The ALMA-Herschel Synergies

Other Astronomical News The ALMA-Herschel Synergies Paola Andreani1 2 Tom Wilson 5σ 1 hour Point Source 100 PACS (1h,5σ) 1 INAF – Osservatorio Astronomico di SPIRE Trieste, Italy ) 10 2 ESO Jy (m ty 1 z = 0.1 One of the ESO-ESA science planning working groups has studied joint op- 0.1 z = 1 portunities offered by Herschel and Flux Densi z = 2 ALMA in the infrared and submillimetre 0.01 HIFI z = 3 bands. A brief summary of the report z = 5 SPIRE edited by David Elbaz and Tom Wilson PACS z = 7 is given here. 0.001 3 4 5 6 7 8 9 10 z = 12 1 cm 1 mm 102 µm 101 µm The ESA/Herschel Satellite and the Ata- Observed Wavelength cama Large Millimeter Array (ALMA) are two large projects in astronomy to in- Figure 1: A plot of the emission from the starburst sensitivity of ALMA. The lower dashed curve is for vestigate the submillimetre and Far Infra- galaxy M82 for different redshifts, z. The horizontal the 64-antenna ALMA and the upper dashed curve axis is observed wavelength, the vertical axis is for a 6-antenna ALMA. PACS, SPIRE and HIFI Red (FIR) range. Herschel covers the predicted flux density in mJy. The crosses show the are Herschel receiver bands. The ALMA bands are wavelength range from 60 to 625 μm sensitivity of the Herschel bolometers. The dashed shown numbered. (480–5000 GHz), while ALMA, an inter- lines at the left side of this diagram show the 5s national project in which ESO has the European leadership, covers the range interstellar dust. The cocoons of form- M82, where the broadband radiation 320 μm to 1 cm (30–950 GHz). Both ing objects are deeply embedded within peaks in the FIR/submm. This is mostly Herschel and ALMA will come into oper- gaseous dusty clouds where optical due to thermal radiation from dust. This ation in similar timeframes. ALMA is extinction can be extremely large and continuum radiation is consistent with planned to be completed in 2012, but prevents the study of these fundamental temperatures in the range 10–100 K. In the ‘early science’ operation will begin well processes with traditional optical tele- FIR/submm/mm there are also spectral before this time. The launch of the scopes. However, cool material emits lines, mostly from molecular species, al- Herschel satellite is planned for August submm and FIR radiation. By exploring though there are prominent atomic fine 2007 with an expected lifetime longer this wavelength range we can directly structure lines of various ionisation stages than three years. Thus there should be measure physical phenomena associated of oxygen, carbon, silicon and nitrogen. an overlap in the time when both are in with the formation process itself. The Objects like M82 were much more fre- operation1. third question may seem less fundamen- quent in the past. With the full ALMA we tal, but since FIR/submm telescopes expect to detect ‘M82-like’ objects even Although the two facilities overlap in measure radiation from dust, an accurate at redshifts up to 12. As Figure 1 shows, wavelength range they are ‘complemen- characterisation of dust properties is a if this SED is shifted in redshift, we wit- tary’. They will lead to major advances prerequisite for answering the other two ness a peculiar effect, called the ‘nega- in many fields of astronomy, especially questions. tive K-correction’, which greatly facilitates those related to the origins of planets, the detection of high-redshift objects stars and galaxies. The crucial questions In the local Universe 30 % of the galax- at FIR/submm wavelength. The thermal are: (1) How do galaxies form? (2) How ies emit in the FIR/submm because they spectrum and characteristics of dust do stars form? and (3) What is the life are dust enshrouded and forming stars. emission makes the observed flux density cycle of a dust grain, and how does this This fraction grows steeply up to redshift constant at Herschel and ALMA wave- depend on environment? The birth of z = 1−2 and flattens off at earlier times, length range over a wide value of red- planets, stars and galaxies is hidden by to z > 6, as inferred from the evolution of shifts. This Figure shows that the broad- the cosmic luminosity density. This means band emission of sources such as 1 A description of the bilateral (North America-Eu- that at redshifts larger than 1 the popu- M82 can be detected with Herschel and rope) ALMA is at http://www.alma.nrao.edu/ lation of galaxies dominating cosmic en- the early science ALMA even at high red- projectbk/construction/. Accounts of ALMA science are in Shaver (1996) and Wootten (2001). ergetics is that of dusty starburst galax- shifts. The web site for the Herschel project, including all ies, i.e. objects that are rapidly forming instruments, is http://www.rssd.esa.int/Herschel/. stars. Our knowledge of the star-formation proc- Accounts of Herschel and ALMA, some plans ess is still very limited. Figure 2 shows for Herschel science, ALMA science and their synergies are to be found in the Proceedings of “The Figure 1 shows the Spectral Energy Dis- a sketch of the four stages of star forma- Dusty and Molecular Universe” (ed. A. Wilson 2005). tribution (SED) of the starburst galaxy tion, from the collapse of a molecular The Messenger 123 – March 2006 59 Other Astronomical News Andreani P. and Wilson T., The ALMA-Herschel Synergies Figure 2: A sketch of the development of a low-mass and 6 and Band 3 in the bilateral ALMA project. With protostar and its disc (after Charles Lada, Figures: the addition of Band 5 and Bands 4, 8 and 10, the Michiel Hogerheijde). Above on the left side are coverage of ALMA receiver bands provides a solid shown the wavelength coverage of the Herschel in- block in the uppermost part of the figure under struments PACS, SPIRE and HIFI. The ALMA re- ‘ALMA’. These will also fill the longer wavelength part ceiver bands from left to right are Band 9, Bands 7 of Herschel HIFI coverage, marked ’HIFI’. cloud to the formation of a star surround- PACS SPIRE Class O ed by a disc. Cloud collapse requires (main accretion phase) HIFI Size: 10 000 AU high interstellar gas densities and low ALMA t = 0 kinetic temperatures. The starting point is a gravitationally-bound ‘pre-stellar core’. 1 18 −2 Class I For column densities N > 10 cm and 0 densities n > 102 cm−3, interstellar gas ??? (late accretion phase) –1 Size: 8000 AU consists mostly of molecular hydrogen, –2 4 5 F t = 10 –10 yr H2 and helium. This is a molecular cloud. g Lo 2 The H2 molecule does not produce emission lines if kinetic temperatures are 0 below ~ 100 K and there are no shock –2 Class II waves. Then the abundances of the H (optically thick discs) 2 0 1 2 3 Size: 200 AU molecules must be traced indirectly. At Log (µm) t = 105–106 yr high density, in cold clouds, grain properties change and constituents of the gas 1 will condense onto grains. From millime- 0 tre-submm maps the mass distribution of –1 Star Disc –2 Class III pre-stellar cores is remarkably similar to F the Initial Mass Function. These pre-stel- g (debris discs?) Lo 0 Size: 200 AU lar cores begin to collapse as the result of t = 106–107 yr processes which may involve ambipolar –1 Star diffusion, the dissipation of turbulence, or –2 Disc an outside impulse. Once begun, the 0 1 2 3 gravitational collapse is rapid, ending in Log (µm) the formation of a hydrostatically-sup- ported protostar in the centre. During the istry. The higher angular resolution of of abundances on scales finer than a few main accretion phase, the central object ALMA images will help to refine the analy- arc seconds and thus the true source plus an accretion disc gradually builds sis of models based on Herschel data. averaged abundances of species which up its mass from a surrounding envelope The final result will be the distribution of are those needed for chemistry models. of matter while progressively warming. H2, selected atoms, molecules and dust, The protostar evolves from the Class 0 as well as their dynamics. ALMA data alone and Herschel data alone phase, in which the mass of the envel- will be a great step forward. A combined ope is much greater than the mass of the The Herschel PACS and SPIRE bolom- ALMA-Herschel data set will be a tremen- protostar + disc, through the Class I stage, eter systems are well suited to surveying dous advance. A number of conditions in which the mass of the protostar + disc rather large regions of the sky, where- must be fulfilled to combine Herschel and becomes greater than the mass of the as ALMA can provide high sensitivity, ALMA data sets. First, the calibrations surrounding envelope, to the Class II high angular resolution images in spectral for both instruments and cross calibration stage, in which material in the envelope line and continuum, but these will usual- must be well determined and consistent. becomes sufficiently rarified that the ly be limited to a few arc minutes in size, This will require a rather extensive set of protostar becomes visible to traditional at most. ALMA and Herschel/HIFI are Herschel measurements and subsquent- optical telescopes. These phases can heterodyne instruments, and will be able ly, accurate models of the calibration be distinguished by the shape of the FIR/ to resolve even the narrowest lines in sources.

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