The Plateau de Bure Interferometer: Gazing At New Spectral Horizons

We are pleased to announce first light in the frequency bands 129¹

GHz (Band 2) and

GHz (extended Band 3) with the Plateau de Bure interferometer. The two additional RF bands of the new generation receivers (Fig. 1) are now available for astronomical research.

First fringes were observed at 146 GHz with the six-element array on December 15, 2007 towards the high mass star forming region W3(OH) (Fig. 2). By opening Band 2 to astronomical research, the interferometer provides for the first time the opportunity to observe in the

GHz frequency window, and this with excellent sensitivity, as apparent from the noise performance of Band 2 (Fig. 3).

**Figure 1:** The cryostat interior of one of the heterodyne receivers installed in the cabins of the six Plateau de Bure antennas. All six receiver units are currently equipped with dual polarization mixers with an IF bandwidth of 4 GHz for operation in the GHz (3 mm band), GHz (2 mm band) and GHz (1.3 mm band). Work is underway to equip the units with a 0.8 mm RF channel. The channels are optically coupled to the antenna by reflection off two aluminium mirrors (the 2 mm band mirrors are at the top left, the 3 mm band mirrors at the top right). The corresponding beams are laterally displaced ( $\sim 180\hbox {$^{\prime \prime }$}$ on the sky) from the radio axis of the antenna. The photo is courtesy of B.Pissard (IRAM).

**Figure 2:** PdBI first light observations in the continuum at 145 GHz (Band 2) towards the center of IC342. The map at 2 mm shows the free-free radiation from ionized gas and dust emission in the mini-spiral at the center of this galaxy. Located at 1.8 Mpc, IC342 is a nearby galaxy of the northern sky and can be studied in great detail with an instrument such as the PdBI. These observations were made in configuration C in the nights of Dec 28 and Dec 31, 2007. The inset at the lower left shows the synthesized beam ( $\sim 1{\farcs}7$ ). The image is courtesy of N.Rodriguez (IRAM).

**Figure 3:** Median noise performance for the 12 mixers (six antennas, dual polarization) of the recently installed 2 mm band. The laboratory data for LSB and USB tuning, and first on-telescope receiver noise data (triangles) are plotted as a function of sky frequency. The data are courtesy of J-Y. Chenu and A. Castro-Carrizo (IRAM).

Not much later, another milestone was reached. On January 07, 2008, the Plateau de Bure interferometer detected first fringes at 267 GHz on the circumbinary ring in the well-known T Tauri star GG Tau (Fig. 4). The very good sensitivity of Band 3 (

GHz with a median SSB Trec of 70 K at 267 GHz) and the extended frequency coverage result from two major modifications: an improved noise performance of the IF amplifier circuitry and the transition to a new generation of frequency multipliers in the LO system. The enhanced capabilities of the PdBI receivers are particularly gratifying as they are likely to trigger a flood of exciting observations e.g. in the line transitions of HCO

and HCN

, in the Milky Way and in nearby galaxies.

**Figure:** First interferometric observations at 267 GHz (Band 3) of the thermal dust emission in the ring surrounding GG Tau, a young binary system in Taurus. The deprojected map of GG Tau ( $i=37^\circ$ ) shows a ring with an inner and outer radius of $\sim 1{\farcs}3$ and $\sim 1{\farcs}9$ , and is in excellent agreement with earlier measurements at 220 GHz by Guilloteau et al. (1999). The data presented here were observed in the nights of Jan 7 and Jan 20, 2008, in the C-configuration and in a subarray of the PdBI. The figure is courtesy of V. Piétu (IRAM).

The analysis of recent test data, and of the few science programs that we have already observed, demonstrate the potential and the new capabilities of the Plateau de Bure interferometer at 2 mm and 1.1 mm, indicating that the performance goals were largely achieved in both frequency bands. The early results of two science demonstration programs are shown in Figure 2 and 4. They illustrate the good performance of the new system. These achievements are the result of years of work of the receiver group, the mechanical construction and workshop group, the SIS group, the computer group, the scientific software group, the science operation group and the telescope operators.

We are now on the way to reach the next two milestones: (1) complete the construction of WideX, a broadband correlator with 4 GHz frequency coverage in each polarization and a channel spacing of 2 MHz and (2) equip the cryostats with Band 4 for operations in the

GHz band. Both projects are progressing well, and we are confident to bring them to first light in 2009.

In closing, we would like to invite you to submit projects for the new bands at the March 13, 2008 deadline, and look forward to a year of smooth operations and exciting science results with the new capabilities of the Plateau de Bure array.