Space and High Energy Astrophysics

Between 1993 July 1 and 1996 June 30 there were 230 papers published in the major astronomical journals with IRAS data a major enough theme that the word IRAS appeared in the title. Some of the most notable advances from IRAS are selected below

On the origin of the 40-120 micron emission of galaxy disks: A comparison with H-alpha fluxes

A comparison of 40 to 120 micron Infrared Astronomy Satellite (IRAS) fluxes with published H alpha and UBV photometry shows that the far infrared emission of galaxy disks consists of at least two components: a warm one associated with OB stars in HII-regions and young star-forming complexes, and a cooler one from dust in the diffuse, neutral interstellar medium, heated by the more general interstellar radiation field of the old disk population (a cirrus-like component). Most spiral galaxies are dominated by emission from the cooler component in this model. A significant fraction of the power for the cool component must originate with non-ionizing stars. For a normal spiral disk there is a substantial uncertainty in a star formation rate derived using either the H alpha or the far infrared luminosity

The SWIRE SIRTF Legacy Program: Studying the Evolutionary Mass Function and Clustering of Galaxies

The SIRTF Wide-area Infrared Extragalactic (SWIRE) survey is a "Legacy Program" using 851 hours of SIRTF observing time to conduct a set of large-area (67 sq. deg. split into 7 fields) high Galactic latitude imaging surveys, achieving 5-sigma sensitivities of 0.45/2.75/17.5 mJy at 24/70/160 micron with MIPS and of 7.3/9.7/27.5/32.5 microJy at 3.6/4.5/5.8/8.0 micron with IRAC. These data will yield highly uniform source catalogs and high-resolution calibrated images, providing an unprecedented view of the universe on co-moving scales up to several hundreds Mpc and to substantial cosmological depths (z\simeq 2.5 for luminous sources). SWIRE will, for the first time, study evolved stellar systems (from IRAC data) versus active star-forming systems and AGNs (from MIPS data) in the same volume, generating catalogues with of order of 2 million infrared-selected galaxies. These fields will have extensive data at other wavebands, particularly in the optical, near-IR and X-rays. SWIRE will provide a complement to smaller and deeper observations in the SIRTF Guaranteed Time and the Legacy Program GOODS, by allowing the investigation of the effect of environment on galaxy evolution. We expand here on capabilities of SWIRE to study with IRAC the evolution of the bright end of the galaxy mass function as a function of cosmic time.Comment: 6 pages, Latex, special macros. To appear in the Proceedings of the ESO Workshop "The Mass of Galaxies at Low and High Redshift", R. Bender and A. Renzini Eds., Springer-Verlag Series "ESO Astrophysics Symposia

Infrared polarization studies of protostars

The thesis to be proved is that the mechanism of optical interstellar polarization, preferential extinction by magnetically aligned aspherical grains, can be extrapolated successfully to explain the large infrared polarizations observed in molecular cloud protostellar sources. Observations of the linear polarization of 13 sources between 1.65 μm and 4.8 μm and of the circular polarization of 9 sources at 2.2 μm have been obtained. Analogously to the optical interstellar case, the data have been modelled using the Rayleigh approximation to calculate extinction and phase -lag efficiencies for a number of grain models. The models successfully account for the high infrared ellipticities (ratio of linear to circular polarization) observed in the molecular clouds, with twists in the grain alignment of about 40°; smaller than those required to explain optical interstellar circular polarization. A shortcoming in the model linear polarization at X < 3 μm is attributable to a failure in the Rayleigh approximation. A dissimilarity in the polarization through the ice band between two of the protostellar sources can be understood by differing grain compositions. A correlation between the position angles of polarization of the protostars and the nearby interstellar field stars can be interpreted by saying the Galactic magnetic field permeates the dense molecular clouds. There are indications that the polarization mechanism even operates in the enhanced density regions of the clouds; then a more efficient alignment mechanism than paramagnetic relaxation is required. It is suggested that this may be "pinwheeling," possibly accompanied by super-paramagnetism of the grains. The twists in the magnetic field lines implied by the model may arise in the collapse process of the rotating, magnetized clouds

Resolution of the Compact Radio Continuum Sources in Arp220

We present 2 cm and 3.6 cm wavelength very long baseline interferometry images of the compact radio continuum sources in the nearby ultra-luminous infrared galaxy Arp220. Based on their radio spectra and variability properties, we confirm these sources to be a mixture of supernovae (SNe) and supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both wavelengths. The SNe generally only have upper size limits. In contrast all the SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent with them having just entered their Sedov phase while embedded in an interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the diameter-luminosity correlation for SNRs (and so also on the diameter-surface brightness relation) and extend these correlations to very small sources. The data are consistent with the relation L {\propto} D^{-9/4}. Revised equipartition arguments adjusted to a magnetic field to relativistic particle energy density ratio of 1% combined with a reasonable synchrotron-emitting volume filling factor of 10% give estimated magnetic field strengths in the SNR shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from compression of ambient ISM fields and must instead be internally generated. We set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in relativistic particles, 2%-20% of the explosion kinetic energy, is consistent with estimates from models that fit the IR-radio correlation in compact starburst galaxies.Comment: 16 pages, 5 figure

Characteristics of UGC galaxies detected by IRAS

Infrared Astronomy Satellite (IRAS) detection rates at 60 microns were determined for the Uppsala General Catalog of Galaxies (Nilson 1973; the UCG). Late-type spirals, characterized by a normal IR/B ratio of approximately 0.6, are detected to a velocity of approximately 6000 km/s for L sub B = L sub *. Contrary to the situation for IRAS-selected galaxy samples, little evidence was found for a correlation between IR/B and 60/100 microns in this large optically-selected sample. Thus a significant fraction of the IRAS-measured far-infrared flux from normal spirals must originate in the diffuse interstellar medium, heated by the interstellar radiation field. Support was not found for Burstein and Lebofsky's (1986) conclusion that spiral disks are optically thick in the far-infrared

Space and High Energy Astrophysics

Between 1993 July 1 and 1996 June 30 there were 230 papers published in the major astronomical journals with IRAS data a major enough theme that the word IRAS appeared in the title. Some of the most notable advances from IRAS are selected below

Global properties of the nearby spiral M101

M101 (NGC 5457) is a classic Sc I spiral galaxy located suffiently nearby, 6.8 Mpc, that its structure can be studied even with the coarse angular resolution of the Infrared Astronomy Satellite (IRAS). The global infrared properties of M101 are addressed including the radial dependence of its infrared emission