MERLIN/VLBI National Facility


Number 1: September 1999

  1. Call for Proposals
  2. General News
  3. Recent Science: Supernova Remnants and HII regions in NGC2146
  4. Summer Engineering Work
  5. MERLIN: A Vision for the Future
  6. Subscription to the MERLIN mailing list

This is the first of a regular biannual newsletter that will describe activities at the MERLIN/VLBI National Facility (NF). In the past there has been no clear mechanism for disseminating information to the NF's user community; this is being rectified through this newsletter and also through the publication of a more detailed biennial report. Copies of the first edition of this report, covering the period 1997-1998, were made available at NAM and will be distributed further within the next two weeks.

Call for Proposals

There are two proposal deadlines approaching that are relevant to the NF. The first is the deadline for Semester 00A on MERLIN. Note that the MERLIN deadline is non-standard and after the busy time of October 1, this is possible because the NF staff process the proposals for the PPARC secretariat.

The proposal deadline is 9th October 1999. All details are in the MERLIN web area, specifically:

MERLIN will be offering 22GHz, 5GHz, and 1.4-1.7GHz for Semester 00A (although it is not yet clear if the Lovell telescope will be available for 1.4-1.7GHz in 00A). Proposal forms, information on MERLIN Key Programmes, and further general information can be obtained via:

ftp:, Directory: /pub/merlin/semester_00A

The second is the autumn deadline for European and global VLBI:

The proposal deadline is 1st October 1999. All details may be found in the EVN USERS GUIDE at:

The provisional plan for EVN observing sessions in 2000 is given below, proposals received by October 1st will be considered for scheduling in any of the sessions listed.

2000 Session 1 Feb 10 - Mar 02 6 cm, (+MERLIN), 18/21 cm, + ??
2000 Session 2 May 25 - Jun 15 6 cm, 18/21 cm, + ??
2000 Session 3 Sep 06 - Sep 27 6 cm, 18/21 cm, + ??
2000 Session 4 Nov 08 - Nov 25 6 cm, 18/21 cm, + ??

The ?? signify wavelengths to be allocated based on proposal pressure.

General News

At the time of writing PPARC have not yet notified us of our final budget for the next grant period. As some of you may be aware there is worry on that front since the rate of salary increases is greater than the inflation indexation rate applied to all PPARC grants. This, combined with the lack of inflationary increases for the NF grant in the past means that the operational, non-staff costs for the NF are under severe pressure. At the June meeting of the GBFC the NF's problems were recognized and a short-term financial band-aid was promised which may help the situation. However in the longer term other measures are needed and GBFC decided that the NF operation should be reviewed next summer in order to evaluate the efficiencies resulting from the restructuring programme initiated two years ago and determine a new base budget for continued operation and development.

There is some good news: GBFC allowed the NF to appoint a data archivist whose job is to produce an archive of all MERLIN data. This will be accomplished in three phases: first the archive database and web interface will be created; secondly, an existing pipeline reduction script written in AIPS will be used to start populating the database with calibrated data and initial images from new observations; finally the archivist will move backwards through the existing archive of raw data and produce a full archive. The existing rules regarding open access to MERLIN data will remain in place, that is: MERLIN data becomes publicly available 12 months after the end of the semester in which the observations were made. Dr. Anita Richards has been appointed as the archivist from October 1, 1999.

In addition, we welcome Brian Smith to Jodrell Bank. Bryan joins us on an EU funded programme to perform development work on broadband fibre optic connections. Specifically, he is modelling the transmission characteristics of such fibres and assessing the feasibility of transmitting several Gbit/sec over lengths of 100s and 1000s of km.

Recent Science: Supernova Remnants and HII regions in NGC2146

NGC2146 is a starburst galaxy at a distance of 14.5 Mpc. The nuclear region is highly obscured in the optical, but radio observations with the VLA (Glendenning & Kronberg 1986, BAAS 18, 1006) revealed a populations of about 20 point sources which were interpreted as radio supernovae (RSN) or supernovae remnants (SNR). MERLIN + VLA observations at 5 GHz with 150 mas resolution show that 7 of the sources are resolved in to diffuse structures, some of which have shell-like morphologies.

(Click on image for full version)
Additional 1.4 and 1.6 GHz MERLIN data have shown 9 sources coincident in position with the ones detected at 5 GHz and allowed us to compute their spectral indices. Three of them show values consistent with synchrotron emission from SNR or RSN, but the others present a very steep inverted spectrum. Most of the sources with inverted spectrum are unresolved with diameters < 9 pc and TB > 1000 K, but two are resolved in amorphous structures with sizes 10 and 20 pc. We claim that the positive spectral index sources are optically thick compact or "ultra dense" HII regions with ne > 103 cm-3, ionized by 103 O6 equivalent stars.

Such massive regions of current star-formation have been seen in other galaxies (NGC5253 and Henize 2-10), but, interestingly, are not seen in M82; this last galaxy has in fact many similarities with NGC2146. The present starburst in NGG2146 may lie in a temporally intermediate phase between the very young ones such as in NGC5253 and older examples like M82 (the ageing of the bursts refers to the sequence presented in Rieke et al., 1988, ApJ 325, 679).

The inverted spectra could alternatively be caused by strong free-free absorption at 18cm caused by foregroung ionized gas with very high emission measures (> 108 pc cm-6). The MERLIN observations also reveal a very compact steep-spectrum source close to the centre of the galaxy which is, up to now, considered a candidate for a RSN, because of its steep nonthermal spectrum and its high brightness temperature. HI absorption has been detected against this source using MERLIN at 21cm, implying a column depth of 25 x 1021 cm-2.

These observations demonstrate the current ability of MERLIN to resolve SNR and star-forming regions in increasingly distant galaxies. The proposed upgrade for MERLIN, including the resurfaced Lovell Telescope, will allow populations of SNR and HII regions to be studied in many more galaxies at greater distances.

PI: A. Tarchi (Bonn), Co-I's: N. Neininger, U. Klein (Bonn), A. Greve (IRAM), B. Glendenning (NRAO), A. Pedlar (JBO), S. Garrington, T. Muxlow (MERLIN/VLBI NF).

Summer Engineering Work

The so-called "shutdown" of the National Facility for about three months each summer is, in fact, the busiest time of the year for the professional engineering staff who operate the system. With the telescopes in 24-hour operation throughout the rest of the year, this is the only opportunity to undertake much of the essential maintenance and to implement many of the developments required by our policy of continuous improvement in the quality of the science output.

MkII Multiple Receiver System
(Click on image for larger version)
Our main task this summer has been the complete replacement of the servo, drive and control system on the Darnhall telescope which is one of the 25-m diameter VLA-style dishes located about 30km from Jodrell Bank. This is part of the Restructuring initiative, which also provides for similar work to be carried out on the Knockin and Pickmere telescopes in summer 2000. The existing (original) control systems on these three telescopes are more than 20 years old, obsolete and beginning to cause trouble, so the replacement is timely. It is interesting to note that even electric motor technology has advanced since the telescopes were built, with the result that the new motors are substantially smaller than the originals.

At the time of writing, the new hardware is installed and commissioning is almost complete. The revised telescope control software has also been written and installed. Testing of the complete system is due to begin shortly. Also ready for testing is a new version of the Mk2 telescope control software which will permit full remote control of the receiver changing carousel in the focus cabin during the next 5/22GHz observing session early in the new year. However, final commissioning of this system has been deferred until after the September VLBI session to permit completion of improvements to the elevation drive and new focus access arrangements required by the Health & Safety Executive. The construction of the carousel was a direct scientific benefit of the restructuring programme which was otherwise aimed at decreasing the running costs of current observations. It was designed and commissioned in-house in less than 12 months.

The Lovell Telescope, which is an important element of many National Facility observations, has been the subject of major engineering work this summer, including:

The University has made a bid to the Joint Infrastructure Fund for a grant to renew the whole of the reflecting surface using the technique successfully tested this summer. If the bid is successful, we plan to complete the work in two, extended summer maintenance periods.

The bearings we have replaced were installed when the Telescope was first constructed in the early 1950s. We hope that the new ones last equally well.

MERLIN: A Vision for the Future

As you will read in the biennial report mentioned above we have a plan for the future development of MERLIN which will, when completed, transform MERLIN into a new instrument with more than an order of magnitude improvement over the current sensitivity and three times the current resolution for high quality imaging. This transformation is especially relevant as we enter a new era of high resolution astronomy, with infrared adaptive optics already demonstrated on GEMINI and with NGST and ALMA on the horizon, MERLIN is uniquely placed to provide complementary radio observations. The development plan for MERLIN capitalizes on this position.

The plan is discussed in more detail at

Briefly, the development is envisaged in three stages:

  1. Array Upgrade: new 12-15 GHz receivers, full frequency flexibility, replacement of the Defford telescope
  2. Prototype Fibre Link: development and testing of a broad-band fibre link between Jodrell Bank and Cambridge
  3. Array broad-banding: the linking of all telescopes to Jodrell Bank with optical fibres and the installation of a new broad-band correlator

Along with the concurrent resurfacing of the Lovell telescope, these upgrades will provide, without significant increase in running costs, a 25-fold sensitivity increase at the prime operating frequency of 5 GHz with 50 mas resolution and the ability to carry out reliable imaging at 22-24 GHz with 8 mas resolution.

The exact sequencing of the components of the development is not critical, nor is the division into three stages.

The unique combination of resolution, sensitivity and frequency coverage of an upgraded MERLIN will open up a spectacular range of new possibilities in both galactic and extra-galactic astronomy. In particular it will bring within reach the thermal and ionized universe of stars, nebulae and active galaxies in a wavelength regime unobscured by dust. Stellar astronomy in particular will be revolutionised by the ability to detect thermal emission from warm gas and dust with linear resolution of a few AU or better. MERLIN will therefore provide complementary science to ALMA (which studies cold/cool material at somewhat lower resolution) and NGST. Other MERLIN opportunities include fundamental astrometry with a positional accuracy < 0.1 mas; planetary searches; studies of star forming regions and active stars; methanol and excited state OH masers, extragalactic SNR and starburst galaxies, AGN and a wide range of cosmological studies.

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MERLIN/VLBI National Facility, Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL, United Kingdom
Telephone: +44 (0)1477 571321, Facsimile: +44 (0)1477 571618