Mm-Wave VLBI with ALMA and Radio Telescopes Around the World Held at ESO Headquarters, Garching, Germany, 27–28 June 2012

Astronomical News

Report on the ESO Workshop mm-wave VLBI with ALMA and Radio Telescopes around the World held at ESO Headquarters, Garching, Germany, 27–28 June 2012

Heino Falcke1 quarters in Garching to attend a one and cient phasing of all antennas [Doeleman, Robert Laing 2 a half day workshop on mm-wave VLBI Baudry]. This would effectively allow Leonardo Testi 2 with ALMA and other telescopes. In the the array to be used as a giant single dish Anton Zensus 3 following report on the workshop, the in parallel to its normal interferometric names of speakers are given in square mode. Recording ALMA data and corre- brackets in the text and all of their talks lating it with those of other (sub)milli- 1 Radboud University Nijmegen and are linked to the conference webpage1. metre telescopes would then provide the ASTRON, Dwingeloo, the Netherlands standard ALMA data products as well 2 ESO The reason why the millimetre-wave as super-high resolution images from the 3 Max-Planck-Institut für Radioastrono- region is so important for ultra-high-reso- VLBI mode. mie, Bonn, Germany lution imaging is that heterodyne receiv- ers can still be used and quantum effects ALMA’s wide frequency range (35– do not yet limit our ability to do interfer- 950 GHz, once all of the planned receiver Very long baseline interferometry at ometry, as they do at shorter wave- bands are completed) will enable a broad millimetre/submillimetre wavelengths lengths. The addition of ALMA offers a range of VLBI science to be pursued (mm-VLBI) offers the highest achiev- leap in sensitivity, thus making many by astronomers in Europe and around the able spatial resolution at any wave- more and weaker sources accessible to world. It was therefore timely to bring length in astronomy and the inclusion of ultra-high-resolution studies and greatly together interested European astrono- ALMA into a global network will bring improving image fidelity for the brighter mers to discuss their scientific interests unprecedented sensitivity. The work- objects. Only sources with flux densities and ambitions for this new ALMA observ- shop on mm-VLBI reviewed the broad above several hundred milliJansky (mJy) ing capability. The meeting followed range of science topics, from imag- have been observable so far; this thresh- immediately after the Garching ALMA ing the event horizon of the black hole old could be reduced by more than an Community Days. The first part of the at the centre of the Galaxy, through order of magnitude if ALMA were to be meeting included reviews of current activ- masers in the Milky Way and distant included. ities and achievements in mm-VLBI, galaxies to jets in radio galaxies. Plans including the Event Horizon Telescope were laid to develop a science case The phasing up of ALMA as part of a Collaboration, currently being run by and a European organisation to pro- VLBI array had been envisaged from the Haystack Observatory, and the Global mote mm-VLBI including ALMA. beginning of the project and is incorpo- Millimeter VLBI Array coordinated at rated into the system design, but had the Max-Planck-Institut für Radioastrono- been deferred on cost grounds, and in mie in Bonn. The second part concerned The huge potential of very long base- the interests of completing the instrument the potential science that will be enabled line interferometry (VLBI) at millimetre and for stand-alone operation. The ALMA by a VLBI array including ALMA (Figure 1). submillimetre wavelengths is that it Development Steering Committee is now The final discussion centred on upcom- enables the highest resolution imaging considering a proposal by a team of in- ing developments worldwide. Following currently possible at any wavelength stitutions led by the MIT Haystack Ob- in astronomy. The spatial resolution of an servatory in the USA to upgrade the Figure 1. Scientists from all across Europe discuss- interferometer in radians is given by the ALMA correlator and to create the opera- ing mm-wave VLBI science with ALMA in the ESO ratio of the observing wavelength to tional infrastructure needed for the effi- auditorium. the separation of the telescopes. For a wavelength of 1 mm and a separation of 9000 km, this resolution is about 20 microarcseconds. Using ALMA as a phased array to form part of a global VLBI network will offer unprecedented sensitivity at very high angular resolution. This capability will allow the shadow of a black hole, the relativistic jet flows in active galactic nuclei (AGN) and the dusty winds near stellar surfaces to be imaged; and it may potentially even improve measurements of the Hubble constant.

While ALMA is nearing completion and the first observing cycle is under way, astronomers are already thinking ahead along these lines. To discuss the oppor- tunities from a European perspective, over 65 scientists gathered at ESO Head-

50 The Messenger 149 – September 2012 on from the meeting, a science case will be developed, incorporating the ideas presented and discussed.

Approaching the event horizon

One major science case that has been driving the development of sub-mm VLBI has been the possibility of imaging the innermost parts of accretion discs and jets around supermassive black holes and in particular imaging the shadow of the event horizon in the black hole at the Galactic Centre [Falcke, Doeleman]. The shapes and sizes of the shadow and the photon ring surrounding it (e.g., Figure 2) are precisely predicted by Einstein’s theory of general relativity (GR), so this observation would provide tests Figure 2. Simulated images at 345 GHz of the black Mościbrodzka et al. (2009). Left: Face-on orientation; not only of the black hole paradigm but hole in the Galactic Centre using a VLBI array involv- Right: Edge-on orientation. The black hole shadow ing ALMA and other telescopes. The input model is and photon ring are much more difficult to detect, also of GR in its strong-field limit. based on general relativistic magneto-hydrodynamic but still visible in the edge-on case. Figure updated simulations of plasma around a Kerr black hole from from Falcke et al. (2011). The Galactic Centre, Sgr A*, is the most favourable source for this experiment, Powerful jets mation. VLBI at 230 GHz and above since the angular size of its black hole should ultimately reveal the region where shadow on the sky is the largest known The other source of great interest in the jet is formed, thereby addressing one and has optically thin millimetre- and this respect is the black hole in the cen- of the fundamental problems of astro- submillimetre emission on event horizon tral galaxy of the Virgo cluster, M87. physics: jets and discs are ubiquitous in scales. Most importantly, the mass of This black hole is a thousand times fur- astronomical objects ranging from young this source has been determined accu- ther away, but also a thousand times stars to supermassive black holes. rately by near-infrared imaging of the more massive then Sgr A*, so their event stars in the central region of the Milky horizons are comparable in angular size. Moreover, the study of relativistic jets with Way. Ultimately, the mass constraints will In M87, the radio emission is undoubtedly VLBI has recently received additional improve even further with near-infrared produced by a relativistic plasma jet (Fig- attention in connection with observations interferometry, especially the GRAVITY ure 3). Recent mm-VLBI observations of blazars made by the Fermi satellite. project (Eisenhauer et al., 2011). There will [Krichbaum, Hada] indicate that this jet Multi-wavelength campaigns suggest be extremely interesting synergies forms on the scale of a few Schwarzs- that bright gamma flares are directly con- between ALMA VLBI and VLTI in the area child radii, thus imposing extreme con- nected to outbursts and component of strong-field GR studies [Eisenhauer]. straints on all possible models for jet for- ejections seen at millimetre wavelengths.

Figure 3. Left: 86 GHz VLBI image of al., 2011). Right: A 43 GHz VLBA 0 *09$*+] the jet in M87 (from Krichbaum et al., image from Walker et al. (2008), clearly 2007). The jet extends down to some revealing the limb-brightened structure tens of Schwarzschild radii (Hada et also seen at 86 GHz.

1 2 345 R

L@ 10 HN M

S M87 VLBA 43 GHz 8 a56

DBKHM@ C ) 6 as UD (m et fs

1DK@SH 4 Of c

l De 2 &RUH ´ 56

0 PDV

SF l –2 l l l l l 50–5 –10–15 1DK@SHUD1 L@R RA Offset (mas)

The Messenger 149 – September 2012 51 Astronomical News Falcke H. et al., Report on the ESO Workshop “mm-wave VLBI with ALMA”

Surprisingly, these flares seem to come Figure 4. Position– &'Y velocity plot showing from rather large distances from the &'Y the observed 22 GHz

&'Y Richards A. black hole, i.e., regions that can still be water vapour emission imaged by mm-VLBI even in more dis- R from the oxygen-rich tant sources, where the event horizon is AGB star IK Tau (colour- L@ too small to resolve [Giroletti, Orienti]. coded by velocity), together with the predicted locations of water O@MRHNM maser emission at 321 Masers in stars, star-forming regions and and 325 GHz from mod- DNEDW AGN SQ els by Malcolm Gray. DM &'Y

Another fascinating science driver at QN millimetre wavelengths is high-brightness SE ERD

line emission. Many of the lines com- .E &'Y monly detected by ALMA are produced in extended and diffuse regions with low brightness temperatures and are not detectable at high spatial resolution. This changes, however, when one looks at 5+21JLR maser emission or at absorption lines towards compact continuum sources [Impellizzeri]. etry of masers [Brunthaler] is also needed wavelengths has been given by the first in the Galactic Centre, where the accu- ALMA test observations of 321 and Well-known examples of maser sources racy of the basic reference frame for cal- 658 GHz water masers with two-kilometre occur in evolved stars, where H20 and culating stellar orbits is limited by errors baselines (Figure 5). The observations SiO masers provide impressively detailed in cross-referencing the radio and optical are part of the initial tests on the longer tracers of the dynamics of the molecular reference frame using SiO masers. baselines and demonstrate the use of and dusty winds blown away from the old compact spectral line sources for calibra- star into the interstellar medium [Colomer, Masers are also seen in star-forming tion. The targets chosen for these initial Richards]. Quite a number of maser lines regions and young stellar objects, par- tests are well known high-mass star- of water and other molecules have fre- ticularly in regions excited by shocks forming regions, which contain bright quencies above 35 GHz. (A list of maser [Goddi]. They may trace radial infall as maser sources, and analysis of the test transitions relevant to VLBI is given in the well as outflow. Again VLBI allows pre- data is still in progress. These early re- contribution by E. Humphreys linked to cise measurement of the dynamics of sults clearly show the potential of ALMA the conference web page). Some of these these processes. and, in the future mm-VLBI, for high transitions are expected to come from angular resolution imaging of bright the most excited molecules close to the A foretaste of the potential of high-reso maser sources. Additional tests are cur- stellar surface (Figure 4). Precise astrom- lution maser observations at millimetre rently being planned targeting masers

W49 H20 maser 321 GHz !@RDKHMDRTOSNL !@RDKHMDRTOSNJL

ǥ ǥ Goddi C.

ǥ ǥ S@ @S HNM @S HNM @ C DBKHM DBKHM ǥ ǥ RS

# Figure 5. ALMA test # D S observations of water- ) ) maser emission from W49N at 321 GHz. ǥ ǥ , + Left: baselines up to 400 metres, with a beam of 0.55 by 0.44 arcseconds. goǥ %HDP ǥ ´ǥ goǥ %HDP ǥ ´ ǥ Right: baselines up to 2 kilometres with G L R R R R R R G L R R R R R R a beam of 0.26 by )1HFGS@RBDMRHNM )1HFGS@RBDMRHNM 0.16 arcseconds.

52 The Messenger 149 – September 2012 in late-type stars. A full characterisation mally located pulsar could, e.g., measure extra level of complexity, such as a yet- of submillimetre maser sources across the black hole mass to one part in a mil- to-be achieved form of dynamic schedul- the sky may also generate an alternative lion and provide independent tests of GR. ing to take account of changing weather network of (bright) phase calibrators for conditions. high frequency observations. Organisational challenges The community’s enthusiasm about the Extragalactic maser sources are also groundbreaking possibilities offered known [Humphreys]. A famous example VLBI inherently requires many telescopes by mm-VLBI with ALMA was clearly evi- is the nearby active galaxy NGC 4258, to operate together. While the ALMA dent during the workshop: some of the where a warped molecular disc sur- phasing project is under way, other milli- expected science results may indeed rounds the central supermassive black metre telescopes will also be needed become transformational. There was gen- hole. VLBI proper motion observations of to act as part of a global array. Existing eral agreement at the conclusion of the 22 GHz water masers in the disc have activities at 3 mm (the Very Long Base- meeting that, from a European perspec- been used to obtain a precise geometric line Array and the Global Millimeter VLBI tive, the ultimate goal of the planning pro- distance to this galaxy, making it a cru- Array) and at 1 mm (the Event Horizon cess should be a facility accessible to cial component of the extragalactic Telescope project) have laid the ground- the general user, with time made avail distance ladder. Currently the water work for future coordinated observa- able through open competition. maser cosmology project is extending tions. Some of the most outstanding sci- this technique to larger distances, ence goals, like imaging the black hole On the way towards this goal, the scien- thereby providing an independent meas- shadow, are unlikely to be achieved with tific needs of the user community for urement of the Hubble constant and a just a few baselines. As ALMA reaches mm-VLBI with ALMA need to be com- direct determination of black hole masses full operation, there will be financial pres- piled as a necessary input to the approval (important for the black hole mass– sures to close down existing facilities, process for the ALMA phasing project. velocity dispersion relation). Observations but on the positive side, new facilities are Some of the technical challenges of this of higher frequency water maser transi- emerging such as the Large Millimeter project are being addressed by the inter- tions (e.g., 183 or 321 GHz) have the Telescope (LMT) in Mexico [Hughes] and national multi-partner phasing-project potential to improve our understanding the Long Latin American Millimeter Array led by the Haystack Observatory: the of the accretion disc physics and geom (LLAMA) in Argentina [Cseh]. In Europe, task for the nascent European ALMA etry. Particularly intriguing is the specula- the Northern Extended Millimeter Array VLBI user community is to organise itself tion that super-masers might exist. In (NOEMA) project, an upgrade of the Insti- to prepare for the successful use of this those systems the millimetre-wave emis- tut de Radioastronomie Millimétrique exciting new capability. sion could be more intense than the well (IRAM) Plateau de Bure Interferometer known 22 GHz line and perhaps detecta- is going ahead [Bremer] and large single The meeting was co-sponsored by ESO ble to greater distances. dishes (IRAM 30-metre, Effelsberg, and Radionet3, an EU Integrated Infra- Sardinia Radio Telescope, Yebes, Onsala structure Initiative to coordinate access, and Metsähovi) will play an important role development and training for radio as- Pulsars, microquasars and gamma-ray at 3 and 7 mm wavelengths. In the USA, tronomy facilities in Europe 2. bursts the Very Long Baseline Array (VLBA) is crucial, as well as the Combined Array Finally, there are many other compact for Research in Millimeter-wave Astron- References sources that are potential targets for omy (CARMA) [Brinkerink], the Submilli Eisenhauer, F. et al. 2011, The Messenger, 143, 16 mm-wave VLBI. In particular, transient meter Telescope (SMT) and Submillimeter Falcke, H. et al. 2011, IAU Symp., 275, 68 sources such as supernovae, gamma-ray Array (SMA) at higher frequencies. Hada, K. et al. 2011, Nature, 477, 185 bursts and microquasars should be very Maitra, D. 2011, IAU Symp., 275, 82 interesting, since the highest frequencies Overall the meeting demonstrated that Krichbaum, T. et al. 2007, Exploring the Cosmic Frontier: Astrophysical Instruments for the 21st probe the earliest phases of any eruption. European astronomers have a broad Century, 189 range of science interests beyond the ini- Mościbrodzka, M. et al. 2009, ApJ, 706, 497 At first sight, pulsar science is an unex- tial key projects that will be addressed Walker, R. C. et al. 2008, Extragalactic Jets: Theory pected field for ALMA, since the radio by a phased ALMA facility alone and in and Observation from Radio to Gamma Ray, ASP Conference Series, 386, 87 spectra of pulsars are usually very steep. coordinated mm-VLBI observations. This Nevertheless, there is some indication might, for example, be organised follow- of spectral flattening at millimetre wave- ing the established examples of the Euro- Links lengths and a phased ALMA should be pean VLBI Network (EVN) supported 1 Workshop webpage: http://www.astro.ru.nl/mmV- able to observe pulsed emission [Kramer]. by the Joint Institute for VLBI in Europe LBI2012 Of particular interest would be searches (JIVE) [van Langevelde], as well as the 2 Radionet3: http://www.radionet-eu.org for pulsars in the Galactic Centre, where Global Millimeter VLBI Array. The inclu- low-frequency pulses suffer from huge sion of ALMA and the extension to higher dispersion and scattering, to the extent frequencies will bring a very substantial that they become undetectable. An opti- improvement in data quality, but also an

The Messenger 149 – September 2012 53