19 research outputs found
High resolution radio continuum survey of M33: I. The radio maps
We study the exponential scale length of total radio emission, the spectral
index distribution, and the linear radio polarization in the Scd galaxy M33.
Observations were carried out using the 3.6 cm dual channel and the 6.2 cm four
channel receivers of the 100--m Effelsberg telescope along with the L-band VLA
D--array at 20 cm. High spatial resolution and sensitivity in both total and
linearly polarized radio continuum emission from M33 were achieved. We found
considerable extended emission, not only from the main arms I S and I N, but
also from the weaker arms. The large--scale magnetic field exhibits
well--ordered spiral structure with almost the same orientation as that of the
optical spiral arms, however, it does not show a clear structural correlation
or anti--correlation with the optical arms. There is a north-south asymmetry in
polarization that is frequency-dependent. We found that the ring mean spectral
index versus radius increases faster beyond = 4 kpc. At each wavelength,
the exponential scale length is larger inside than outside = 4 kpc. From
the larger scales lengths at 4 kpc, we conclude that star forming
regions are mainly spread over the region 4 kpc without a dominant
nuclear concentration. Furthermore, at 4 kpc, a spatial correlation
between cosmic rays and star forming regions may exist. From the behaviour of
the mean spectral indices obtained from different pairs of the radio continuum
data at 3.6, 6.2, and 20 cm, we confirm that a decrease in the thermal fraction
causes an increase in the spectral index. The frequency-dependent asymmetry in
the polarization hints to an asymmetry in Faraday depolarization.Comment: 16 pages, 13 figures, accepted for publication in the Astronomy and
Astrophysics Journa
High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission
We determine the variation in the nonthermal radio spectral index in the
nearby spiral galaxy M33 at a linear resolution of 360 pc. We separate the
thermal and nonthermal components of the radio continuum emission without the
assumption of a constant nonthermal spectral index. Using the Spitzer FIR data
at 70 and 160 m and a standard dust model, we deredden the H
emission. The extinction corrected H emission serves as a template for
the thermal free-free radio emission. Subtracting from the observed 3.6 cm and
20 cm emission (Effelsberg and the VLA) this free-free emission, we obtain the
nonthermal maps. A constant electron temperature used to obtain the thermal
radio intensity seems appropriate for M~33 which, unlike the Milky Way, has a
shallow metallicity gradient. For the first time, we derive the distribution of
the nonthermal spectral index across a galaxy, M33. We detect strong nonthermal
emission from the spiral arms and star-forming regions. Wavelet analysis shows
that at 3.6 cm the nonthermal emission is dominated by contributions from
star-forming regions, while it is smoothly distributed at 20 cm. For the whole
galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm,
respectively. The thermal emission is slightly stronger in the southern than in
the northern half of the galaxy. We find a clear radial gradient of mean
extinction in the galactic plane. The nonthermal spectral index map indicates
that the relativistic electrons suffer energy-loss when diffusing from their
origin in star-forming regions towards interarm regions and the outer parts of
the galaxy. We also conclude that the radio emission is mostly nonthermal at R
5 kpc in M33.Comment: 15 pages, 14 figures, accepted for publication in the Astronomy and
Astrophysics journa
Aperture Synthesis Observations of CO, HCN, and 89GHz Continuum Emission toward NGC 604 in M 33: Sequential Star Formation Induced by Supergiant Hii region
We present the results from new Nobeyama Millimeter Array observations of
CO(1-0), HCN(1-0), and 89-GHz continuum emissions toward NGC 604, known as the
supergiant H ii region in a nearby galaxy M 33. Our high spatial resolution
images of CO emission allowed us to uncover ten individual molecular clouds
that have masses of (0.8 -7.4) 10M_{\sun } and sizes of 5 -- 29 pc,
comparable to those of typical Galactic giant molecular clouds (GMCs).
Moreover, we detected for the first time HCN emission in the two most massive
clouds and 89 GHz continuum emission at the rims of the "H shells".
Three out of ten CO clouds are well correlated with the H shells both
in spatial and velocity domains, implying an interaction between molecular gas
and the expanding H ii region. Furthermore, we estimated star formation
efficiencies (SFEs) for each cloud from the 89-GHz and combination of
H and 24-m data, and found that the SFEs decrease with
increasing projected distance measured from the heart of the central OB star
cluster in NGC 604, suggesting the radial changes in evolutionary stages of the
molecular clouds in course of stellar cluster formation. Our results provide
further support to the picture of sequential star formation in NGC604 initially
proposed by Tosaki et al. (2007) with the higher spatially resolved molecular
clouds, in which an isotropic expansion of the H ii region pushes gases outward
and accumulates them to consecutively form dense molecular clouds, and then
induces massive star formations.Comment: 23 pages, 8 figures, accepted for publication in Ap
High-resolution radio continuum survey of M33: III. Magnetic fields
Using the linearly polarized intensity and polarization angle data at 3.6,
6.2 and 20 cm, we determine variations of Faraday rotation and depolarization
across the nearby galaxy M33. A 3-D model of the regular magnetic field is
fitted to the observed azimuthal distribution of polarization angles. Faraday
rotation, measured between 3.6 and 6.2 cm at a linear resolution of 0.7 kpc,
shows more variation in the south than in the north of the galaxy. About 10% of
the nonthermal emission from M33 at 3.6 cm is polarized. We estimate the
average total and regular magnetic field strengths in M33 as ~ 6.4 and 2.5
G, respectively. Under the assumption that the disk of M33 is flat, the
regular magnetic field consists of horizontal and vertical components: however
the inferred vertical field may be partly due to a galactic warp. The
horizontal field is represented by an axisymmetric (m=0) mode from 1 to 3 kpc
radius and a superposition of axisymmetric and bisymmetric (m=0+1) modes from 3
to 5 kpc radius. An excess of differential Faraday rotation in the southern
half together with strong Faraday dispersion in the southern spiral arms seem
to be responsible for the north-south asymmetry in the observed wavelength
dependent depolarization. The presence of an axisymmetric m=0 mode of the
regular magnetic field in each ring suggests that a galactic dynamo is
operating in M33. The pitch angles of the spiral regular magnetic field are
generally smaller than the pitch angles of the optical spiral arms but are
twice as big as simple estimates based on the mean-field dynamo theory and
M33's rotation curve. Generation of interstellar magnetic fields from turbulent
gas motions in M33 is indicated by the equipartition of turbulent and magnetic
energy densities.Comment: 17 pages, 12 figures, accepted for Astronomy and Astrophysics
publicatio
The spiral galaxy M33 mapped in the FIR by ISOPHOT: A spatially resolved study of the warm and cold dust
The Sc galaxy M33 has been mapped with ISOPHOT in the far-infrared, at 60,
100, and 170mue. The spatial resolution of these FIR maps allows the separation
of spiral arms and interarm regions and the isolation of a large number of
star-forming regions. The spectral energy distribution in the FIR indicates a
superposition of two components, a warm one originating from dust at ~45K, and
a cold one, at ~16K. The warm component is concentrated towards the spiral arms
and the star-forming regions, and is likely heated by the UV radiation from OB
stars. The cold component is more smoothly distributed over the disk, and
heated by the diffuse interstellar radiation. For the about 60 star-forming
regions detected the H-alpha/FIR flux ratio increases significantly with the
distance from the galaxy center, probably due to decreasing extinction. An
anti-correlation of F_Ha/F_60 with F_170 suggests the intrinsic extinction to
be related to the cold dust surface brightness according to
A_V/S_170~0.03mag/MJy*sr. For the total galaxy the star formation rate (SFR)
derived from the FIR is in agreement with that derived from the de-extincted
H-alpha emission. For individual star-forming regions, a consistency between
SFRs derived from the optical and from the FIR requires only a fraction of the
UV radiation to be absorbed locally. The individual star-forming regions also
show a local radio-FIR correlation. This local correlation is, however, due to
quite different components than to those that lead to the well-known global
radio-FIR correlation for entire galaxies.Comment: 10 pages, 9 figures; the full resolution images of Fig.1 can be
obtained by anonymous ftp from ftp.mpia.de /pub/iso-arc
The Hard X-ray Luminosity of OB Star Populations: Implications for the Contribution of Star Formation to the Cosmic X-ray Background
We present an empirical analysis of the integrated X-ray luminosity arising
from populations of OB stars. In particular, we utilize results from the
All-Sky Monitor on RXTE, along with archival data from previous missions, to
assess the mean integrated output of X-rays in the 2-10 keV band from accreting
early-type binaries within 3 kpc of the Sun. Using a recent OB star census of
the Solar neighborhood, we then calculate the specific X-ray luminosity per O
star from accretion-powered systems. We also assess the contribution to the
total X-ray luminosity of an OB population from associated T Tauri stars,
stellar winds, and supernovae. We repeat this exercise for the major Local
Group galaxies, concluding that the total X-ray luminosity per O star spans a
broad range from 2 to 20e34 erg/s. Contrary to previous results, we do not find
a consistent trend with metallicity; in fact, the specific luminosities for M31
and the SMC are equal, despite having metallicities which differ by an order of
magnitude. In light of these results, we assess the fraction of the observed
2-10 keV emission from starburst galaxies that arises directly from their OB
star populations, concluding that, while binaries can explain most of the hard
X-ray emission in many local starbursts, a significant additional component or
components must be present in some systems. A discussion of the nature of this
additional emission, along with its implications for the contribution of
starbursts to the cosmic X-ray background, concludes our report.Comment: aastex, 30 pages including 2 tables and 1 figure. To appear in Ap
The Magnetized Universe
Cosmology, high-energy physics and astrophysics are converging on the study
of large-scale magnetic fields. While the experimental evidence for the
existence of large-scale magnetization in galaxies, clusters and superclusters
is rather compelling, the origin of the phenomenon remains puzzling especially
in light of the most recent observations. The purpose of the present review is
to describe the physical motivations and some of the open theoretical problems
related to the existence of large-scale magnetic fields.Comment: 147 pages, 10 included figures. Few corrected typos and added
reference
Astrophysical magnetic fields and nonlinear dynamo theory
The current understanding of astrophysical magnetic fields is reviewed,
focusing on their generation and maintenance by turbulence. In the
astrophysical context this generation is usually explained by a self-excited
dynamo, which involves flows that can amplify a weak 'seed' magnetic field
exponentially fast. Particular emphasis is placed on the nonlinear saturation
of the dynamo. Analytic and numerical results are discussed both for small
scale dynamos, which are completely isotropic, and for large scale dynamos,
where some form of parity breaking is crucial. Central to the discussion of
large scale dynamos is the so-called alpha effect which explains the generation
of a mean field if the turbulence lacks mirror symmetry, i.e. if the flow has
kinetic helicity. Large scale dynamos produce small scale helical fields as a
waste product that quench the large scale dynamo and hence the alpha effect.
With this in mind, the microscopic theory of the alpha effect is revisited in
full detail and recent results for the loss of helical magnetic fields are
reviewed.Comment: 285 pages, 72 figures, accepted by Phys. Re
Radiokontinuumsstrahlung der Spiralgalaxie M33
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