1,603 research outputs found
The Discovery of 1000 km/s Outflows in Massive Post-starburst Galaxies at z=0.6
Numerical simulations suggest that active galactic nuclei (AGNs) play an
important role in the formation of early-type galaxies by expelling gas and
dust in powerful galactic winds and quenching star formation. However, the
existence of AGN feedback capable of halting galaxy-wide star formation has yet
to be observationally confirmed. To investigate this question, we have obtained
spectra of 14 post-starburst galaxies at z~0.6 to search for evidence of
galactic winds. In 10/14 galaxies we detect Mg II 2796,2803 absorption lines
which are blueshifted by 490 - 2020 km/s with respect to the stars. The median
blueshift is 1140 km/s. We hypothesize that the outflowing gas represents a
fossil galactic wind launched near the peak of the galaxy's activity, a few 100
Myr ago. The velocities we measure are intermediate between those of luminous
starbursts and broad absorption line quasars, which suggests that feedback from
an AGN may have played a role in expelling cool gas and shutting down star
formation.Comment: 5 pages, 2 figures, accepted to ApJ Letter
Unobscured Type 2 Active Galactic Nuclei
Type 2 active galactic nuclei (AGNs) with intrinsically weak broad emission lines (BELs) would be exceptions to the unified model. After examining a number of proposed candidates critically, we find that the sample is contaminated significantly by objects with BELs of strengths indicating that they actually contain intermediate-type AGNs, plus a few Compton-thick sources as revealed by extremely low ratios of X-ray to nuclear IR luminosities. We develop quantitative metrics that show two (NGC 3147 and NGC 4594) of the remaining candidates to have BELs 2-3 orders of magnitude weaker than those of typical type 1 AGNs. Several more galaxies remain as candidates to have anomalously weak BELs, but this status cannot be confirmed with the existing information. Although the parent sample is poorly defined, the two confirmed objects are well under 1% of its total number of members, showing that the absence of a BEL is possible, but very uncommon in AGN. We evaluate these two objects in detail using multi-wavelength measurements including new IR data obtained with Spitzer and ground-based optical spectropolarimeteric observations. They have little X-ray extinction with N_H < ~10^(21) cm^(–2). Their IR spectra show strong silicate emission (NGC 4594) or weak aromatic features on a generally power-law continuum with a suggestion of silicates in emission (NGC 3147). No polarized BEL is detected in NGC 3147. These results indicate that the two unobscured type 2 objects have circumnuclear tori that are approximately face-on. Combined with their X-ray and optical/UV properties, this behavior implies that we have an unobscured view of the nuclei and thus that they have intrinsically weak BELs. We compare their properties with those of the other less-extreme candidates. We then compare the distributions of bolometric luminosities and accretion rates of these objects with theoretical models that predict weak BELs
Geant4 Simulation of a filtered X-ray Source for Radiation Damage Studies
Geant4 low energy extensions have been used to simulate the X-ray spectra of
industrial X-ray tubes with filters for removing the uncertain low energy part
of the spectrum in a controlled way. The results are compared with precisely
measured X-ray spectra using a silicon drift detector. Furthermore, this paper
shows how the different dose rates in silicon and silicon dioxide layers of an
electronic device can be deduced from the simulations
Improved PISO algorithms for modeling density varying flow in conjugate fluid–porous domains
Two modified segregated PISO algorithms are proposed, which are constructed to avoid the development of spurious oscillations in the computed flow near sharp interfaces of conjugate fluid–porous domains. The new collocated finite volume algorithms modify the Rhie–Chow interpolation to maintain a correct pressure–velocity coupling when large discontinuous momentum sources associated with jumps in the local permeability and porosity are present. The Re-Distributed Resistivity (RDR) algorithm is based on spreading flow resistivity over the grid cells neighboring a discontinuity in material properties of the porous medium. The Face Consistent Pressure (FCP) approach derives an auxiliary pressure value at the fluid–porous interface using momentum balance around the interface. Such derived pressure correction is designed to avoid spurious oscillations as would otherwise arise with a strictly central discretization. The proposed algorithms are successfully compared against published data for the velocity and pressure for two reference cases of viscous flow. The robustness of the proposed algorithms is additionally demonstrated for strongly reduced viscosity, i.e., higher Reynolds number flows and low Darcy numbers, i.e., low permeability of the porous regions in the domain, for which solutions without unphysical oscillations are computed. Both RDR and FCP are found to accurately represent porous media flow near discontinuities in material properties on structured grids
Local Luminous Infrared Galaxies. III. Co-evolution of Black Hole Growth and Star Formation Activity?
Local luminous infrared (IR) galaxies (LIRGs) have both high star formation
rates (SFR) and a high AGN (Seyfert and AGN/starburst composite) incidence.
Therefore, they are ideal candidates to explore the co-evolution of black hole
(BH) growth and star formation (SF) activity, not necessarily associated with
major mergers. Here, we use Spitzer/IRS spectroscopy of a complete
volume-limited sample of local LIRGs (distances of <78Mpc). We estimate typical
BH masses of 3x10^7 M_sun using [NeIII]15.56micron and optical [OIII]5007A gas
velocity dispersions and literature stellar velocity dispersions. We find that
in a large fraction of local LIRGs the current SFR is taking place not only in
the inner nuclear ~1.5kpc region, as estimated from the nuclear 11.3micron PAH
luminosities, but also in the host galaxy. We next use the ratios between the
SFRs and BH accretion rates (BHAR) to study whether the SF activity and BH
growth are contemporaneous in local LIRGs. On average, local LIRGs have SFR to
BHAR ratios higher than those of optically selected Seyferts of similar AGN
luminosities. However, the majority of the IR-bright galaxies in the RSA
Seyfert sample behave like local LIRGs. Moreover, the AGN incidence tends to be
higher in local LIRGs with the lowest SFRs. All this suggests that in local
LIRGs there is a distinct IR-bright star forming phase taking place prior to
the bulk of the current BH growth (i.e., AGN phase). The latter is reflected
first as a composite and then as a Seyfert, and later as a non-LIRG optically
identified Seyfert nucleus with moderate SF in its host galaxy.Comment: Accepted for publication in Ap
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