In order to estimate the uncertainty of measured offsets between PdBI positions and near-infrared/optical objects, we have to take several possible uncertainties into account. Due to the phase calibration of the mm frame based on bright radio calibrators, we assume that there is no systematic offset between these two frames. Thus, we conclude that the differences between the VLA and PdBI positions are consistent with the relatively low S/N of the VLA detections. The uncertainty between the radio and the optical/near-infrared frames consists of the systematic offset between the radio and the optical/near-infrared frames and the random uncertainties both of the radio and optical/near-infrared positions. Thus, the total error is dominated by the random errors of the individual measurements and is typically <100.
2.3
Interferometry
To obtain subarcsecond-accurate positions — the key element of the identification work — we carried out observations both at the Plateau de Bure mm interferometer and the Very Large Array cm interferometer.
2.3.1
1.2 mm Interferometry
At the time of proposing a PdBI follow-up (September 2000), neither radio interfero- metric nor MAMBO mm photometric mode observations were available to select targets for mm interferometry. Therefore, the PdBI source selection was only based on the bolometer map and on optical/NIR information. We considered only sources for which inspection of the partial data set shows the MAMBO detection to be reliable and proposed to observe interesting and/or peculiar mm sources selected from the cluster Abell 2125 and the NDF. Five sources were observed, four being in the NDF and one in the field of Abell 2125, the securely radio-identified source J151427+6615 (Bertoldi et al., 2000). These are some of the brightest sources in the MAMBO bolometer mm map. The ob- servations were carried out with the PdBI in winter 2000/2001 in order to determinate absolute source positions, test for possible multiplicity or spatial extent, and confirm the MAMBO fluxes. The observations were performed in the 5D configuration, with the re- ceivers tuned to 238.46 GHz. The phase center was at the nominal MAMBO bolometer position and we integrated between nine and 38 hours per source, reaching an rms of
Figure 2.5 “Dirty” 1.26 mm interferometric maps for the four PdBI detections. From top to bottom: MM J120546–0741.5, MM J120539–0745.4, MM J120517–0743.1, and
MM J154127+6615. Each image is 3500 ×3500 and oriented such that north is at the
top and east is to the left. The plus indicates the phase center, placed at the position of the mm source originally estimated from the bolometer map. The contour levels are spaced by 1 mJy and the solid and dotted lines represent positive and negative contours respectively. The dirty beams are shown to the right, where the three NDF sources show considerable side lobes because of their equatorial declination. The beam profiles for these three sources are similar because of the similar declination and UV coverage.
2.3. INTERFEROMETRY 27
Table 2.2. PdBI positions and fluxes of the five MAMBO 1.2 mm galaxies
Object Obs. Time RA (J2000) DEC (J2000) Flux
(1) (2) (3) (4) (5) MM J120546–0741.5 9 12h05m46.s59±0s.02 −07◦41034.003±0.0037 6.1±1.2 MM J120539–0745.4 14 12h05m39.s47±0s.02 −07◦45027.000±0.0033 6.4±1.0 MM J120517–0743.1 38 12h05m17.s86±0s.02 −07◦43008.005±0.0024 2.6±0.5 MM J120518–0742.6 30 - - <1.9 (3σ) J154127+6615 20 15h41m26.s96±0s.02 66◦1403700.62±000.20 3.2±0.5
Note. — Col (2) — Units of observing time are hours. Col. (3) — Units of right ascension are hours, minutes and seconds. Col. (4) — Units of declination are degrees, arcminutes, and arcseconds. Col. (5) — Units of flux are mJy.
0.5−1.2 mJy. We used 3C273, 1124–186, 1637+574 and 1458+718 to calibrate the tem-
poral variation of amplitude and phase. 3C273, 3C345, 3C454.4 were used to calibrate the IF bandpass. The absolute flux scale was determined with observations of MWC349,
CRL618 and 3C273, and has an uncertainty of ∼ 20 %. We calibrated and reduced the
data at IRAM, Grenoble using the GAG software packages CLIC and GRAPHIC. The final naturally weighted dirty maps are shown in Fig. 2.5 along with the dirty beams.
The FWHM of the beam for MM J120546–0741.5 is 3.400×2.800 with PA = −21◦ at
1.26 mm; beams for the other NDF sources are similar. The beam for the Abell 2125 source is 3.000×2.500 with PA = 84◦ at 1.26 mm.
Four sources are detected at the > 5σ level. We derived the positions and total
1.26 mm flux densities listed in Table 2.2 from point-source fits to the calibrated visibili- ties in the UV plane. The positional accuracy quoted in Table 2.2 is the sum in quadrature of the statistical error of the point source fit with an astrometric systematic uncertainty of 0.200 (e.g., Downes et al., 1999). The signal-to-noise ratio of our data does not enable
us to put strong constraints on the source size at 1.26 mm. No detection is resolved in
our ∼300 beam data, however. The PdBI map of MM J120546–0741.5 suggests a possible
second source near 12h05m46s.7 −7◦4102800 but its significance is only ∼ 3−4σ and no
additional supporting evidence is given by a radio or optical/NIR counterpart. Obser- vations at 95 GHz (3.16 mm) were obtained in parallel. None of the sources is detected at this frequency, consistent with dust emission. The only nondetection was the source
MM J120518–0742.6 where no tentative signal was detected after 30 hours of observations on the nominal bolometer map position at 12h05m18s.7−7◦4203900.7.
2.3.2
1.4 GHz Interferometry
The NTT Deep Field was observed at 1.4 GHz with the VLA in April and May, 2001, for 15 hours in the B configuration (10 km maximum baseline). Standard wide- field imaging mode was employed (50 MHz total bandwidth with two polarizations and 16 spectral channels). The source 3C 286 was used for absolute gain calibration and 1224+035 was used for phase and bandpass calibration. The data were also self-calibrated using sources in the NDF itself. Images were synthesized and deconvolved using the wide- field imaging capabilities in the AIPS task IMAGR, and the primary beam correction was
applied to the final image. The FWHM of the Gaussian CLEAN beam was 700 ×500
with a PA = 0◦. The rms noise in the final image is between 13 and 15 µJy over the
150 ×150 region covered by MAMBO. The variation of the rms noise of the map was not
only caused by the usual beam attenuation and UV plane coverage, but was also due to residual calibration errors because of a relatively large number of bright sources (2 sources of ∼> 60 mJy/beam) and one spatial grouping of fainter but still relatively bright sources to the northeast of the center of the map. These residual calibration errors manifested themselves as uneven noise in a series of stripes in the final map. About one-third of area of the final VLA map was affected by this striping. We describe in § 3.2.1 how this was dealt with in identifying radio counterparts to the MAMBO 1.2 mm detections. A
catalog for the whole observed NDF at 1.4 GHz down to ≥ 5σ (66 µJy) was generated
with AIPS.