3,406 research outputs found
Finding Young Stellar Populations in Elliptical Galaxies from Independent Components of Optical Spectra
Elliptical galaxies are believed to consist of a single population of old
stars formed together at an early epoch in the Universe, yet recent analyses of
galaxy spectra seem to indicate the presence of significant younger populations
of stars in them. The detailed physical modelling of such populations is
computationally expensive, inhibiting the detailed analysis of the several
million galaxy spectra becoming available over the next few years. Here we
present a data mining application aimed at decomposing the spectra of
elliptical galaxies into several coeval stellar populations, without the use of
detailed physical models. This is achieved by performing a linear independent
basis transformation that essentially decouples the initial problem of joint
processing of a set of correlated spectral measurements into that of the
independent processing of a small set of prototypical spectra. Two methods are
investigated: (1) A fast projection approach is derived by exploiting the
correlation structure of neighboring wavelength bins within the spectral data.
(2) A factorisation method that takes advantage of the positivity of the
spectra is also investigated. The preliminary results show that typical
features observed in stellar population spectra of different evolutionary
histories can be convincingly disentangled by these methods, despite the
absence of input physics. The success of this basis transformation analysis in
recovering physically interpretable representations indicates that this
technique is a potentially powerful tool for astronomical data mining.Comment: 12 Pages, 7 figures; accepted in SIAM 2005 International Conference
on Data Mining, Newport Beach, CA, April 200
Spatially Resolved Stellar Populations of Eight GOODS-South AGN at z~1
We present a pilot study of the stellar populations of 8 AGN hosts at z~1 and
compare to (1) lower redshift samples and (2) a sample of nonactive galaxies of
similar redshift. We utilize K' images in the GOODS South field obtained with
the laser guide star adaptive optics (LGSAO) system at Keck Observatory. We
combine this K' data with B, V, i, and z imaging from the ACS on HST to give
multi-color photometry at a matched spatial resolution better than 100 mas in
all bands. The hosts harbor AGN as inferred from their high X-ray luminosities
(L_X > 10^42 ergs/s) or mid-IR colors. We find a correlation between the
presence of younger stellar populations and the strength of the AGN, as
measured with [OIII] line luminosity or X-ray (2-10 keV) luminosity. This
finding is consistent with similar studies at lower redshift. Of the three Type
II galaxies, two are disk galaxies and one is of irregular type, while in the
Type I sample there only one disk-like source and four sources with smooth,
elliptical/spheroidal morphologies. In addition, the mid-IR SEDs of the strong
Type II AGN indicate that they are excited to LIRG (Luminous InfraRed Galaxy)
status via galactic starbursting, while the strong Type I AGN are excited to
LIRG status via hot dust surrounding the central AGN. This supports the notion
that the obscured nature of Type II AGN at z~1 is connected with global
starbursting and that they may be extincted by kpc-scale dusty features that
are byproducts of this starbursting.Comment: 56 pages, 39 figures, accepted to A
Spatially Resolved Stellar Populations of Eight GOODS-South Active Galactic Nuclei at z ~ 1
We present a pilot study of the stellar populations of eight active galactic nucleus (AGN) hosts at z ~ 1 and compare with (1) lower redshift samples and (2) a sample of nonactive galaxies of similar redshift. We utilize K' images in the Great Observatories Origins Deep Survey South field obtained with the laser guide star adaptive optics system at Keck Observatory. We combine these K' data with B, V, i, and z imaging from the Advanced Camera for Surveys on Hubble Space Telescope to give multicolor photometry at a matched spatial resolution better than 100 mas in all bands. The hosts harbor AGNs as inferred from their high X-ray luminosities (LX > 10^42 erg s^â1) or mid-IR colors. We find a correlation between the presence of younger stellar populations and the strength of the AGN, as measured with [O III] line luminosity or X-ray (2-10 keV) luminosity. This finding is consistent with similar studies at lower redshift. Of the three Type II galaxies, two are disk galaxies and one is of irregular type, while in the Type I sample there are only one disk-like source and four sources with smooth, elliptical/spheroidal morphologies. In addition, the mid-IR spectral energy distributions of the strong Type II AGNs indicate that they are excited to Luminous InfraRed Galaxy (LIRG) status via galactic starbursting, while the strong Type I AGNs are excited to LIRG status via hot dust surrounding the central AGN. This supports the notion that the obscured nature of Type II AGNs at z ~ 1 is connected with global starbursting and that they may be extincted by kpc-scale dusty features that are by-products of this starbursting
The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions
The mean ages of early-type galaxies obtained from the analysis of optical
spectra, give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr.
Independent age determinations are possible by using infrared spectra (5-21
microns), which we have obtained with the Infrared Spectrograph on the Spitzer
Observatory. This age indicator is based on the collective mass loss rate of
stars, where mass loss from AGB stars produces a silicate emission feature at
9-12 microns. This feature decreases more rapidly than the shorter wavelength
continuum as a stellar population ages, providing an age indicator. From
observations of 30 nearby early-type galaxies, 29 show a spectral energy
distribution dominated by stars and one has significant emission from the ISM
and is excluded. The infrared age indicators for the 29 galaxies show them all
to be old, with a mean age of about 10 Gyr and a standard deviation of only a
few Gyr. This is consistent with the ages inferred from the values of M/L_B,
but is inconsistent with the ages derived from the optical line indices, which
can be much younger. All of these age indicators are luminosity-weighted and
should be correlated, even if multiple-age components are considered. The
inconsistency indicates that there is a significant problem with either the
infrared and the M/L_B ages, which agree, or with the ages inferred from the
optical absorption lines.Comment: Accepted for publication in Ap
Stellar populations of bulges at low redshift
This chapter summarizes our current understanding of the stellar population
properties of bulges and outlines important future research directions.Comment: Review article to appear in "Galactic Bulges", Editors: Laurikainen
E., Peletier R., Gadotti D., Springer Publishing. 34 pages, 12 figure
Precise Black Hole Masses From Megamaser Disks: Black Hole-Bulge Relations at Low Mass
The black hole (BH)-bulge correlations have greatly influenced the last
decade of effort to understand galaxy evolution. Current knowledge of these
correlations is limited predominantly to high BH masses (M_BH> 10^8 M_sun) that
can be measured using direct stellar, gas, and maser kinematics. These objects,
however, do not represent the demographics of more typical L< L* galaxies. This
study transcends prior limitations to probe BHs that are an order of magnitude
lower in mass, using BH mass measurements derived from the dynamics of H_2O
megamasers in circumnuclear disks. The masers trace the Keplerian rotation of
circumnuclear molecular disks starting at radii of a few tenths of a pc from
the central BH. Modeling of the rotation curves, presented by Kuo et al.
(2010), yields BH masses with exquisite precision. We present stellar velocity
dispersion measurements for a sample of nine megamaser disk galaxies based on
long-slit observations using the B&C spectrograph on the Dupont telescope and
the DIS spectrograph on the 3.5m telescope at Apache Point. We also perform
bulge-to-disk decomposition of a subset of five of these galaxies with SDSS
imaging. The maser galaxies as a group fall below the M_BH-sigma* relation
defined by elliptical galaxies. We show, now with very precise BH mass
measurements, that the low-scatter power-law relation between M_BH and sigma*
seen in elliptical galaxies is not universal. The elliptical galaxy M_BH-sigma*
relation cannot be used to derive the BH mass function at low mass or the
zeropoint for active BH masses. The processes (perhaps BH self-regulation or
minor merging) that operate at higher mass have not effectively established an
M_BH-sigma* relation in this low-mass regime.Comment: 21 pages, 14 figures, accepted for publication in the Astrophysical
Journa
Emission lines and optical continuum in low-luminosity radio galaxies
We present spectroscopic observations of a complete sub-sample of 13
low-luminosity radio galaxies selected from the 2Jy sample. The underlying
continuum in these sources is carefully modelled in order to make a much-needed
comparison between the emission line and continuum properties of FRIs with
those of other classes of radio sources. We find that 5 galaxies in the sample
show a measurable UV excess: 2 of the these sources are BL Lacs and in the
remaining 3 galaxies we argue that the most likely contributor to the UV excess
is a young stellar component. Excluding the BL Lacs, we therefore find that
\~30% of the sample show evidence for young stars, which is similar to the
results obtained for higher luminosity samples. We compare our results with
far-infrared measurements in order to investigate the far-infrared-starburst
link. The nature of the optical-radio correlations is investigated in light of
this new available data and, in contrast to previous studies, we find that the
FRI sources follow the correlations with a similar slope to that found for the
FRIIs. Finally, we compare the luminosity of the emission lines in the FRI and
BL Lac sources and find a significant difference in the [OIII] line
luminosities of the two groups. Our results are discussed in the context of the
unified schemes.Comment: 18 pages, 31 figures, MNRAS in press, (all enquiries to Clive
Tadhunter ([email protected])
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