.17 1.05 1.05 1.00 0.03 0.09 0.93 1.03 0.90 0.19 0.53 16 18 1.04 1.04 1.00 0.15 0.02 1.07 1.05 1.02 0.02 0.97 0.86 1.03 0.84 0.15 1.10 P504/ P681... 15 16 1.06 1.06 0.99 0.23 0.03 1.21 1.05 1.15 0.05 3.07 0.88 1.04 0.84 0.24 0.64 16 17 1.36 1.06 1.29 0.29 0.99 1.24 1.06 1.17 0.05 3.64 0.77 1.03 0.75 0.19 1.30 17 18 1.38 1.05 1.31 0.26 1.20 1.39 1.07 1.30 0.05 6.52 1.18 1.04 1.13 0.22 0.59 16 18 1.37 1.05 1.30 0.19 1.55 1.32 1.06 1.24 0.03 7.39 1.00 1.04 0.96 0.15 0.26 P445/ P740... 15 16 1.16 1.09 1.07 0.25 0.27 1.06 1.07 0.99 0.04 0.15 0.89 1.05 0.85 0.29 0.52 16 17 0.97 1.08 0.90 0.17 0.59 1.04 1.08 0.97 0.03 1.06 0.93 1.05 0.89 0.24 0.46 17 18 1.13 1.07 1.06 0.20 0.29 1.10 1.09 1.01 0.04 0.23 0.90 1.06 0.86 0.18 0.79 16 18 1.06 1.07 0.99 0.13 0.08 1.08 1.09 0.99 0.03 0.32 0.91 1.05 0.87 0.15 0.91 P386/ P739... 15 16 0.69 1.11 0.63 0.14 2.59 0.87 1.09 0.80 0.04 5.41 1.06 1.06 0.99 0.36 0.01 16 17 0.81 1.09 0.74 0.16 1.63 0.91 1.10 0.83 0.03 5.53 0.81 1.06 0.76 0.23 1.05 17 18 0.96 1.08 0.89 0.18 0.62 1.08 1.11 0.97 0.03 0.79 0.92 1.07 0.86 0.18 0.80 16 18 0.89 1.09 0.82 0.12 1.45 1.01 1.11 0.91 0.02 4.00 0.88 1.07 0.82 0.14 1.24 P330/ P738... 15 16 0.93 1.10 0.85 0.21 0.74 1.05 1.08 0.97 0.04 0.60 1.50 1.05 1.42 0.54 0.78 16 17 1.47 1.08 1.36 0.29 1.22 1.22 1.09 1.12 0.05 2.53 0.97 1.05 0.92 0.25 0.33 17 18 1.51 1.07 1.41 0.25 1.62 1.37 1.10 1.25 0.04 5.64 1.24 1.06 1.16 0.25 0.66 16 18 1.49 1.08 1.39 0.19 2.03 1.30 1.10 1.19 0.03 5.96 1.14 1.06 1.07 0.18 0.40 P275/ P737... 15 16 0.79 1.01 0.79 0.18 1.17 0.88 1.01 0.88 0.04 2.72 0.54 1.00 0.54 0.20 2.36 16 17 0.75 1.00 0.75 0.18 1.44 0.86 1.00 0.86 0.04 3.86 0.67 1.00 0.67 0.18 1.82 17 18 0.74 1.00 0.74 0.16 1.60 0.93 1.00 0.93 0.03 2.22 0.67 1.00 0.66 0.13 2.53 1355
1356 PARKER, HUMPHREYS, & LARSEN Vol. 126 TABLE 2 Continued Plate Pair Blue Intermediate Red O Mag. Bin R obs R mod R s Rs s R obs R mod R s Rs s R obs R mod R s Rs s 16 18 0.74 1.00 0.74 0.12 2.13 0.90 1.00 0.90 0.02 4.12 0.67 1.00 0.67 0.11 3.11 P225/ P736... 15 16 0.73 1.06 0.70 0.21 1.44 0.98 1.05 0.93 0.05 1.45 1.07 1.03 1.04 0.40 0.10 16 17 1.01 1.05 0.97 0.21 0.16 1.09 1.05 1.04 0.04 1.03 0.78 1.03 0.76 0.23 1.06 17 18 1.07 1.04 1.03 0.20 0.13 1.15 1.06 1.09 0.04 2.41 0.87 1.04 0.84 0.18 0.88 16 18 1.04 1.04 1.00 0.15 0.01 1.13 1.06 1.07 0.03 2.55 0.84 1.04 0.82 0.14 1.31 P178/ P735... 15 16 0.79 0.99 0.80 0.26 0.76 0.87 0.99 0.88 0.05 2.47 0.97 0.99 0.98 0.36 0.05 16 17 0.99 0.99 1.00 0.22 0.00 0.91 0.99 0.92 0.04 2.22 0.89 0.99 0.90 0.23 0.46 17 18 0.94 0.98 0.96 0.20 0.22 0.88 0.98 0.89 0.05 2.35 0.78 0.99 0.78 0.13 1.66 16 18 0.96 0.98 0.98 0.15 0.17 0.89 0.99 0.90 0.03 3.18 0.81 0.99 0.82 0.11 1.58 P444/ P680... 15 16 1.22 1.12 1.09 0.24 0.38 1.03 1.12 0.92 0.04 2.02 1.63 1.18 1.38 0.42 0.90 16 17 1.16 1.11 1.04 0.19 0.21 1.00 1.13 0.88 0.03 4.28 1.56 1.16 1.35 0.27 1.27 17 18 1.24 1.14 1.09 0.16 0.54 1.09 1.14 0.95 0.03 1.57 1.21 1.12 1.08 0.15 0.52 16 18 1.21 1.13 1.06 0.12 0.54 1.05 1.14 0.93 0.02 3.63 1.31 1.14 1.15 0.13 1.15 P385/ P679... 15 16 1.14 1.04 1.10 0.26 0.37 1.06 1.04 1.02 0.05 0.51 1.22 1.08 1.13 0.37 0.35 16 17 1.29 1.04 1.24 0.26 0.93 1.08 1.04 1.04 0.04 1.06 1.00 1.08 0.92 0.21 0.39 17 18 0.96 1.04 0.92 0.17 0.49 1.25 1.05 1.20 0.04 5.32 1.33 1.07 1.24 0.18 1.34 16 18 1.07 1.04 1.03 0.14 0.22 1.19 1.05 1.13 0.03 5.00 1.23 1.08 1.14 0.14 0.97 P329/ P678... 15 16 1.18 1.09 1.08 0.26 0.30 0.94 1.08 0.87 0.04 3.08 0.58 1.05 0.56 0.20 2.17 16 17 1.28 1.08 1.19 0.27 0.70 1.02 1.09 0.93 0.04 1.60 0.72 1.05 0.69 0.20 1.52 17 18 1.09 1.07 1.02 0.20 0.09 1.12 1.10 1.02 0.04 0.44 1.20 1.07 1.12 0.18 0.68 16 18 1.17 1.08 1.09 0.16 0.55 1.07 1.10 0.98 0.03 0.68 1.00 1.06 0.95 0.14 0.37 P274/ P677... 15 16 1.00 1.02 0.98 0.23 0.09 0.99 1.01 0.97 0.05 0.55 1.03 1.01 1.02 0.39 0.04 16 17 0.96 1.01 0.95 0.23 0.20 0.91 1.02 0.90 0.04 2.42 0.88 1.01 0.87 0.23 0.55 17 18 0.80 1.01 0.79 0.15 1.33 0.92 1.02 0.90 0.04 2.66 1.12 1.01 1.11 0.22 0.51 16 18 0.86 1.01 0.85 0.13 1.13 0.92 1.02 0.90 0.03 3.60 1.03 1.01 1.02 0.16 0.12 P224/ P676... 15 16 1.19 1.06 1.12 0.32 0.38 1.19 1.05 1.13 0.06 2.23 0.94 1.03 0.92 0.31 0.28 16 17 1.29 1.05 1.23 0.32 0.72 1.20 1.06 1.13 0.05 2.56 1.00 1.03 0.97 0.25 0.13 17 18 0.97 1.04 0.93 0.20 0.37 1.16 1.06 1.09 0.04 2.09 1.26 1.04 1.21 0.23 0.91 16 18 1.09 1.05 1.04 0.17 0.23 1.18 1.06 1.11 0.03 3.26 1.16 1.04 1.11 0.17 0.66 P177/ P675... 15 16 0.95 1.00 0.95 0.24 0.19 1.02 1.00 1.02 0.05 0.48 1.25 1.00 1.25 0.42 0.60 16 17 1.08 1.00 1.08 0.24 0.33 1.00 1.00 1.00 0.04 0.10 1.18 1.00 1.18 0.30 0.60 17 18 1.18 0.99 1.19 0.24 0.78 1.13 1.00 1.14 0.05 2.58 1.43 1.00 1.43 0.26 1.66 16 18 1.13 0.99 1.14 0.17 0.82 1.07 1.00 1.08 0.03 2.18 1.33 1.00 1.33 0.20 1.68 compared with their matched fields in quadrant IV. A comparison between the observed and predicted color-magnitude diagrams (CMDs) and the expected star counts from GALMOD shows that the asymmetry in the counts is due to an excess in the quadrant I fields rather than a deficiency in the star counts in quadrant IV. Figure 4 shows several observed CMDs compared with the model-predicted distribution. In the next section, we discuss the spatial distribution of the star-count excess and the evidence for an asymmetry in the stellar distribution in quadrant I versus quadrant IV. 4.1. Quadrant I versus Quadrant IV Above the Plane In our blue color group, the halo/thick-disk stars make their greatest contribution to the counts fainter than 16th magnitude. For many of the fields, the 15 16 mag bin contains only a very small fraction of halo/thick-disk stars. Therefore, in Figure 9 we show the results for the combined magnitude bin 16 18 mag for the blue color range. We adopt a 1.5, or 87% confidence, as evidence for a statistically significant deviation and use three significance ranges, 1.5 s < 2, 2 s < 3 (greater than 95% confidence), and s 3 (greater than 99% confidence). A plus or minus sign indicates an excess or deficiency in the star 4. STAR-COUNT EXCESS AND THE ASYMMETRY To investigate the significance and spatial extent of the star-count excess in quadrant I, we discuss several maps in l and b of the significance parameter s for each color range. Fig. 9. Map in l and b of the significance parameter (s) for the combined 16 18 mag bin for the blue color group.
No. 3, 2003 ASYMMETRIC THICK DISK. I. 1357 counts and a blank field indicates that the deviation in the expected ratio is not significant, s < 1.5. Because of the relatively small sample sizes, the errors in the ratios for many of the fields are relatively large (0.1 0.2), and the resulting significance parameter is small. Although a number of fields in quadrant I show evidence for an excess of star counts in the blue color range, it is not uniform. We find that the number of fields having an excess and the significance of the excess increases with fainter magnitudes. This suggests that there is more influence from the halo/thick-disk population. We conclude that the star-count excess among the blue stars is clumpy, although there are a number of fields with a significant excess of star counts in the region b 30 40 from l 20 to 50, corresponding to the direction where Larsen & Humphreys (1996) first reported the excess in the faint blue stars. There are several additional fields in this region that show an excess of blue star counts but are not statistically significant because of the large error in the ratios. The intermediate color range is centered on the maximum of the stellar color distribution, and the number of stars in this sample is much greater than in either the blue or red groups. It also overlaps the blue color bin. Fainter than magnitude 16, the halo and thick-disk stars contribute about 50% of the expected counts in this color range. It is unfortunate that these two populations cannot be separated more completely by color. However, this color range contains the majority of thick-disk stars and is therefore important for distinguishing which population is responsible for excess and for establishing the boundaries of the asymmetry. In Figure 10, we use the intermediate color range to illustrate the variation of the ratio R s with magnitude. The size of the excess and the spatial extent of the asymmetry both clearly increase with fainter magnitudes. Figure 11 shows the significance of the star-count ratio summed over the 16 18 mag range, similar to Figure 9 for the blue color group. We follow the criteria for the significance described above but note that because of the large sample size, the counting statistics yield much smaller errors for the ratios (0.02 0.05). Consequently, the significance, s, is quite large for many fields, and small differences between the observed and model ratios can yield a statistically significant result, even when R s is near one. For this reason, fields with statistically significant results but with 0.9 R s 1.1 are shown as a white box with a large gray circle in the center. The intermediate color range reveals a clearly recognizable region in quadrant I with a significant excess or asymmetry in the star counts compared with the complementary fields in quadrant IV. The region of the asymmetry Fig. 10. Maps in l and b of the significance parameter (s) for the intermediate color group for the O-magnitude bins (a) 15 16, (b) 16 17, and (c) 17 18. Fig. 11. Map in l and b of the significance parameter (s) for the combined O = 16 18 mag bin for the intermediate color group. Compare with Fig. 9.