Abstract. High resolution, mid-infrared (MIR) images of nine nearby active galaxies are presented. The data were obtained with the TIMMI 2 instrument mounted at the ESO 3.6 m telescope using a set of N-band narrow filters. The resulting images have an angular resolution of 0.6 −1 . The MIR emission has been resolved in four galaxies: NGC 253, NGC 1365, NGC 1808 and NGC 7469. The images show a circumnuclear population of unknown MIR sources in NGC 1365 and NGC 1808, coincident with radio sources. These MIR/radio sources are interpreted in terms of embedded young star clusters. A high-resolution MIR map of NGC 253 is also presented, and enables the identification of a previously unknown MIR counterpart to the radio nucleus. Extended MIR emission is detected in NGC 7469, and concurs with previous observations in the NIR and radio. For this source, an interesting morphological difference between the 10.4 µm and the 11.9 µm emission is observed, suggesting the presence of a dust-rich micro-bar. Our MIR images of Circinus do not show resolved emission from the nucleus down to an angular scale of 0.5 . In the case of NGC 2992, an upper limit to the extended MIR emission can be set. We provide new MIR flux measurements for the unresolved AGN in NGC 5995, IZw1 and IIZw136.
Abstract. We revisit in this paper the location of the various components observed in the AGN of NGC 1068. Discrepancies between previously published studies are explained, and a new measurement for the absolute location of the K-band emission peak is provided. It is found to be consistent with the position of the central engine as derived by Gallimore et al. (1997 and Kishimoto (1999). A series of map overlays is then presented and discussed. Model predictions of dusty tori show that the nuclear unresolved NIR-MIR emission is compatible with a broad range of models: the nuclear SED alone does not strongly constrain the torus geometry, while placing reasonable constraints on its size and thickness. The extended MIR emission observed within the ionizing cone is shown to be well explained by the presence of optically thick dust clouds exposed to the central engine radiation and having a small covering factor. Conversely, a distribution of diffuse dust particles within the ionizing cone is discarded. A simple model for the H 2 and CO emission observed perpendicularly to the axis of the ionizing cone is proposed. We show that a slight tilt between the molecular disc and the Compton thick central absorber naturally reproduces the observed distribution of H 2 of CO emission.
We report on the discovery of several compact regions of mid-infrared emission in the starforming circum nuclear disk of the starburst/Seyfert2 galaxy NGC7582. The compact sources do not have counterparts in the optical and near-infrared, suggesting that they are deeply embedded in dust. We use the [NeII]12.8 micron line emission to estimate the emission measure of the ionized gas, which in turn is used to assess the number of ionizing photons. Two of the brighter sources are found to have ionizing fluxes of ~2.5x10^52, whereas the fainter ones have ~1x10^52 photons/s. Comparing with a one Myr old starburst, we derive stellar masses in the range (3-5)x10^5 Msun, and find that the number of O-stars in each compact source is typically (0.6-1.6)x10^3. We conclude that the compact mid-infrared sources are likely to be young, embedded star clusters, of which only a few are known so far. Our observation highlights the need for high resolution mid-infrared imaging to discover and study embedded star clusters in the proximity of active galactic nuclei.Comment: 6 pages, 2 figures, accepted for publication in MNRAS Letter
We set out to quantify the number density of quiescent massive compact galaxies at intermediate redshifts. We determine structural parameters based on i-band imaging using the CFHT equatorial SDSS Stripe 82 (CS82) survey (∼ 170 sq. degrees) taking advantage of an exquisite median seeing of ∼ 0. ′′ 6. We select compact massive (M ⋆ > 5 × 10 10 M ⊙ ) galaxies within the redshift range of 0.2 < z < 0.6. The large volume sampled allows to decrease the effect of cosmic variance that has hampered the calculation of the number density for this enigmatic population in many previous studies. We undertake an exhaustive analysis in an effort to untangle the various findings inherent to the diverse definition of compactness present in the literature. We find that the absolute number of compact galaxies is very dependent on the adopted definition and can change up to a factor of > 10. We systematically measure a factor of ∼ 5 more compacts at the same redshift than what was previously reported on smaller fields with HST imaging, which are more affected by cosmic variance. This means that the decrease in number density from z ∼ 1.5 to z ∼ 0.2 might be only of a factor of ∼ 2 − 5, significantly smaller than what previously reported. This supports progenitor bias as the main contributor to the size evolution. This milder decrease is roughly compatible with the predictions from recent numerical simulations. Only the most extreme compact galaxies, with R eff < 1.5 × M ⋆ /10 11 M ⊙ 0.75 and M ⋆ > 10 10.7 M ⊙ , appear to drop in number by a factor of ∼ 20 and hence likely experience a noticeable size evolution.
Abstract. New near-IR long slit spectroscopic data obtained with ISAAC on VLT/ANTU (ESO/Paranal) complement and extend our previously published near-IR data (Alloin et al. 2001) to produce Br γ and H 2 emission line maps and line profile grids of the central 4 × 4 region surrounding the central engine of NGC 1068. The seeing quality together with the use of an 0.3 wide slit and 0.3 slit position offsets allow one to perform 2D-spectroscopy at a spatial resolution ≈ 0.5 . Slit orientations (PA = 102• and PA = 12 • ) were chosen so as to match respectively the equatorial plane and the axis of the suspected molecular/dusty torus in NGC 1068. The selected wavelength range from 2.1 to 2.2 µm is suitable to detect and analyze the Br γ and H 2 emission lines at a spectral resolution corresponding to 35 km s −1 . An asymmetric distribution of H 2 emission around the continuum peak is observed. No H 2 emission is detected at the location of the strong 2.2 µm continuum core (coincident within error-bars with the central engine location), while two conspicuous knots of H 2 emission are detected at about 1 on each side of the central engine along PA = 90• , with a projected velocity difference of 140 km s −1 : this velocity jump has been interpreted in Alloin et al. (2001) as the signature of a rotating disk of molecular material. From this new data set, we find that only very low intensity Br γ emission is detected at the location of the two main knots of H 2 emission. Another knot with both H 2 and Br γ emission is detected to the North of the central engine, close to the radio source C where the small scale radio jet is redirected and close to the brightest [OIII] cloud NLR-B. It has a counterpart to the South, placed almost symmetrically with respect to the central engine, although mainly visible in the Br γ emission. The northern and southern knots appear to be related to the ionization cone. At the achieved spectral resolution, the H 2 emission line profiles appear highly asymmetric with their low velocity wing being systematically more extended than their high velocity wing. A simple way to account for the changes of the H 2 line profiles (peak-shift with respect to the systemic velocity, width, asymmetry) over the entire 4 × 4 region, is to consider that a radial outflow is superimposed over the emission of the rotating molecular disk. We present a model of such a kinematical configuration and compare our predicted H 2 emission profiles to the observed ones. Excitation of the H 2 line is briefly discussed: X-ray irradiation from the central engine is found to be the most likely source of excitation. Given the fact that the material obscuring our direct view toward the central engine is Compton thick (N H ≥ 10 24 cm −2 ), the observed location of the main H 2 knots at a distance of 70 pc from the central engine suggests that the rotating molecular disk is warped.
Cluster formation and gas dynamics in the central regions of barred galaxies are not well understood. This paper reviews the environment of three 10 7 M ⊙ clusters near the inner Lindblad resonance of the barred spiral NGC 1365. The morphology, mass, and flow of HI and CO gas in the spiral and barred regions are examined for evidence of the location and mechanism of cluster formation. The accretion rate is compared with the star formation rate to infer the lifetime of the starburst. The gas appears to move from inside corotation in the spiral region to looping filaments in the interbar region at a rate of ∼ 6 M ⊙ yr −1 before impacting the bar dustlane somewhere along its length. The gas in this dustlane moves inward, growing in flux as a result of the accretion to ∼ 40 M ⊙ yr −1 near the ILR. This inner rate exceeds the current nuclear star formation rate by a factor of 4, suggesting continued buildup of nuclear mass for another ∼ 0.5 Gyr. The bar may be only 1-2 Gyr old. Extrapolating the bar flow back in time, we infer that the clusters formed in the bar dustlane outside the central dust ring at a position where an interbar filament currently impacts the lane. The ram pressure from this impact is comparable to the pressure in the bar dustlane, and both are comparable to the pressure in the massive clusters. Impact triggering is suggested. The isothermal assumption in numerical simulations seems inappropriate for the rarefraction parts of spiral and bar -2gas flows. The clusters have enough lower-mass counterparts to suggest they are part of a normal power law mass distribution. Gas trapping in the most massive clusters could explain their [NeII] emission, which is not evident from the lower-mass clusters nearby.
Context. In a previous work, three bright mid-infrared/radio sources were discovered in the nuclear region of starburst/AGN galaxy NGC 1365. Aims. The present study aims to confirm that these sources are indeed young and massive "embedded" clusters, and derive their physical parameters, such as extinction, age and mass. Methods. Using ISAAC and VISIR at the VLT we obtained maps and low resolution spectra in the near-and mid-infrared. The resulting datasets are first interpreted by comparing the observations with images and spectra of the close-by young cluster R136 in the Large Magellanic Cloud and then by using model predictions for both the nebular emission lines and the spectral energy distribution of the sources. Results. We produce maps of the region containing the three sources in the R, J, Ks, L bands and at 12.8 μm and perform their accurate relative positioning. We also provide spectra in the ranges 1.8−2.4 μm, 3.3−4.0 μm, 8.1−9.3 μm and 10.4−13.2 μm. The spectral energy distribution of the three sources rises with wavelength. Emission lines from ionised hydrogen and molecular hydrogen are detected, as well as PAH emission. A conspicuous [NeII] 12.8 μm line is also observed, while neither the [ArIII] 8.9 μm nor the [SIV] 10.4 μm lines are detected. This provides a stringent constraint on the age of the sources: we argue that they are relatively evolved young clusters (6−8 Myr). Owing to their ionising photon emission rates and ages, they must be extremely massive clusters (of the order of 10 7 M ). Their mid-infrared spectral energy distribution suggests the presence of two components: (1) an optically thin component, with a continuum comparable to that of R136; and (2) an optically thick component which might be related to subsequent or on-going episodes of star formation. We anticipate that these sources are good candidates for evolution according to a bi-modal hydrodynamical regime, in which matter is trapped at the centre of a compact and massive cluster and generates further star formation.
Abstract. An absolute calibration of the DENIS photometric system is presented. It includes the determination of the overall transmission profiles in the 3 bands, namely i, J and K s , combining contributions from atmosphere, telescope mirrors, instrument lenses and dichroics, filters, and detectors. From these normalized profiles, isophotal and effective wavelengths are computed, using the same synthetic Vega spectrum as that used to support the absolute calibration of many other ground-based and spaceborne photometric systems. Flux densities at zero magnitude are derived and integrated to give in-band fluxes, which are used to compute theoretical zero-points and compare them to observed ones, yielding estimates of the overall throughput of the whole system.Send offprint requests to: P. Fouqué
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