68 research outputs found
Megamaser Disks in Active Galactic Nuclei
Recent spectroscopic and VLBI-imaging observations of bright extragalactic
water maser sources have revealed that the megamaser emission often originates
in thin circumnuclear disks near the centers of active galactic nuclei (AGNs).
Using general radiative and kinematic considerations and taking account of the
observed flux variability, we argue that the maser emission regions are clumpy,
a conclusion that is independent of the detailed mechanism (X-ray heating,
shocks, etc.) driving the collisionally pumped masers. We examine scenarios in
which the clumps represent discrete gas condensations (i.e., clouds) and do not
merely correspond to velocity irregularities in the disk. We show that even two
clouds that overlap within the velocity coherence length along the line of
sight could account (through self-amplification) for the entire maser flux of a
high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and
we suggest that cloud self-amplification likely contributes also to the flux of
the background-amplifying ``systemic'' features in these objects. Analogous
interpretations have previously been proposed for water maser sources in
Galactic star-forming regions. We argue that this picture provides a natural
explanation of the time-variability characteristics of extragalactic megamaser
sources and of their apparent association with Seyfert 2-like galaxies. We also
show that the requisite cloud space densities and internal densities are
consistent with the typical values of nuclear (broad emission-line region-type)
clouds.Comment: 55 pages, 7 figures, AASTeX4.0, to appear in The Astrophysical
Journal (1999 March 1 issue
Water maser emission and the parsec-scale jet in NGC 3079
We have conducted VLBI observations of water maser and radio continuum
emission in the nucleus of NGC 3079. The 22 GHz maser emission arises in
compact clumps, distributed along an axis that is aligned with the major axis
of the galactic disk. The velocities of the masers are consistent with their
lying in the inner parsec of a molecular disk rotating in the same sense as the
rest of the galaxy. However, the velocity field has a significant
non-rotational component, which may indicate supersonic turbulence. The bright
maser emission is not coincident with any detected compact 22 GHz continuum
source, suggesting the high apparent luminosity of the maser may not due to
beamed amplification of continuum emission. We observed two compact continuum
sources that have inverted spectra between 5 and 8 GHz, and steep spectra
between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the
class of compact symmetric gigahertz-peaked spectrum (GPS) radio sources. We
detected a third continuum component that lies along the same axis as the other
two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser
emission was detected near this axis, which may indicate a second population of
masers associated with the jet.Comment: 31 pages, includes 8 figures. To appear in ApJ, Vol 495, 10 March
1998. Full-resolution figures and color plate available at
http://cfa-www.harvard.edu/~trotter/ngc3079.htm
High Energy gamma-rays From FR I Jets
Thanks to Hubble and Chandra telescopes, some of the large scale jets in
extragalactic radio sources are now being observed at optical and X-ray
frequencies. For the FR I objects the synchrotron nature of this emission is
surely established, although a lot of uncertainties - connected for example
with the particle acceleration processes involved - remain. In this paper we
study production of high energy gamma-rays in FR I kiloparsec-scale jets by
inverse-Compton emission of the synchrotron-emitting electrons. We consider
different origin of seed photons contributing to the inverse-Compton
scattering, including nuclear jet radiation as well as ambient, stellar and
circumstellar emission of the host galaxies. We discuss how future detections
or non-detections of the evaluated gamma-ray fluxes can provide constraints on
the unknown large scale jet parameters, i.e. the magnetic field intensity and
the jet Doppler factor. For the nearby sources Centaurus A and M 87, we find
measurable fluxes of TeV photons resulting from synchrotron self-Compton
process and from comptonisation of the galactic photon fields, respectively. In
the case of Centaurus A, we also find a relatively strong emission component
due to comptonisation of the nuclear blazar photons, which could be easily
observed by GLAST at energy ~10 GeV, providing important test for the
unification of FR I sources with BL Lac objects.Comment: 39 pages, 6 figures included. Modified version, accepted for
publication in Astrophysical Journa
Blue Stragglers in Galactic Open Clusters and the Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic open clusters
are studies in this work, where twenty-seven Galactic open clusters of ages >=
1Gyr are selected as the working sample. Based on the photometric observations
of these open clusters, synthetic integrated spectrum has been made for the
stellar population of each cluster. The effects of blue straggler stars (BSSs)
on the conventional simple stellar population (SSP) model are analyzed on an
individual cluster base. It is shown that the BSSs, whose holding positions in
the color-magnitude diagrams (CMDs) cannot be predicted by the current
single-star evolution theory, present significant modifications to the
integrated properties of theoretical SSP model. The synthesized integrated
spectral energy distributions (ISEDs) of our sample clusters are dramatically
different from the SSPs based on isochrone only. The BSSs corrected ISEDs of
stellar populations show systematic enhancements towards shorter wavelength in
the spectra. When measured with wide-band colors in unresolvable conditions,
the age of a stellar population can be seriously under-estimated by the
conventional SSP model. Therefore, considering the common existence of BSS
component in real stellar populations, a considerable amount of alternations on
the conventional ISEDs should be expected when applying the technique of
evolutionary population synthesis (EPS) to more complicated stellar systems.Comment: 45 pages, 21 figures Accepted for publication in ApJ (Feburary 1,
2005 issue
The Survey for Ionization in Neutral Gas Galaxies: I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG), a
census of star formation in HI-selected galaxies. The survey consists of
H-alpha and R-band imaging of a sample of 468 galaxies selected from the HI
Parkes All Sky Survey (HIPASS). The sample spans three decades in HI mass and
is free of many of the biases that affect other star forming galaxy samples. We
present the criteria for sample selection, list the entire sample, discuss our
observational techniques, and describe the data reduction and calibration
methods. This paper focuses on 93 SINGG targets whose observations have been
fully reduced and analyzed to date. The majority of these show a single
Emission Line Galaxy (ELG). We see multiple ELGs in 13 fields, with up to four
ELGs in a single field. All of the targets in this sample are detected in
H-alpha indicating that dormant (non-star forming) galaxies with M(HI) > ~3e7
M_sun are very rare. A database of the measured global properties of the ELGs
is presented. The ELG sample spans four orders of magnitude in luminosity
(H-alpha and R-band), and H-alpha surface brightness, nearly three orders of
magnitude in R surface brightness and nearly two orders of magnitude in H-alpha
equivalent width (EW). The surface brightness distribution of our sample is
broader than that of the Sloan Digital Sky Survey spectroscopic sample, the
(EW) distribution is broader than prism-selected samples, and the morphologies
found include all common types of star forming galaxies (e.g. irregular,
spiral, blue compact dwarf, starbursts, merging and colliding systems, and even
residual star formation in S0 and Sa spirals). (abridged)Comment: 28 pages, ApJS, in press. Full resolution version with all panels of
Fig. 8 available at http://sungg.pha.jhu.edu/publications.html . On line data
available at http://sungg.pha.jhu.edu/PubData/ . Author list corrected. Wrong
value for f_ap used in eq. 7 now corrected; typos corrected, non-used
references replaced, others update
Evidence for a Geometrically Thick Self-Gravitating Accretion Disk in NGC 3079
We have mapped, for the first time, the full velocity extent of the water
maser emission in NGC 3079. The largely north-south distribution of emission,
aligned with a kpc-scale molecular disk, and the segregation of blue- and
red-shifted emission on the sky are suggestive of a nearly edge-on molecular
disk on pc-scales. Positions and line-of-sight velocities of blue- and
red-shifted maser emission are consistent with a central mass of ~2 x 10^6 Msun
enclosed within a radius of ~0.4 pc. The corresponding mean mass density of
10^6.8 Msun pc^-3 is suggestive of a central black hole, which is consistent
with the detection of hard X-ray excess (20-100 keV) and an Fe Kalpha line from
the nucleus. Because the rotation curve traced by the maser emission is flat,
the mass of the pc-scale disk is significant with respect to the central mass.
Since the velocity dispersion of the maser features does not decrease with
radius and constitutes a large fraction of the orbital velocity, the disk is
probably thick and flared. The rotation curve and the physical conditions
necessary to support maser emission imply a Toomre Q-parameter that is << 1.
Thus, the disk is most likely clumpy, and we argue that it is probably forming
stars. Overall, the accretion disk in NGC 3079 stands in contrast to the
compact, thin, warped, differentially rotating disk in the archetypal maser
galaxy NGC 4258 (abridged).Comment: 41 pages, 13 figures, to appear in the 2005 January 10 issue of the
Astrophysical Journal. High resolution versions of the figures and of the
paper are available at
http://cfa-www.harvard.edu/~pkondratko/publications/NGC3079
The Influence of Bars on Nuclear Activity
We test ideas on fueling of galactic nuclei by bar-driven inflow by comparing
the detection rate and intensity of nuclear H II regions and AGNs among barred
and unbarred galaxies in a sample of over 300 spirals selected from our recent
optical spectroscopic survey of nearby galaxies. Among late-type spirals
(Sc-Sm), but not early-type (S0/a-Sbc), we observe in the barred group a very
marginal increase in the detection rate of H II nuclei and a corresponding
decrease in the incidence of AGNs. The minor differences in the detection
rates, however, are statistically insignificant, most likely stemming from
selection effects and not from a genuine influence from the bar. The presence
of a bar seems to have no noticeable impact on the likelihood of a galaxy to
host either nuclear star formation or an AGN. The nuclei of early-type barred
spirals do exhibit measurably higher star-formation rates than their unbarred
counterparts, as indicated by either the luminosity or the equivalent width of
H-alpha emission. By contrast, late-type spirals do not show such an effect.
Bars have a negligible effect on the strength of the AGNs in our sample,
regardless of the Hubble type of the host galaxy. This result confirms similar
conclusions reached by other studies based on much smaller samples.Comment: To appear in the Astrophysical Journal. LaTex, 31 pages including 6
postscript figures and 3 tables. AAStex macros include
Kinematics of elliptical galaxies with a diffuse dust component
Observations show that early-type galaxies contain a considerable amount of
interstellar dust, most of which is believed to exist as a diffusely
distributed component. We construct a four-parameter elliptical galaxy model in
order to investigate the effects of such a smooth absorbing component on the
projection of kinematic quantities, such as the line profiles and their
moments. We investigate the dependence on the optical depth and on the dust
geometry. Our calculations show that both the amplitude and the morphology of
these quantities can be significantly affected. Dust effects should therefore
be taken in consideration when interpreting photometric and kinematic
properties, and correlations that utilize these quantities.Comment: 12 pages, 9 figures, accepted for publication in MNRA
MCAO near-IR photometry of the Globular Cluster NGC 6388: MAD observations in crowded fields
Deep photometry of crowded fields, such as Galactic Globular Clusters, is
severely limited by the actual resolution of ground-based telescopes. On the
other hand, the Hubble Space Telescope does not provide the near-infrared (NIR)
filters needed to allow large color baselines. In this work we aim at
demonstrating how ground based observations can reach the required resolution
when using Multi-Conjugated Adaptive Optic (MCAO) devices in the NIR, such as
the experimental infrared camera (MAD) available on the VLT. This is
particularly important since these corrections are planned to be available on
all ground--based telescopes in the near future. We do this by combining the
infrared photometry obtained by MAD/VLT with ACS/HST optical photometry of our
scientific target, the bulge globular cluster NGC 6388, in which we imaged two
fields. In particular, we constructed color-magnitude diagrams with an
extremely wide color baseline in order to investigate the presence of multiple
stellar populations in this cluster. From the analysis of the external field,
observed with better seeing conditions, we derived the deepest optical-NIR CMD
of NGC 6388 to date. The high-precision photometry reveals that two distinct
sub-giant branches are clearly present in this cluster. We also use the CMD
from the central region to estimate the distance ((m-M)=15.33) and the
reddening (E(B-V)=0.38) for this cluster. We estimate the age to be ~11.5+/-
1.5 Gyr. The large relative-age error reflects the bimodal distribution of the
SGB stars. This study clearly demonstrates how MCAO correction in the NIR bands
implemented on ground based telescopes can complement the high-resolution
optical data from HST.Comment: 9 pages, 10 figures. Accepted for publication on A. &
New Young Star Candidates in CG4 and Sa101
The CG4 and Sa101 regions together cover a region of ~0.5 square degree in
the vicinity of a "cometary globule" that is part of the Gum Nebula. There are
seven previously identified young stars in this region; we have searched for
new young stars using mid- and far-infrared data (3.6 to 70 microns) from the
Spitzer Space Telescope, combined with ground-based optical data and
near-infrared data from the Two-Micron All-Sky Survey (2MASS). We find infrared
excesses in all 6 of the previously identified young stars in our maps, and we
identify 16 more candidate young stars based on apparent infrared excesses.
Most (73%) of the new young stars are Class II objects. There is a tighter
grouping of young stars and young star candidates in the Sa101 region, in
contrast to the CG4 region, where there are fewer young stars and young star
candidates, and they are more dispersed. Few likely young objects are found in
the "fingers" of the dust being disturbed by the ionization front from the
heart of the Gum Nebula.Comment: Accepted for publication in A
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