Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stecf.org/coordination/esa_eso/alma-herschel/report.pdf Дата изменения: Thu Sep 4 17:58:31 2008 Дата индексирования: Tue Oct 2 00:59:25 2012 Кодировка: Поисковые слова: comet tail


Report by the ESA-ESO Working Group on The Herschel-ALMA Synergies

Introduction & Background
Following an agreement to cooperate on science planning issues, the executives of the European Southern Observatory (ESO) and the European Space Agency (ESA) Science Programme and representatives of their science advisory structures have met to share information and to identify potential synergies within their future projects. The agreement arose from their joint founding membership of EIROforum (http://www.eiroforum.org) and a recognition that, as pan-European organisations, they served essentially the same scientific community. At a meeting at ESO in Garching during September 2003, it was agreed to establish a number of working groups that would be given the task of exploring these synergies in important areas of mutual interest and to make recommendations to both organisations. The chair and co-chair of each group were to be chosen by the executives but thereafter, the groups would be free to select their membership and to act independently of the sponsoring organisations.

Terms of Reference and Composition
The goals set for the working group were to provide:

· A survey of the scientific areas covered by the Herschel and ALMA missions. This
survey will comprise: a. a review of the methods used in those areas; b. a survey of the use of ALMA and Herschel in those areas and how these missions compete with or complement each other; c. for each area, a summary of the potential targets, accuracy and sensitivity limits, and scientific capabilities and limitations.

· An examination of the role of ESO and ESA in running these facilities that will:
a. identify areas in which current and planned ESA and ESO facilities will contribute to science areas covered by Herschel and ALMA; b. analyse the expected scientific returns and risks of each; c. identify areas of potential scientific overlap, and thus assess the extent to which the facilities complement or compete; d. identify open areas which merit attention by one or both organisations (for example, follow-up observations by ESO to maximise the return from Herschel); e. conclude on the scientific case for coordination of ALMA and Herschel activities. i


Edited by T. L. Wilson (ESO) and D. Elbaz (CEA) The contributors to the scientific content of this report are: P. Andreani (Trieste), D. Bockelee-Morvan (Paris), J. Cernicharo (Madrid), P. Cox (Grenoble), C. De Breuck (ESO), E. van Dishoeck (Leiden), D. Elbaz (CEA, Saclay), M. Gerin (Paris), R. Laing (ESO), E. Lellouch (CEA, Saclay), G. L. Pilbratt (ESA), P. Schilke (Bonn), T. L. Wilson and M. Zwaan (ESO). R. Fosbury and W. Freudling (ST-ECF) provided coordination, A. Rhodes and J. Walsh proof-read the final version of this document. Technical production was done by R. Y. Shida (ESA/Hubble), M. Kornmesser (ESA/Hubble) and L. L. Christensen (ESA/Hubble). W. Fusshoeller (MPIfR Bonn), E. Janssen (ESO), J. Vernet (ESO) and W. Freudling provided figures.

ii


Contents
1 Executive Summary ..................................................................................... 1 2 Introduction .................................................................................................. 3 3 Descriptions of Herschel and ALMA in the Literature ............................. 5
3.1 The ALMA Project ............................................................................................. 5 3.2 Herschel .............................................................................................................. 9

4 Comparison of the ALMA and Herschel................................................... 13
4.1 Comparison of the Properties of ALMA and Herschel ..................................... 13 4.2 Comparisons of Observational Techniques ...................................................... 14 4.2.1 4.2.3 Broadband Measurements................................................................. 14 Spectral Line Measurements ............................................................ 15

5 Common Calibration of ALMA and Herschel data ................................ 19
5.1 Data Taking Procedures .................................................................................... 19 5.2 Qualitative Summary of the Comparison.......................................................... 20

6 Specific Examples of Synergies Between ALMA and Herschel .............. 21
6.1 Herschel and the origin of the cosmic infrared background ............................. 21 6.2 Galaxy and large-scale structure formation ...................................................... 22 6.3 High Redshift Objects ....................................................................................... 23 6.4 Active Galactic Nuclei ...................................................................................... 27 6.5 Normal Spirals and Low Surface Brightness Galaxies ..................................... 28 6.6.1 Protostars........................................................................................... 31

6.7 Selected Galactic Sources ................................................................................. 36 iii


6.7.1 6.7.2

Spectral Line Measurements ............................................................. 36 Continuum Measurements ................................................................ 38

6.8 Large Scale Surveys of our Galaxy................................................................... 38 6.9 Evolved Stars ................................................................................................... 39 6.9.1 6.9.2 6.9.3 6.9.4 Spectral Lines ................................................................................... 39 Continuum Emission......................................................................... 41 Solar System Objects ........................................................................ 41 Modelling and Analysis of the Data.................................................. 42

7 Summary ...................................................................................................... 45 8 Recommendations for the Future .............................................................. 47
8.1 Allocation of Time ............................................................................................ 47 8.2 Supporting Observations ................................................................................... 48 8.3 Supporting Data ................................................................................................ 48 8.4 Calibration......................................................................................................... 49 8.5 Data Archives .................................................................................................... 49 8.6 Action Items for the Organisations ................................................................... 49

Abbreviations .................................................................................................... 51 References ......................................................................................................... 53

iv


v


vi


1

Executive Summary

The Herschel Satellite and the Atacama Large Millimeter Array (ALMA) are two very large sub-mm and far infrared (FIR) astronomy projects that are expected to come into operation in this decade. This report contains descriptions of these instruments, emphasising the overlaps in wavelength range and additional complementarities. A short rationale for studying sub-mm and far infrared astronomy is given. Following this, brief presentations of Herschel and ALMA are presented, with references to more detailed documents and use cases. Emphasis is placed on the synergies between these facilities, and the challenges of comparing data produced using both. Specific examples of projects are given for a number of areas of astronomical research where these facilities will lead to dramatic improvements. This report is addressed to an audience of non-specialist astronomers who may be interested in extending their areas of research by making use of Herschel and ALMA instruments. ALMA has a small instantaneous field of view, but allows high angular resolution images of selected sources. The Herschel satellite has two multi-beam bolometer systems, PACS and SPIRE. These have larger fields of view than ALMA, but with lower angular resolutions. Thus the SPIRE and PACS cameras provide the opportunity to cover large areas of the sky rather quickly. Measurements with ALMA would be follow-ups, while Herschel SPIRE and PACS can provide finding lists for ALMA, or for shorter wavelength measurements of source emission to give complete Spectral Energy Distributions (SEDs). Since Herschel will be above the atmosphere, measurements can be made in wavelength regions where astronomical signals cannot reach the Earth's surface. This is especially the case for wavelengths shorter than 300 µm. Herschel HIFI is a heterodyne instrument, so is especially well suited to high resolution spectroscopy of molecules such as water vapour. The number of sources that can be measured with HIFI will be more limited than PACS and SPIRE since HIFI is a single pixel instrument. On the Earth's surface, most of the water vapour lines are blocked by the atmosphere. The few that do reach the surface are nearly all strong masers. One exception seems to be the water vapour line at 183 GHz. Six antennas of ALMA will be equipped with receivers to image the 313-220 transition of water vapour at 183 GHz and the same transition from the oxygen-18 isotope of water vapour at 203 GHz. Such high angular resolution images will complement the Herschel HIFI data. A particularly important condition for combining ALMA and Herschel data involves a common source sample and consistent calibration. This calibration programme will require a fairly extensive set of Herschel and ALMA measurements, in addition to accurate models of the calibration sources. These sources will have to be more compact planets in the outer part of the Solar System or asteroids. For comparisons with ALMA, PACS and SPIRE calibrations will be more complex than HIFI calibrations. Operationally, Herschel should take the lead in initiating projects. For extragalactic sources, PACS and SPIRE could survey large regions, providing finding surveys for ALMA. The ALMA follow-ups could be redshift determinations, an extension of the SED's to longer 1


wavelengths, and high resolution imaging. For an efficient synergy, a significant amount of Herschel time should be devoted to legacy projects early in the life of the satellite. For galactic sources, sensitivity is less critical. There could be very large-scale surveys or deep targeted surveys of selected objects. Before full ALMA operation, ESO should consider carrying out large, very deep surveys with ground-based sub-mm single dishes equipped with large bolometer cameras. For ESA, the most pressing need is for a coordination of large surveys planned in the Herschel guaranteed time. This includes both large scale and targeted surveys. ESA must have a clear policy of data rights. Ideally ESA should provide access to the Herschel data during the satellite's lifetime. This includes data files, calibration information and a pipeline for data reduction. For both organisations, the most important task is to coordinate the large surveys that are planned in the Herschel guaranteed time by means of dedicated conferences or workshops.

2


2

Introduction

The Herschel satellite and the Atacama Large Millimeter Array (ALMA) are two large astronomical instruments designed to explore the "cool Universe", that is, to study cool gas and dust. Such cool material is associated with objects in formation or dust enshrouded sources. These include the earliest evolutionary stages of galaxies, stars and planets; these are deeply hidden within dust clouds where optical extinction can be extremely large. At far infrared and sub-mm wavelengths, the extinction is not only much smaller, but we can also directly measure the physical phenomena associated with the formation process itself.

Figure 1: The Spectral Energy Distribution (SED) of a nearby starburst galaxy, M82 (from Genzel, 1991). Above the SED, we show the total wavelength and frequency ranges covered by the Herschel and ALMA instruments (Band 3 to 10, see Fig. 2).

Herschel will cover the wavelength ranges from 60 to 625 µm where the broadband dust emission peaks, allowing one to measure total bolometric luminosities and to characterise dust temperatures. ALMA will ultimately cover the range from 320 µm (~950 GHz) to 1 cm (~30 GHz) and will nearly always measure radiation from the optically thin Rayleigh-Jeans part of the spectrum and so can provide measurements of dust masses. Using assumed dustto-gas ratios, it is possible to use dust masses to determine the total mass of the interstellar medium. With ALMA one can measure transitions of molecules with permanent dipole moments. In the sub-mm wavelength range, emission lines arise from gas with densities higher than those emitted at longer wavelengths. These results can be used to estimate molecular 3


abundances and H2 densities. A typical Spectral Energy Distribution (SED) is shown in Fig. 1. We also show the operational wavelength ranges of the two instruments. Both Herschel and ALMA will come into operation at similar times. ALMA should be completed in 2012, but "early science" operation will begin well before this. The official launch date of the Herschel satellite is early 2008 and it has an expected lifetime of more than 3 years. Thus there should be a period during which both are in operation together. This will allow for the simultaneous measurement of astronomical sources with rapid time variations. However, even if the measurements do not overlap, one can combine results from these facilities to improve the analysis of sources with slow or no time variations. Hence it is important to establish regions of common ground and to develop strategies that will optimise the opportunities to exploit the distinctive features of each instrument and define combined programmes aimed at specific science goals. This report reviews the capabilities of each instrument and compares their performance and approach to common observational techniques in Chapters 3 and 4. If the observational areas of the two instruments are to overlap, some form of common calibration is desirable and methods for this are discussed in Chapter 5, before specific examples of potential common science programmes are given in Chapter 6. Recommendations for the future, addressed separately and jointly to ESO and ESA, are presented in Chapter 8.

4


3

Descriptions of Herschel and ALMA in the Literature

A description of the bilateral (North America-Europe) ALMA project is at http://www.alma. nrao.edu/projectbk/construction/. Accounts of ALMA science are in Wootten (2001) and Shaver (1995). The website for the Herschel project, including all instruments, is http://www. rssd.esa.int/Herschel/. In particular, descriptions of the science that will be carried out by Herschel/SPIRE (Spectral & Photometric Imaging Receiver) are found in the SPIRE web page: http://www.ssd.rl.ac.uk/SPIRE/Science.htm, with PACS (Photodetector Array Camera & Spectrometer) in the PACS web page: http://pacs.ster.kuleuven.ac.be/ and for the Herschel HIFI in the web page: http://www.sron.nl/divisions/lea/hifi Accounts of Herschel and ALMA, some plans for Herschel science, ALMA science and their synergies are to be found in the Proceedings of The Dusty and Molecular Universe (ed. A. Wilson, 2005). The Molecular Universe Research Training Network is involved directly in Herschel projects, as well as laboratory measurements and analysis tools to support Herschel observing projects. A description of the network, including links to Herschel activities is at http://molecular-universe.obspm.fr/.

3.1

The ALMA Project

The characteristics of ALMA are determined by three primary science goals. The first two are specific science cases. The first is detecting a Milky Way type galaxy at redshift z = 3, and the second is imaging planet-forming regions in a protostellar disk at 140 pc, the distance to the nearest star-forming regions. These determine the sensitivity and highest angular resolution of ALMA. The third requirement demands that ALMA produce high quality images at high spatial resolutions. In the millimetre and sub-mm wavelength range, the Earth's atmosphere has a significant influence on the measurements of astronomical sources. To minimise this influence, ALMA is located on a dry site at an elevation of 5 km. A sketch of the frequency coverage of the planned set of receivers is shown in Fig. 2. Parameters of ALMA as known at the end of 2005 are given in Tables 1 and 2. In the bilateral ALMA baseline plan, receiver bands 3, 6, 7 and 9 will be provided.

5


Figure 2: The numbered horizontal lines at the top show the coverage of ALMA receiver bands 3 to 10. Receiver bands 3, 6, 7 and 9 are in the North American-European bilateral plan. Receiver bands 4, 8 are to be contributed by Japan. There are plans for Japan to provide Band 10, dependent on R & D work. Bands 1 and 2 are not yet planned. Six receivers for Band 5 will be provided by EC funding in FP6. At any time, observations can be carried out with only one receiver band. The coloured curves shows three atmospheric transmissions for total perceptible water vapour (pwv) amounts of 0.2, 0.5 and 1 mm for Mauna Kea (from http://www.submm.caltech.edu/cso/weather/atplot.shtml).

It is now certain that Japan will enter the ALMA project. Receiver Bands 4, 8 will also be provided by Japan. There are plans for Japan to provide Band 10 eventually, after further research and development. Japan will also provide the "ALMA Compact Array" (ACA) that consists of twelve 7 m antennas to provide measurements of more extended structure and four 12 m antennas, to be used in the single dish mode to measure the total flux density of a source. The parameters of the ALMA project given in Table 1 reflect the result of a rebaseline process for the bilateral array. At present 50 antennas are planned, with an increase to 64 antennas if funding allows. An interactive sensitivity calculator for all ALMA Bands except 1, 2 and 10 is at: http://www.eso.org/projects/alma/science/bin/sensitivity.html. This calculator takes into account the effect of the Earth's atmosphere.

6


Table 1: ALMA Antenna Arrays and Configurations Array Number of Antennas Total Collecting Area Array Configurations (dimension of region filled) Compact filled Largest extent Total Number of Antenna stations Antenna Diameter Surface accuracy Field of View 3mm 0.5 mm
a) b)

Bilateral 50, up to 64 5654-7237 m
2

Compact (ACA) 16 915 m
2

150 m 18.5 km 175 12 m
a)

35 m 22 4 x 12 mb) ;12 x 7 m
a)

25 µm

12 m: 25 m; 7 m: 20 m

50" 8.3"

50"; 85" 8.3"; 14"

Transportable by especially constructed vehicles Fixed in position
a

Table 2: ALMA Front Ends

Band 1 (1cm) Band 2 (4mm) Band 3 (3mm) Band 4 (2mm) Band 5 (1.8mm) Band 6 (1.3mm) Band 7 (0.9mm) Band 8 (0.6mm) Band 9 (0.5mm) Band 10 (0.3mm)
a

31.3-45 GHz 67-90 GHz 84-116 GHz 125-163 GHz 163-211 GHz 211-275 GHz 275-373 GHz 385-500 GHz 602-720 GHz 787-950 GHz

HEMT HEMT SIS SIS SIS SIS SIS SIS SIS SIS
c,d e f d d e d g

b

All with dual polarisation; each polarisation feeds four IF sections, each with a bandwidth of 2 GHz, giving a total bandwidth of 8 GHz in each polarisation. Also uncooled 183 GHz water vapour monitors provide data for phase corrections b High Electron Mobility Transistor receiver c Superconductor-Insulator-Superconductor mixer receivers. d In the bilateral baseline plan e Contribution from Japan f Funded by EC for 6 antennas g Contribution from Japan; depends on R&D programme

7


The angular resolution is generally given by the expression = 0.2 / B Here is in arc seconds, is the observing wavelength in millimetres and B is the largest separation of the antennas in km. For = 3.5 mm and B = 0.15 km, = 4" while for = 0.5 mm, and B = 14.5 km, = 0.005". This relation also allows a calculation of the field of view (FOV) of ALMA, if one uses the diameter of the antenna for the term B. For a 12 m antenna at 3.5 mm, the FOV will be 58" while for the ACA 7 metre antennas the FOV is 100". For each receiver band, there are four intermediate frequency (IF) sub-bands, each with a total bandwidth of 2 GHz. In each, the ALMA spectrometer provides a range of velocity resolutions. The lowest resolution is 128 channels of 18.9 MHz, covering 2 GHz for each of the 2 polarisations. The spectrometer has the property that the product of total bandwidth (in GHz) with the number of channels is 256. These spectrometer windows can be placed nearly anywhere in each 2 GHz wide IF sub-band. For spectroscopic surveys one can measure 4 sub-bands of a given velocity resolution simultaneously within an 8 GHz wide region. The finest resolution will be 3.1 kHz; this corresponds to 0.01 kms-1 at 100 GHz, the middle of receiver Band 3. This velocity resolution corresponds to 10% of the expected collapse speed of a molecular cloud that will form a low mass star. Since ALMA is made up of individual antennas, it is possible, indeed desirable, to begin "early science" measurements before full completion in 2012. In the early science mode, at least 6 antennas will be available. Thus the ALMA images will be less detailed compared to those possible with the full ALMA and the sensitivity will be about a factor of 8 lower. Examples of ALMA scientific projects with a 64 antenna bilateral array can be found in the Design Reference Science Plan (DRSP) at http://www.strw.leidenuniv.nl/~alma/drsp. shtml. The DRSP contains examples of prototype projects to demonstrate the capabilities of ALMA. The DRSP consists of a mix of small and large programmes, written by researchers currently active in the field. Some of the larger programmes could evolve into "legacy" or "key programmes", but could also be carried out as a number of smaller programmes by different groups with essentially the same science goals but different sources. The DRSP contributions do not reserve research areas for the authors, but are meant to be examples to provide estimates of integration times, calibration accuracies and noise limits. At longer wavelengths where the FOV is fairly large, ALMA can be used to carry out blind surveys, but since the field of view of the antennas is small, a more efficient approach would be to use ALMA for imaging selected sources. There are many methods to select such sources. One is to use the Atacama Pathfinder Experiment (APEX) 12 m sub-mm antenna to survey selected regions. APEX could be used to find star-forming regions in the galactic plane or survey high galactic latitude regions. Other instruments that could be used to provide finding lists are the James Clerk Maxwell (JCMT) 15 m sub-mm antenna on Mauna Kea or the 50 m Large Millimeter Telescope (LMT) in Mexico. The sources found would be measured in follow-up surveys with ALMA.

8


3.2

Herschel

Herschel is a cornerstone mission of the European Space Agency. A recent report on the Herschel mission has been given by Pilbratt (2005). The Herschel satellite will orbit at the Lagrange point L2, and will have three instruments mounted on a single 3.5 metre passively cooled antenna. The broadband instrument systems consist of the Photodetector Array Camera & Spectrometer, PACS (Poglitsch et al., 2005) and Bolometer cameras, the Spectral & Photometric Imaging Receiver, SPIRE (Griffin et al., 2005). The PACS instrument consists of photometers that can perform measurements at 75, 110 or 170 µm. We list some properties of the PACS broadband instrument in Table 3. The SPIRE instrument consists of cameras that can perform measurements at 250, 360 or 520 µm. We list some properties of the SPIRE instrument in Table 4. The PACS and SPIRE beamsizes are compared with an image of the Hubble Deep Field image in Fig. 3. This gives a qualitative estimate of source confusion due to the angular resolution of PACS and SPIRE. For very deep integrations at shorter wavelengths there should be little source confusion, but confusion will be a consideration at longer wavelengths (see section 6.1). Both SPIRE and PACS can be used as low resolution spectrometers. The PACS spectrometer can be used for simultaneous measurements at 57-105 m and 105-210 m. See Table 5 for a list of some of the PACS spectrometer properties. The SPIRE spectrometer can perform measurements at 200-300 or 300-670 µm. We list a summary of the low resolution Herschel spectrometers in Table 6. There is a single pixel heterodyne system, Heterodyne Instrument for the Far Infrared, Herschel HIFI (Graauw & Helmich, 2001). Herschel HIFI is a single pixel high resolution spectrometer.
Table 3: Some Characteristics of the PACS Photometer Central Wavelength (m) 75 110 170 Angular Resolution (") 5.4 8.0 12.2 Number of pixels 64 by 32 64 by 32 32 by 16 Field of View (') 1.75 by 3.5 1.75 by 3.5 1.75 by 3.5

Table 4: Some Characteristics of SPIRE Central Wavelength (m) 250 360 520 Pixel Size (") 12 18 24 Angular Resolution (") 17 24 35 Number of pixels 139 88 43 Field of View (') 4 by 8 4 by 8 4 by 8

9


Figure 3: The Hubble Deep Field (angular resolution 0.05") is shown on the right. On the left are the Full Width to Half Power beamsizes of the PACS (three highest) and SPIRE instruments (three lowest). Table 5: Some Characteristics of the PACS Spectrometer Central Wavelength (m) 72 105 210 Angular Resolution (") 9.4 9.4 9.4 Number of pixels Field of View (") Spectral Resolution (approximate) 1800 1000 2000

16 x 25 16 x 25 16 x 25

47 by 47 47 by 47 47 by 47

Since Herschel is above the atmosphere, signals are not blocked by terrestrial spectral features. This location is ideal for the measurement of water vapour lines that are usually absorbed by the atmosphere even at the ALMA site. Since the Herschel HIFI receivers are double sideband mixers, the receiver noise for spectral line measurements is twice the double sideband (DSB) system noise. To relate the measured temperatures to flux densities, there is a correction for antenna efficiency but no correction for the Earth's atmosphere (see caption of Fig. 4). Thus, the Herschel HIFI receiver noise temperatures, shown in Fig. 4, are all important for sensitivity. As with ALMA, the velocity resolution of Herschel HIFI can be extremely high. The finest spectrometer resolution is 140 kHz, corresponding to 0.1 kms-1 at 480 GHz with finer values at higher frequencies.

10


Table 6: Some Characteristics of the Herschel Spectrometers System PACS SPIRE SPIRE HIFI HIFI Wavelength Limits (m) 57-210 200-300 300-670 157-212 240-625 Resolution (kms-1) 80-300 300-15000 480-24000 0.02-0.2 0.1-0.6 Spectral Coverage (kms-1) 700-3000 2500-125000 4000-200000 960-2500 850-625 Field of View (') 0.8 2.6 2.6 0.2 0.8

Figure 4: The Double Sideband (DSB) noise temperatures in Kelvin for Herschel HIFI receivers plotted versus sky frequency. The receiver bands are numbered above the horizontal lines at bottom of plot. For a 3.5 m antenna, the relation between flux density S in Jy and antenna temperature, T, in K is S = 287.3TA/, where the Herschel antenna efficiency is A. The finest frequency resolution of the Herschel HIFI spectrometer is 140 kHz, or 0.042 kms-1 at 1 THz. The 1-sigma uncertainty for a 140 kHz spectrometer resolution is TRMS = 5.34 x 10-3 T(DSB) Hz-1/2, Jackson (2005).

11


12


4

Comparison of the ALMA and Herschel

This section makes a brief comparison between the fundamental properties of ALMA and Herschel, demonstrating that they will have angular resolutions comparable with (ALMA) or approaching (Herschel) those of today's cutting edge telescopes, while operating at wavelengths not covered by current instruments. In addition, two principal observing strategies are presented.

4.1

Comparison of the Properties of ALMA and Herschel

In Fig. 5 we show the angular resolutions of the Very Large Array (VLA), ALMA, Herschel, the James Webb Space Telescope (JWST), the Very Large Telescope (VLT), the VLT Interferometer (VLTI) and the Hubble Space Telescope (HST). The range of angular resolutions for ALMA and the VLA correspond to the different configurations. This two-dimensional plot does not contain the frequency resolutions of the instruments. These are included in the projected three-dimensional illustration shown in Fig. 6.

Figure 5: A plot of the angular resolutions of the VLA, ALMA, Herschel instruments, VLT, the VLT Interferometer, VLTI, JWST and HST. Below the region marked "ALMA" are the ALMA receiver bands.

13


Figure 6: A three-dimensional plot of frequency, angular resolution and resolution for the instruments compared in Fig. 5.

From Fig. 6, we see that Herschel HIFI approaches the velocity resolution capable with ALMA, while for all other instruments, the velocity resolutions are much lower.

4.2
4.2.1

Comparisons of Observational Techniques
Broadband Measurements

ALMA will have a much larger collecting area than Herschel. However, at specific frequencies, usually near water vapour lines, the Earth's atmosphere attenuates signals, perhaps completely blocking these. For the SPIRE bolometer array, there is a significant overlap with ALMA receiver bands 8 to 10. The PACS bolometer array provides shorter wavelength data needed to determine SEDs. In Fig. 7 we show the coverage on a spectrum of the starburst galaxy M82 for different redshifts.

14


Figure 7: A plot of the emission from the starburst galaxy M82 for different redshifts, z. The horizontal axis is observed wavelength, the vertical axis is predicted flux density in mJy. The crosses show the sensitivity of the Herschel bolometers, taken from Griffin et al., 2005. The dashed lines in the left side of this diagram show the 5-sigma sensitivity of ALMA. The lower dashed curve is for the 64 antenna ALMA and the upper curve for a 6-antenna ALMA. The ALMA receiver bands are shown numbered above the horizontal axis. Cf Fig. 1 for line identifications. The coverages of the Herschel instruments are shown as horizontal lines (HIFI, SPIRES, PACS).

Even at redshifts of 3 < z < 5, the broadband emission of sources such as M82 can be detected with Herschel and the early science ALMA. To compare ALMA with Herschel data accurately, there are two important considerations. The first is the signal-to-noise ratio. From Fig. 7, we see that SPIRE and PACS will be able to obtain good measurements of M82-like sources for redshifts up to z = 2 in 1 hour. For higher redshifts, much longer integration times are needed. The second consideration is concerned with systematic effects. For example, the bandwidths of PACS and SPIRE detectors are much larger than the bandwidth of ALMA receivers. The most obvious requirement of a calibration plan must be to reduce or eliminate systematic effects such as the very different bandwidths and angular