138 research outputs found
Spatial Distributions of Cold and Warm Interstellar Dust in M101 Resolved with AKARI/Far-Infrared Surveyor (FIS)
The nearby face-on spiral galaxy M101 has been observed with the Far-Infrared
Surveyor (FIS) onboard AKARI. The far-infrared four-band images reveal fine
spatial structures of M101, which include global spiral patterns, giant HII
regions embedded in outer spiral arms, and a bar-like feature crossing the
center. The spectral energy distribution of the whole galaxy shows the presence
of the cold dust component (18 K) in addition to the warm dust component (55
K). The distribution of the cold dust is mostly concentrated near the center,
and exhibits smoothly distributed over the entire extent of the galaxy, whereas
the distribution of the warm dust indicates some correlation with the spiral
arms, and has spotty structures such as four distinctive bright spots in the
outer disk in addition to a bar-like feature near the center tracing the CO
intensity map. The star-formation activity of the giant HII regions that
spatially correspond to the former bright spots is found to be significantly
higher than that of the rest of the galaxy. The latter warm dust distribution
implies that there are significant star-formation activities in the entire bar
filled with molecular clouds. Unlike our Galaxy, M101 is a peculiar normal
galaxy with extraordinary active star-forming regions.Comment: 18 pages, 9 figures, accepted for publication in PASJ AKARI special
issu
Emission from Dust in Galaxies: Metallicity Dependence
Infrared (IR) dust emission from galaxies is frequently used as an indicator
of star formation rate (SFR). However, the effect of the dust-to-gas ratio
(i.e., amount of the dust) on the conversion law from IR luminosity to SFR has
not so far been considered. Then, in this paper, we present a convenient
analytical formula including this effect. In order to obtain the dependence on
the dust-to-gas ratio, we extend the formula derived in our previous paper, in
which a theoretical formula converting IR luminosity to SFR was derived. That
formula was expressed as , where f is
the fraction of ionizing photons absorbed by hydrogen, is the
efficiency of dust absorption for nonionizing photons, is the cirrus
fraction of observed dust luminosity, and is the observed
luminosity of dust emission in the 8-1000-m range. Our formula explains
the IR excess of the Galaxy and the Large Magellanic Cloud. In the current
paper, especially, we present the metallicity dependence of our conversion law
between SFR and . This is possible since both f and can
be estimated via the dust-to-gas ratio, which is related to metallicity. We
have confirmed that the relation between the metallicity and the dust-to-gas
ratio is applied to both giant and dwarf galaxies. Finally, we apply the result
to the cosmic star formation history. We find that the comoving SFR at z=3
calculated from previous empirical formulae is underestimated by a factor of
4-5.Comment: 8 pages LaTeX, to appear in A&
Planetary Formation Scenarios Revistied: Core-Accretion Versus Disk Instability
The core-accretion and disk instability models have so far been used to
explain planetary formation. These models have different conditions, such as
planet mass, disk mass, and metallicity for formation of gas giants. The
core-accretion model has a metallicity condition ([Fe/H] > −1.17 in the
case of G-type stars), and the mass of planets formed is less than 6 times that
of the Jupiter mass MJ. On the other hand, the disk instability model does not
have the metallicity condition, but requires the disk to be 15 times more
massive compared to the minimum mass solar nebulae model. The mass of planets
formed is more than 2MJ. These results are compared to the 161 detected planets
for each spectral type of the central stars. The results show that 90% of the
detected planets are consistent with the core-accretion model regardless of the
spectral type. The remaining 10% are not in the region explained by the
core-accretion model, but are explained by the disk instability model. We
derived the metallicity dependence of the formation probability of gas giants
for the core-accretion model. Comparing the result with the observed fraction
having gas giants, they are found to be consistent. On the other hand, the
observation cannot be explained by the disk instability model, because the
condition for gas giant formation is independent of the metallicity.
Consequently, most of planets detected so far are thought to have been formed
by the core-accretion process, and the rest by the disk instability process.Comment: accepted for publication in The Astrophysical Journa
Mapping of Large Scale 158 micron [CII] Line Emission: Orion A
We present the first results of an observational programme undertaken to map
the fine structure line emission of singly ionized carbon ([CII] 157.7409
micron) over extended regions using a Fabry Perot spectrometer newly installed
at the focal plane of a 100cm balloon-borne far-infrared telescope. This new
combination of instruments has a velocity resolution of ~200 km/s and an
angular resolution of 1.5'. During the first flight, an area of 30'x15' in
Orion A was mapped. The observed [CII] intensity distribution has been compared
with the velocity-integrated intensity distributions of 13CO(1-0), CI(1-0) and
CO(3-2) from the literature. The observed line intensities and ratios have been
analyzed using the PDR models by Kaufman et al. 1999 to derive the incident UV
flux and volume density at a few selected positions.Comment: To appear in Astronomy & Astrophysic
SPICA--A Large Cryogenic Infrared Space Telescope: Unveiling the Obscured Universe
Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in the Milky Way and of galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. Such a facility is essential to address key astrophysical questions, especially concerning galaxy evolution and the development of planetary systems. SPICA is a mission concept aimed at taking the next step in mid- and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultrasensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. Rather than using liquid cryogen, a combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With cooling not dependent on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. The combination of low telescope background and instruments with state-of-the-art detectors means that SPICA can provide a huge advance on the capabilities of previous missions. The SPICA instrument complement offers spectral resolving power ranging from R 50 through 11000 in the 17-230 ?m domain as well as R 28.000 spectroscopy between 12 and 18 ?m. Additionally SPICA will be capable of efficient 30-37 ?m broad band mapping, and small field spectroscopic and polarimetric imaging in the 100-350 ?m range. SPICA will enable far infrared spectroscopy with an unprecedented sensitivity of 5×10?20 W/m2 (5 /1hr) - at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems
A New Galactic Extinction Map of the Cygnus Region
We have made a Galactic extinction map of the Cygnus region with 5' spatial
resolution. The selected area is 80^\circ to 90^\circ in the Galactic longitude
and -4^\circ to 8^\circ in the Galactic latitude. The intensity at 140 \mum is
derived from the intensities at 60 and 100 \mum of the IRAS data using the
tight correlation between 60, 100, and 140 \mum found in the Galactic plane.
The dust temperature and optical depth are calculated with 5' resolution from
the 140 and 100 \mum intensity, and Av is calculated from the optical depth. In
the selected area, the mean dust temperature is 17 K, the minimum is 16 K, and
the maximum is 30 K. The mean Av is 6.5 mag, the minimum is 0.5 mag, and the
maximum is 11 mag. The dust temperature distribution shows significant spatial
variation on smaller scales down to 5'. Because the present study can trace the
5'-scale spatial variation of the extinction, it has an advantage over the
previous studies, such as the one by Schlegel, Finkbeiner, & Davis, who used
the COBE/DIRBE data to derive the dust temperature distribution with a spatial
resolution of 1^\circ. The difference of Av between our map and Schlegel et
al.'s is \pm 3 mag. A new extinction map of the entire sky can be produced by
applying the present method.Comment: 27 pages, 14 figures, accepted for publication in Ap
Wide-Area Mapping of 155 Micron Continuum Emission from the Orion Molecular Cloud Complex
We present the results of a wide-area mapping of the far-infrared continuum
emission toward the Orion complex by using a Japanese balloon-borne telescope.
The 155-um continuum emission was detected over a region of 1.5 deg^2 around
the KL nebula with 3' resolution similar to that of the IRAS 100-um map.
Assuming a single-temperature model of the thermal equilibrium dust, maps of
the temperature and the optical depth were derived from the 155 um intensity
and the IRAS 100 um intensity. The derived dust temperature is 5 - 15 K lower
and the derived dust optical thickness were derived from the 155-um intensity
and the IRAS 100-um intensity. The derived dust temperature is 5 - 15 K lower
and the derived dust optical depth is 5 - 300 times larger than those derived
from the IRAS 60 and 100-um intensities due to the significant contribution of
the statistically heated very small grains to the IRAS 60-um intensity. The
optical-thickness distribution shows a filamentary dust ridge that has a 1.5
degrees extent in the north - south direction and well resembles the
Integral-Shaped Filament (ISF) molecular gas distribution. The gas-to-dust
ratio derived from the CO molecular gas distribution along the ISF is in the
range 30 - 200, which may be interpreted as being an effect of CO depletion due
to the photodissociation and/or the freezing on dust grains.Comment: 23 pages, 7 figures, 1 table, to appear in PASJ, Vol. 56, No.
Far-infrared all sky diffuse mapping with AKARI
We discuss the capability of AKARI in recovering diffuse far-infrared
emission, and examine the achieved reliability. Critical issues in making
images of diffuse emission are the transient response and long-term stability
of the far-infrared detectors. Quantitative evaluation of these characteristics
are the key to achieving sensitivity comparable to or better than that for
point sources (< 20 -- 95 MJy sr-1). We describe current activity and progress
toward the production of high quality images of the diffuse far-infrared
emission using the AKARI all-sky survey data.Comment: 4 pages, 8 figures, to appear in the Proceedings of the Conference
"AKARI, a light to illuminate the misty Universe", Fukutake Hall, The
University of Tokyo, Japan, 16-19 February 200
Observing H2 Emission in Forming Galaxies
We study the H2 cooling emission of forming galaxies, and discuss their
observability using the future infrared facility SAFIR. Forming galaxies with
mass >10^11 Msun emit most of their gravitational energy liberated by
contraction in molecular hydrogen line radiation, although a large part of
thermal energy at virialization is radiated away by the H Ly alpha emission.
For more massive objects, the degree of heating due to dissipation of kinetic
energy is so great that the temperature does not drop below 10^4 K and the
gravitational energy is emitted mainly by the Ly alpha emission. Therefore, the
total H2 luminosity attains the peak value of about 10^42 ergs/s for forming
galaxies whose total mass 10^11 Msun. If these sources are situated at redshift
z=8, they can be detected by rotational lines of 0-0S(3) at 9.7 micron and
0-0S(1) at 17 micron by SAFIR. An efficient way to find such H2 emitters is to
look at the Ly alpha emitters, since the brightest H2 emitters are also
luminous in the Ly alpha emission.Comment: 20 pages, 7 figures, ApJ accepte
AKARI Far-Infrared All Sky Survey
We demonstrate the capability of AKARI for mapping diffuse far-infrared
emission and achieved reliability of all-sky diffuse map. We have conducted an
all-sky survey for more than 94 % of the whole sky during cold phase of AKARI
observation in 2006 Feb. -- 2007 Aug. The survey in far-infrared waveband
covers 50 um -- 180 um with four bands centered at 65 um, 90 um, 140 um, and
160 um and spatial resolution of 3000 -- 4000 (FWHM).This survey has allowed us
to make a revolutionary improvement compared to the IRAS survey that was
conducted in 1983 in both spatial resolution and sensitivity after more than a
quarter of a century. Additionally, it will provide us the first all-sky survey
data with high-spatial resolution beyond 100 um. Considering its extreme
importance of the AKARI far-infrared diffuse emission map, we are now
investigating carefully the quality of the data for possible release of the
archival data. Critical subjects in making image of diffuse emission from
detected signal are the transient response and long-term stability of the
far-infrared detectors. Quantitative evaluation of these characteristics is the
key to achieve sensitivity comparable to or better than that for point sources
(< 20 -- 95 [MJy/sr]). We describe current activities and progress that are
focused on making high quality all-sky survey images of the diffuse
far-infrared emission.Comment: To appear in Proc. Workshop "The Space Infrared Telescope for
Cosmology & Astrophysics: Revealing the Origins of Planets and Galaxies".
Eds. A.M. Heras, B. Swinyard, K. Isaak, and J.R. Goicoeche
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