E. Krügel

Dust in starburst nuclei and ULIRGs: SED models for observers

Aims. We provide a library of some 7000 SEDs for the nuclei of starburst and ultra luminous galaxies. Its purpose is to quickly ob tain estimates of the basic parameters, such as luminosity, size and dust or gas mass and to predict the flux at yet unobserved wavelengths . The procedure is simple and consists of finding an element in the library that m atches the observations. The objects may be in the local universe or at high z. Methods. We calculate the radiative transfer in spherical symmetry for a stellar cluster permeated by an interstellar medium with standard (Milky Way) dust properties. The cluster contains two stellar populations: old bulge stars and OB stars. Because the latter are young, a certain fraction of them will be embedded in compact clouds which constitute hot spots that determine the MIR fluxes. Results. We present SEDs for a broad range of luminosities, sizes and obscurations. We argue that the assumption of spherical symmetry and the neglect of clumpiness of the medium are not severe shortcomings for computing the dust emission. The validity of the approach is demonstrated by matching the SED of seven of the best studied galaxies, including M82 and Arp220, by library elements. In all cases, one finds an element which fits the observed SED very well, and the p arameters defining the element are in full accord with what is known about the galaxy from detailed studies. We also compare our method of computing SEDs with other techniques described in the literature.

Mid–infrared emission of galactic nuclei - TIMMI2 versus ISO observations and models

We investigate the mid-infrared radiation of galaxies that are powered by a starburst or by an AGN. For this end, we compare the spectra obtained at different spatial scales in a sample of infrared bright galaxies. ISO observations which include emission of the nucleus as well as most of the host galaxy are compared with TIMMI2 spectra of the nuclear region. We find that ISO spectra are generally dominated by strong PAH bands. However, this is no longer true when inspecting the mid-infrared emission of the pure nucleus. Here PAH emission is detected in starbursts whereas it is significantly reduced or completely absent in AGNs. A physical explanation of these new observational results is presented by examining the temperature fluctuation of a PAH after interaction with a photon. It turns out that the hardness of the radiation field is a key parameter for quantifying the photo-destruction of small grains. Our theoretical study predicts PAH evaporation in soft X-ray environments. Radiative transfer calculations of clumpy starbursts and AGN corroborate the observational fact that PAH emission is connected to starburst activity whereas PAHs are destroyed near an AGN. The radiative transfer models predict for starbursts a much larger mid-infrared size than for AGN. This is confirmed by our TIMMI2 acquisition images: We find that the mid-infrared emission of Seyferts is dominated by a compact core while most of the starbursts are spatially resolved.

ISOCAM survey and dust models of 3CR radio galaxies and quasars

We present a survey of all 3CR sources imaged with ISOCAM onboard the Infrared Space Observatory (ISO). For the source, we present spatially integrated mid--infrared (MIR, 5 - 18mic.) fluxes measured from newly calibrated ISOCAM images. In total, we detected 68 AGN of the 3CR catalogue, at redshifts z<2.5. The one with the highest redshift is 4C+72.26 at z = 3.53. ISOCAM data are combined with other photometric measurements to construct the spectral energy distribution (SED) from optical to radio wavelengths. In order to describe dust emission we apply new radiative transfer models. By varying three parameters, luminosity, effective size and extinction, we obtain a fit to the SED for our objects. In the MIR the hot dust component is mainly due to small grains and PAHs. In the models, a type 1 AGN is represented by a compact dust distribution, the dust is therefore very warm and emission of PAHs is weak because of photo--destruction. In AGNs of type 2, the dust is relatively colder but PAH bands are strong.

PAHs in circumstellar disks around Herbig Ae/Be stars

We investigate the presence and properties of PAHs on the surface of circumstellar disks around Herbig Ae/Be stars by comparing the predictions of disk models with observations. We present results of a radiation transfer code for disks heated by the central star, in hydrostatic equilibrium in the vertical direction (flared disks). The dust is a mixture of large grains in thermal equilibrium, transiently heated small grains and PAHs. Special attention is given to the influence of the stellar, disk and PAH properties on the strength of the PAH emission lines and their spatial distribution. The models predict an infrared SED showing PAH features at 3.3, 6.2, 7.7, and 11.3 µm clearly visible above the continuum, and with some of them very strong. The PAH emission, spatially extended, comes mostly from the outer disk region (R ∼ 100 AU) while the continuum emission at similar wavelengths, mostly due to warm large grains, is confined to the innermost disk regions (R ∼ few AU). We compare the model results to infrared observations from ISO and ground-based telescopes of some thirty Herbig Ae/Be stars. Most of the observed PAH features in objects with spectral type later than B9 are well described by our disk models and we discuss in some detail the PAH characteristics one can derive from the existing data. Objects with strong radiation field (generally earlier than about B9) have the 3.3 µm feature (often the only one observed) much weaker than predicted, and we discuss possible explanations (dissipation of the disk, photoevaporation or modification of the PAH properties).

High mass star formation in the infrared dark cloud G11.11-0.12

We report detection of moderate to high-mass star formation in an infrared dark cloud (Gl 1.11-0.12) where we discovered class II methanol and water maser emission at 6.7 GHz and 22.2 GHz, respectively. We also observed the object in ammonia inversion transitions. Strong emission from the (3,3) line indicates a hot (60 K) compact component associated with the maser emission. The line width of the hot component (4 km s -1 ), as well as the methanol maser detection, are indicative of high mass star formation. To further constrain the physical parameters of the source, we derived the spectral energy distribution (SED) of the dust continuum by analysing data from the 2MASS survey, HIRAS, MSX, the Spitzer Space Telescope, and interferometric 3 mm observations. The SED was modelled in a radiative transfer program: a) the stellar luminosity equals ∼1200 L ○. corresponding to a ZAMS star of 8 M ○. ; b) the bulk of the envelope has a temperature of 19 K; c) the mass of the remnant protostellar cloud in an area 8 x 10 17 cm or 15 across amounts to 500 M ○. , if assuming standard dust of the diffuse medium, and to about 60 M ○. , should the grains be fluffy and have ice mantles; d) the corresponding visual extinction towards the star, Av, is a few hundred magnitudes. The near IR data can be explained by scattering from tenuous material above a hypothetical disk. The class II methanol maser lines are spread out in velocity over 11 km s -1 . To explain the kinematics of the masing spots, we propose that they are located in a Kepler disk at a distance of about 250 AU. The dust temperatures there are around 150 K, high enough to evaporate methanol-containing ice mantles.

High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission

Context. Constraints on the origin and propagation of cosmic rays can be achieved by studying the variation in the spectral index of the synchrotron emission across external galaxies. Aims. We determine the variation in the nonthermal radio spectral index in the nearby spiral galaxy M 33 at a linear resolution of 360 pc. Methods. We separated the thermal and nonthermal components of the radio continuum emission without the assumption of a constant nonthermal spectral index. Using the Spitzer FIR data at 70 and 160 µm and a standard dust model, we dereddened the Hα emission. The extinction corrected Hα emission serves as a template for the thermal free-free radio emission. Subtracting this free-free emission from the observed 3.6 cm and 20 cm emission (Effelsberg and the VLA), we obtained the nonthermal maps. A constant electron temperature used to obtain the thermal radio intensity seems appropriate for M 33, which, unlike the Milky Way, has a shallow metallicity gradient. Results. For the first time, we derive the distribution of the nonthermal spectral index across a galaxy, M 33. We detect strong nonthermal emission from the spiral arms and starforming regions. Wavelet analysis shows that at 3.6 cm the nonthermal emission is dominated by contributions from starforming regions, while it is smoothly distributed at 20 cm. For the whole galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm, respectively. The thermal emission is slightly stronger in the southern than in the northern half of the galaxy. We find a clear radial gradient of mean extinction in the galactic plane. Conclusions. The nonthermal spectral index map indicates that the relativistic electrons suffer energy loss when diffusing from their origin in starforming regions towards interarm regions and the outer parts of the galaxy. We also conclude that the radio emission is mostly nonthermal at R > 5 kpc in M 33.

Dust and CO emission towards the centers of normal galaxies, starburst galaxies and active galactic nuclei , I. New data and updated catalogue

Aims. The amount of interstellar matter in a galaxy determines its evolution, star formation rate and the activity phenomena in the nucleus. We therefore aimed at obtaining a data base of the 12 CO line and thermal dust emission within equal beamsizes for galaxies in a variety of activity stages. Methods. We have conducted a search for the 12 CO (1–0) and (2–1) transitions and the continuum emission at 1300 µ mt owards the centers of 88 galaxies using the IRAM 30 m telescope (MRT) and the Swedish ESO Submillimeter Telescope (SEST). The galaxies are selected to be bright in the far infrared (S 100 µm > 9 Jy) and optically fairly compact (D25 ≤ 180 �� ). We have applied optical spectroscopy and IRAS colours to group the galaxies of the entire sample according to their stage of activity into three sub-samples: normal, starburst and active galactic nuclei (AGN). The continuum emission has been corrected for line contamination and synchrotron contribution to retrieve the thermal dust emission. For the latter we have determined the radio spectral indices of the individual sources and extrapolated the synchrotron emission corresponding to our millimeter beams to 1300 µm. Results. We present new observational data for the 12 CO (1–0) and (2−1) transitions and the thermal dust emission at 1300 µm for 88 galaxies. In conjunction with our previous data, the new observations are used to compile an updated catalogue for a total of 160 galaxies.

The destruction and survival of polycyclic aromatic hydrocarbons in the disks of T Tauri stars

In Spitzer observations of Tauri stars and their disks, features of polycyclic aromatic hydrocarbons (PAHs) are detected in less than 10% of the objects, although the stellar photosphere is sufficiently hot to excite PAHs. To explain the deficiency, we discuss PAH destruction by photons, assuming that the star has beside its photospheric emission also a far ultraviolet (FUV), an extreme ultraviolet (EUV) and an X-ray component with a fractional luminosity of 1%, 0.1% and 0.025%, respectively. We consider as a PAH destruction process unimolecular dissociation and present a simplified scheme to estimate the location from the star at which the molecules become photo-stable. We find that soft photons with energies below ∼20 eV dissociate PAHs only up to short distances from the star (r < 1 AU); whereas dissociation by hard photons (EUV and X-ray) is so efficient that it would destroy all PAHs (from regions in the disk where they could be excited). As a possible path for PAH-survival we suggest turbulent motions in the disk. They can replenish or remove PAHs from the reach of hard photons. For standard disk models, where the surface density changes like r −1 and the mid plane temperature like r −0.5 , the critical vertical velocity for PAH survival is proportional to r −3/4 and equals ∼ 5m / sa t 10 AU, which is in the range of expected velocities in the surface layer. The uncertainty in the parameters is large enough to explain both detection and non-detection of PAHs. Our approximate treatment also takes into account the presence of gas which is ionized at the top of the disk and neutral at lower levels.

Dust emission in star-forming dwarf galaxies: General properties and the nature of the submm excess

We studied the global characteristics of dust emission in a large sample of emission-line star-forming galaxies. The sample consists of two subsamples. One subsample (SDSS sample) includes ~4000 compact star-forming galaxies from the SDSS, which were also detected in all four bands at 3.4, 4.6, 12, and 22 mum of the WISE all-sky survey. The second subsample (Herschel sample) is a sample of 28 compact star-forming galaxies observed with Herschel in the FIR range. Data of the Herschel sample were supplemented by the photometric data from the Spitzer observations, GALEX, SDSS, WISE, 2MASS, NVSS, and FIRST surveys, as well as optical and Spitzer spectra and data in sub-mm and radio ranges. It is found that warm dust luminosities of galaxies from the SDSS sample and cold and warm dust luminosities of galaxies from the Herschel sample are strongly correlated with Hbeta luminosities, which implies that one of the main sources of dust heating in star-forming galaxies is ionising UV radiation of young stars. Using the relation between warm and cold dust masses for estimating the total dust mass in star-forming galaxies with an accuracy better than ~0.5 dex is proposed. On the other hand, it is shown for both samples that dust temperatures do not depend on the metallicities. The dust-to-neutral gas mass ratio strongly declines with decreasing metallicity, similar to that found in other studies of local emission-line galaxies, high-redshift GRB hosts, and DLAs. On the other hand, the dust-to-ionised gas mass ratio is about one hundred times as high implying that most of dust is located in the neutral gas. It is found that thermal free-free emission of ionised gas in compact star-forming galaxies might be responsible for the sub-mm emission excess. This effect is stronger in galaxies with lower metallicities and is also positively affected by an increased star-formation rate.

Nuclear activity in nearby galaxies Mid-infrared imaging with the VLT

Aims. Dust enshrouded activity can be studied ideally by mid-infrared (MIR) observations. To explore the active galactic nuclei (AGN) versus star forming origin of the nuclear MIR emission of galaxies, observations of high spatial resolution are required. We report on 11.3 μm observations with VISIR at the VLT, which reach 0.35 �� spatial resolution (FWHM). Methods. During the scientific verification of VISIR, we observed a sample of 36 nearby galaxies with a range of optically classified nuclear activity: 17 black hole driven AGN, 10 starbursts (SB), and 9 quiescent spirals. Results. 16 objects out of 17 AGN are detected and unresolved, 5 out of 10 SB are detected and resolved with structured emission up to a few arcsec, while for 5 targets out of 10 SB and all 9 quiet nuclei low upper limits are provided. The morphology of the resolved SB nuclei is similar to that observed at radio frequencies. The compactness of AGN and the extent of SB nuclei is consistent with the predictions of radiative transfer models and Spitzer spectra observed at much lower spatial resolution than with VISIR. We introduce the nuclear MIR surface brightness, S , as a quantitative measurement for AGN and SB activity. Although one is unable to distinguish between AGN and SB activity using the nuclear MIR surface brightness derived from 4 m class telescopes, our data indicate that, out to a distance of 100 Mpc, the MIR surface brightness acquired by 8 m class telescopes can differentiate AGN from SB behavior.