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Authors
C. Vlahakis
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The Sombrero galaxy (NGC 4594) is an Sa galaxy with a symmetric dust ring. We have used the Large APEX BOlometer CAmera (LABOCA) at 870 micron and the MAx-Planck Millimeter BOlometer (MAMBO-2) at 1.2 mm to detect the dust ring for the first time at submillimetre and millimetre wavelengths. We have constructed a model of the galaxy to separate the active galactic nucleus (AGN) and dust ring components. The ring radius at both 870 micron and 1.2 mm agrees well with the radius determined from optical absorption and atomic gas studies. The spectral energy distribution of the ring is well fitted by a single grey-body with dust emissivity index beta=2 and a dust temperature T_d=18.4 K. The dust mass of the ring is found to be 1.6pm0.2x10^7Msun which, for a Galactic gas-to-dust ratio, implies a gas mass that is consistent with measurements from the literature.
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Abridged: We investigated the detailed radio structure of the nucleus of the Sombrero galaxy using high-resolution, quasi-simultaneous, multi-frequency, phase-referencing VLBA observations. We obtained the VLBI images toward this nucleus, with unprecedented sensitivities and resolutions, at the seven frequencies between 1.4 and 43 GHz, where those at 15, 24 and 43 GHz are the first clear VLBI detections. At 43 GHz, the nuclear structure was imaged on a linear scale under 100 Schwarzschild radii. For the first time, we have discovered the presence of the extended structure in this nucleus, which is directing from the radio core in two sides toward northwest/southeast directions. The nuclear structure shows a clear spatial gradient on the radio spectra, which is similar to that commonly seen in more luminous AGN with powerful relativistic jets. Moreover, the radio core shows a frequency-dependent size with an elongated shape, and the position of the core also tends to be frequency dependent. A set of these new findings provide evidence that the central engine of the Sombrero is powering radio jets. Based on the observed brightness ratio of jet-to-counter jet, core position shift and its comparison with a theoretical model, we constrained the following fundamental physical parameters for the M 104 jets: (1) the northern side is the approaching jet, whereas the southern side is receding: (2) the inclination angle of the jet is relatively close to our line-of-sight, probably less than ~25degrees: (3) the jet intrinsic velocity is highly sub-relativistic at a speed less than ~0.2c. The derived pole-on nature of the M 104 jet is in accordance with the previous argument that M 104 contains a true type II AGN, i.e., the broad line region of this nucleus is actually absent or intrinsically weak, if the plane of the presumed circumnuclear torus is perpendicular to the axis of the radio jets.
We report far-infrared (FIR) imaging of the Seyfert 2 galaxy NGC 3081 in the range 70-500 micron, obtained with an unprecedented angular resolution, using the Herschel Space Observatory instruments PACS and SPIRE. The 11 kpc (~70 arcsec) diameter star-forming ring of the galaxy appears resolved up to 250 micron. We extracted infrared (1.6-500 micron) nuclear fluxes, that is active nucleus-dominated fluxes, and fitted them with clumpy torus models, which successfully reproduce the FIR emission with small torus sizes. Adding the FIR data to the near- and mid-infrared spectral energy distribution (SED) results in a torus radial extent of Ro=4(+2/-1) pc, as well as in a flat radial distribution of the clouds (i.e. the q parameter). At wavelengths beyond 200 micron, cold dust emission at T=28+/-1 K from the circumnuclear star-forming ring of 2.3 kpc (~15 arcsec) in diameter starts making a contribution to the nuclear emission. The dust in the outer parts of the galaxy is heated by the interstellar radiation field (19+/-3 K).
We examine Herschel Space Observatory images of one nearby prototypical outer ring galaxy, NGC 1291, and show that the ring becomes more prominent at wavelengths longer than 160um. The mass of cool dust in the ring dominates the total dust mass of the galaxy, accounting for at least 70% of it. The temperature of the emitting dust in the ring (T=19.5+/-0.3K) is cooler than that of the inner galaxy (T=25.7+/-0.7K). We discuss several explanations for the difference in dust temperature, including age and density differences in the stellar populations of the ring versus the bulge.
We discuss results from sensitive, wide-field imaging of the millimeter extragalactic background using the Max-Planck Millimeter Bolometer array (MAMBO) at the IRAM 30m telescope.
The nature of the Sombrero galaxy (M 104 = NGC 4594) has remained elusive despite many observational studies at a variety of wavelengths. Here we present Hubble Space Telescope imaging of two fields at $sim$16 and 33 kpc along the minor axis to examine stellar metallicity gradients in the extended spheroid. We use this imaging, extending more than 2 mag below the tip of the red giant branch (TRGB), in combination with artificial star tests to forward model observed color-magnitude diagrams (CMDs), measuring metallicity distribution functions (MDFs) at different radii along the minor axis. An important and unexpected result is that the halo of the Sombrero is strikingly metal-rich: even the outer field, located at $sim$17 effective radii of the bulge, has a median metallicity [Z/H]$sim$-0.15 and the fraction of stars with [Z/H]<-1.0 is negligible. This is unprecedented among massive galaxy halos studied to date, even among giant ellipticals. We find significant radial metallicity gradients, characterized by an increase in the fraction of metal-poor stars with radius and a gradient in median metallicity of $sim$-0.01 dex/kpc. The density profile is well fit by power laws with slopes that exhibit a dependence on metallicity, with flatter slopes for more metal-poor stars. We discuss our results in the context of recent stellar MDF studies of other nearby galaxies and potential formation scenarios for the Sombrero galaxy.
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