126 research outputs found
The parsec scale region of Active Galactic Nuclei in the IR
First results from the AGN-Heidelberg program aimed at spatially resolving
the central pc region of the closest Active Galactic Nuclei are presented. The
core region of prototype active nuclei are clearly unveiled at IR waves and at
distances from the nucleus - few pc- where circumnuclear starforming regions
appear not to be present. Within that perspective, classical active nuclei as
Circinus and NGC 1097, reveal with unprecedented detail clear channels of
material being driven to the core whereas others as Centaurus A and NGC 1566,
show a "clean" core environment. At the very center, a central compact region
of about 2 pc scale is resolved in Circinus but not in the other cases
challenging thus the universal presence of the putative obscuring torus.Comment: 4 pages, 6 color figures, To appear in the Proceedings of the IAU
Symp. 222: "The Interplay among Black Holes, Stars and ISM in Galactic
Nuclei" held in Gramado, Brazil, March 200
Classification and Redshift Estimation in Multi-Color Surveys
We present a photometric method for identifying stars, galaxies and quasars
in multi-color surveys and estimating multi-color redshifts for the
extragalactic objects. We use a library of >65000 color templates for
comparison with observed objects. The method was originally developed for the
Calar Alto Deep Imaging Survey (CADIS), but is now used in a variety of survey
projects. We checked its performance by spectroscopy of CADIS objects, where it
provides high reliability (6 mistakes among 151 objects with R<24), especially
for the quasar selection, and redshifts accurate within sigma_z = 0.03 for
galaxies and sigma_z = 0.1 for quasars. For an optimization of future surveys,
a few model surveys are compared, which use the same amount of telescope time
but different filter sets. In summary, medium-band surveys perform superior to
broad-band surveys although they collect less photons. A full account of this
work is already in print.Comment: 7 pages, 2 figures, proceedings of MPA/ESO/MPE Joint Astronomy
Conference Mining THE SKY held in Garching, Germany, July 31 - Aug 4, 200
Resolving the obscuring torus in NGC 1068 with the power of infrared interferometry: Revealing the inner funnel of dust
We present new interferometric data obtained with MIDI (MID infrared
Interferometric instrument) for the Seyfert II galaxy NGC 1068, with an
extensive coverage of sixteen uv points. These observations resolve the nuclear
mid-infrared emission from NGC 1068 in unprecedented detail with a maximum
resolution of 7 mas. For the first time, sufficient uv points have been
obtained, allowing us to generate an image of the source using maximum entropy
image reconstruction. The features of the image are similar to those obtained
by modelling. We find that the mid-infrared emission can be represented by two
components, each with a Gaussian brightness distribution. The first, identified
as the inner funnel of the obscuring torus, is hot (800K), 1.35 parsec long,
and 0.45 parsec thick in FWHM at a PA=-42 degrees (from north to east). It has
an absorption profile different than standard interstellar dust and with
evidence for clumpiness. The second component is 3 by 4 pc in FWHM with T=300K,
and we identify it with the cooler body of the torus. The compact component is
tilted by 45 degrees with respect to the radio jet and has similar size and
orientation to the observed water maser distribution. We show how the dust
distribution relates to other observables within a few parsecs of the core of
the galaxy such as the nuclear masers, the radio jet, and the ionization cone.
We compare our findings to a similar study of the Circinus galaxy and other
relevant studies. Our findings shed new light on the relation between the
different parsec-scale components in NGC 1068 and the obscuring torus.Comment: Accepted to MNRA
Classification and Redshift Estimation in Multi-Color Surveys
We present a photometric method for identifying stars, galaxies and quasars
in multi-color surveys and estimating multi-color redshifts for the
extragalactic objects. We use a library of >65000 color templates for
comparison with observed objects. The method was originally developed for the
Calar Alto Deep Imaging Survey (CADIS), but is now used in a variety of survey
projects. We checked its performance by spectroscopy of CADIS objects, where it
provides high reliability (6 mistakes among 151 objects with R<24), especially
for the quasar selection, and redshifts accurate within sigma_z = 0.03 for
galaxies and sigma_z = 0.1 for quasars. For an optimization of future surveys,
a few model surveys are compared, which use the same amount of telescope time
but different filter sets. In summary, medium-band surveys perform superior to
broad-band surveys although they collect less photons. A full account of this
work is already in print.Comment: 7 pages, 2 figures, proceedings of MPA/ESO/MPE Joint Astronomy
Conference Mining THE SKY held in Garching, Germany, July 31 - Aug 4, 200
Resolving the nucleus of Centaurus A at mid-IR wavelengths
We have observed Centaurus A with the MID-infrared Interferometric instrument
(MIDI) at the Very Large Telescope Interferometer (VLTI) at resolutions of 7 -
15 mas (at 12.5 micron) and filled gaps in the (u,v) coverage in comparison to
earlier measurements. We are now able to describe the nuclear emission in terms
of geometric components and derive their parameters by fitting models to the
interferometric data. With simple geometrical models, the best fit is achieved
for an elongated disk with flat intensity profile with diameter 76 +/- 9 mas x
35 +/- 2 mas (1.41 +/- 0.17 pc x 0.65 +/- 0.03 pc) whose major axis is oriented
at a position angle (PA) of 10.1 +/- 2.2 degrees east of north. A point source
contributes 47 +/- 11 % of the nuclear emission at 12.5 micron. There is also
evidence that neither such a uniform nor a Gaussian disk are good fits to the
data. This indicates that we are resolving more complicated small-scale
structure in AGNs with MIDI, as has been seen in Seyfert galaxies previously
observed with MIDI. The PA and inferred inclination i = 62.6 +2.1/-2.6 degrees
of the dust emission are compared with observations of gas and dust at larger
scales.Comment: Accepted for the PASA special issue on Centaurus
Observing the Seyfert 2 nucleus of NGC 1068 with the VLT Interferometer
Dusty tori have been suggested to play a crucial role in determining the
physical characteristics of active galactic nuclei (AGN), but investigation of
their properties has stalled for lack of high resolution mid-IR imaging.
Recently, a long-awaited breakthrough in this field was achieved: NGC 1068, a
nearby AGN, was the first extragalactic object to be observed with a mid-IR
interferometer, thereby obtaining the needed angular resolution to study the
alleged torus.
In this proceeding, first the field of AGN research is briefly reviewed, with
an emphasis on models of dusty tori. Second, the general properties of the key
object NGC 1068 are discussed. Third, the MIDI mid-IR interferometric data set
is presented together with a first attempt to interpret this data in the
context of tori models. Fourth, preliminary MIDI interferometric spectra of the
nucleus of the nearby starbursting galaxy Circinus are presented. Finally, we
briefly discuss the prospects of ESA's Darwin mission for observing nearby and
distant AGN. This mission will allow detailed mapping of tori of low luminosity
AGN such as NGC 1068 up to redshifts of 1 - 2 and more luminous AGN up to
redshift of 10 and beyond (abridged).Comment: 10 pages, 3 figures, published in Proceedings of SPIE Volume 5491
"New Frontiers in Stellar Interferometry
The merger rate of massive galaxies
We calculate the projected two point correlation function for samples of
luminous and massive galaxies in the COMBO-17 photometric redshift survey,
focusing particularly on the amplitude of the correlation function at small
projected radii and exploring the constraints such measurements can place on
the galaxy merger rate. For nearly volume-limited samples with 0.4<z<0.8, we
find that 4+/-1% of luminous M_B<-20 galaxies are in close physical pairs (with
real space separation of <30 proper kpc). The corresponding fraction for
massive galaxies with M_*>2.5e10 M_sun is 5+/-1%. Incorporating close pair
fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid
evolution of the merger fraction of massive galaxies between z=0.8 and the
present day. Assuming that the major merger timescale is of order the dynamical
timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%)
of all galaxies with present-day masses M_*>5e10 M_sun (remnants of mergers
between galaxies with M_*>2.5e10 M_sun) have undergone a major merger since
z=0.8(1): major mergers between massive galaxies are a significant driver of
galaxy evolution over the last eight billion years.Comment: ApJ, in press. 8 pages, 3 figures. Expanded discussion section with
explicit discussion of merger fraction vs. close pair fraction. Change of
typical close pair timescale results in increased inferred merger rat
The stellar masses of 25000 galaxies at 0.2<z<1.0 estimated by the COMBO-17 survey
We present an analysis of stellar mass estimates for a sample of 25000
galaxies from the COMBO-17 survey over the interval 0.2<z<1.0. We have
developed, implemented, and tested a new method of estimating stellar
mass-to-light ratios, which relies on redshift and spectral energy distribution
(SED) classification from 5 broadband and 12 medium band filters. We find that
the majority (>60%) of massive galaxies with M_* > 10^{11} solar masses at all
z<1 are non-star-forming; blue star-forming galaxies dominate at lower masses.
We have used these mass estimates to explore the evolution of the stellar mass
function since z=1. We find that the total stellar mass density of the universe
has roughly doubled since z~1. Our measurements are consistent with other
measurements of the growth of stellar mass with cosmic time and with estimates
of the time evolution of the cosmic star formation rate. Intriguingly, the
integrated stellar mass of blue galaxies with young stars has not significantly
changed since z~1, even though these galaxies host the majority of the star
formation: instead, the growth of the total stellar mass density is dominated
by the growth of the total mass in the largely passive galaxies on the red
sequence.Comment: Astronomy and Astrophysics in press. 15 pages, 12 figure
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