72 research outputs found
Physical Conditions in Barnard's Loop, Components of the Orion-Eridanus Bubble, and Implications for the WIM Component of the ISM
We have supplemented existing spectra of Barnard's Loop with high accuracy
spectrophotometry of one new position. Cloudy photoionization models were
calculated for a variety of ionization parameters and stellar temperatures and
compared with the observations. After testing the procedure with recent
observations of M43, we establish that Barnard's Loop is photoionized by four
candidate ionizing stars, but agreement between the models and observations is
only possible if Barnard's Loop is enhanced in heavy elements by about a factor
of 1.4. Barnard's Loop is very similar in properties to the brightest
components of the Orion-Eridanus Bubble and the Warm Ionized Medium (WIM). We
are able to establish models that bound the range populated in low-ionization
color-color diagrams (I([SII])/I(H{\alpha}) versus I([NII])/I(H{\alpha})) using
only a limited range of ionization parameters and stellar temperatures.
Previously established variations in the relative abundance of heavy elements
render uncertain the most common method of determining electron temperatures
for components of the Orion-Eridanus Bubble and the WIM based on only the
I([NII])/I(H{\alpha}) ratio, although we confirm that the lowest surface
brightness components of the WIM are on average of higher electron temperature.
The electron temperatures for a few high surface brightness WIM components
determined by direct methods are comparable to those of classical bright H II
regions. In contrast, the low surface brightness HII regions studied by the
Wisconsin H{\alpha} Mapper are of lower temperatures than the classical bright
HII regions
The Chandra ACIS Survey of M33 (ChASeM33): The final source catalog
This study presents the final source catalog of the Chandra ACIS Survey of
M33 (ChASeM33). With a total exposure time of 1.4 Ms, ChASeM33 covers ~70% of
the D25 isophote (R\approx4kpc) of M33 and provides the deepest, most complete,
and detailed look at a spiral galaxy in X-rays. The source catalog includes 662
sources, reaches a limiting unabsorbed luminosity of ~2.4x10^(34) erg/s in the
0.35-8.0keV energy band, and contains source positions, source net counts,
fluxes and significances in several energy bands, and information on source
variability. The analysis challenges posed by ChASeM33 and the techniques
adopted to address these challenges are discussed. To constrain the nature of
the detected X-ray source, hardness ratios were constructed and spectra were
fit for 254 sources, followup MMT spectra of 116 sources were acquired, and
cross-correlations with previous X-ray catalogs and other multi-wavelength data
were generated. Based on this effort, 183 of the 662 ChASeM33 sources could be
identified. Finally, the luminosity function for the detected point sources as
well as the one for the X-ray binaries in M33 is presented. The luminosity
functions in the soft band (0.5-2.0 keV) and the hard band (2.0-8.0 keV) have a
limiting luminosity at the 90% completeness limit of 4.0x10^(34) erg/s and
1.6x10^(35) erg/s (for D=817kpc), respectively, which is significantly lower
than what was reported by previous X-ray binary population studies in galaxies
more distant than M33. The resulting distribution is consistent with a dominant
population of high mass X-ray binaries as would be expected for M33.Comment: 186 pages, 11 figures, 10 tables. Accepted for publication in the
ApJS. For a high resolution version of the paper, see
http://hea-www.harvard.edu/vlp_m33_public
An unusual high-ionization nebula in NGC55
We report on the detection of a previously unknown extended low-density
nebula of very high temperature and excitation, TR001507.7-391206, located
above the midplane of NGC55. The nebular emission line spectrum, in which HeII
is present, is consistent with photoionization by about 3 very hot massive O3
or WR-type stars. There are no indications for shock ionization. The faint blue
optical continuum and XMM-Newton EPIC-pn and OM-data also support our
assumption that this nebula is not a supernova remnant. Galactic nebulae
harboring massive Wolf-Rayet (WR) stars may be appropriate examples, in
particular the highly ionized nebula G2.4+1.4 around WR 102 (aka Sk4 or
LSS4368). The relatively large diameter of the inhomogeneously expanding nebula
in NGC55 of about 58pc, compared to 11pc of G2.4+1.4, would also be consistent
with the number of ionizing stars.Comment: 8 pages, 6 figures, 3 tables, A&A in pres
The Integrated Polarization of Spiral Galaxy Disks
We present integrated polarization properties of nearby spiral galaxies at
4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a
function of inclination. Spiral galaxies in our sample have observed integrated
fractional polarization in the range < 1% to 17.6%. At inclinations less than
50 degrees, the fractional polarization depends mostly on the ratio of random
to regular magnetic field strength. At higher inclinations, Faraday
depolarization associated with the regular magnetic field becomes more
important. The observed degree of polarization is lower (<4%) for more luminous
galaxies, in particular those with L_{4.8} > 2 x 10^{21} W/Hz. The polarization
angle of the integrated emission is aligned with the apparent minor axis of the
disk for galaxies without a bar. In our axially symmetric models, the
polarization angle of the integrated emission is independent of wavelength.
Simulated distributions of fractional polarization for randomly oriented spiral
galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization
measurements, e.g. with the SKA, of unresolved spiral galaxies allow
statistical studies of the magnetic field in disk galaxies using large samples
in the local universe and at high redshift. As these galaxies behave as
idealized background sources without internal Faraday rotation, they can be
used to detect large-scale magnetic fields in the intergalactic medium.Comment: 13 pages, 6 figures; Accepted for publication in The Astrophysical
Journa
Searching for the pulsar in G18.95-1.1: Discovery of an X-ray point source and associated synchrotron nebula with Chandra
Using the Chandra X-ray Observatory, we have pinpointed the location of a
faint X-ray point source (CXOUJ182913.1-125113) and an associated diffuse
nebula in the composite supernova remnant G18.95-1.1. These objects appear to
be the long-sought pulsar and its wind nebula. The X-ray spectrum of the point
source is best described by an absorbed powerlaw model with Gamma=1.6 and an
N_H of ~1x10^(22) cm^(-2). This model predicts a relatively low unabsorbed
X-ray luminosity of about L_X (0.5-8.0keV) = 4.1x10^(31)D_2^2 erg s^(-1), where
D_2 is the distance in units of 2kpc. The best-fitted model of the diffuse
nebula is a combination of thermal (kT = 0.48keV) and non-thermal (1.4 < Gamma
< 1.9) emission. The unabsorbed X-ray luminosity of L_X = 5.4x10^(33)D_2^2 erg
s^(-1) in the 0.5-8keV energy band seems to be largely dominated by the thermal
component from the SNR, providing 87% of L_X in this band. No radio or X-ray
pulsations have been reported for CXOUJ182913.1-125113. If we assume an age of
~5300yr for G18.95-1.1 and use the X-ray luminosity for the pulsar and the wind
nebula together with the relationship between spin-down luminosity (via
magnetic dipole radiation) and period, we estimate the pulsar's period to be P
= 0.4s. Compared to other rotation-powered pulsars, a magnetic field of
2.2x10^(13)G is implied by its location in the P-Pdot diagram, a value which is
close to that of the quantum critical field.Comment: 8 pages, 3 Figures, accepted for publication in Ap
An ultraluminous supersoft source with a 4 hour modulation in NGC 4631
Context. Supersoft X-ray sources (SSSs) are characterised by very low
temperatures (< 100 eV). Classical SSSs have bolometric luminosities in the
range of 10^36-10^38 erg/s and are modelled with steady nuclear burning of
hydrogen on the surfaces of white dwarfs. However, several SSSs have been
discovered with much higher luminosities. Their nature is still unclear. Aims.
We report the discovery of a 4h modulation for an ultraluminous SSS in the
nearby edge-on spiral galaxy NGC 4631, observed with XMM-Newton in 2002 June.
Temporal and spectral analysis of the source is performed. Methods. We use a
Lomb-Scargle periodogram analysis for the period search and evaluate the
confidence level using Monte-Carlo simulations. We measure the source
temperature, flux and luminosity through spectral fitting. Results. A
modulation of 4.2+-0.4 h (3 sigma error) was found for the SSS with a
confidence level >99%. Besides dips observed in the light curve, the flux
decreased by a factor of 3 within ~10h. The spectrum can be described with an
absorbed blackbody model with kT~67eV. The absorbed luminosity in the 0.2-2 kev
energy band was 2.7x10^38 erg/sec while the bolometric luminosity was a hundred
time higher (3.2x10^40 erg/s), making the source one of the most luminous of
its class, assuming the best fit model is correct. Conclusions. This source is
another very luminous SSS for which the standard white dwarf interpretation
cannot be applied, unless a strong beaming factor is considered. A stellar-mass
black hole accreting at a super Eddington rate is a more likely interpretation,
where the excess of accreted matter is ejected through a strong optically-thick
outflow. The 4 h modulation could either be an eclipse from the companion star
or the consequence of a warped accretion disk.Comment: 4 pages, 4 figures, accepted as a Letter in Astronomy & Astrophysic
The magnetic fields of large Virgo cluster spirals: Paper II
The Virgo cluster of galaxies provides excellent conditions for studying
interactions of galaxies with the cluster environment. Both the high-velocity
tidal interactions and effects of ram pressure stripping by the intracluster
gas can be investigated in detail. We extend our systematic search for possible
anomalies in the magnetic field structures of Virgo cluster spirals in order to
characterize a variety of effects and attribute them to different disturbing
agents. Six angularly large Virgo cluster spiral galaxies (NGC4192, NGC4302,
NGC4303, NGC4321, NGC4388, and NGC4535) were targets of a sensitive total power
and polarization study using the 100-m radio telescope in Effelsberg at 4.85GHz
and 8.35GHz (except for NGC4388 observed only at 4.85GHz, and NGC4535 observed
only at 8.35GHz). Magnetic field structures distorted to various extent are
found in all galaxies. Three galaxies (NGC4302, NGC4303, and NGC4321) show some
signs of possible tidal interactions, while NGC4388 and NGC4535 have very
likely experienced strong ram-pressure and shearing effects, respectively,
visible as distortions and asymmetries of polarized intensity distributions. As
in our previous study, even strongly perturbed galaxies closely follow the
radio-far-infrared correlation. In NGC4303 and NGC4321, we observe symmetric
spiral patterns of the magnetic field and in NGC4535 an asymmetric pattern.
Magnetic fields allow us to trace even weak interactions that are difficult to
detect with other observations. Our results show that the degree of distortions
of a galaxy is not a simple function of the distance to the cluster center but
reflects also the history of its interactions. The angle between the velocity
vector and the rotation vector of a galaxy may be a general parameter that
describes the level of distortions of galactic magnetic fields.Comment: 12 pages, 18 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
The warm ionized medium in spiral galaxies
This article reviews observations and models of the diffuse ionized gas that
permeates the disk and halo of our Galaxy and others. It was inspired by a
series of invited talks presented during an afternoon scientific session of the
65th birthday celebration for Professor Carl Heiles held at Arecibo Observatory
in August 2004. This review is in recognition of Carl's long standing interest
in and advocacy for studies of the ionized as well as the neutral components of
the interstellar medium.Comment: 29 pages, 19 figures; accepted by Reviews of Modern Physic
An H-alpha survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new H-alpha imaging survey,
aiming at the detection of extraplanar diffuse ionized gas in halos of
late-type spiral galaxies. We have investigated a sample of 74 nearby edge-on
spirals, covering the northern and southern hemisphere. In 30 galaxies we
detected extraplanar diffuse emission at mean distances of |z|~1-2 kpc.
Individual filaments can be traced out to |z|<= 6 kpc in a few cases. We find a
good correlation between the FIR flux ratio (S_60/S_100) and the SFR per unit
area (L_FIR/D^2_25), based on the detections/non-detections. This is actually
valid for starburst, normal and for quiescent galaxies. A minimal SFR per unit
area for the lowest S_60/S_100 values, at which extended emission has been
detected, was derived, which amounts to (dE/dt)_A25^thres = (3.2+-0.5)x10E+40
erg/s/kpc^2. There are galaxies where extraplanar emission was detected at
smaller values of L_FIR/D^2_25, however, only in combination with a
significantly enhanced dust temperature. The results corroborate the general
view that the gaseous halos are a direct consequence of SF activity in the
underlying galactic disk.Comment: LaTeX, 12 pages, 7 figures, accepted for publication in A&
The Chandra Local Volume Survey: The X-ray Point Source Catalog of NGC 300
We present the source catalog of a new Chandra ACIS-I observation of NGC 300
obtained as part of the Chandra Local Volume Survey. Our 63 ks exposure covers
~88% of the D25 isophote (R~6.3 kpc) and yields a catalog of 95 X-ray point
sources detected at high significance to a limiting unabsorbed 0.35-8 keV
luminosity of ~10^36 erg s^-1. Sources were cross-correlated with a previous
XMM-Newton catalog, and we find 75 "X-ray transient candidate" sources that
were detected by one observatory, but not the other. We derive an X-ray scale
length of 1.7+/-0.2 kpc and a recent star formation rate of 0.12 Msun yr^-1, in
excellent agreement with optical observations. Deep, multi-color imaging from
the Hubble Space Telescope, covering ~32% of our Chandra field, was used to
search for optical counterparts to the X-ray sources, and we have developed a
new source classification scheme to determine which sources are likely X-ray
binaries, supernova remnants, and background AGN candidates. Finally, we
present the X-ray luminosity functions (XLFs) at different X-ray energies, and
we find the total NGC 300 X-ray point source population to be consistent with
other late-type galaxies hosting young stellar populations (<50 Myr). We find
XLF of sources associated with older stellar populations has a steeper slope
than the XLF of X-ray sources coinciding with young stellar populations,
consistent with theoretical predictions.Comment: 23 pages, 13 figures. Accepted to Ap
- …