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Report of the Cosmology and Extragalactic Working Group

L. Blitz (U. Maryland), W.B. Burton (Leiden), L.F. Brown (Conn. College), R.L. Brown (NRAO), J.E. Carlstrom (Chair, Caltech), D. Downes (IRAM), D.E. Hogg (NRAO), J.D. Kenney (Yale), M.L. Kutner (NRAO), K.Y. Lo (U. Illinois), J.M. Moran (Harvard), S.T. Myers (U. Penn), F.N. Owen (NRAO), S.J.E. Radford (NRAO), N.Z. Scoville (Caltech), R.P.J. Tilanus (JACH), C.D. Wilson (McMaster), G. Wynn-Williams (U. Hawaii)

Overview

The MMA will have unsurpassed sensitivity and imaging capabilities for molecular spectroscopic studies of external galaxies. Additionally, its high brightness sensitivity and large continuum bandwidth will allow fundamental cosmological problems to be addressed through observations of structure in the Cosmic Microwave Background radiation and through observations of dust in the most distant galaxies in the Universe. The early Universe will be investigated by imaging the small scale anisotropies imprinted in the Cosmic Microwave Background at which have led to the structures on the scales of clusters and galaxies. The first galaxies formed with be detected by MMA observations of their dust emission. The molecular interstellar medium of ultraluminous galaxies will be imaged to and in normal galaxies to , allowing their subsequent evolution to be followed. The dynamics of interacting, merging, spiral, and barred galaxies will be revealed by detailed imaging. Individual star forming regions in nearby galaxies of all morphological types will be investigated. MMA observations of the Magellanic Clouds offer the opportunity to investigate the star formation process for a large population of clouds with different environmental conditions all at the same distance and at the resolution of an individual star forming core.

Highlights

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  1. Cosmology and the Early Universe

  2. Epoch of Galaxy Formation and Subsequent Galaxy Evolution

  3. Imaging Gas in Galaxies

  4. Galactic Nuclei

Cosmology and the Early Universe

The MMA will be able to image anisotropies in the Cosmic Microwave Background (CMB) at angular scales up to 2 with unsurpassed brightness sensitivity, obtaining 2 K RMS for a 1 beam in only 1 hr. These fluctuations are generated by the seeds in the early universe () that have grown into galaxies and clusters of galaxies. The MMA is the only instrument with the sensitivity, frequency coverage, and range of angular resolutions needed to accurately measure the extremely weak () fluctuations on these scales.

Even stronger (K) distortions in the CMB are caused by scattering of the CMB photons by ionized gas in the hot () atmospheres in massive clusters of galaxies -- this is known as the Sunyaev--Zel'dovich Effect. The MMA will provide detailed imaging and polarimetry of the Sunyaev--Zel'dovich Effect in distant () clusters. Multi-frequency observations from 26-350 GHz can be used to compute the cluster radial peculiar velocities, independent of the distance to the cluster. Combining the MMA images with high resolution x--ray maps (e.g. AXAF) leads to a determination of the expansion rate (H) and deceleration parameter (q) of the universe that is independent of the standard distance scale ladder. X--ray and Sunyaev--Zel'dovich measurements can also inventory the mass of the hot gaseous cluster medium, providing crucial limits linking the predictions of big-bang nucleosynthesis with the observed baryonic masses.

The local Hubble flow and perturbations to it can be measured, in principle, by using the Tully--Fisher relation. The T--F relation is now determined from HI emission in relatively nearby galaxies which are subject to local velocity perturbations. However, the MMA is much more sensitive to CO emission from an external galaxy than any telescope is to HI emission; the sensitivity of the MMA to CO emission in a external galaxy is times the sensitivity of the VLA to HI emission per unit mass of gas, and is a few hundred times more sensitivity than the 300 m Arecibo telescope to HI emission. This increased sensitivity will enable the MMA to extend T--F to much larger distances than possible using HI. The CO emission is less susceptible than HI to inducing scatter in the T--F relationship as a result of warping and non-circular rotations beyond the edge of the stellar disk. Furthermore, CO more faithfully follows the distribution of the luminous matter in galaxies than does HI. Once T--F is calibrated, one can measure accurately velocity perturbations to the local Hubble flow.

Epoch of Galaxy Formation and Subsequent Galaxy Evolution

Galaxy formation and evolution are, at best, poorly understood. Since QSOs and galaxies have been directly observed at , the first galaxies must have formed earlier than that. On the other hand, the size of cosmic background radiation distortions suggests galaxies are unlikely to have formed before . Pinpointing the initial epoch of galaxy formation within this broad range, however, has not been possible. All galaxies identified so far have characteristics, such as active nuclei or metal enrichment, that suggest they have already undergone significant evolution. Extensive searches in the optical for more primitive systems have turned up nothing, which may mean we do not have a good idea of a protogalaxy's properties. Furthermore, the epoch when the majority of galaxies formed is not necessarily the same as when the first galaxies formed.

Many influences, both internal and external, on galaxy evolution can be identified, but their relative importance is unclear. It is clear, though, that the relationship between the interstellar medium and star formation and how it is affected by other factors are crucial points to be understood.

The Millimeter Array should make major contributions to the study of galaxy formation and evolution. Some examples:

Imaging Gas in Galaxies: Star Formation, Galactic Structure, and Evolution

The MMA will make large-scale, fully-sampled images of the molecular gas in galaxies, images whose resolution and detail can surpass that of optical images. These maps will give the information on both parsec and kiloparsec scales needed to explore the relationship between star formation, gas density and gas kinematics. In this way, we can identify the reasons why gas concentrations develop (for example, in spiral arms and at the ends of bars), and why some gas concentrations continue to condense, to the point of star formation, while others do not. These images will also reveal the structure and kinematics of the faint emission between spiral arms and bars, which is crucial for understanding the life cycle of molecular clouds, galaxy dynamics, and radial gas flows.

The MMA will be the premier instrument for studying the central gas concentrations of galaxies. Most of the gas driven towards the central parts of spiral galaxies by bars and interactions eventually settles into a molecular disk, spanning the circumnuclear (inner kpc) region. These circumnuclear gas disks, an order of magnitude more compact than the disks of spiral galaxies, constitute a previously unrecognized major component of galaxies. They spawn circumnuclear starbursts and can drive galaxy evolution by building compact stellar disks, and perhaps ultimately bulges. CO galaxy images made with the MMA will show the circumnuclear gas morphologies of large numbers of galaxies, revealing trends along the Hubble sequence and with environmental conditions. It will be possible to address whether spiral galaxies evolve along the Hubble sequence from Sc toward Sa, as the result of gaseous inflow driven by bars or interactions.

The key to understanding why starbursts happen lies in understanding the dynamical conditions which allow gas to reach the circumnuclear regions, and the star formation laws which control the conversion of gas into stars. The MMA can answer these questions by revealing the gas distributions and kinematics of starbursts with unprecedented accuracy. MMA maps of starbursts will reveal the clumpiness of the molecular ISM, which may determine whether most of the gas in the burst is converted into stars, or whether most gets blown out in a wind. They will also show how the molecular ISM changes as a starburst evolves, and whether there are outflows of molecular gas.

The MMA will also be able to study the formation of globular clusters. Many starburst galaxies contain luminous compact clusters of young stars. With masses as large as 10 M, and sizes of just a few pc, these are surely young globular clusters. The MMA can resolve the dense, ultramassive (10-10 M) molecular clouds which give birth to globular clusters in the nearest starburst galaxies, and probe the physical conditions in the gas which form clusters much larger than any presently forming in the Milky Way.

The large-area, high-resolution images from the MMA will allow us to study individual molecular clouds in a wide range of galactic environments, from elliptical to irregular galaxies, out to a distance of 100 Mpc. To understand star formation, we need to understand how the process occurs on small scales, which may depend on the properties of the molecular cloud in which the stars form. Observations of molecular cloud populations are required to understand the formation and evolution of clouds. The capability of the MMA to rapidly produce maps of many molecular lines is essential for measuring the physical and chemical properties of a large sample of clouds. One important question is whether molecular clouds have the same basic properties in all environments, or whether such factors as location in an arm and interarm region, the local rate of star formation, or the morphological type of the host galaxy produce fundamental changes in the molecular cloud population.

The Magellanic Clouds will provide a unique opportunity to study star formation in another galaxy if the MMA is sited in the southern hemisphere. At the distances of the Large Magellanic and Small Magellanic Clouds, 1 corresponds to 0.25 and 0.35 pc, respectively, the size of the star forming core of molecular clouds in the Milky Way. The MMA will thus be ideally suited for imaging structures from Giant Molecular Clouds (GMCs) to individual star forming cores and outflows. Also, since all of the objects within one of the galaxies are at the same distance, it is possible to make meaningful comparisons of the conditions within clouds and their star formation histories. In particular, the role of the environment in determining the properties of molecular clouds can be addressed directly. The environment within the Magellanic Clouds is completely different from the Milky Way, especially in terms of metallicity, radiation field and external pressure on clouds. By studying a statistically significant number of clouds in the LMC and SMC, and comparing them with the Milky Way, we can see how these environmental factors affect the properties of the molecular clouds the formation of stars within the clouds.

Galactic Nuclei

The MMA is a superb instrument for studying galaxy nuclei of all luminosities. In our own Galactic Center we should be able to detect the continuum emission from the accretion disk around Sgr A, as well as map the kinematics of the infalling neutral gas on a scale of cm. With the same angular resolution as the HST () the MMA can resolve circumnuclear disks out to the Virgo cluster and beyond, providing information on the geometry, physical conditions and kinematics of the gas that fuels active galactic nuclei. For example, radio observations of the masers in the nucleus of NGC4258 trace a Keplerian disk of 0.1 pc scale, bound by a central mass of . It is probable that this disk extends to larger radii and merges with an obscuring molecular torus, which can be detected by the MMA. At 230 GHz and a resolution of (3 pc at 6 Mpc distance) the MMA has a sensitivity of 0.3 K in 12 hours at a spectral resolution of 10 . The gas velocity on this scale is expected to be 300 , and, in analogy with the circumnuclear disk in our Galaxy, should have a brightness temperature of at least 10 K in the CO line. On larger scales, the MMA can probe the molecular line and dust emission from starburst regions of galaxies at distances up to 60 Mpc at resolutions of 30 pc. These maps will provide unique images of the processes triggered by galaxy collisions and the feeding of AGNs inside starburst regions. Non-thermal processes in the lobes and jets of extended radio galaxies will also be studied, including magnetic fields, electron energy distributions and reacceleration processes, as well as the origin of depolarization in radio lobes. Finally, as the prime component in a world-wide millimeter-wave VLBI network, the MMA would allow us to map the structure of AGNs with a resolution of order 10 micro arcseconds, the highest resolution achievable in astronomy.

Comments on MMA Specifications

  1. Sensitivity. The projected sensitivity of the MMA greatly exceeds that of any existing mm-wave telescope. The current design calls for 40 telescopes of 8 m diameter, giving a total collecting area of 2000 m. The large number of baselines ensures fast imaging speed and high image fidelity. The collecting area is roughly 2 -- 3 times larger than planned expanded mm--wave arrays and comparable to the largest existing and planned single dish mm--wave telescopes. The large increase in sensitivity projected for the MMA is due to extremely low--noise, broadband receivers, clean optics, and most importantly, its location on an extremely dry site. All of the science projects outlined above will benefit from increased sensitivity. It is unlikely that the MMA receivers can be improved much beyond the design specification, and therefore only by increasing the collecting area will a further increase in sensitivity be realized. Since it is not feasible to increase substantially the collecting area by the addition of more telescopes, we suggest that an increase in the diameter of the telescopes be considered subject to the following constraints:

    a.
    The accuracy of the telescopes remain adequate for high fidelity mosaicing imaging at 345 GHz. This also preserves the submillimeter performance of the telescopes.

    b.
    The number of telescopes could be reduced, but not at the expense of a significant reduction in image fidelity.

    c.
    The cost of the array remain within the bounds of the current proposal. We enthusiastically endorse the MMA and do not advocate altering the scope of the project in such a manner that a new proposal need be submitted to the NSF; construction of the MMA should not be delayed. We do request, however, the optimization of the total collection area versus the number of telescopes be reconsidered. The telescopes are the most basic component of the MMA and their size is the one thing that cannot be changed at a later date. Other components of the MMA (e.g. receivers, correlators etc.) could be compromised now and upgraded at a later date, thus freeing more resources for the construction of the telescopes.

    We do point out, however, that Cosmic Microwave Background science with the MMA will be compromised if the diameter of the telescopes is increased much beyond 10 m.

  2. Frequency coverage. The bulk of the science will be carried out within the 70 -- 380 GHz band. We do not require continuous frequency coverage, particularly near strong atmospheric lines. Additionally, we recommend strongly the following:
    a.
    The centimeter--wave band is important for Cosmic Microwave Background experiments, with the Ka band (26 -- 36 GHz) being preferred. We recommend that this band be included in the original set of receivers. The Q band (36 -- 55 GHz) could be omitted, but space should allocated for its implementation at a later date.
    b.
    The submillimeter windows will be important to increase the sensitivity to dust continuum emission, to measure dust color temperatures, and to allow observations of high excitation molecular transitions, as well as redshifted CII emission. The dust color temperatures will be particularly important for studying distant galaxies. We recommend that the receivers be designed with room for mixers to cover the 600, 450, and 350 m bands, but that only the 450 m band be included with the original suite of mixers.

  3. Site. The Chilean site is favored since it appears better than Mauna Kea, allows expansion of the baselines, and provides access to the Magellanic clouds.

  4. Imaging capabilities. It is desired to make large mosaics of nearby galaxies with a limited number of spectral channels (). This requires good total power capabilities. Mosaicing and total power capabilities are also required for CMB anisotropy and Sunyaev--Zel'dovich experiments. We hope that this can be done without a nutating subreflector, but urge that this be demonstrated before abandoning nutating subreflectors.

  5. Correlator. Extragalactic work does not require many subbands or high frequency resolution. However, the total bandwidth should be maximized for both continuum sensitivity and for line searches. We desire 8 GHz or more bandwidth, but only modest frequency resolution.

  6. Simultaneous frequencies. Simultaneous dual frequency observations are not required; the fast time multiplexing scheme that was discussed by the MDC receiver group is satisfactory.

  7. Polarimetery. Polarimetry is desired for continuum observations. High purity polarization capabilities need only be possible at one frequency within each mixer band.

  8. Baselines. Longer baselines (10 km) would be particularly useful for studies of the gas and dust emission associated with AGN.


kweather@aoc.nrao.edu