The nucleus of the nearby galaxy, NGC 1097, is known to host a young, compact (r < 9 parsec) nuclear star cluster as well as a low-luminosity active galactic nucleus (AGN). It has been suggested both that the nuclear stellar cluster is associated with a dusty torus, and that low-luminosity AGN like NGC 1097 do not have the torus predicted by the unified model of AGN. To investigate these contradictory possibilities we have acquired Gemini/T-ReCS 11.7 micron and 18.3 micron images of the central few hundred parsecs of this galaxy at < 45 parsec angular resolution, in which the nucleus and spectacular, kiloparsec-scale star-forming ring are detected in both bands. The small-scale mid-infrared (mid-IR) luminosity implies thermal emission from warm dust close to the central engine of this galaxy. Fitting of torus models shows that the observed mid-IR emission cannot be accounted for by dust heated by the central engine. Rather, the principal source heating the dust in this object is the nuclear star cluster itself, suggesting that the dust that we detect is not the torus of AGN unified schemes (although it is also possible that the dusty starburst itself could provide the obscuration invoked by the unified model). Comparison of Spitzer/IRS and Gemini/GNIRS spectra shows that, although polyaromatic hydrocarbon emission (PAH) bands are strong in the immediate circumnuclear region of the galaxy, PAH emission is weak or absent in the central 19 parsecs. The lack of PAH emission can probably be explained largely by destruction/ionization of PAH molecules by hard photons from the nuclear star cluster. If NGC 1097 is typical, PAH emission bands may not be a useful tool with which to find very compact nuclear starbursts even in low-luminosity AGN.

In order to test the silicate-core/organic-mantle model of galactic interstellar dust, we have performed spectropolarimetry of the 3.4 μm C–H bond stretch that is characteristic of aliphatic hydrocarbons, using the nucleus of the Seyfert 2 galaxy, NGC 1068, as a bright, dusty background source. Polarization calculations show that if the grains in NGC 1068 had the properties assigned by the core-mantle model to dust in the Galactic diffuse interstellar medium (ISM), they would cause a detectable rise in polarization over the 3.4 μm feature. No such increase is observed. We discuss modifications to the basic core-mantle model, such as changes in grain size or the existence of additional nonhydrocarbon aligned grain populations, that could better fit the observational evidence. However, we emphasize that the absence of polarization over the 3.4 μm band in NGC 1068—and, indeed, in every line of sight examined to date—can be readily explained by a population of small, unaligned carbonaceous grains with no physical connection to the silicates.

To search for thermal emission from a dusty AGN torus associated with the compact starburst in the nucleus of the LINER/Seyfert 1 galaxy NGC 1097, we have performed mid-IR imaging at high (<= 0.5") angular resolution. Emission from warm dust is detected, but fitting clumpy torus models shows that the major source heating the dust is the nuclear stellar cluster rather than the weak active nucleus. This could suggest that the dust is not directly associated with the AGN, consistent with calculations and other observations suggesting that low-luminosity AGN do not host obscuring tori. However, it does not rule out that the dusty starburst itself could be the obscurer of the unified model.

Aliphatic hydrocarbons exhibit an absorption feature at 3.4 μm, observed toward sources that sample diffuse regions of the interstellar medium (ISM). The absorbers responsible for this feature are assumed to reside in some component of interstellar dust, but the physical nature of the particles (size, shape, structure, etc.) is uncertain. Observations of interstellar polarization provide discrimination. Since the grains that carry the silicate absorption feature are known to be aligned, polarization across the 3.4 μm hydrocarbon feature can be used to test the silicate-core organic refractory mantle grain theory. Although the 3.4 μm feature has been observed to be devoid of polarization for one line of sight toward the Galactic center, a corresponding silicate polarization measurement for the same line of sight was not available. Here, we present spectropolarimetric observations of GCS 3-II and GCS 3-IV toward the Galactic center, where the 9.7 μm silicate polarization has been previously observed. We show that polarization is not detected across the 3.4 μm feature to a limit of 0.06% ± 0.13% (GCS 3-II) and 0.15% ± 0.31% (GCS 3-IV), well below the lowest available prediction of polarization on the basis of the core-mantle model. We conclude that the hydrocarbons in the diffuse ISM do not reside on the same grains as the silicates, and likely form a separate population of small grains.

We present spectropolarimetry of the 3.4 mu m solid hydrocarbon absorption feature arising in the ISM toward the Quintuplet member GCS3-II, permitting for the first time a comparison of the hydrocarbon and silicate polarizations in the same line of sight. The hydrocarbon band is not measurably polarized, strengthening our previous conclusion that the silicates and carbonaceous grains form distinct populations.

We present new spectroscopic observations of the 3.4μm absorption feature in a Seyfert 2 galaxy and a ULIRG. A signature of C–H bonds in aliphatic hydrocarbons, the 3.4μm feature indicates the presence of organic material in Galactic and extragalactic dust. The feature in these galaxies closely resembles that seen in the Galactic diffuse ISM and in newly-formed dust in a protoplanetary nebula. This similarity implies a common composition for the hydrocarbon component of interstellar dust in a range of galaxy types, and one which is resistant to processing in the interstellar and/or circumnuclear medium.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present contemporary optical and infrared spectroscopic observations of the type IIn SN 1998S covering the period between 3 and 127 days after discovery. During the first week the spectra are characterized by prominent broad H, He and C III/N III emission lines with narrow peaks, superimposed on a very blue continuum (T similar to 24 000 K). In the following two weeks the C III/N III emission vanished, together with the broad emission components of the H and He lines. Broad, blueshifted absorption components appeared in the spectra. The temperature of the continuum also dropped to similar to 14000 K. By the end of the first month the spectrum comprised broad, blueshifted absorptions in H, He, Si II, Fe II and Sc II. By day 44, broad emission components in H and He reappeared in the spectra. These persisted to as late as days similar to 100-130, becoming increasingly asymmetric. We agree with Leonard et al. that the broad emission lines indicate interaction between the ejecta and circumstellar material (CSM) emitted by the progenitor. We also agree that the progenitor of SN 1998S appears to have gone through at least two phases of mass-loss, giving rise to two CSM zones. Examination of the spectra indicates that the inner zone extended to less than or equal to 90 au, while the outer CSM extended from 185 an to over 1800 au.We also present high-resolution spectra obtained at days 17 and 36. These spectra exhibit narrow P Cygni H I and He I lines superimposed on shallower, broader absorption components. Narrow lines of [N II], [O III], [Ne III] and [Fe III] are also seen. We attribute the narrow lines to recombination and heating following ionization of the outer CSM shell by the UV/X-ray flash at shock breakout. Using these lines, we show that the outer CSM had a velocity of 40-50 kin s(-1). Assuming a constant velocity, we can infer that the outer CSM wind commenced more than 170 years ago, and ceased about 20 years ago, while the inner CSM wind may have commenced less than 9 years ago. During the era of the outer CSM wind the outflow from the progenitor was high - at least similar to2 x 10(-5) M. yr(-1). This corresponds to a mass-loss of at least similar to0.003 M., suggesting a massive progenitor. The shallower, broader absorption is of width similar to 350 km s(-1) and may have arisen from a component of the outer CSM shell produced when the progenitor was going through a later blue supergiant phase. Alternatively, it may have been produced by the acceleration of the outer CSM by the radiation pressure of the UV precursor.We also describe and model first-overtone emission in carbon monoxide observed in SN 1998S. We deduce a CO mass of similar to 10(-3) M. moving at similar to 2200 km s(-1), and infer a mixed metal/He core of about 4 M., again indicating a massive progenitor. Only three core-collapse supernovae have been observed in the K band at post-100 days, and all three have exhibited emission from CO.

Spectropolarimetry of the 3.4 μm aliphatic C-H stretch feature, generally attributed to carbonaceous dust in the diffuse interstellar medium, has been carried out in the line of sight from the Galactic center source Sagittarius A IRS 7. The feature is unpolarized (Δp/Δτ<0.2): the upper limit for polarization is well below that expected on the basis of a model in which the carrier molecules are associated with the aligned silicate component of interstellar dust, for example, as an organic or carbonaceous mantle on a silicate core. The simplest explanation is that the 3.4 μm carrier resides in a population of small, nonpolarizing carbonaceous grains, physically separate from the silicates and sharing many characteristics with the carriers of the 217.5 nm extinction bump.

The dynamics of a conductive meniscus subjected to an external field are examined with a view toward drop-on-demand applications. A low-order model suitable for exploring parametric effects is developed. Equilibrium and stability criteria are derived for the purpose of estimating electric field requirements. The criteria are expressed in terms of dimensionless pressures due to electric stress, gravity, and hydrostatic head, normalized to surface tension pressure. Eigenvalues of the linearized model yield a formula for characteristic frequency analogous to Rayleigh's formula for a free drop and, in addition, indicate that most liquids will show underdamped behavior. Requirements for critical damping are derived and illustrated with root locus plots.

In Paper I we used JHKL photometry of a sample of interacting galaxies to argue that interactions induce a burst of star formation in the nucleus of one member of the interacting pair with nearly 100 per cent efficiency. We have followed up this result by mid-infrared (10 and 20 µm) observations of a large fraction of this sample. We have added to these results existing mid-infrared data in the literature to obtain a comprehensive picture of the evidence for mid-infrared activity in the nuclear regions of interacting galaxies. We show that this information, combined with radio and optical data, provides a consistent picture of interaction-induced bursts of star formation in the central regions of these galaxies. Using a simple analytical starburst model we derive some of the quantitative features of these interaction-induced starbursts: they are unusually efficient in using available gas, and the initial mass function is heavily biased toward massive stars. The supernova rate and radio luminosity implied by this model are consistent with the observations. Finally we stress that, if the observed rate of interactions is representative, interaction-induced starbursts are likely to have occurred in the evolution of most galaxies. Such events may be related to other forms of activity in galactic nuclei.

The subset of galaxy–galaxy interactions which have resulted in a merger are, as a class, ultraluminous IR galaxies. Their IR luminosities span a narrow range which overlaps with the most luminous Seyfert galaxies. However, in contrast with Seyfert galaxies, the available optical, IR, and radio properties of mergers show no evidence for a compact non-thermal central source, and are easily understood in terms of a burst of star formation of extraordinary intensity and spatial extent: they are 'super starbursts.' We argue that super starbursts occur in the evolution of most mergers, and discuss the implications of super starbursts for the suggestion that mergers evolve into elliptical galaxies. Finally, we note that merger-induced shocks are likely to leave the gas from both galaxies in dense molecular form which will rapidly cool, collapse, and fragment. Thus a merger might in fact be expected to result in a burst of star formation of exceptional intensity and spatial extent, i.e. a super starburst.

In the course of checking preliminary lists of IRAS (Infrared Astronomical Satellite) sources to see whether any of the interacting galaxies that we have been studying1 were included, we discovered that the position of the IRAS source 1650+024PO4 (ref. 2) is within a few arc seconds of one of our programme interacting galaxies, NGC6240. Examination of the Palomar sky survey prints shows that NGC6240 is the only non-stellar object within the 1 arc min IRAS error box for this source. Photometry at 10 and 20 µm from the United Kingdom IR Telescope (UKIRT) confirms that this galaxy is among the most luminous IR galaxies yet discovered. An interaction-induced burst of star formation is the most likely source of the strong IR emission.