186 research outputs found
Evidence for an Additional Heat Source in the Warm Ionized Medium of Galaxies
Spatial variations of the [S II]/H-Alpha and [N II]/H-Alpha line intensity
ratios observed in the gaseous halo of the Milky Way and other galaxies are
inconsistent with pure photoionization models. They appear to require a
supplemental heating mechanism that increases the electron temperature at low
densities n_e. This would imply that in addition to photoionization, which has
a heating rate per unit volume proportional to n_e^2, there is another source
of heat with a rate per unit volume proportional to a lower power of n_e. One
possible mechanism is the dissipation of interstellar plasma turbulence, which
according to Minter & Spangler (1997) heats the ionized interstellar medium in
the Milky Way at a rate ~ 1x10^-25 n_e ergs cm^-3 s^-1. If such a source were
present, it would dominate over photoionization heating in regions where n_e <
0.1 cm^-3, producing the observed increases in the [S II]/H-Alpha and [N
II]/H-Alpha intensity ratios at large distances from the galactic midplane, as
well as accounting for the constancy of [S II]/[N II], which is not explained
by pure photoionization. Other supplemental heating sources, such as magnetic
reconnection, cosmic rays, or photoelectric emission from small grains, could
also account for these observations, provided they supply to the warm ionized
medium ~ 10^-5 ergs s^-1 per cm^2 of Galactic disk.Comment: 10 pages, 1 figur
WHAM Observations of H-Alpha, [S II], and [N II] toward the Orion and Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium
A large portion of the Galaxy (l = 123 deg to 164 deg, b = -6 deg to -35
deg), which samples regions of the Local (Orion) spiral arm and the more
distant Perseus arm, has been mapped with the Wisconsin H-Alpha Mapper (WHAM)
in the H-Alpha, [S II] 6716, and [N II] 6583 lines. Several trends noticed in
emission-line investigations of diffuse gas in other galaxies are confirmed in
the Milky Way and extended to much fainter emission. We find that the [S
II]/H-Alpha and [N II]/H-Alpha ratios increase as absolute H-Alpha intensities
decrease. For the more distant Perseus arm emission, the increase in these
ratios is a strong function of Galactic latitude and thus, of height above the
Galactic plane. The [S II]/[N II] ratio is relatively independent of H-Alpha
intensity. Scatter in this ratio appears to be physically significant, and maps
of it suggest regions with similar ratios are spatially correlated. The Perseus
arm [S II]/[N II] ratio is systematically lower than Local emission by 10%-20%.
With [S II]/[N II] fairly constant over a large range of H-Alpha intensities,
the increase of [S II]/H-Alpha and [N II]/H-Alpha with |z| seems to reflect an
increase in temperature. Such an interpretation allows us to estimate the
temperature and ionization conditions in our large sample of observations. We
find that WIM temperatures range from 6,000 K to 9,000 K with temperature
increasing from bright to faint H-Alpha emission (low to high [S II]/H-Alpha
and [N II]/H-Alpha) respectively. Changes in [S II]/[N II] appear to reflect
changes in the local ionization conditions (e.g. the S+/S++ ratio). We also
measure the electron scale height in the Perseus arm to be 1.0+/-0.1 kpc,
confirming earlier, less accurate determinations.Comment: 28 pages, 10 figures. Figures 2 and 3 are full color--GIFs provided
here, original PS figures at link below. Accepted for publication in ApJ.
More information about the WHAM project can be found at
http://www.astro.wisc.edu/wham/ . REVISION: Figure 6, bottom panel now
contains the proper points. No other changes have been mad
The Origin of the Dust Arch in the Halo of NGC 4631: An Expanding Superbubble?
We study the nature and the origin of the dust arch in the halo of the
edge-on galaxy NGC 4631 detected by Neininger & Dumke (1999). We present CO
observations made using the new On-The-Fly mapping mode with the FCRAO 14m
telescope, and find no evidence for CO emission associated with the dust arch.
Our examination of previously published HI data shows that if previous
assumptions about the dust temperature and gas/dust ratio are correct, then
there must be molecular gas associated with the arch, below our detection
threshold. If this is true, then the molecular mass associated with the dust
arch is between 1.5 x 10^8 M(sun)and 9.7 x 10^8 M(sun), and likely towards the
low end of the range. A consequence of this is that the maximum allowed value
for the CO-to-H_2 conversion factor is 6.5 times the Galactic value, but most
likely closer to the Galactic value. The kinematics of the HI apparently
associated with the dust arch reveal that the gas here is not part of an
expanding shell or outflow, but is instead two separate features (a tidal arm
and a plume of HI sticking out into the halo) which are seen projected together
and appear as a shell. Thus there is no connection between the dust "arch" and
the hot X-ray emitting gas that appears to surround the galaxy Wang et al.
(2001).Comment: 14 pages, including 4 figures. Accepted by A.J. for March 200
The Wisconsin H-Alpha Mapper Northern Sky Survey
The Wisconsin H-Alpha Mapper (WHAM) has surveyed the distribution and
kinematics of ionized gas in the Galaxy above declination -30 degrees. The WHAM
Northern Sky Survey (WHAM-NSS) has an angular resolution of one degree and
provides the first absolutely-calibrated, kinematically-resolved map of the
H-Alpha emission from the Warm Ionized Medium (WIM) within ~ +/-100 km/s of the
Local Standard of Rest. Leveraging WHAM's 12 km/s spectral resolution, we have
modeled and removed atmospheric emission and zodiacal absorption features from
each of the 37,565 spectra. The resulting H-Alpha profiles reveal ionized gas
detected in nearly every direction on the sky with a sensitivity of 0.15 R (3
sigma). Complex distributions of ionized gas are revealed in the nearby spiral
arms up to 1-2 kpc away from the Galactic plane. Toward the inner Galaxy, the
WHAM-NSS provides information about the WIM out to the tangent point down to a
few degrees from the plane. Ionized gas is also detected toward many
intermediate velocity clouds at high latitudes. Several new H II regions are
revealed around early B-stars and evolved stellar cores (sdB/O). This work
presents the details of the instrument, the survey, and the data reduction
techniques. The WHAM-NSS is also presented and analyzed for its gross
properties. Finally, some general conclusions are presented about the nature of
the WIM as revealed by the WHAM-NSS.Comment: 42 pages, 14 figures (Fig 6-9 & 14 are full color); accepted for
publication in 2003, ApJ, 149; Original quality figures (as well as data for
the survey) are available at http://www.astro.wisc.edu/wham
Optical Spectroscopy of Diffuse Ionized Gas in M31
We have obtained sensitive long-slit spectra of Diffuse Ionized Gas (DIG) in
the Andromeda Galaxy, M31, covering the wavelength range of 3550-6850 Angs. By
co-adding extracted DIG spectra, we reached a 1 sigma uncertainty of 9.3E-19
ergs/s/cm^{2}/arcsec^{2} corresponding to .46 pc/cm^{6} in Emission Measure. We
present average spectra of DIG at four brightness levels with Emission Measures
ranging from 9 to 59 pc/cm^{6}. We present the first measurements of
[OII]\lambda3727 and [OIII]\lambda5007 of the truly diffuse ionized medium in
the disk of an external spiral galaxy. We find that I_[OII]/I_H\alpha=.9-1.4.
The [OIII] line is weak (I_[OIII]/I_H\beta = .5), but stronger than in the
Galactic DIG. Measurements of [NII]\lambda6583 and
[SII](\lambda6717+\lambda6731) are also presented. The [SII] lines are clearly
stronger than typical HII regions (I_[SII]/I_H\alpha = .5 compared to .2).
Overall, the line ratios are in agreement with predictions of photoionization
models for diffuse gas exposed to a dilute stellar radiation field, but the
line ratios of the DIG in M31 are somewhat different than observed for Galactic
DIG. The differences indicate a less diluted radiation field in the DIG of
M31's spiral arms compared to DIG in the Solar Neighborhood of the Milky Way.
We have also detected HeI\lambda5876 emission from the brightest DIG in M31.
The HeI line appears to be stronger than in the Galactic DIG, possibly
indicating that most of the Helium in the bright DIG in M31 is fully ionized.
However, this result is somewhat tentative.Comment: Fig. 5 corrected and other minor changes. Paper accepted to ApJ. 21
pages, Latex, incl. 5 fig. & 1 tab., submitted to Ap
Characterisation of nanoparticles by means of high-resolution SEM/EDS in transmission mode
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd
Extraplanar Emission-Line Gas in Edge-On Spiral Galaxies. II. Optical Spectroscopy
The results from deep long-slit spectroscopy of nine edge-on spiral galaxies
with known extraplanar line emission are reported. Emission from Halpha, [N II]
lambda 6548, 6583, and [S II] lambda 6716, 6731 is detected out to heights of a
few kpc in all of these galaxies. Several other fainter diagnostic lines such
as [O I] lambda 6300, [O III] lambda 4959, 5007, and He I lambda 5876 are also
detected over a smaller scale. The relative strengths, centroids and widths of
the various emission lines provide constraints on the electron density,
temperature, reddening, source(s) of ionization, and kinematics of the
extraplanar gas. In all but one galaxy, photoionization by massive OB stars
alone has difficulties explaining all of the line ratios in the extraplanar
gas. Hybrid models that combine photoionization by OB stars and another source
of ionization such as photoionization by turbulent mixing layers or shocks
provide a better fit to the data. The (upper limits on the) velocity gradients
measured in these galaxies are consistent with the predictions of the galactic
fountain model to within the accuracy of the measurements.Comment: 25 pages + several jpg figures. Accepted for publication in the
Astrophysical Journal, Vol. 592, July 20 200
A high spatial resolution X-ray and H-alpha study of hot gas in the halos of star-forming disk galaxies. I. Spatial and spectral properties of the diffuse X-ray emission
We present arcsecond resolution Chandra X-ray and ground-based optical
H-alpha imaging of a sample of ten edge-on star-forming disk galaxies (seven
starburst and three ``normal'' spiral galaxies), a sample which covers the full
range of star-formation intensity found in disk galaxies. We use the
unprecedented spatial resolution of the Chandra X-ray observatory to robustly
remove point sources, and hence obtain the X-ray properties of the diffuse
thermal emission alone. The X-ray observations are combined with
comparable-resolution H-alpha and R-band imaging, and presented as a mini-atlas
of images on a common spatial and surface brightness scale. The vertical
distribution of the halo-region X-ray surface brightness is best described as
an exponential, with the observed scale heights lying in the range H_eff = 2 --
4 kpc. The ACIS X-ray spectra of extra-planar emission from all these galaxies
can be fit with a common two-temperature spectral model with an enhanced
alpha-to-iron element ratio. This is consistent with the origin of the X-ray
emitting gas being either metal-enriched merged SN ejecta or shock-heated
ambient halo or disk material with moderate levels of metal depletion onto
dust. The thermal X-ray emission observed in the halos of the starburst
galaxies is either this pre-existing halo medium, which has been swept-up and
shock heated by the starburst-driven wind, or wind material compressed near the
walls of the outflow by reverse shocks within the wind. In either case the
X-ray emission provides us with a powerful probe of the properties of gaseous
halos around star-forming disk galaxies.Comment: To appear in April 2004 edition of ApJS. For high resolution version,
see http://proteus.pha.jhu.edu/~dks/ Accepted version, now has nuclear and
total diffuse emission fluxes and luminosities, a few other minor change
The P2X(7) receptor tracer [C-11]SMW139 as an in vivo marker of neuroinflammation in multiple sclerosis: a first-in man study
Purpose:
The novel PET tracer [11C]SMW139 binds with high affinity to the P2X7 receptor, which is expressed on pro-inflammatory microglia. The purposes of this first in-man study were to characterise pharmacokinetics of [11C]SMW139 in patients with active relapsing remitting multiple sclerosis (RRMS) and healthy controls (HC) and to evaluate its potential to identify in vivo neuroinflammation in RRMS. /
Methods:
Five RRMS patients and 5 age-matched HC underwent 90-min dynamic [11C]SMW139 PET scans, with online continuous and manual arterial sampling to generate a metabolite-corrected arterial plasma input function. Tissue time activity curves were fitted to single- and two-tissue compartment models, and the model that provided the best fits was determined using the Akaike information criterion. /
Results:
The optimal model for describing [11C]SMW139 kinetics in both RRMS and HC was a reversible two-tissue compartment model with blood volume parameter and with the dissociation rate k4 fixed to the whole-brain value. Exploratory group level comparisons demonstrated an increased volume of distribution (VT) and binding potential (BPND) in RRMS compared with HC in normal appearing brain regions. BPND in MS lesions was decreased compared with non-lesional white matter, and a further decrease was observed in gadolinium-enhancing lesions. In contrast, increased VT was observed in enhancing lesions, possibly resulting from disruption of the blood-brain barrier in active MS lesions. In addition, there was a high correlation between parameters obtained from 60- to 90-min datasets, although analyses using 60-min data led to a slight underestimation in regional VT and BPND values. /
Conclusions:
This first in-man study demonstrated that uptake of [11C]SMW139 can be quantified with PET using BPND as a measure for specific binding in healthy controls and RRMS patients. Additional studies are warranted for further clinical evaluation of this novel neuroinflammation tracer
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