474 research outputs found
Three-Dimensional Simulations of a Starburst-Driven Galactic Wind
We have performed a series of three-dimensional simulations of a
starburst-driven wind in an inhomogeneous interstellar medium. The introduction
of an inhomogeneous disk leads to differences in the formation of a wind, most
noticeably the absence of the ``blow-out'' effect seen in homogeneous models. A
wind forms from a series of small bubbles that propagate into the tenuous gas
between dense clouds in the disk. These bubbles merge and follow the path of
least resistance out of the disk, before flowing freely into the halo.
Filaments are formed from disk gas that is broken up and accelerated into the
outflow. These filaments are distributed throughout a biconical structure
within a more spherically distributed hot wind. The distribution of the
inhomogeneous interstellar medium in the disk is important in determining the
morphology of this wind, as well as the distribution of the filaments. While
higher resolution simulations are required in order to ascertain the importance
of mixing processes, we find that soft X-ray emission arises from gas that has
been mass-loaded from clouds in the disk, as well as from bow shocks upstream
of clouds, driven into the flow by the ram pressure of the wind, and the
interaction between these shocks.Comment: 37 pages, 16 figures, mpg movie can be obtained at
http://www.mso.anu.edu.au/~jcooper/movie/video16.mpg, accepted for
publication in Ap
Nuclear activity and star formation properties of Seyfert 2 galaxies
In order to characterize the amount of recent or ongoing stellar formation in
the circumnuclear region of active galaxies on a statistically sound basis, we
have studied the stellar component of the nuclear spectra in three different
samples of galaxies, namely Seyfert 2 galaxies (hereafter S2G), star-forming
galaxies (SFG) and passive normal galaxies (NG), i.e., no emission lines
observed, using Sloan Digital Sky Survey data (SDSS) (Adelman-McCarthy, 2008).
The stellar component of the observed spectra has been extracted using
STARLIGHT (Cid Fernandes et al., 2004), which fits an observed spectrum with a
model (template) spectrum obtained by combining a library of pre-defined simple
stellar populations spectra, with distinct ages and metallicities. The
resulting template spectra for the different samples of galaxies have been
compared to determine the features of the stellar emission component and to
evaluate the presence and intensity of the star formation in the nuclear
regions of different families of galaxies. From a first qualitative analysis it
results that the shape of the Spectral Energy Distribution (SED) of S2G and NG
is very similar, while that of SFG is characterized by a strong blue excess.
The presence of the 4000 A break in the spectra of S2G and NG together with the
lack of a strong blue continuum clearly indicate the absence of ongoing star
formation in the circumnuclear regions of S2G and obviously of NG. Anyway
traces of a recent star formation history are evident in the spectra of S2G
galaxies, which show a 4000 A break systematically shallower than in NG.Comment: Proceeding of the VII Serbian Conference on Spectral Line Shapes in
Astrophysic
Self-energy corrections in an antiferromagnet -- interplay of classical and quantum effects on quasiparticle dispersion
Self-energy corrections due to fermion-magnon interaction are studied in the
antiferromagnetic state of the Hubbard model within the rainbow
(noncrossing) approximation in the full range from weak to strong coupling.
The role of classical (mean-field) features of fermion and magnon dispersion,
associated with finite , are examined on quantum corrections to
quasiparticle energy, weight, one-particle density of states etc. A finite-
induced classical dispersion term, absent in the model, is found to play
an important role in suppressing the quasiparticle weight for states near , as seen in cuprates. For intermediate , the renormalized AF band
gap is found to be nearly half of the classical value, and the weak coupling
limit is quite non-trivial due to strongly suppressed magnon amplitude. For
finite , the renormalized AF band gap is shown to vanish at a critical
interaction strength , yielding a spin fluctuation driven first-order AF
insulator - PM metal transition. Quasiparticle dispersion evaluated with the
same set of Hubbard model cuprate parameters, as obtained from a recent magnon
spectrum fit, provides excellent agreement with ARPES data for .Comment: 11 pages, 17 figure
1ES 1927+654: Persistent and rapid X-ray variability in an AGN with low intrinsic neutral X-ray absorption and narrow optical emission lines
We present X-ray and optical observations of the X-ray bright AGN 1ES
1927+654. The X-ray observations obtained with ROSAT and Chandra reveal
persistent, rapid and large scale variations, as well as steep 0.1-2.4 keV
(Gamma = 2.6 +/- 0.3) and 0.3-7.0 keV (Gamma = 2.7 +/- 0.2) spectra. The
measured intrinsic neutral X-ray column density is approximately 7e20cm^-2. The
X-ray timing properties indicate that the strong variations originate from a
region, a few hundred light seconds from the central black hole, typical for
type 1 AGN. High quality optical spectroscopy reveals a typical Seyfert 2
spectrum with some host galaxy contamination and no evidence of Fe II
multiplets or broad hydrogen Balmer wings. The intrinsic optical extinction
derived from the BLR and NLR are A_V >= 3.7 and A_V=1.7, respectively. The
X-ray observations give an A_V value of less than 0.58, in contrast to the
optical extinction values. We discuss several ideas to explain this apparent
difference in classification including partial covering, an underluminous BLR
or a high dust to gas ratio.Comment: 8 pages including 10 figures. Accepted for publication in Astronomy
and Astrophysic
The location of an active nucleus and a shadow of a tidal tail in the ULIRG Mrk 273
Analysis of data from the Chandra X-ray Observatory for the double nucleus ULIRG Mrk 273 reveals an absorbed hard X-ray source coincident with the southwest nucleus, implying that this unresolved, near-infrared source is where an active nucleus resides, while the northern nuclear region contains a powerful starburst that dominates the far infrared luminosity. There is evidence of a slight image extension in the 6â7 keV band, where an Fe K line is present, towards the northern nucleus. A large-scale, diffuse emission nebula detected in soft X-rays contains a dark lane that spatially coincides with a high surface-brightness tidal tail extending ~50 arcsec (40 kpc) to the south. The soft X-ray source is likely located behind the tidal tail, which absorbs X-ray photons along the line of sight. The estimated column density of cold gas in the tidal tail responsible for shadowing the soft X-rays is N_H â„ 6 Ă 10^(21) cm^(-2), consistent with the tidal tail having an edge-on orientation
SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy
While performing a survey to detect recoiling supermassive black holes, we
have identified an unusual source having a projected offset of 800 pc from a
nearby dwarf galaxy. The object, SDSS J113323.97+550415.8, exhibits broad
emission lines and strong variability. While originally classified as a
supernova (SN) because of its nondetection in 2005, we detect it in recent and
past observations over 63 yr and find over a magnitude of rebrightening in the
last 2 years. Using high-resolution adaptive optics observations, we constrain
the source emission region to be <12 pc and find a disturbed host-galaxy
morphology indicative of recent merger activity. Observations taken over more
than a decade show narrow [O III] lines, constant ultraviolet emission, broad
Balmer lines, a constant putative black hole mass over a decade of observations
despite changes in the continuum, and optical emission-line diagnostics
consistent with an active galactic nucleus (AGN). However, the optical spectra
exhibit blueshifted absorption, and eventually narrow Fe II and [Ca II]
emission, each of which is rarely found in AGN spectra. While this peculiar
source displays many of the observational properties expected of a potential
black hole recoil candidate, some of the properties could also be explained by
a luminous blue variable star (LBV) erupting for decades since 1950, followed
by a Type IIn SN in 2001. Interpreted as an LBV followed by a SN analogous to
SN 2009ip, the multi-decade LBV eruptions would be the longest ever observed,
and the broad Halpha emission would be the most luminous ever observed at late
times (>10 yr), larger than that of unusually luminous supernovae such as SN
1988Z, suggesting one of the most extreme episodes of pre-SN mass loss ever
discovered.Comment: Accepted for publication in MNRA
STIS ultraviolet/optical spectroscopy of `warm' ultraluminous infrared galaxies
(Abridged) We present high spatial resolution ultraviolet and optical
spectroscopy, obtained using the Space Telescope Imaging Spectrograph on board
the Hubble Space Telescope, of nuclear structures within four `warm'
Ultraluminous Infrared Galaxies (ULIRGs). We find an AGN in at least three, and
probably all four of our sample, hosted in a compact, optically luminous
`knot'. In three cases these knots were previously identified as a putative AGN
nucleus from multiband optical imaging. Three of the sample also harbor a
starburst in one or more knots, suggesting that the optically luminous knots
seen in local ULIRGs are the most likely sites of the dust-shrouded starburst
and AGN activity that power the infrared emission. The four AGN have a diverse
range of properties; two are classical narrow line AGN, one shows both broad
and narrow lines and evidence for lines of sight from the narrow through to the
broad line regions, and one is plausibly a FeLoBAL AGN. The probable presence
in one object of an FeLoBAL AGN, which are extremely rare in the QSO
population, supports the idea that LoBAL AGN may be youthful systems shrouded
in gas and dust rather than AGN viewed along a certain line of sight. The three
starbursts for which detailed constraints are possible show a smaller range in
properties; all three bursts are young with two having ages of ~4Myr and the
third having an age of 20Myr, suggesting that ULIRGs undergo several bursts of
star formation during their lifetimes. None of the starbursts show evidence for
Initial Mass Function slopes steeper than about 3.3. The metallicities of the
knots for which metallicities can be derived are all at least 1.5 times the
Solar value. The properties of one further starburst knot are consistent with
it being the forming core of an elliptical galaxy.Comment: ApJ, accepte
Nuclear activity in galaxy pairs: a spectroscopic analysis of 48 UZC-BGPs
Galaxy pairs are ideal sites in which to investigate the role of interaction
on nuclear activity. For this reason we have undertaken a spectroscopic survey
of a large homogeneous sample of galaxy pairs (UZC-BGP) and we present the
results of the nuclear spectral classification of 48 pairs (more than half of
the whole sample). The fraction of emission line galaxies is extremely large,
especially among spirals (84 % and 95 %, for early and late spirals
respectively). SB is the most frequent type of nuclear activity encountered (30
% of galaxies) while AGNs are only 19%. The fractions raise to 45 % and 22 %
when considering only spirals. Late spirals are characterized by both an
unusual increase (35 %) of AGN activity and high luminosity (44 % have M_B
<-20.0 + 5log h). LLAGNs are only 8% of the total number of galaxies, but this
activity could be present in another 10 % of the galaxies (LLAGN candidates).
Absorption line galaxies reside mostly (61 %) in S0 galaxies and display the
lowest B luminosity in the sample, only 18 % of them have M_B < -20 + 5 log h,
but together with LLAGNs they are the most massive galaxies in the sample.
Intense-SB nuclei are found in galaxy pairs with galaxy-galaxy projected
separations up to 160 h^{-1} kpc suggesting that in bright isolated galaxy
pairs interaction may be at work and effective up to that distance. AGNs are
characterized by an advanced morphology while SB phenomenon occurs with the
same frequency in early and late spirals. LLAGNs and LLAGN candidates do not
always show similar properties, a finding which might confirm the heterogeneous
nature of this class of objects. Half LLAGNs are hosted in galaxies showing
visible signs of interaction with fainter companions, suggesting that minor
interactions might be a driving mechanism for a relevant fraction of LLAGNs.Comment: 19 pages, 11 figures, accepted by A&
SCG0018-4854: a young and dynamic compact group I. Kinematical analysis
Compact groups of galaxies are in particular good laboratories for studying
galaxy interactions and their effects on the evolution of galaxies due to their
high density and low velocity dispersion. SCG0018-4854 is a remarkably high
galaxy density and low velocity dispersion group with evidence of a recent
interaction. We obtained VLT FORS2 optical observations and we present
spectroscopic and photometric evidence of how dramatically galaxy interactions
have affected each of the four member galaxies. We found peculiar kinematics
for each galaxy and evidence of recent star formation. In particular, the gas
and stellar radial velocity curves of two galaxies are irregular with a level
of asymmetry similar to that of other interacting galaxies. We discovered the
presence of a bar for NGC 92 therefore revising a previous morphological
classification and we obtained spectroscopic confirmation of a galactic-scale
outflow of NGC 89. Peculiar kinematics and dynamic consideration lead to a
rough estimate of the age of the latest interaction: 0.2-0.7 Gyr, suggesting
that SCG0018-4854 is a young and dynamical group.Comment: 11 pages, 12 figures, to be published in A&
The Broad Absorption Line Tidal Disruption Event iPTF15af: Optical and Ultraviolet Evolution
We present multi-wavelength observations of the tidal disruption event (TDE)
iPTF15af, discovered by the intermediate Palomar Transient Factory (iPTF)
survey at redshift . The optical and ultraviolet (UV) light curves
of the transient show a slow decay over five months, in agreement with previous
optically discovered TDEs. It also has a comparable black-body peak luminosity
of erg/s. The inferred temperature
from the optical and UV data shows a value of (35) K. The
transient is not detected in X-rays up to erg/s within
the first five months after discovery. The optical spectra exhibit two distinct
broad emission lines in the He II region, and at later times also H
emission. Additionally, emission from [N III] and [O III] is detected, likely
produced by the Bowen fluorescence effect. UV spectra reveal broad emission and
absorption lines associated with high-ionization states of N V, C IV, Si IV,
and possibly P V. These features, analogous to those of broad absorption line
quasars (BAL QSOs), require an absorber with column densities cm. This optically thick gas would also explain the
non-detection in soft X-rays. The profile of the absorption lines with the
highest column density material at the largest velocity is opposite that of BAL
QSOs. We suggest that radiation pressure generated by the TDE flare at early
times could have provided the initial acceleration mechanism for this gas.
Spectral UV line monitoring of future TDEs could test this proposal.Comment: 20 pages, 12 figures, published in Ap
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