ALMA (the Atacama Large Millimeter Array) which will become operational by 2012, will dominate the prospects in the field of interstellar molecules in the next decades, by providing orders of magnitude gain in sensitivity through the whole millimetre and submillimetre domain accessible from the ground. In the meantime, the very next years will see significant upgrades of millimetre facilities and the full harvest of results of mid-infrared spectroscopy (H2, PAHs, etc.) with current infrared space observatories; however, this immediate future will be mainly marked by breakthroughs in submillimetre astronomy, with the space observatory Herschel and the new generation of submillimetre cameras. In parallel to ALMA, other breakthroughs are expected in space mid-infrared spectroscopy with the James Webb Space Telescope (JWST), from the ground with the extremely large telescopes, and later in radio with the Square Kilometer Array (SKA).
12.1. Waiting for ALMA: ongoing studies and submillimetre breakthroughs
The infrared space observatory Spitzer is expected to remain in full operation until early 2009. Many of its results are still to come out or even to be observed. It will continue to provide exquisite imaging details on mid-infrared properties of local galaxies, calling for new molecular observations at other , especially of starburst regions. It will also discover a large number of new high redshift ULIRGs, and among them, new prominent, rare sources for follow-up millimetre molecular studies. However, as discussed in Sections 8 and 10, the most important results of Spitzer for extragalactic molecules are produced by its Infra-Red Spectrometer (IRS), mainly on rotational lines of H2 and aromatic bands of PAHs. One may thus expect comprehensive studies on both topics. H2 lines will provide detailed information on the `warm' (a few 102 K) molecular gas in local galaxies. They will give information about its distribution in starburst regions, and cast new light on various kinds of shocks and postshock regions, including cooling flows and extra-galactic shocks (Section 10). The amazing sensitivity of the IRS for low resolution spectroscopy will be fully exploited for extensive studies of PAH features, both locally and at high redshift (Section 8). In nearby galaxies, sensitive comparative studies of small variations in PAH features will confirm, refine and exploit their diagnostic power in various conditions of physical parameters - especially UV radiation, interstellar chemistry and metallicity. The Magellanic Clouds and nearby starbursts will be particularly useful to complement Galactic PAH sources with widely different conditions. Similar comparative studies will address the molecular compounds in dust in the Magellanic Clouds and other nearby galaxies with various metallicities. The Japanese infrared space observatory, AKARI (ASTRO-F), will complement and extend Spitzer results (see e.g. Matsuhara et al. 2006). In parallel, near-IR and redshifted UV molecular observations, mostly of H2, will fully exploit the capabilities of the large park of 8-10 m optical telescopes. The adaptive optics breakthrough in near-IR high angular resolution should mostly address AGN (see Sections 10 and 11). HST should benefit from the UV spectrometer COS after 2008, adding outstanding UV capabilities for studying molecules (H2, PAHs, etc., Snow 2005) to those of NICMOS in the near-infrared.
Waiting for ALMA and its first antennae by 2010, millimetre and radio observations of interstellar molecules, especially at high redshift, will continue with the current facilities with significant upgrades. Interferometers will mainly benefit from improvements of their receivers and correlators, with wider bandwidths and frequency coverage, and better sensitivities. The Californian interferometers, Owens Valley and BIMA will gain from their merging into CARMA (Scott & Pound 2006). However, the IRAM interferometer (Fig. 7a) has still the largest collecting area. It has just increased its receiver capabilities in a significant way 3. The extension of VLA capabilities (EVLA, http://www.aoc.nrao.edu/evla/, Butler 2004) will boost observations of low-J lines of high-z CO. Large single dishes will benefit from similar developments of their receiver and frequency coverage, especially the 100 m Green Bank Telescope (GBT, http://www.gb.nrao.edu/gbt/, Mason 2004), and later the 50 m LMT-GTM in construction in Mexico (http://www.lmtgtm.org/). One breakthrough soon to be expected is the direct blind determination of redshifts from CO lines of SMGs with spectrometers with very broad bandwidths (Baker et al. 2007).
The observations of high redshift molecules will certainly increase using the expected breakthrough in the identification of large numbers of high-z ULIRGs with the new generation of submillimetre cameras. Arrays, such as SCUBA2 to be soon in operation (e.g. Doug 2004), will have thousands of Transition Edge Superconducting (TES) detectors. They will increase the submm mapping speed by a factor of ~ 100. The 2-year survey programme of SCUBA2 aims to observe ~ 20 square degrees, and to detect ~ 104 high-z ULIRGs, to be compared with the few hundreds currently known. Such very large samples will change the prospects of millimetre studies of high-z molecules, focussing on prominent exceptional objects, especially lensed ones, allowing deeper chemistry probes, and various statistical, environmental and clustering studies.
The space submillimetre observatory, Herschel, to be launched in 2008, will extend such surveys to the whole submillimetre and far-infrared range, providing much more detailed information about far-infrared luminosities, star formation rates and redshifts. More importantly for the exploration of the molecular world, Herschel will really open up the submillimetre window for sensitive observations in its whole range with its heterodyne instrument HIFI (e.g. Lis 2004, Greve et al. 2006c). Compared to ALMA, Herschel will be completely unaffected by atmospheric effects, but both its sensitivity and angular resolution will be severely hampered by differences of three orders of magnitude in collecting area and baseline/diameter with the Herschel 3.5 m mirror. Both handicaps will cumulate for the observation of extragalactic molecular lines. The results to be expected should thus be limited to strong lines of most abundant molecules, such as high-J lines of CO, HCN, HCO+, CS, etc. (Greve et al. 2006c, see also Iono et al. 2007), lines of H2O and isotopomers including HDO, and of a few other abundant hydrids such as OH, CH, CH+, H2D, NH3, as well as related fine structure lines of CI and CII, in prominent starburst and AGN sources. The same applies to low resolution spectroscopy with the other instruments PACS and SPIRE of Herschel; and also to the airborne telescope SOFIA (http://sofia.arc.nasa.gov/, Erickson 2005), with its 2.5 m mirror, whose various instrument capabilities in the mid- and far-IR may well address molecular lines in prominent extragalactic sources.
12.2. The ALMA revolution
12.2.1. Overview of the ALMA project. The `Atacama Large Millimeter Array', ALMA, now extended into the `Enhanced Atacama Large Millimeter/Submillimeter Array' with Japan and other East Asian parties, is a world-wide project (http://www.eso.org/projects/alma/, http://www.alma.nrao.edu/, http://www.nro.nao.ac.jp/alma/E/ and Conicyt Chile). It will represent a jump of almost two orders of magnitude in sensitivity and angular resolution as compared with present millimetre/submillimetre interferometers, and will thus undoubtedly produce a major step in astrophysics. The main objectives will be the origins of galaxies, stars and planets. ALMA will be able to detect dust-enshrouded star-forming galaxies at redshifts z 10, both in the emission of dust and spectral lines (CO and other species, including C+). ALMA will also allow enormous gains for the observation of the molecular gas in local and intermediate redshift galaxies of various types.
The ALMA interferometer, to be completed by 2012, will be installed in an exceptional site for submillimetre observations, at Chajnantor, Atacama, Chile, at 5000 m elevation, with baselines up to 14 km (Fig. 7b). Together with the ALMA Compact Array (ACA, driven by NAOJ Japan), it will include 54x12m-dishes and 12x7m ACA-dishes (total collecting area 6500 m2), providing a very good coverage in the uv interferometric plane, and allowing high sensitivity fast mapping with angular resolution better than 0.1", and an ultimate angular resolution better than 0.01". It will eventually operate in at least eight frequency bands, covering the main atmospheric windows between 4 mm and 0.4 mm (84 to 720 GHz), and will be equipped with very broad-band heterodyne receivers close to quantum-limit sensitivity, and a huge correlator (16 GHz, 4096 channels).
Figure 7. (left) The IRAM interferometer at Plateau de Bure in the French Alps, at 2500 m elevation - 6 × 15m-dishes. (right) Artist view of the ALMA interferometer, to be completed by 2012, at Chajnantor, Atacama, Chile, at 5000 m elevation - 54 × 12m-dishes and 12 × 7m-dishes - with baselines up to 14 km.
Such capabilities will deeply renew all the fields of millimetre astronomy, with a sensitive extension to the submillimetre range (see e.g. the ALMA Design Reference Science Plan (DRSP), http://www.strw.leidenuniv.nl/~alma/drsp11.shtml, and its future updates). As concerns local galaxies, one may say that many of the goals of current millimetre astronomy in distant sources of the Milky Way, such as the central regions, will become accessible in nearby galaxies. Similarly, detailed studies currently carried out in nearby galaxies will be easy in rare, relatively distant local galaxies such as ULIRGs. It is thus easy to figure out the enormous impact of ALMA for all kinds of detailed studies of the molecular gas in local galaxies, and more generally our global knowledge of the interstellar molecular medium, without being restricted to a single galaxy. Routine high sensitivity imaging of nearby galaxies with a resolution approaching 1 pc will well resolve the giant molecular clouds, revealing many details and the dynamics of various kinds of structures, such as condensations, hot spots, outflows, photodissociation regions, shocks, turbulence, etc. The various aspects of star formation in various types of galaxies can then be addressed in much more detail than currently. Comparisons of molecular abundances and interstellar chemistry, both at galactic scales and for specific regions, will be extended to a variety of galaxy types and element abundances. Dedicated observations of isotopomers will address isotope ratios and nucleosynthesis at galactic scales in this rich diversity of environments. A particular interest will focus on the specificity of the chemistry in AGN and under strong X-ray irradiation.
12.2.2. ALMA capabilities at high redshift It is well known that the past history of star formation in the Universe, in dusty and molecular starbursts, is one of the two main driver goals for ALMA. The expected leap forward may be summarized as a sensitivity for dust detection at 850 µm ~ 50 times better than SCUBA and more than 10 times SCUBA2, no limitation by source confusion, and a sensitivity for lines an order of magnitude better than the upgraded IRAM interferometer. ALMA will be able to detect dust in luminous infrared galaxies (LIRGs with LFIR of a few 1011 L, SFR of a few 10 M / yr) and map CO lines in ULIRGs, at any redshift where they might exist; i.e. the first dusty starbursts in the Universe. One may expect that the major breakthroughs of ALMA in this field will include (Blain 1999 & 2006, Blain et al. 2002, Combes 2005, Omont 2004, Wootten 2001, 2004, Takeuchi et al. 2001, Walter & Carilli 2007, Carilli et al. 2007, Combes, 2007, papers by Bertoldi, Walter, etc. in Bachiller et al. 2007):
Comprehensive studies of high-z dusty starbursts in a few ALMA ultra-deep fields, with detections up to hundreds of them per field of view, determination of their redshift and luminosity distributions by multi- mm/submm observations, and CO detection in a number of them 4. A clever use of 'gravitational telescopes` by clusters will be needed to carry out comprehensive studies of high-z starbursts down to ~ 10 M / yr and detect weaker ones; and in particular to address the earliest starbursts in the Universe.
Complementary observations of deep fields observed by other instruments or with multi- coverage. In particular, combined projects by ALMA and JWST should be unique for unveiling the first major starbursts in the Universe, clumping of starburst galaxies in proto-clusters, deepest studies of highly lensed fields, observations of highest-z Gamma-Ray Bursts, their host galaxy and their environments, etc.
Sensitive mapping of star-forming galaxies at all redshifts, both in the dust continuum and mainly in CO, C+ and CI lines. The combination of sensitivity, high angular resolution and heterodyne velocity profiles will provide a rich information about detailed structure, star formation, mass of molecular gas, ionization, and their spatial distribution; dynamics, rotation, dynamical masses; mergers and companions; outflows; etc. One will derive a complete picture of the properties and evolution of starburst galaxies through the whole history of the Universe. One may also say that we will in particular build a complete coverage of the history of all major phases of star formation in Milky Way-like galaxies in the last ten billion years or so. Many dedicated observations of this type will address single or multiple objects discovered at other wavelengths, peculiar, rare, prominent, gravitationaly lensed, hosts of various types of AGN, etc.
Very fast complete frequency coverage of an atmospheric frequency window, allowing blind redshift determination from CO lines, as well as multi-line detections in strong sources. One may thus expect a full development of interstellar chemistry, including isotopomers, in prominent sources at intermediate redshifts, and at all redshifts through absorption line studies with numerous continuum background sources of modest strength.
12.3. Accompanying- and post-ALMA: JWST, extremely large telescopes and SKA
It is certainly hazardous to try to make predictions 15-20 year from now in a fast moving field which is only 35 year old. We will thus just concentrate on the three major fields where projects, as costly as or even more costly than ALMA, are decided or in a pre-decision phase in domains of interest for extragalactic molecules: JWST in near/mid-IR, the ground-based 30-40 m class telescopes in optical/near-IR, and SKA in radio. However, for all of them, despite their importance for molecules, their main drivers are not centred on molecules as for ALMA.
This leaves aside other important fields, such as UV spectroscopy where no major project following FUSE and HST/COS, is yet decided. It is of course impossible to anticipate about the follow-up of more or less serendipitous discoveries that one may expect in the next five years, and then especially with ALMA and JWST. It is even hard to predict the evolution of eventual molecular studies in fields yet unexplored, such as the reionization epoch and the formation of the very first galaxies, as well as of long pending problems such as the diffuse insterstellar bands.
12.3.1. JWST and ground-based extremely large telescopes Current instrumentation technology may already allow a gain of several orders of magnitude with respect to present facilities for spectroscopy in the whole infrared range, with tremendous potential impact on molecular studies, especially at high redshift. This will be achieved in the near- and mid-infrared up to 27 µm, by the 6m James Webb Space Telescope (JWST, Gardner et al. 2006). One may thus expect much deeper studies, with better angular resolution, than current ones for vibration lines of hot molecular gas, mostly H2, in various types of local galaxies, mainly starbursts, AGN, cooling flows, etc. For this purpose, JWST will be complemented by the new generation 30-40 m ground telescopes 5, especially for high angular resolution near-IR spectroscopy of molecules, mainly H2, in various extragalactic shocks and in the galactic nuclei of starburst galaxies and AGN.
The Mid-InfraRed Instrument (MIRI) of JWST will have orders of magnitude improvements in sensitivity, spatial and/or spectral resolution compared with other facilities and will be a unique facility for astrochemistry in the next decade. MIRI will expand tremendously the capabilities of other facilities, with e.g. two orders of magnitude more sensitive, one order of magnitude larger spatial resolution and a factor of 4-6 higher spectral resolution than Spitzer (van Dishoeck et al. 2005). One may thus expect much more comprehensive studies in local galaxies of warm H2 rotational lines, PAH emission and molecular features in dust. Even more important will be the extensions at high redshift allowing the comparison of the evolution in the history of galaxies of these essential molecular features of the interstellar medium. H2 lines in particular could trace any kind of warm molecular gas in high-z starbursts, AGN, mergers, outflows, and various shocks.
The 30-40 m telescopes will be unique for high-sensitivity spectroscopy of redshifted UV molecular lines, especially H2 in Damped Lyman- systems (DLAs) on the line of sight of QSOs and GRBs at very high z. This will be a unique tool for studying H2 at high z and the conditions in DLAs, as well as the possible variation of me/mp, with extensions to other molecules such as CO, and studying H2 in GRBs and their host galaxies (Theuns & Srianand 2006, Campana et al. 2007).
There are also projects, some of them less advanced, to take advantage of the extraordinary possible sensitivity for far-infrared spectroscopy in space. A first step could be the Japan-led SPICA project with a 3m-class telescope (Matsumoto 2005). It could be followed by a larger far-IR telescope, such as the SAFIR project (Benford et al. 2004), and eventually by a far-IR space interferometer, but it is difficult to anticipate what could be the exact time-scale for such projects.
12.3.2. SKA, mega-masers and cold molecular gas at very large redshift The Square Kilometer Array (SKA) will provide two orders of magnitude increase in collecting area over existing telescopes in the cm range, allowing for study of the HI content of galaxies to cosmologically significant distances (i.e. to z 2 rather than z ~ 0.2). Radio studies of molecular lines, especially at high z and mega-masers, will benefit from the same sensitivity gain which is larger than brought by ALMA for millimetre astronomy. Among the five key science projects of SKA (see e.g. Carilli & Rawlings 2004), observations of high-z molecules are quoted in `Galaxy evolution and cosmology' for the precise measurement of H0 using extragalactic water masers, and in `Probing the dark ages' for the incomparable sensitivity of the SKA enabling studies of the molecular gas, dust, and star formation activity in the first galaxies. As discussed e.g. by Carilli & Blain (2002) and Blain, Carilli & Darling (2004), the detection of redshifted low-J lines of CO will be carried out by the SKA at z 2 at a rate at least comparable to that of ALMA for higher-J lines. SKA will be complementary to ALMA for studying the cold gas and resolving its morphology, and also for detecting OH and H2O mega-masers and redshifted low-J lines of molecules such as HCN, HCO+, CS, CN, etc., whose high-J lines are not easily excited.
3 The new generation of receivers of the IRAM Plateau de Bure interferometer are more sensitive than the previous ones by a factor ~ 5 in the continuum and 1.5-2.0 in the line detection. The available bandwidth is currently 2 GHz and will be increased to 4 GHz with a new generation of correlators. Back.
4 However, because of its small field of view (~ 25" at 1 mm), ALMA will have a limited speed for wide surveys, an order of magnitude less than SCUBA2, and a larger factor compared with future projects with larger telescope and TES camera such as CCAT (e.g. Blain 2006). But the sensitivity of such instruments will be severely limited by source confusion contrary to ALMA. Back.
5 Such projects presently include: the European Extremely Large Telescope (E-ELT, https://www.eso.org/projects/e-elt/); the Giant Magellan Telescope (GMT, http://www.gmto.org/) and the Thirty Meter Telescope (TMT, http://www.tmt.org/). They will allow deeper, higher-resolution spectroscopy in the atmospheric windows with the sensitivity needed for extragalactic observations. Back.