I. Introduction


2. The 2MASS Sky

b. 2MASS Extended Sources in the Zone of Avoidance

The so-called "Zone of Avoidance" (ZoA) refers to the area contained within 5° or 10° of the disk plane of the Milky Way. Gas and dust obscures starlight within this region and screens nearly all background extragalactic objects from traditional optical-wavelength surveys; as such, the Galactic plane has historically been avoided by astronomers. 2MASS penetrates this zone with its near-infrared wavelength detectors, providing a "window" that is 90% free of obscuration (relative to that of optical wavelengths). The large nearby spiral galaxy (Figure 1) is just one example of a new 2MASS discovery of objects hidden beyond our Galaxy.

Figure 1

The major limitation that 2MASS suffers in the ZoA is "confusion" noise from starlight that, like background galaxies, is suddenly revealed in the near-infrared. The confusion noise becomes appreciable within an area ±5° of the Galactic plane. The end result is a loss of sensitivity, as the surface density of stars increases exponentially, and a profusion of multiple surface groupings of stars (doubles, triples, etc.) that can, under relatively rare circumstances, mimic galaxies and be picked up by the automated extended source detection and extraction routines (see Section IV.5). The typical reliability of the 2MASS Extended Source Catalog in the Galactic plane ranges from 20 to 80% (compared to >98% well out of the plane), where the primary contaminant is multiple stellar groups.

To demonstrate the ability of 2MASS to find galaxies in the ZoA, probable extended sources (most of which are galaxies or H II regions) from a small area of the sky are given in Table 1 and are shown below. The area corresponds to 53° < glon < 59°, and -2.5° < glat < 4.5°, giving a total of ~40 square degrees. This targeted region is illustrated in the "sky" plot of all 2MASS galaxies in the First Incremental Data Release, shown in Figure 2. The area is populated by both Galactic H II regions, planetary nebulae and extragalactic objects (galaxies). The left panel of the figure shows all of the sources. White dots represent new (previously uncatalogued) extended sources (most of which are probably galaxies), green dots represent galaxies found in NED (thus, previously catalogued), the dashed red line represents the Galactic plane at glat=0°, and the two dotted red lines represents glat=±5°. The right panel shows a subregion near the Galactic plane, defined by 53° < glon < 59°, -2.5° < glat < 4°. Verified extended sources (images shown below) are represented with blue filled circles. White points within the blue box represent false extended sources (typically multiple stars).

Figure 2

Figure 3 shows a mosaic of 45´´×45´´ JHKs three-color-composite "Postage Stamp Images" of extended sources, located within the area 53° < glon < 59°, -2.5° < glat < 4° and listed in Table 1. Figure 4 shows a much larger view of one of these fields.

Figure 3Figure 4

Explanation of Figures and Text

The 2MASS images represent a "three-color" RGB composite of the J-band (blue), H-band (green), and Ks-band (red) images.

The DSS image refers to the STScI Digitized Sky Survey. These images are based on photographic (optical-wavelength) data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of Caltech, AAO, RAO and ESO. The images from both surveys range in size from 30´´ to 60´´ in width. The position coordinates correspond to the intensity-weighted centroid of the object. The equatorial coordinates are given in units of degrees, as are the galactic coordinates (glon, glat).

The integrated fluxes come from the elliptical isophotal (Ks-band 20 mag per sq. arcsec isophote) aperture measurements. The size of the object refers to the semi-major axis of this elliptical aperture. The measurement uncertainties encorporate the Poisson flux error, including the confusion noise. N.B.: We caution, however, that every source in the table is contaminated by flux from foreground stars, which we have attempted to minimize. The error due to this contamination far outweighs the quoted flux uncertainties (and remains largely unknown).

The Galactic dust extinction is derived from UC Berkeley composite COBE/DIRBE/IRAS/ISSA maps with the zodiacal foreground and confirmed point sources removed (see Schlegel et al. 1998, ApJ, 500, 525). The extinction is computed using the DIRBE 100 and 240 micron data, inferring the dust temperature, then using the 100 micron map to convert to a grid proportional to dust column density. More information can be found here.

NED refers to the NASA Extragalactic Database, a general all-purpose database warehouse of galaxies, including classic catalogs (e.g., NGC) and the latest deep surveys. SIMBAD refers to the database "Set of Identifications, Measurements, and Bibliography for Astronomical Data", maintained by Centre de Donnees Astronomiques de Strasbourg, France. It contains catalogs and information relevant to Galactic objects (e.g., stars, H II regions, planetary nebulae, etc).

2MASS J+H+Ks DSS Legend
(288.569336, 20.708765); glon = 53.9250, glat = 4.4976
Estimated Extinction (mag): AV = 4.2 (AK = 0.42)
Size: 10.8", semi-major axis
Flux (J,H,Ks mag): 13.84, 12.95, 12.54
Color (J-H, H-Ks, J-Ks): 0.89, 0.41, 1.30
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(290.385803, 23.333241); glon = 57.0397, glat = 4.2188
Estimated Extinction (mag): AV = 4.0 (AK = 0.40)
Size: 14.2", semi-major axis
Flux (J,H,Ks mag): 13.69, 12.56, 12.02
Color (J-H, H-Ks, J-Ks): 1.13, 0.54, 1.67
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(289.078400, 20.192081); glon = 53.6856, glat = 3.836
Estimated Extinction (mag): AV = 4.8 (AK = 0.48)
Size: 15.2", semi-major axis
Flux (J,H,Ks mag): 13.02, 12.17, 11.73
Color (J-H, H-Ks, J-Ks): 0.85, 0.44, 1.28
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(290.951813, 23.331978); glon = 57.2822, glat = 3.7588
Estimated Extinction (mag): AV = 4.5 (AK = 0.45)
Size: 10.6", semi-major axis
Flux (J,H,Ks mag): 13.84, 12.87, 12.40
Color (J-H, H-Ks, J-Ks): 0.97, 0.47, 1.44
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(290.366516, 22.092396); glon = 55.9313, glat = 3.6554
Estimated Extinction (mag): AV = 4.9 (AK = 0.49)
Size: 11.4", semi-major axis
Flux (J,H,Ks mag): 13.37, 12.30, 11.82
Color (J-H, H-Ks, J-Ks): 1.07, 0.48, 1.55
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(290.201202, 21.631920); glon = 55.4515, glat = 3.5761
Estimated Extinction (mag): AV = 5.8 (AK = 0.58)
Size: 12.2", semi-major axis
Flux (J,H,Ks mag): 13.94, 12.91, 12.38
Color (J-H, H-Ks, J-Ks): 1.03, 0.53, 1.57
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(290.669281, 22.371202); glon = 56.3096, glat = 3.5382
Estimated Extinction (mag): AV = 4.4 (AK = 0.43)
Size: 21", semi-major axis
Flux (J,H,Ks mag): 12.96, 12.08. 11.73
Color (J-H, H-Ks, J-Ks): 0.88, 0.35, 1.23
NED: no entry
SIMBAD: IRAS 19206+2216
Probable Nature: ???
(290.386078, 21.550463); glon = 55.4598, glat = 3.3860
Estimated Extinction (mag): AV = 5.8 (AK = 0.58)
Size: 14.2", semi-major axis
Flux (J,H,Ks mag): 13.68, 12.55, 12.07
Color (J-H, H-Ks, J-Ks): 1.13, 0.48, 1.61
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(291.046387, 22.137291); glon = 56.2669, glat = 3.1202
Estimated Extinction (mag): AV = 4.8 (AK = 0.48)
Size: 14", semi-major axis
Flux (J,H,Ks mag): 12.90, 11.99, 11.53
Color (J-H, H-Ks, J-Ks): 0.91, 0.45, 1.36
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy or double star
(296.078552, 18.258453); glon = 55.1629, glat = -2.8896
Estimated Extinction (mag): AV = 2.3 (AK = 0.23)
Size: 18.4", semi-major axis
Flux (J,H,Ks mag): 12.14, 11.22, 10.79
Color (J-H, H-Ks, J-Ks): 0.93, 0.42, 1.35
NED: IRAS 19420+1808
Probable Nature: galaxy
(290.638367, 20.906759); glon = 55.0007, glat = 2.8766
Estimated Extinction (mag): AV = 6.4 (AK = 0.64)
Size: 16.8", semi-major axis
Flux (J,H,Ks mag): 13.03, 12.02, 11.55
Color (J-H, H-Ks, J-Ks): 1.01, 0.47, 1.48
NED: no entry
SIMBAD: no entry
Probable Nature: ???
(290.598999, 20.605236); glon = 54.7169, glat = 2.7675
Estimated Extinction (mag): AV = 6.66 (AK = 0.67)
Size: 20.6", semi-major axis
Flux (J,H,Ks mag): 13.08, 11.89, 11.51
Color (J-H, H-Ks, J-Ks): 1.19, 0.38, 1.57
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(291.544556, 21.983480); glon = 56.3495, glat = 2.6406
Estimated Extinction (mag): AV = 5.6 (AK = 0.56)
Size: 13.4", semi-major axis
Flux (J,H,Ks mag): 14.00, 12.43, 11.88
Color (J-H, H-Ks, J-Ks):2.12, 0.55, 1.97
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(296.424805, 19.622694); glon = 56.5102, glat = -2.4931
Estimated Extinction (mag): AV = 3.5 (AK = 0.35)
Size: 20", semi-major axis
Flux (J,H,Ks mag): 12.59, 11.69, 11.29
Color (J-H, H-Ks, J-Ks): 0.90, 0.39, 1.30
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(291.128784, 20.791735); glon = 55.1151, glat = 2.4181
Estimated Extinction (mag): AV = 8.0 (AK = 0.80)
Size: 56", semi-major axis
Flux (J,H,Ks mag): 10.37, 9.66, 8.72
Color (J-H, H-Ks, J-Ks): 0.71, 0.95, 1.66
SIMBAD: PKS B1922+206, HII region
Probable Nature: H II region
(296.438080, 19.900749); glon = 56.7574, glat = -2.3648
Estimated Extinction (mag): AV = 3.7 (AK = 0.37)
Size: 8.2", semi-major axis
Flux (J,H,Ks mag): 13.66, 12.64, 12.10
Color (J-H, H-Ks, J-Ks): 1.02, 0.54, 1.56
NED: radio source WSRTGP 1943+1945
SIMBAD:IRAS 19435+1946
Probable Nature: galaxy
(291.149567, 20.695066); glon = 55.0390, glat = 2.3554
Estimated Extinction (mag): AV = 8.6 (AK = 0.86)
Size: 10.8", semi-major axis
Flux (J,H,Ks mag): 13.66, 12.47, 11.95
Color (J-H, H-Ks, J-Ks): 1.19, 0.52, 1.71
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy or double star
(291.159607, 20.580065); glon = 54.9420, glat = 2.2928
Estimated Extinction (mag): AV = 7.9 (AK = 0.79)
Size: 7", semi-major axis
Flux (J,H,Ks mag): 14.25, 12.78, 12.13
Color (J-H, H-Ks, J-Ks): 1.47, 0.65, 2.12
NED: radio source WSRTGP 1922+2029 within 1'
Probable Nature: galaxy
(295.401276, 18.500484); glon = 55.0543, glat = -2.2115
Estimated Extinction (mag): AV = 3.8 (AK = 0.38)
Size: 15.4", semi-major axis
Flux (J,H,Ks mag): 13.39, 12.33, 11.84
Color (J-H, H-Ks, J-Ks): 1.06, 0.49, 1.55
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(290.233856, 18.692518); glon = 52.8647, glat = 2.1737
Estimated Extinction (mag): AV = 9.4 (AK = 0.94)
Size: 24.6", semi-major axis
Flux (J,H,Ks mag): 13.07, 13.72, 11.05
Color (J-H, H-Ks, J-Ks): -0.66, 2.68, 2.02
NED: no entry
SIMBAD: IRAS 19186+1835
Probable Nature: ??
(291.381317, 20.190228); glon = 54.6967, glat = 1.9252
Estimated Extinction (mag): AV = 9.6 (AK = 0.96)
Size: 7.8", semi-major axis
Flux (J,H,Ks mag): 14.14, 12.70, 12.10
Color (J-H, H-Ks, J-Ks): 1.44, 0.60, 2.04
NED: no entry
SIMBAD: no entry
Probable Nature: galaxy
(293.655212, 23.829866); glon = 58.9035, glat = 1.8186
Estimated Extinction (mag): AV = 7.1 (AK = 0.71)
Size: 25", semi-major axis
Flux (J,H,Ks mag): 12.04, 11.40, 10.80
Color (J-H, H-Ks, J-Ks): 0.64, 0.60, 1.24
NED: no entry
SIMBAD: IRAS 19325+2343
Probable Nature: H II region
(297.110077, 22.143667); glon = 59.0131, glat = -1.7794
Estimated Extinction (mag): AV = 5.2 (AK = 0.52)
Size: 8.2", semi-major axis
Flux (J,H,Ks mag): 13.43, 12.91, 12.03
Color (J-H, H-Ks, J-Ks): 0.51, 0.89, 1.40
NED: PN G059.0-01.7
Probable Nature: Planetary Nebula
(292.117401, 19.508265); glon = 54.4262, glat = 0.9918
Estimated Extinction (mag): AV = 9.9 (AK = 0.99)
Size: 35.8", semi-major axis
Flux (J,H,Ks mag): 11.80, 10.57, 9.71
Color (J-H, H-Ks, J-Ks): 1.23, 0.86, 2.09
NED: radio source MG2 J192832+1930
within 1" of object position
Probable Nature: galaxy or H II region
(294.701691, 23.144455); glon = 58.7735, glat = 0.6464
Estimated Extinction (mag): AV = 12.2 (AK = 1.22)
Size: 45", semi-major axis
Flux (J,H,Ks mag): 11.99, 10.45, 9.31
Color (J-H, H-Ks, J-Ks): 1.54, 1.15, 2.68
NED: radio source IRAS 19366+2301
Probable Nature: H II region
(294.631073, 20.422840); glon = 56.3702 glat = -0.6327
Estimated Extinction (mag): AV = 13.61 (AK = 1.36)
Size: 7", semi-major axis
Flux (J,H,Ks mag): 14.69, 12.92, 10.79
Color (J-H, H-Ks, J-Ks): 1.77, 2.13, 3.90
NED: no entry
SIMBAD: IRAS 19363+2018
Probable Nature: ??
(294.127625, 20.548206); glon = 56.2487, glat = -0.1601
Estimated Extinction (mag): AV = 23.85 (AK = 2.39)
Size: 28", semi-major axis
Flux (J,H,Ks mag): 11.32, 10.08, 10.46
Color (J-H, H-Ks, J-Ks): 1.24, -0.38, 0.86
NED: radio source WSRTGP 1934+2026
within 1" of object position
Probable Nature: H II region
(292.323456, 17.939505); glon = 53.1418, glat = 0.0700
Estimated Extinction (mag): AV = 25.6 (AK = 2.56)
Size: 28", semi-major axis
Flux (J,H,Ks mag): 16.32, 12.99, 10.55
Color (J-H, H-Ks, J-Ks): 3.33, 2.44, 5.78
NED: no entry
SIMBAD: IRAS 19270+1750 ?
Probable Nature: H II region
(292.337189, 17.955620); glon = 53.1622, glat = 0.0663
Estimated Extinction (mag): AV = 25.7 (AK = 2.57)
Size: 7", semi-major axis
Flux (J,H,Ks mag): 15.24, 13.38, 11.69
Color (J-H, H-Ks, J-Ks): 1.86, 1.70, 3.56
NED: no entry
SIMBAD: no entry
Probable Nature: ??
(292.594238, 18.339275); glon = 53.6160, glat = 0.0360
Estimated Extinction (mag): AV = 26.4 (AK = 2.64)
Size: 34", semi-major axis
Flux (J,H,Ks mag): 14.59, 12.58, 10.80
Color (J-H, H-Ks, J-Ks): 2.01, 1.78, 3.79
NED: no entry
SIMBAD: radio source F3R 1872
Probable Nature: H II region
(294.669067, 21.823648); glon = 57.6081, glat = 0.0240
Estimated Extinction (mag): AV = 24.3 (AK = 2.43)
Size: 24", semi-major axis
Flux (J,H,Ks mag): 15.61, 13.45, 11.97
Color (J-H, H-Ks, J-Ks): 2.16, 1.48, 3.64
NED: no entry
SIMBAD: IRAS 19365+2142
Probable Nature: ??

[Last Updated: 1999 Mar 26; by T.H. Jarrett, T. Chester, R. Hurt, R. Cutri, R. Tam, S. Schneider, and J. Huchra]



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