382 research outputs found
VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve
We present VLT/VIMOS IFU observations of an occulting galaxy pair previously
discovered in HST observations. The foreground galaxy is a low-inclination
spiral disk, which causes clear attenuation features seen against the bright
bulge and disk of the background galaxy. We find redshifts of and z=0.065 for the foreground and background galaxy respectively.
This relatively small difference does not rule out gravitational interaction
between the two galaxies. Emission line ratios point to a star-forming, not
AGN-dominated foreground galaxy.
We fit the Cardelli, Clayton & Mathis (CCM) extinction law to the spectra of
individual fibres to derive slope () and normalization (). The
normalization agrees with the HST attenuation map and the slope is lower than
the Milky Way relation (), which is likely linked to the spatial
sampling of the disk. We speculate that the values of point to either
coherent ISM structures in the disk larger than usual ( kpc) or higher
starting values of , indicative of recent processing of the dust.
The foreground galaxy is a low stellar mass spiral () with a high dust content (). The dust disk geometry visible in the HST image would explain the
observed SED properties of smaller galaxies: a lower mean dust temperature, a
high dust-to-stellar mass ratio but relatively little optical attenuation.
Ongoing efforts to find occulting pairs with a small foreground galaxies will
show how common this geometry is.Comment: 16 pages, 3 tables, 13 figures, accepted for publication in MNRA
Towards an Axiomatization of Simple Analog Algorithms
International audienceWe propose a formalization of analog algorithms, extending the framework of abstract state machines to continuous-time models of computation
Molecular Gas Dynamics in NGC 6946: a Bar-driven Nuclear Starburst "Caught in the Act"
We present high angular resolution ~1" and 0.6" mm-interferometric
observations of the 12CO(1-0) and 12CO(2-1) line emission in the central 300pc
of the late-type spiral galaxy NGC6946. The data, obtained with the IRAM
Plateau de Bure Interferometer (PdBI), allow the first detection of a molecular
gas spiral in the inner ~10" (270pc) with a large concentration of molecular
gas (M(H_2) ~1.6x10^7M_sun) within the inner 60pc. This nuclear clump shows
evidence for a ring-like geometry with a radius of ~10pc as inferred from the
p-v diagrams. Both the distribution of the molecular gas as well as its
kinematics can be well explained by the influence of an inner stellar bar of
about 400pc length. A qualitative model of the expected gas flow shows that
streaming motions along the leading sides of this bar are a plausible
explanation for the high nuclear gas density. Thus, NGC6946 is a prime example
of molecular gas kinematics being driven by a small-scale, secondary stellar
bar.Comment: accepted for publication in the Astrophysical Journal; 47 pages, 17
figures, 1 tabl
Elusive Active Galactic Nuclei
A fraction of active galactic nuclei do not show the classical Seyfert-type
signatures in their optical spectra, i.e. they are optically "elusive". X-ray
observations are an optimal tool to identify this class of objects. We combine
new Chandra observations with archival X-ray data in order to obtain a first
estimate of the fraction of elusive AGN in local galaxies and to constrain
their nature. Our results suggest that elusive AGN have a local density
comparable to or even higher than optically classified Seyfert nuclei. Most
elusive AGN are heavily absorbed in the X-rays, with gas column densities
exceeding 10^24 cm^-2, suggesting that their peculiar nature is associated with
obscuration. It is likely that in elusive AGN, the nuclear UV source is
completely embedded and the ionizing photons cannot escape, which prevents the
formation of a classical Narrow Line Region. Elusive AGN may contribute
significantly to the 30 keV bump of the X-ray background.Comment: accepted for publication in MNRAS Letters, 6 pages, 3 figures, typos
and references correcte
Electrostatically gated membrane permeability in inorganic protocells
Although several strategies are now available to produce functional microcompartments analogous to primitive cell-like structures, little progress has been made in generating protocell constructs with self-controlled membrane permeability. Here we describe the preparation of water-dispersible colloidosomes based on silica nanoparticles and delineated by a continuous semipermeable inorganic membrane capable of self-activated, electrostatically gated permeability. We use crosslinking and covalent grafting of a pH-responsive copolymer to generate an ultrathin elastic membrane that exhibits selective release and uptake of small molecules. This behaviour, which depends on the charge of the copolymer coronal layer, serves to trigger enzymatic dephosphorylation reactions specifically within the protocell aqueous interior. This system represents a step towards the design and construction of alternative types of artificial chemical cells and protocell models based on spontaneous processes of inorganic self-organization
N-band Imaging of Seyfert Nuclei and the MIR-X-ray correlation
We present new mid-infrared (N-band) images of a sample of eight nearby
Seyfert galaxies. In all of our targets, we detect a central unresolved source,
which in some cases has been identified for the first time. In particular, we
have detected the mid-infrared emission from the active nucleus of NGC 4945,
which previously remained undetected at any wavelength but hard X-rays. We also
detect circumnuclear extended emission in the Circinus galaxy along its major
axis, and find marginal evidence for extended circumnuclear emission in NGC
3281.
The high spatial resolution (1.7") of our data allows us to separate the flux
of the nuclear point sources from the extended circumnuclear starburst (if
present). We complement our sample with literature data for a number of
non-active starburst galaxies, and relate the nuclear N-band flux to published
hard (2-10 kev) X-ray fluxes. We find tight and well-separated correlations
between nuclear N-band flux and X-ray flux for both Seyfert and starburst
nuclei which span over 3 orders of magnitude in luminosity. We demonstrate that
these correlations can be used as a powerful classification tool for galactic
nuclei.
For example, we find strong evidence against NGC 1808 currently harbouring an
active Seyfert nucleus based on its position in the mid-infrared-X-ray diagram.
On the other hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.Comment: 31 pages, incl. 4 figures, uses AASTex. Replaced with accepted
version after minor modifications. To appear in Ap
Galaxies with unusually high abundances of molecular hydrogen
A sample of 66 galaxies from the catalog of Bettoni et al. (CISM) with
anomalously high molecular-to-atomic hydrogen mass ratios (M_{mol}/M_{HI}>2) is
considered. The sample galaxies do not differ systematically from other
galaxies in the catalog with the same morphological types, in terms of their
photometric parameters, rotational velocities, dust contents, or the total mass
of gas in comparison with galaxies of similar linear sizes and disk angular
momentum. This suggests that the overabundance of is due to transition of
HI to H_2. Galaxies with bars and active nuclei are found more frequently among
galaxies which have M_{mol} estimates in CISM. In a small fraction of galaxies,
high M_{mol}/M_{HI} ratios are caused by the overestimation of M_{mol} due to a
low conversion factor for the translation of CO-line intensities into the
number of H_2 molecules along the line of sight. It is argued that the
"molecularization" of the bulk of the gas mass could be due 1) to the
concentration of gas in the inner regions of the galactic disks, resulting to a
high gas pressure and 2) to relatively low star-formation rate per unit mass of
molecular gas which indeed takes place in galaxies with high M_{mol}/M_{HI}
ratios.Comment: 11 pages,7 figures, published in Astronomy Report
The NICMOS Snapshot Survey of nearby Galaxies
We present ``snapshot'' observations with the NearInfrared Camera and
MultiObject Spectrometer (NICMOS) on board the Hubble Space Telescope (HST) of
94 nearby galaxies from the Revised Shapley Ames Catalog. Images with 0.2 as
resolution were obtained in two filters, a broad-band continuum filter (F160W,
roughly equivalent to the H-band) and a narrow band filter centered on the
Paschen alpha line (F187N or F190N, depending on the galaxy redshift) with the
51x51 as field of view of the NICMOS camera 3. A first-order continuum
subtraction is performed, and the resulting line maps and integrated Paschen
alpha line fluxes are presented. A statistical analysis indicates that the
average Paschen alpha surface brightness {\bf in the central regions} is
highest in early-type (Sa-Sb) spirals.Comment: Original contained error in flux calibration. Table 1 now has correct
Paschen Alpha fluxes. 14 pages LaTeX with JPEG and PS figures. Also available
at http://icarus.stsci.edu/~boeker/publications.htm
- …