5,925 research outputs found
The Discovery of an Active Galactic Nucleus in the Late-type Galaxy NGC 3621: Spitzer Spectroscopic Observations
We report the discovery of an Active Galactic Nucleus (AGN) in the nearby SAd
galaxy NGC 3621 using Spitzer high spectral resolution observations. These
observations reveal the presence of [NeV] 14 um and 24 um emission which is
centrally concentrated and peaks at the position of the near-infrared nucleus.
Using the [NeV] line luminosity, we estimate that the nuclear bolometric
luminosity of the AGN is ~ 5 X 10^41 ergs s^-1, which corresponds based on the
Eddington limit to a lower mass limit of the black hole of ~ 4 X 10^3 Msun.
Using an order of magnitude estimate for the bulge mass based on the Hubble
type of the galaxy, we find that this lower mass limit does not put a strain on
the well-known relationship between the black hole mass and the host galaxy's
stellar velocity dispersion established in predominantly early-type galaxies.
Mutli-wavelength follow-up observations of NGC 3621 are required to obtain more
precise estimates of the bulge mass, black hole mass, accretion rate, and
nuclear bolometric luminosity. The discovery reported here adds to the growing
evidence that a black hole can form and grow in a galaxy with no or minimal
bulge.Comment: 5 pages, 7 figures, Accepted for publication in ApJ Letter
The Starburst Nature of Lyman-Break Galaxies: Testing UV Extinction with X-rays
We derive the bolometric to X-ray correlation for a local sample of normal
and starburst galaxies and use it, in combination with several UV reddening
schemes, to predict the 2--8 keV X-ray luminosity for a sample of 24
Lyman-break galaxies in the HDF/CDF-N. We find that the mean X-ray luminosity,
as predicted from the Meurer UV reddening relation for starburst galaxies,
agrees extremely well with the Brandt stacking analysis. This provides
additional evidence that Lyman-break galaxies can be considered as scaled-up
local starbursts and that the locally derived starburst UV reddening relation
may be a reasonable tool for estimating the UV extinction at high redshift. Our
analysis shows that the Lyman-break sample can not have far-IR to far-UV flux
ratios similar to nearby ULIGs, as this would predict a mean X-ray luminosity
100 times larger than observed, as well as far-IR luminosities large enough to
be detected in the sub-mm. We calculate the UV reddening expected from the
Calzetti effective starburst attenuation curve and the radiative transfer
models of Witt & Gordon for low metallicity dust in a shell geometry with
homogeneous or clumpy dust distributions and find that all are consistent with
the observed X-ray emission. Finally, we show that the mean X-ray luminosity of
the sample would be under predicted by a factor of 6 if the the far-UV is
unattenuated by dust.Comment: 7 pages, 3 figures. Accepted for publication in A
Spitzer Uncovers Active Galactic Nuclei Missed by Optical Surveys in 7 Late-type Galaxies
We report the discovery using Spitzers high resolution spectrograph of 7
Active Galactic Nuclei (AGN) in a sample of 32 late-type galaxies that show no
definitive signatures of AGN in their optical spectra. Our observations suggest
that the AGN detection rate in late-type galaxies is possibly 4 times larger
than what optical spectroscopic observations alone suggest. We demonstrate
using photoionization models with an input AGN and an extreme EUV-bright
starburst ionizing radiation field that the observed mid-infrared line ratios
cannot be replicated unless an AGN contribution, in some cases as little as 10%
of the total galaxy luminosity, is included. These models show that when the
fraction of the total luminosity due to the AGN is low, optical diagnostics are
insensitive to the presence of the AGN. In this regime of parameter space, the
mid-infrared diagnostics offer a powerful tool for uncovering AGN missed by
optical spectroscopy. The AGN bolometric luminosities in our sample range from
~3 X 10^41 - ~2 X 10^43 ergs s^-1, which, based on the Eddington limit,
corresponds to a lower mass limit for the black hole that ranges from ~3 X
10^3Mdot to as high as ~1.5 X 10^5Mdot. These lower mass limits however do not
put a strain on the well-known relationship between the black hole mass and the
host galaxy's stellar velocity dispersion established in predominantly
early-type galaxies. Our findings add to the growing evidence that black holes
do form and grow in low-bulge environments and that they are significantly more
common than optical studies indicate.Comment: 8 figures, 17 pages, astro-ph\0801.2766 (Abel & Satyapal 2008; ApJ
accepted) and this posting designed to form a two-part investigatio
The relation between star formation rate and accretion rate in LINERs
It is argued that there is a linear correlation between star formation rate
(SFR) and accretion rate for normal bright active galactic nuclei (AGNs).
However, it is still unclear whether this correlation holds for LINERs, of
which the accretion rates are relatively lower than those of normal bright
AGNs. The radiatively inefficient accretion flows (RIAFs) are believed to be
present in these LINERs. In this work, we derive accretion rates for a sample
of LINERs from their hard X-ray luminosities based on spectral calculations for
RIAFs. We find that LINERs follow the same correlation between star formation
rate and accretion rate defined by normal bright AGNs, when reasonable
parameters are adopted for RIAFs. It means that the gases feed the black hole
and star formation in these low-luminosity LINERs may follow the same way as
that in normal bright AGNs, which is roughly consistent with recent numerical
simulations on quasar evolution.Comment: 15 pages, 3 figures, accepted for publication in PASP, in pres
Feedback in the local LBG Analog Haro 11 as probed by far-UV and X-ray observations
We have re-analyzed FUSE data and obtained new Chandra observations of Haro
11, a local (D_L=88 Mpc) UV luminous galaxy. Haro 11 has a similar far-UV
luminosity (10^10.3 L_\odot), UV surface brightness (10^9.4 L_\odot kpc^-2),
SFR, and metallicity to that observed in Lyman Break Galaxies (LBGs). We show
that Haro 11 has extended, soft thermal (kT~0.68 keV) X-ray emission with a
luminosity and size which scales with the physical properties (e.g. SFR,
stellar mass) of the host galaxy. An enhanced alpha/Fe, ratio of ~4 relative to
solar abundance suggests significant supernovae enrichment. These results are
consistent with the X-ray emission being produced in a shock between a
supernovae driven outflow and the ambient material. The FUV spectra show strong
absorption lines similar to those observed in LBG spectra. A blueshifted
absorption component is identified as a wind outflowing at ~200-280 km/s.
OVI\lambda\lambda1032,1038 emission, the dominant cooling mechanism for coronal
gas at T~10^5.5 K is also observed. If associated with the outflow, the
luminosity of the OVI emission suggests that <20% of the total mechanical
energy from the supernovae and solar winds is being radiated away. This implies
that radiative cooling through OVI is not significantly inhibiting the growth
of the outflowing gas. In contradiction to the findings of Bergvall et al 2006,
we find no convincing evidence of Lyman continuum leakage in Haro 11. We
conclude that the wind has not created a `tunnel' allowing the escape of a
significant fraction of Lyman continuum photons and place a limit on the escape
fraction of f_{esc}<2%. Overall, both Haro 11 and a previously observed LBG
analogue VV 114, provide an invaluable insight into the X-ray and FUV
properties of high redshift LBGs.Comment: Accepted for publication in ApJ, 40 pages, 17 figure
Properties of H II Regions in the Centers of Nearby Galaxies
As part of an optical spectroscopic survey of nearby, bright galaxies, we
have identified a sample of over 200 emission-line nuclei having optical
spectra resembling those of giant extragalactic H II regions. Such "H II
nuclei," powered by young, massive stars, are found in a substantial fraction
of nearby galaxies, especially those of late Hubble type. This paper summarizes
the observational characteristics of H II nuclei, contrasts the variation of
their properties with Hubble type, and compares the nuclear H II regions with
those found in galaxy disks. Similarities and differences between H II nuclei
and luminous starburst nuclei are additionally noted.Comment: To appear in the Astrophysical Journal. LaTex, 37 pages including 15
postscript figures. AAStex macros include
Matter wave functions and Yukawa couplings in F-theory Grand Unification
We study the local structure of zero mode wave functions of chiral matter
fields in F-theory unification. We solve the differential equations for the
zero modes derived from local Higgsing in the 8-dimensional parent action of
F-theory 7-branes. The solutions are found as expansions both in powers and
derivatives of the magnetic fluxes. Yukawa couplings are given by an overlap
integral of the three wave functions involved in the interaction and can be
calculated analytically. We provide explicit expressions for these Yukawas to
second order both in the flux and derivative expansions and discuss the effect
of higher order terms. We explicitly describe the dependence of the couplings
on the U(1) charges of the relevant fields, appropriately taking into account
their normalization. A hierarchical Yukawa structure is naturally obtained. The
application of our results to the understanding of the observed hierarchies of
quarks and leptons is discussed.Comment: Latex, 51 pages, 4 figures, typos corrected, note adde
Star Formation Histories of Nearby Elliptical Galaxies. II. Merger Remnant Sample
This work presents high spectroscopic observations of a sample of six
suspected merger remnants, selected primarily on the basis of H{\sc i} tidal
debris detections. Single stellar population analysis of these galaxies
indicates that their ages, metallicities, and -enhancement ratios are
consistent with those of a representative sample of nearby elliptical galaxies.
The expected stellar population of a recent merger remnant, young age combined
with low [/Fe], is not seen in any H{\sc i}-selected galaxy. However,
one galaxy (NGC~2534), is found to deviate from the -plane in the sense
expected for a merger remnant. Another galaxy (NGC~7332), selected by other
criteria, best matches the merger remnant expectations.Comment: 12 pages, 10 figures, accepted by A
Equality of opportunity and educational achievement in Portugal
Portugal has one of the highest levels of income inequality in Europe, and low wages and unemployment are concentrated among low skill individuals. Education is an important determinant of inequality. However, there are large differences in the educational attainment of different individuals in the population, and the sources of these differences emerge early in the life-cycle when families play a central role in individual development. We estimate that most of the variance of school achievement at age 15 is explained by family characteristics. Observed school inputs explain very little of adolescent performance. Children from highly educated parents benefit of rich cultural environments in the home and become highly educated adults. Education policy needs to be innovative: (1) it needs to explicitly recognize the fundamental long run role of families on child development; (2) it needs to acknowledge the failure of traditional input based policies
- …