99 research outputs found
AKARI Far-Infrared Source Counts in the Lockman Hole
We report initial results of far-infrared observations of the Lockman hole
with Far-Infrared Surveyor (FIS) onboard the AKARI infrared satellite. On the
basis of slow scan observations of a 0.6 deg x 1.2 deg contiguous area, we
obtained source number counts at 65, 90 and 140 um down to 77, 26 and 194 mJy
(3 sigma), respectively. The counts at 65 and 140 um show good agreement with
the Spitzer results. However, our 90 um counts are clearly lower than the
predicted counts by recent evolutionary models that fit the Spitzer counts in
all the MIPS bands. Our 90 um counts above 26 mJy account for about 7% of the
cosmic background. These results provide strong constraints on the evolutionary
scenario and suggest that the current models may require modifications.Comment: 25 pages, 8 figures, Publications of the Astronomical Society of
Japan, in pres
Spectral Energy Distributions of Dusty Galaxies
We present a SED model of dusty galaxies, in which the equation of radiative
transfer is solved by assuming spherical symmetry. The temperature fluctuation
of very small dust particles is calculated consistently with the radiative
transfer. The adopted dust model consists of graphite and silicate grains and
PAHs, whose relative fractions are determined for each MW, LMC and SMC type
extinction curve. This model allows us to derive the intrinsic SEDs of stellar
populations embedded in dusty ISM, which are very important indicators for the
age of stellar populations. Therefore, the evolutionary phase of starburst
galaxies which have frequently very dusty ISM can be investigated with this SED
model. We show that the SEDs of Arp220 and M82 can both be explained by the
same single stellar population, despite the significant differences in the SEDs
and the infrared luminosities. The apparent difference between their SEDs is
mainly caused by the difference in the optical depth. In contrast, the SED of
prototypical star-forming ERO, HR10, indicates that this galaxy is relatively
old comparing to Arp220 and M82. It is found that, in the case of optically
thin limit like elliptical galaxies, the optical depth cannot be inferred only
from the SED, due to a degeneracy between the optical depth, galactic size, and
the spatial distribution of dust; the latter two are important for estimating
the average temperature of dust grains in elliptical galaxies. When the
observed size of elliptical galaxies is adopted for the model geometry, SEDs
can be used to constrain the spatial distribution of dust in elliptical
galaxies.Comment: 36 pages, 21 figures, submitted to PAS
Galaxy Colours in the AKARI Deep SEP Survey
We investigate the segregation of the extragalactic population via colour
criteria to produce an efficient and inexpensive methodology to select specific
source populations as a function of far-infrared flux. Combining galaxy
evolution scenarios and a detailed spectral library of galaxies, we produce
simulated catalogues incorporating segregation of the extragalactic population
into component types (Normal, star-forming, AGN) via color cuts. As a practical
application we apply our criteria to the deepest survey to be undertaken in the
far-infrared with the AKARI (formerly ASTRO-F) satellite. Using the
far-infrared wavebands of the Far-Infrared Surveyor (FIS, one of the
focal-plane instruments on AKARI) we successfully segregate the normal,
starburst and ULIRG populations. We also show that with additional MIR imaging
from AKARI's Infrared Camera (IRC), significant contamination and/or degeneracy
can be further decreased and show a particular example of the separation of
cool normal galaxies and cold ULIRG sources. We conclude that our criteria
provide an efficient means of selecting source populations (including rare
luminous objects) and produce colour-segregated source counts without the
requirement of time intensive ground-based follow up to differentiate between
the general galaxy population.Comment: Accepted for publication in Advances in Space Research. COSPAR,
Beijing, 2006, E1.6 Advances in FIR and Submillimeter Astrophysics. (13
pages, 2 colour figures
Age and Dust Degeneracy for Starburst Galaxies Solved?
A spectral evolution model of galaxies that includes both stellar and dust
effects is newly built. xApplying the model to 22 nearby starburst galaxies, we
have shown that far infrared luminosity of galaxies helps to break the
age-dustiness degeneracy. We have derived a unique solution of age and the
dustiness for each starburst galaxy. The resulting starburst ages and optical
depths are in the range and , respectively. The result is robust and is almost independent of model
assumptions such as dust distributions, extinction curves, and burst strengths.
With the rapidly growing sensitivity of submillimeter detectors, it should
become possible in the near future to determine the age and of
star-forming galaxies at redshifts and beyond. Accurate estimates
of for Lyman-break galaxies and high-z galaxies might require a
substantial revision of the previously claimed picture of star formation
history over the Hubble time.Comment: Latex (aas2pp4) 15 pages, 1 table, 6 figures. Accepted for Ap
North Ecliptic Pole Wide Field Survey of AKARI: Survey Strategy and Data Characteristics
We present the survey strategy and the data characteristics of the North
Ecliptic Pole (NEP) Wide Survey of AKARI. The survey was carried out for about
one year starting from May 2006 with 9 passbands from 2.5 to 24 micron and the
areal coverage of about 5.8 sq. degrees centered on NEP. The survey depth
reaches to 21.8 AB magnitude near infrared (NIR) bands, and ~ 18.6 AB
maggnitude at the mid infrared (MIR) bands such as 15 and 18 micron. The total
number of sources detected in this survey is about 104,000, with more sources
in NIR than in the MIR. We have cross matched infrared sources with optically
identified sources in CFHT imaging survey which covered about 2 sq. degrees
within NEP-Wide survey region in order to characterize the nature of infrared
sources. The majority of the mid infrared sources at 15 and 18 micron band are
found to be star forming disk galaxies, with smaller fraction of early type
galaxies and AGNs. We found that a large fraction (60~80 %) of bright sources
in 9 and 11 micron stars while stellar fraction decreases toward fainter
sources. We present the histograms of the sources at mid infrared bands at 9,
11, 15 and 18 micron. The number of sources per magnitude thus varies as m^0.6
for longer wavelength sources while shorter wavelength sources show steeper
variation with m, where m is the AB magnitude.Comment: 18 pages, 11 figures, to appear in PASJ, Vol. 61, No. 2. April 25,
2009 issu
Evolutionary SED diagnostics of starburst galaxies: signature of bimodality
We construct an evolutionary spectral energy distribution (SED) model of a
starburst region, from the ultraviolet to submillimetre wavelengths. This model
allows us to derive the star formation rate, optical depth by dust and apparent
effective radius of starburst regions at various wavelengths; as a result, the
intrinsic surface brightness of starburst regions can be derived. Using this
SED model, we analyse 16 UV-selected starburst galaxies and 10 ultraluminous
infrared galaxies. The derived star formation rates and optical depths are
compared with emission line measurements and found to be consistent. The
derived apparent effective radii are also consistent with observations. From
the SED analysis, we find a bimodal property of the star formation rate with
the optical depth and the compactness of stellar distributions. While mild
starbursts have a limiting intrinsic surface brightness L(bol) x r_e^{-2} ~
10^{12} Lsun kpc^{-2}, intense starbursts tend to be more heavily obscured and
concentrated within a characteristic scale of r_e ~ 0.3 kpc. We suggest that
the mild starbursts can be triggered by a self-gravitating disc instability in
which feedback is effective in the shallow gravitational potential. On the
other hand, the intense starbursts can be induced via an external dynamical
perturbation like galaxy merging, in which feedback is less effective due to
the deep gravitational potential attained by the large gas concentration within
the central starburst region.Comment: 22 pages, 15 figures, accepted for publication in MNRA
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