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Environment

Deep CCD imaging of the region surrounding the host galaxy of NVSS 2146+82 has revealed a large number of nearby galaxies. These galaxies are near the limiting magnitude of the POSS/DSS images, so NVSS 2146+82 appears to lie in a sparsely populated region of the sky in the DSS. However, photometry from the deeper Palomar 1.52-m images gives tex2html_wrap_inline1466 for these nearby galaxies if they also lie at z = 0.145, indicating a possible association with NVSS 2146+82. In Figure 12, we present a region of the KPNO 4-m image of NVSS 2146+82 that is 0.5 Mpc on a side and that has all identified galaxies with tex2html_wrap_inline1470 (corresponding to tex2html_wrap_inline1472 at z=0.145) circled. These images do not go deep enough to allow accurate identification and photometry of all galaxies to MV = -19, so this sample is not complete. However, even though the sample shown in Figure 12 is probably only complete to MV ~ -20.5, we have identified 34 candidate galaxies surrounding NVSS 2146+82.

  figure499
Figure 12: A view of the field surrounding NVSS 2146+82 from the central region of our KPNO 4 meter image (north is up, east to the left). This field is 0.5 Mpc on a side at the redshift of NVSS 2146+82, and it contains 34 objects down to mv = 21.3 (tex2html_wrap_inline1727 at z = 0.145) that are morphologically identified as galaxies. For reference, the host galaxy of NVSS 2146+82 is marked with hash marks, and the three galaxies in this region that we measured spectroscopic redshifts for are marked with their ID numbers from Table 5

Although there are no previous identifications of the cluster around NVSS 2146+82 (at b = 21.5, it is too close to the Galactic Plane to have been included in the Abell [1958] catalog), there is a Zwicky cluster to the north, with NVSS 2146+82 lying only ~5'  south of the southern border of the Zwicky cluster. The Zwicky cluster 2147.0+8155 (B1950.0 coordinates) is a compact group with 56 members classified as "extremely distant'' or z > 0.22 ([Zwicky et al. 1961]). While this gives a redshift for the Zwicky cluster larger than that of NVSS 2146+82, it is close enough to z = 0.145 (< 400 Mpc more distant) that we may be seeing NVSS 2146+82 in projection against a background rich cluster.

In September of 1998 WIYN/HYDRA spectra were obtained of 46 candidate galactic companions of NVSS 2146+82 to determine their redshifts. The sample of 46 was selected in the following way: (1) We selected all objects morphologically classified as galaxies in the KPNO 4-m frame by FOCAS with aperture magnitudes <21, resulting in an initial sample of 205 galaxies. (2) We divided this group into two subdivisions: the first being all galaxies within 0.5 Mpc of 2146+82 in projected radius, and the second being all those outside of the 0.5 Mpc radius. However, due to exposure time limitations, the available sample taken from the 34 galaxies identified in Figure 12 within 0.5 Mpc of the host was reduced to the 17 brightest galaxies. Fiber placement restrictions allowed us to observe only 11 of these 17 galaxies. Objects from the sample outside of the 0.5 Mpc radius from NVSS 2146+82 were assigned to 35 of the remaining fibers, leaving about 45 fibers on blank sky to allow accurate sky subtraction. Unfortunately, as mentioned in 3.2 above, the weather conditions during some of the queue observing were poor, and this limited the success of the program. There was enough signal-to-noise to identify features in the spectra of only 24 of the 46 objects successfully. We found that 7 of the 24 objects with good spectra were actually misidentified stars.

Nonetheless, from the remaining 17 spectra of galaxies in the field surrounding NVSS 2146+82, we were successful in identifying what we believe to be a true cluster that contains the radio source host galaxy. Figure 13 presents an image with the 17 galaxies with measured redshifts marked. The positions, redshifts, and magnitudes for these objects are listed in Table 5. A quality factor is assigned for each redshift using the 0 (unreliable) to 6 (highly reliable) scale of [Munn et al. (1997)]. The quality is determined using: q = min[6,min(1,Ndef), + 2Ndef + Nprob], where Ndef is the number of spectral features that are accurately identified (less than 5% chance of being incorrect) and Nprob is the number of spectral features that are probably correct (about a 50% chance of being correct). If q > 3 is adopted as the requirement for a reliable redshift, 5 of the 17 galaxies have unreliable redshifts. The histogram plotted in Figure 14 is a redshift distribution for the 17 galaxies, and it shows that 50% (6) of the reliable redshifts fall in the range of z = 0.135 - 0.148, with 5 of those having redshifts of z = 0.144 - 0.148.

  figure506
Figure 13: The full field that we observed with the KPNO 4 meter surrounding NVSS 2146+82. In this image, the 17 galaxies with spectroscopic redshifts are circled and identified with their ID number from Table 5. Those objects with reliable redshifts in the range 0.135 < z < 0.149 are marked with arrows. NVSS 2146+82 is the galaxy just outside of the southeast edge of the circle surrounding galaxy 5.

Extrapolating the redshift distribution for the sample of galaxies identified around NVSS 2146+82 from the redshift distribution of the 17 reliable galaxy spectra suggests that the 2146+82 cluster may be Abell richness class 0 or 1. Of course, the statistics are very uncertain. Of the 11 galaxies within a projected distance of 0.5 Mpc of NVSS 2146+82 that were in the WIYN/HYDRA sample, redshifts were measured for three of them. Two of these have z=0.144-0.145, while the third has z=0.135. We identified features in 21 of the remaining 35 spectra that were measured for objects outside of the projected 0.5 Mpc radius. We found that 7 were misclassified stars, and 3 of the 14 galaxies with reliable redshifts had 0.144 < z < 0.148. Abell's (1958) richness criterion was based on the number of cluster galaxies within the range m3 to m3+2 (m3 is the magnitude of the third brightest cluster member). For the NVSS 2146+82 cluster, m3 should be < 18.3, since the third brightest galaxy of the 7 (which includes NVSS 2146+82) we have found at z = 0.145 has m = 18.3. Of the 205 galaxies originally found in the KPNO 4-m field containing NVSS 2146+82, 123 of these fall within the m3 to m3+2 range used for estimating the Abell richness. If we apply the percentages above to this sample of 123 galaxies, then 3713 might be at the same redshift as NVSS 2146+82. To this point, we have been considering the cluster richness inside of 0.5 Mpc, for comparison with the tex2html_wrap_inline1530 richnesses of Allington-Smith et al. (1993) and Zirbel (1997), and also within an area ~ 3.8 Mpc on a side, which is the size of the KPNO 4-m field at z=0.145. However, we must note that the original richness criterion for Abell class 1 clusters was that 50 or more galaxies were contained in a radius of 3 Mpc for H0 = 50 km sec-1 Mpc-1 ([Abell 1958]). A circle of radius 3h50-1 Mpc at z=0.145 subtends 507 square arcminutes on the sky, nearly twice the amount of area covered in our image. If the calculated optical richness from the 4-m image galaxy sample is taken as a lower limit to the number of galaxies within an Abell radius, the richness class of the group surrounding NVSS 2146+82 appears to be at least Abell class 0.


 

Table 5: Redshifts of Candidate Cluster Members
in the Field of NVSS 2146+82

Galaxy
ID

R.A.2000.0 2000.0 z q  mv

1

21:42:18.5 81:55:34 0.242 5 20.0
2 21:42:56.3 81:48:29 0.378 3 20.2
3 21:42:58.5 81:57:40 0.350 5 19.4
4 21:44:47.8 81:56:15 0.145 6 18.8
5 21:45:24.5 81:55:05 0.144 6 19.3
6 21:45:27.7 81:57:54 0.267 6 19.4
7 21:45:54.8 81:53:23 0.135 6 18.4
8 21:46:08.8 81:48:08 0.123 6 20.1
9 21:46:24.3 81:57:43 0.243 6 18.4
10 21:46:48.6 82:01:46 0.183 2 19.8
11 21:47:05.7 81:52:35 0.144 6 18.3
12 21:47:07.2 81:55:36 0.145 6 18.4
13 21:47:20.4 81:51:40 0.149 1 19.7
14 21:47:24.7 81:50:40 0.173 6 20.0
15 21:47:53.7 81:53:43 0.208 2 19.4
16 21:48:19.9 82:01:03 0.143 1 17.7
17 21:48:44.3 81:56:59 0.148 6 18.9


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Next: X-ray Observations and Constraints Up: Optical Observations of NVSS 2146+82 and its Environs Previous: Optical Properties of the Host Galaxy


Fri Feb 4 16:02:06 EST 2000