STARBURST

SPECIAL ISSUE 1999

Course: Star formation in galaxies: the FIR and submm view

Prof. Alberto Franceschini
University of Padova
ITALY

Space missions such as HST, ISO, SIRFT or FIRST, have been or will be launched to help get a picture of the early Universe, to understand how stars and galaxies formed. Astronomers have gathered an incredible amount of information at different wavelengths, but there appear to be discrepancies in the results when it comes to high redshift studies. Alberto Franceschini, from the University of Padova, and participant to some of these missions, comments on the role of the past and future space missions.

"Indeed the exploitation of the unique capabilities for spatial resolution in the optical of the Hubble Space Telescope through very deep integrations in two areas, the HDF North and South, has produced an unprecedented, clean and sharp view of the distant universe, revealing galaxies with a variety of morphologies over a substantial fraction of the Hubble time. These spectacular capabilities of angular resolution are unique at the moment, and will not be likely superseded by other instruments in the many years to come. However, Space Telescope is based on a relatively old technology, limited as for photon collecting power, detector size and wavelength coverage. The limited sizes of the primary collector and of the detector assemblies imply that only very small sky areas can be covered during the whole lifetime of the observatory (each of the two HDFs of only 4 squared arcminutes size has required 10 days of continuous integration).

The HST Servicing Mission programmed for the mid of 2001 will implement in the focal plane an improved imaging camera (the Advanced Camera for Surveys) and will restore the near-infrared imager. This will allow to survey more extensive areas to similar depths, hence to verify the statistical significance of the results obtained in the Hubble Deep Fields.

However, new breakthroughs in our knowledge of the distant and very distant universe are likely to come from a technological breakthrough in space astronomy: the New Generation Space Telescope, a fantastic step forward with respect to HST. The longer wavelength capabilities of this observatory will allow to circumvent the problem that the optical emission by stellar populations in high redshift galaxies is redshifted into the infrared, which prevents proper characterization of distant galaxies by HST.

If the anticipated performances of NGST will be achieved, many mysteries about the origin of the universe around us - e.g. about the formation of structures at very high redshifts (as traced by stellar emission), and about star and planet formation - will start having an answer."

"Extensive sky surveys have been particularly successful with the mid-IR camera ISOCAM, exploiting among other a superior spatial resolution than the longer-wavelength instrumentation. The main result appears to be the requirement for an extremely high rate of evolution with cosmic time of the IR emissivity of galaxies, needed to explain the very steep observed counts. The populations responsible for these excess counts are mostly found by spectroscopic surveys to be at redshifts from 0.5 to 1.5. It is in this redshift interval that the bulk of the e.m. energy in the universe was produced, presumably by young stars in forming galaxies. Several of us anticipated strong cosmological evolution for IR galaxies, but not at the level observed by ISO. This seems to imply that a substantial fraction (50%, or possibly more) of the radiant energy by young galaxies was absorbed by dust and re-emitted at long wavelengths. A trace of this was also found by the NASA’s mission COBE in the form of a bright diffuse background at far-infrared and sub-mm wavelengths, likely produced by the integrated signals of faint distant galaxies.

"Contrary to what the acronyms might suggest, the two missions are quite different. SIRTF is a NASA follow-up of the ISO mission with much improved and sophisticated IR instrumentation. Because of the limited size of the primary mirror (85 cm), it is likely that SIRTF’s main results will concern the emission properties of galaxies and AGNs from 5 to 50 micron, a spectral region very rich in dust features and spectral lines by a variety of atomic species of various ionization levels. This will allow detailed studies of the physical processes originating the IR emission, in particular those related with stellar populations and nuclear non-thermal activity. Another interesting investigation by SIRTF will be to use the broad emission feature of galaxies at 1.7 micron (due to a minimum in the stellar atmospheric opacity) to measure precise photometric redshifts via multi-band observations in the mid-infrared for a large number of galaxies. This technique, complementary to those currently adopted in the optical (e.g. the "drop-out" and the optical multi-band fitting), has the obvious advantage of being much less prone to the dust extinction and stellar aging effects.

The ESA cornerstone mission FIRST, instead, exploits a much larger primary mirror, which will enable powerful investigations in the mostly unknown spectral domain from 70 to 700 microns. This brackets the huge emission peak due to dust emission, and also includes important emission lines by atoms and molecules, in particular the CII cooling line at 158 micron, which will be observable to very high redshifts. However the major task identified for this space observatory will be to explore the distant universe by means of deep extensive surveys: these will preferentially detect high redshift objects because of the strong, favourable K-correction when observing longwards of the dust emission peak.

FIRST will have sensitive large format detectors, able to simultaneously cover large areas in several broad bands. Again photometric redshifts for tens of thousands of high-redshift sources detected by FIRST over large sky areas will be measurable, hence providing a way to picture the distant universe in a way totally insensitive to dust obscuration. These extensive surveys of FIRST will be perfectly synergic with the planned millimeter arrays (ALMA, MMA), to be operative towards the end of the next decade, whose very high spatial resolution will allow to obtain a detailed physical characterization of the FIRST sources."

"Most of the disagreement is due to the uncertain corrections to apply to the optical fluxes to account for dust extinction in distant galaxies. Recent analyses have shown that often this correction may not even be possible if the analysis is limited to the optical/near-infrared, where stellar emission dominates: in many local starbursts there is evidence for young star clusters completely embedded in optically thick clouds, which essentially do not emit in the optical. Often this optically-hidden emission dominates the bolometric output in luminous, actively star forming galaxies. It is clear that only mid- to far-IR observations are needed to identify and measure these emission components.

The processes bringing to the collapse of an interstellar cloud, fragmentation and the formation of stars, are among the least understood in astrophysics. It is clear, however, that the star formation originates from compression of the interstellar gas, which may be due to a variety of reasons. According to the efficiency of this gas compression, star formation may proceed quiescently, as in the gaseous disk of spirals following the density enhancement in the spiral arms, or may occur as a much more violent event when a large fraction of the whole gaseous content of a galaxy is made to collapse in the inner galactic core by a strong interaction or a merger with another massive system, destroying the rotational ordered motions of the gas. The latter brings to a luminous or ultra-luminous starburst, which in the current cosmogonic scenario is expected to originate the spheroidal (as opposed to the disk) components in galaxies. IRAS and ISO observations have shown that, at increasing gas compression and starburst efficiency, the fraction of the optical light by the young massive stars absorbed by dust and re-emitted in the mid- to far-IR increases, from the few tens percent typical of quiescent spirals up to 99 percent of the most luminous starbursts (e.g. Arp 220).

So, if we expect that optical observations, with suitable extinction corrections, may determine the quiescent phase of star formation, the investigation of the more active star-formation phases is only possible with long-wavelength observations."

ALBERTO FRANCESCHINI was born in Verona (Italy), in 1952. Associate Professor of Astronomy at the University of Padova since 1989, he started to work during the late seventies and early eighties on the origin of the X-ray background, and more generally on the statistical properties of faint extragalactic sources and the contributions of galaxies and AGNs to the cosmic background radiations in the microwave, IR and X-rays. He defined the wavelength intervals and ranges of angular size suitable for investigations of the anisotropies of the cosmic microwave background (CMB), then providing grounds on which two space missions (ESA’s PLANCK and NASA’s MAP) have been planned and developed.

For long time he studied fthe potentialities of tnfrared and millimetric astronomy in the cosmological context, and predicted the existence of a cosmological background at the precise level it was measured years later by the COBE experiment, and the existence of a population of IR-luminous starbursts at high redshifts originating current spheroidal galaxies, a population recently detected by the bolometric imaging array SCUBA on JCMT.

Franceschini has been co-I of the imaging camera CAM of the ISO observatory; scientific expert for the ESA cornerstone mission FIRST and is currently co-I of the FIRST’s bolometric imager and spectrograph (SPIRE). He is also co-I of the Low-Frequency Instrument for the PLANCK Surveyor, dedicated to the study of the CMB small-scale anisotropies.

Member of the IAU and of the Padova Consortium for Astrophysics and Space Sciences (CISAS), since 1998, he is part of the ESA Astronomy Working Group.