C. G. Wynn-Williams

The Deep Silicate Absorption Feature in IRAS 08572+3915 and Other Infrared Galaxies

New mid-infrared (10 and 20 μm) spectrophotometry of the ultraluminous infrared galaxy IRAS 08572+3915 is presented. The 10 μm spectrum reveals a deep silicate absorption feature, while the 20 μm spectrum shows no clear evidence for an 18 μm silicate absorption feature. An interstellar extinction curve is fitted to IRAS 08572+3915 and two other deep silicate infrared galaxies, NGC 4418 and Arp 220. It is found that pure extinction cannot explain the spectral energy distributions of these sources. On the other hand, both the strength of the silicate absorption and the overall spectral energy distributions of the three galaxies agree well with scaled-up models of galactic protostars. From this agreement, we conclude that the infrared emission comes from an optically thick dust shell surrounding a compact power source. The size of the power source is constrained to be smaller than a few parsecs. We argue that a significant portion of the total luminosities of these galaxies arises from an active galactic nucleus deeply embedded in dust.

Infrared emission and mass loss from evolved stars in elliptical galaxies

Small aperture 10.2-micron measurements of normal elliptical galaxies show that for almost all of these galaxies the 12-micron emission seen by IRAS is extended on the scale of the galaxy. NGC 1052 and NGC 3998 are exceptions to this; much of their 10-12-micron emission comes from the inner regions of the galaxies and may be associated with their active nuclei, as is the case for many radio galaxies. The distribution of the IR light and the IR colors of elliptical galaxies suggest that the most plausible source of the 12-micron emission is photospheric and circumstellear emission from cool evolved red giant stars. The 12-micron emission is well in excess of that expected from photospheric emission alone; about 40 percent of it probably comes from circumstellar dust.

The properties and environment of the giant, infrared-luminous galaxy IRAS 09104 + 4109

IRAS 09104 + 4109 is the most luminous object yet discovered by means of the IRAS survey. It is identified with a cD galaxy having a strong emission-line spectrum at a redshift of 0.442 and emits 6 x 10 to the 12th solar luminosities/sq h, 99 percent of which emerges at infrared wavelengths. One component of a double radio source is coincident with the center of the galaxy. The high luminosity of this source may be related to an interaction with one or more members of the rich cluster in which it lies. There is a secondary peak in the emission-line image of the galaxy. Emission lines from both regions are broad but narrower than those characteristic of the only other objects known to have such high luminosities, such as Seyfert 1s and QSOs. It is suggested that the strong infrared excess of IRAS 09104 + 4109 is produced by dust obscuring a broad line region.

Infrared studies of H II regions and OH sources

This paper presents the observational results of a high spatial-resolution mapping and photometric study in the wavelength range from 1.65 to 20 microns of four H II regions and seven OH sources. Infrared emission indicative of the presence of heated dust was found from the compact H II condensations in W51, DR21, and NGC7538. Infrared emission was also found from the positions of maser sources in NGC 7538, W75(N) W75(S) and Sharpless 269, but not from that in W51. An extended infrared source was found coincident with the peculiar OH source OH 0739-14.

A luminous 3 kiloparsec infrared disk in NGC 1068

A 10 micron map of the Seyfert galaxy NGC 1068 and airborne measurements of its angular extent in the far-infrared are presented. It is shown that the infrared emission originates primarily from two physically distinct regions; approximately half of the total infrared luminosity of 3 x 10 to the 11th solar luminosities is associated with the Seyfert nucleus and half with a 3 kpc (35 arc sec) diameter disk surrounding it. It is argued that the disk component of infrared emission originates from an extended but heavily obscured burst of star formation which resembles those seen in some non-Seyfert galaxies. This high-luminosity disk is distinguished more by its large size than by its high surface brightness. On the basis of current evidence it cannot be concluded that the high disk luminosity in NGC 1068 is causally related to its Seyfert activity.

The Kleinmann-Low nebula - An infrared cavity

High resolution 20-30 micron IR continuum emission observations of the Orion-KL region, combined with the recent 3.8-micron polarization results of Werner et al. (1983), yield a self-consistent model of the central 30 arcsec of the nebula. In this model, the geometry of the KL nebula is that of a clumpy cavity rather than that of a number of isolated objects. The cavity is centered on IRc2, which is confirmed to be the source of nearly all the regions luminosity. The model which best fits all the IR and radio data implies that the other peaks in th KL nebula are irregularities in the material at the edge and surrounding the cavity, rather than individual self-luminous sources.

New multiple systems in molecular clouds

Twenty-micron searches of the dense cores of the molecular clouds S140, S255, and Cepheus A have revealed the existence of multiple sources of infrared radiation which seem to be precursors to Trapezium-like clusters of OB stars. Two of the new sources are extended with unusually low color temperatures, less than 100 K. Observations with the Very Large Array with 1 mJy sensitivity have shown that three and possibly four of the infrared sources are associated with small H II regions. Multiple systems appear to be the rule rather than the exception in molecular clouds.

Cold Dust in Galaxies

Models for dust heated by the interstellar radiation field of a normal galaxy show that most of the thermal emission from the dust should be occurring in the submillimeter waveband (100 µm < λ < 1000 µm) (Spitzer 1978; Draine and Lee 1984; Draine and Anderson 1985). The IRAS survey confirmed that because most galaxies are emitting more strongly at 100 µm than at 60 µm, much of a galaxy’s emission should be in this wavelength region. We have used the UKT14 bolometer on the United Kingdom Infrared Telescope to observe the submillimeter emission from a sample of 11 spiral galaxies that are bright at 100 pm. The instrumental beam of the bolometer system has an approximately Gaussian shape with size (FWHM) of ≈80 arcsec. We used a beam separation of 136 arcsec E-W. The flux calibration and the extinction coefficients were obtained from observations of Saturn, Uranus, OMC-1, and IRC+10216. More details are given in Eales, Wynn-Williams, and Duncan (1989).

Infrared emission and star formation in the central regions of the galaxy IC 342

The face-on Scd galaxy IC 342 has been studied at infrared wavelengths between 1.2 μm and 250 μm and at centimeter radio wavelengths. At 10 μm the nucleus is bright and extended on a scale of 200 pc, with a double structure unlike that of the stars seen at 2 μm. The infrared emission between 8 and 250 μm probably arises from heated dust grains in star formation regions in the disk of the galaxy. The radio emission comes from a region with many of the same spatial features as the 10 μm source; it appears to include both thermal and nonthermal sources. IC 342 is intermediate in luminosity between the Galaxy and NGC 253; the difference can probably be accounted for by a difference in the current rate of star formation at their centers.

Infrared and x-ray variability of Cyg X-3

CYGNUS X-3 is a candidate for the radio source which in September 1972 experienced a series of exceptional radio outbursts1. The X-ray emission is chiefly distinguishable as showing periodic intensity variations with a period of 4.8 h (ref. 2); if these variations are caused by an eclipse, the orbital period is the shortest observed up to this time from an X-ray source. The definite identification of the radio source with the X-ray source has remained unconfirmed up to now because of the poor positional accuracy of the X-ray source; the error box in the 3U catalogue is about 1 x 2 arc min (ref. 3). Although there is no visual candidate for Cyg X-3 to a limit of ∼2 X 10−32 W m−2 Hz−1 (V∼23 mag) (ref. 4), at infrared wavelengths Becklin et al.5 found a source coincident to ±2 arc s with the radio position. At 2.2 μm, the flux density is approximately 2 X 10−28 W m−2 Hz−1, whereas the 1.5 to 2.2 µm colour of the infrared source is consistent with a Rayleigh-Jeans spectrum which is reddened by 1.5 mag at 2.2 µm by the interstellar medium. There is no reason to suppose that the source is intrinsically more luminous in the infrared than at visible wavelengths.