NGC 7027: analysis of 7.8–20 micron continuum,silicate and PAH grain distributions

Images of mid-infrared (5 - 20µm) circumstellar dust sources have been obtained with a new 58 × 62 pixel infrared array camera system. A seven-color imaging study of the bright planetary nebula NGC 7027 challenges the assertion that polycyclic aromatic hydrocarbons (PAH) may extend further from the center of the nebula than the continuum emission from silicate dust grains. It appears that the overall distributions are nearly identical, ruling out differences in intensity between the PAH emission and the general “silicate” dust material. A rigorous comparison between the infrared image data and new visible CCD images of NGC 7027 is made.     

Spatially-resolved stellar population properties of the M51–NGC 5195 system from multi-wavelength photometric data

Using multi-band photometric images of M51 and its companion NGC 5195 from ultraviolet to optical and infrared,we investigate spatially resolved stellar population properties of this interacting system with stellar population synthesis models.The observed infrared excess(IRX)is used to constrain dust extinction.Stellar mass is also inferred from the model fitting.By fitting observed spectral energy distributions(SEDs)with synthetical ones,we derive two-dimensional distributions of stellar age,metallicity,dust extinction and stellar mass.In M51,two grand-designed spiral arms extending from the bulge show young age,rich metallicity and abundant dust.The inter-arm regions are filled with older,metalpoorer and less dusty stellar populations.Except for the spiral arm extending from M51 into NGC 5195,the stellar population properties of NGC 5195 are quite featureless.NGC 5195 is much older than M51,and its core is very dusty with AV up to 1.67 mag and dense in stellar mass surface density.The close encounters might drive the dust in the spiral arm of M51 into the center of NGC 5195.     

The infrared emission lines of H2

The photodissociation regions located between ionized regions and molecular clouds are studied by using a one-dimensional model where molecular H₂ are formed on the dust grains, and destructed by photodissociation. The escape probability method is used for the line transfer. The excitation of infrared emission lines of H₂ by UV fluorescence in M17, by shock heating in Orion KL and mainly by UV fluorescence in NGC 2023 are discussed.     

Extended dust emission in NGC 7465

We present SCUBA 850-μm, JCMT  CO( J =2→1)  , B -band imaging and VLA H  i observations of the NGC 7465/4/3 group of galaxies. The 850-μm emission associated with NGC 7465 extends to at least ∼2 R ₂₅ and is well correlated with the H  i . We investigate a range of possible mechanisms by which dust beyond R ₂₅ may be heated to give the observed extended submillimetre emission. By modelling the dust heating by stars in two extreme geometries, we fail to find any reasonable star formation scenario that is consistent with both the 850-μm and optical data. Furthermore, we do not detect any  CO( J =2→1)  emission coincident with the extended dust and atomic gas as would be expected if significant star formation were occurring. We show that shock-heating of dust via cloud–cloud collisions in the stripped interstellar medium of NGC 7465 could be sufficient to explain the extended 850-μm emission and lack of optical emission in the stripped gas, and suggest that cloud–cloud collisions may be an important dust heating mechanism in gas-rich systems.     

The 3.4-μm absorption feature towards three obscured active galactic nuclei

The results of 3–4-μm spectroscopy towards the nuclei of NGC 3094, 7172, and 7479 are reported. In ground-based 8–13-μm spectra, all the sources have strong absorption-like features at ∼10 μm, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4-μm carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3-μm PAH emission feature, while NGC 7172 and 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features, is thought to be dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4-μm carbonaceous dust and 9.7-μm silicate dust absorption     The     ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, our results may provide spectroscopic evidence for the presence of the putative 'dusty tori' in the close vicinity of AGNs.     

Effects of massive star formation on the ISM of dwarf galaxies

We are studying star formation effects on the properties of the ISM in low metallicity environments using mid-infrared (MIR) and far-infrared (FIR) observations of starbursting dwarf galaxies taken with the Infrared Space Observatory (ISO) and the Kuiper Airborne Observatory (KAO). Effects of the hard pervasive radiation field on the gas and dust due to the dust-poor environments are apparent in both the dust and gas components. From a 158 μm [CII] survey we find enhanced I[CII]/FIR ratios in dwarf galaxies and I[CII]/I(CO) ratios up to 10 times higher than those for normal metallicity starburst galaxies. We consider MIR observations in understanding the star formation properties of dwarf galaxies and constraints on the stellar SED. Notably, the strong MIR [NeIII]/[NeII] ratios reveal the presence of current massive stellar populations <5 My old in NGC 1569, NGC 1140 and IIZw40. The MIR unidentified infrared bands (UIBs) are weak, if present at all, as a general characteristic in low metallicity environments, revealing the destruction of the smallest carbon particles (e.g. PAHs) over large spatial scales. This is confirmed with our dust modeling: mass fractions of PAHs are almost negligible compared to the larger silicate grains emitting in the FIR as well as the small carbon grains emitting in the MIR, which appear to be the source of the photoelectric gas heating in these galaxies, in view of the [CII] cooling.     

Large-Scale Structure of the Carina Nebula

Observations obtained with the Midcourse Space Experiment (MSX) satellite reveal for the first time the complex mid-infrared morphology of the entire Carina Nebula (NGC 3372). On the largest size scale of approximately 100 pc, the thermal infrared emission from the giant H ii region delineates one coherent structure: a (somewhat distorted) bipolar nebula with the major axis perpendicular to the Galactic plane. The Carina Nebula is usually described as an evolved H ii region that is no longer actively forming stars, clearing away the last vestiges of its natal molecular cloud. However, the MSX observations presented here reveal numerous embedded infrared sources that are good candidates for sites of current star formation. Several compact infrared sources are located at the heads of dust pillars or in dark globules behind ionization fronts. Because their morphology suggests a strong interaction with the peculiar collection of massive stars in the nebula, we speculate that these new infrared sources may be sites of triggered star formation in NGC 3372.     

Further 2MASS mapping of hot dust in planetary nebulae

We have used 2 Micron All Sky Survey (2MASS) mapping results to investigate the distribution of hot dust continua in 12 planetary nebulae (PNe). The nature of this emission is unclear, but it is possible that where the continuum is extended, as is the case for M 1-12 and NGC 40, then the grains concerned may be very small indeed. The absorption of individual photons by such grains may lead to sharp spikes in temperature, as has previously discussed for several other such outflows. Other sources (such as MaC 1-4, He 2-25, B1 2-1 and K 3-15) appear to be relatively compact, and the high temperatures observed are understandable in terms of more normal heating processes. It is possible that the grains in these cases are experiencing high radiant flux levels.
Finally, it is noted that whilst the core of M 2-2 appears to show hot grain emission, this is less the case for its more extended envelope. The situation may, in this case, be similar to that of NGC 2346, in which much of the emission is located within an unresolved nucleus. Similarly, it is noted that in addition to hot dust and gas thermal continua, the emission in the interior of NGC 40 may be enhanced through rotational–vibrational transitions of H₂, and/or the 2p³P⁰–2s³S transition of He  i .     

Young Populations in the Nuclei of Barred Galaxies

We have conducted UBVRI and H_α CCD photometry of five barred galaxies (NGC 2523, NGC 2950, NGC 3412, NGC 3945 and NGC 5383),along with SPH simulations, in order to understand the origin of young stellar populations in the nuclei of barred galaxies. The H_α emission, which is thought to be emitted by young stellar populations, is either absent or strongly concentrated in the nuclei of early-type galaxies (NGC 2950, NGC 3412 and NGC 3945),while they are observed in the nuclei and circumnuclear regions of intermediate-type galaxies with strong bars (NGC 2523 and NGC 5383).SPH simulations of realistic mass models for these galaxies show that some disc material can be driven into the nuclear region by a strong bar potential. This implies that the young stellar populations in the circumnuclear regions of barred galaxies can be formed out of such gas. The existence of nuclear dust lanes is an indication of an ongoing gas inflow and extremely young stellar populations in these galaxies, because nuclear dust lanes such as those in NGC 5383 are not long-lasting features according to our simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ionized gas characteristics in the cavities of the gas and dust disc of the spiral galaxy NGC 6946

The parameters of the ionized gas in NGC 6946 (in the [NII] λλ6548, 6583, H _α and [SII] λλ6717, 6731 lines) are investigated with the SAO RAS BTA telescope along three positions of the long slit of the SCORPIO focal reducer, passing through a number of large and small cavities of the gaseous disc of the galaxy. These cavities correspond exactly to the cavities in warm dust, visible at 5 − 8μm. We found that everywhere in the direction of NGC 6946 the lines of ionized gas are decomposed into two Gaussians, one of which shows almost constant [SII]/H _α and [NII]/H _α ratios, as well as an almost constant radial velocity within the measurement errors (about −35… − 50 km/s). This component is in fact the foreground radiation from the diffuse ionized gas of our Galaxy, which is not surprising, given the low (12°) latitude of NGC 6946; a similar component is also present in the emission of neutral hydrogen. The analysis of the component of ionized gas, occurring inNGC 6946, has revealed that it shows signs of shock excitation in the cavities of the gaseous disc of the galaxy. This shock excitation is as well typical for the extraplanar diffuse ionized gas (EDIG), observed in a number of spiral galaxies at their high Z-coordinates. This can most likely be explained by low density of the gas in the NGC 6946 disc (with the usual photoionization) inside the cavities, due to what we see the spectral features of the EDIG gas of NGC 6946, projected onto them, and located outside the plane of the galaxy. In the absence of separation of ionized gas into two components by radial velocities, there is an increasing contribution to the integral line parameters by the EDIG of our Galaxy when the gas density in NGC 6946 decreases, which explains some strange results, obtained in the previous studies. Themorphology of warmdust, visible in the infrared range and HI is almost the same (except for the peripheral parts of the galaxy, where there are no sources of dust heating).Moreover, the shock excitation of the ionized gas is detected in the smallest holes, distinguishable only in the IR images.     

Circumnuclear Regions and their Barred Host Galaxies

In this paper we present first results from an observational study of the connection between the global parameters of barred galaxies and the appearance of structures, such as dust lanes and star-forming rings in their circumnuclear regions. We combine near-infrared imaging of the central small-scale core structure with optical imaging of the large-scale discs of these barred galaxies. We present more detailed studies of two of our sample galaxies, NGC1530 and NGC 3504, for which we relate the presence and morphology of dust lanes (with different curvature radii) to the strength of the bar. A relation where strong bars lead to straight dust lanes and ovals or weak bars to curved dust lanes is expected theoretically and is borne out by our preliminary imaging results. This revised version was published online in July 2006 with corrections to the Cover Date.     

The galactic infrared/submillimeter dust radiation

Summary The unprecedented quality of the IRAS data prompted a series of papers reinvestigating the origin of the galactic infrared/submillimeter radiation and the nature of the heating sources of the interstellar dust. The scope of this paper is to review the main results of these new studies. Sect. 1 contains a general introduction to the subject and an overview of the earlier work. In Sect. 2 and 3, we summarize our current knowledge on dust properties and on the interstellar radiation field. Then, we present the recent interpretations of the infrared emission of our Galaxy, which are based on the IRAS data: in Sect. 4 we present the results from the galactic disk; Sect. 5 covers the question of the contribution from small dust particles, and in Sect. 6 we describe the infrared radiation from the galactic center. The nature of the heating sources and the origin of the galactic infrared radiation is then discussed in Sect. 7. Finally, Sect. 8 presents a comparison of our Galaxy with external galaxies     

The infrared variability of a dust model for Seyfert galaxies

Assuming a dust model of Reeset al. (1969), we study the time dependence of the infrared emission from grains which absorb variable ultraviolet radiation. A time scale for the variability is shown to be fairly small compared with the light travel-time across the infrared emitting region, of which size depends on the wavelength strongly. Comparison of calculated infrared light curves with observations of NGC 4151 leads to the conclusion that the variability at 2.2 is consistent with interpretation by the infrared emission on the basis of thermal process.     

The 3.4-μm absorption feature towards the nucleus of NGC 5506

We report the results of 3-μm spectroscopy towards the nucleus of a nearby Seyfert 2 galaxy, NGC 5506. A weak absorption feature of carbonaceous dust is detected at ∼3.4 μm. The optical depth ratio of the 3.4-μm carbonaceous dust absorption to the 9.7-μm silicate dust absorption is smaller by more than a factor of 2 than that in the interstellar medium in our Galaxy. The small ratio is consistent with the idea that the obscuration towards the nucleus of NGC 5506 is caused mainly by dust grains in its host galaxy (type S0/a) and that the contribution of carbonaceous dust grains to the visual extinction there is smaller than that in our Galaxy (type Sb/bc).     

Revisiting the formation history of the minor-axis dust lane galaxy NGC 1947

In this paper, we present a detailed study of the peculiar early-type galaxy NGC 1947. The main goal of this work is to constrain the dynamical status and the formation history of NGC 1947 by comparing the observed properties with the predictions derived from different galaxy formation scenarios. To this aim, we derived the photometric and kinematical properties of NGC 1947. Due to the presence of an extended dust lane, which crosses the galaxy centre along the photometric minor axis, we used near-infrared (NIR) images ( J and K bands) to derive an accurate analysis of the stellar light distribution. Optical images (in the V and R bands) are used to derive the colour profiles and colour maps to study the structure of the dust lane. The observed kinematics confirm the presence of two components with decoupled angular momentum: gas and dust rotate along the minor axis, while the rotation velocities of the stars are observed along the major axis. The complex structure observed in NGC 1947 supports the hypothesis that some kind of interactions happened in the evolution of this object. We analysed two alternatives: a merging process and an accretion event. We discussed how the observed properties strongly suggest that the decoupled ring of gas and dust has been accreted from outside.     

Planetary nebulae NGC 6826 and NGC 2899: early aspherical mass loss?

By systematically searching regions around planetary nebulae (PNe) for signs of interactions of their precursors’ wind with ambient matter we found a number of huge IRAS dust structures. Some of them may be chance projections, but a few appear to be real, like those around NGC 6826 and NGC 2899. In the case of NGC 6826 we noticed a giant (∼2°) bipolar dust emission, whose axis is along the proper motion of the central star. The PN itself is offset in the direction of motion both as to the center of this ∼30 pc large dust structure and to the center of a similarly large new Hα nebula. NGC 2899 was found in the center of a 14×11 pc quadrupolar cavity, whose directions of axes coincide with the directions of the main axes of the optical PN. In both cases, the formation of these structures appears to have commenced in the asymptotic giant branch (AGB) phase.     

SCUBA observations of NGC 1275

Deep SCUBA observations of NGC 1275 at 450 and 850 μm along with the application of deconvolution algorithms have permitted us to separate the strong core emission in this galaxy from the fainter extended emission around it. The core has a steep spectral index and is likely caused primarily by the active galactic nucleus. The faint emission has a positive spectral index and is clearly caused by extended dust in a patchy distribution out to a radius of ∼20 kpc from the nucleus. These observations have now revealed that a large quantity of dust, ∼     (two orders of magnitude larger than that inferred from previous optical absorption measurements), exists in this galaxy. We estimate the temperature of this dust to be ∼20 K (using an emissivity index of     and the gas/dust ratio to be 360. These values are typical of spiral galaxies. The dust emission correlates spatially with the hot X-ray emitting gas, which may be a result of collisional heating of broadly distributed dust by electrons. As the destruction time-scale is short, the dust cannot be replenished by stellar mass loss and must be externally supplied, via either the infalling galaxy or the cooling flow itself.     

Infrared spectroscopy of interplanetary dust in the laboratory

Diagnostic infrared spectra of individual nanogram-sized interplanetary dust particles (IDPs) collected in the Earth's stratosphere have been obtained. A mount containing three crushed “chondritic” IDPs shows features near 1000 and 500 cm^?1, suggestive of crystalline pyroxene, and different from those of crystalline olivine, amorphous olivine, or meteoritic clay minerals. The structural diversity of chondritic IDPs and possible effects of atmospheric heating must be considered when comparing this spectrum with astrophysical spectra of interplanetary and cometary dust. Transmission electron microscope (TEM) and infrared observations are also reported on one member of the rare subset of IDPs which resemble hydrated carbonaceous chondrite matrix material. The infrared spectrum of this particle between 4000 and 400 cm^?1 closely matches that of the C2 meteorite Murchison. TEM observations suggest that this class of particles might serve as a thermometer for the process of heating on atmospheric entry.     

Variability in the Nucleus of NGC 1068

The near-infrared output of the nucleus of NGC 1068 increased by a factor of two over the 18 years between 1976 and 1994 and has recently started to decline. It is not clear whether this event has been the response of an extended dusty region to a single outburst in the central engine or is the result of a continuous change in its XUV output. The integrated energy of the event amounts to well over 1052 erg and thus cannot be due to a single supernova. The variable part of the infrared flux from NGC 1068, observed through an aperture of constant diameter, is found to have a constant spectral shape. Comparison with similar data from other Seyfert galaxies shows that the variable infrared component in NGC 1068 is reddened by about 20 mag in Av if the emission mechanism is similar in all cases. This revised version was published online in July 2006 with corrections to the Cover Date.