18 research outputs found
Cosmic Star Formation Activity at z=2.2 Probed by H-alpha Emission Line Galaxies
We present a pilot narrow-band survey of H-alpha emitters at z=2.2 in the
Great Observatories Origins Deep Survey North (GOODS-N) field with MOIRCS
instrument on the Subaru telescope. The survey reached a 3 sigma limiting
magnitude of 23.6 (NB209) which corresponds to a 3 sigma limiting line flux of
2.5 x 10^-17 erg s^-1 cm^-2 over a 56 arcmnin^2 contiguous area (excluding a
shallower area). From this survey, we have identified 11 H-alpha emitters and
one AGN at z=2.2 on the basis of narrow-band excesses and photometric
redshifts. We obtained spectra for seven new objects among them, including one
AGN, and an emission line above 3 sigma is detected from all of them. We have
estimated star formation rates (SFR) and stellar masses (M_star) for individual
galaxies. The average SFR and M_star is 27.8M_solar yr^-1 and 4.0 x
10^10M_solar, respectivly. Their specific star formation rates are inversely
correlated with their stellar masses. Fitting to a Schechter function yields
the H-alpha luminosity function with log L = 42.82, log phi = -2.78 and alpha =
-1.37. The average star formation rate density in the survey volume is
estimated to be 0.31M_solar yr^-1Mpc^-3 according to the Kennicutt relation
between H-alpha luminosity and star formation rate. We compare our H-alpha
emitters at z=2.2 in GOODS-N with narrow-band line emitters in other field and
clusters to see their time evolution and environmental dependence. We find that
the star formation activity is reduced rapidly from z=2.5 to z=0.8 in the
cluster environment, while it is only moderately changed in the field
environment. This result suggests that the timescale of galaxy formation is
different among different environments, and the star forming activities in high
density regions eventually overtake those in lower density regions as a
consequence of "galaxy formation bias" at high redshifts.Comment: Accepted for publication in PASJ Subaru Special Issue, 11 pages, 10
figure
Evaluation of heat extraction through sapphire fibers for the GW observatory KAGRA
Currently, the Japanese gravitational wave laser interferometer KAGRA is
under construction in the Kamioka mine. As one main feature, it will employ
sapphire mirrors operated at a temperature of 20K to reduce the impact from
thermal noise. To reduce seismic noise, the mirrors will also be suspended from
multi-stage pendulums. Thus the heat load deposited in the mirrors by
absorption of the circulating laser light as well as heat load from thermal
radiation will need to be extracted through the last suspension stage. This
stage will consist of four thin sapphire fibers with larger heads necessary to
connect the fibers to both the mirror and the upper stage. In this paper, we
discuss heat conductivity measurements on different fiber candidates. While all
fibers had a diameter of 1.6mm, different surface treatments and approaches to
attach the heads were analyzed. Our measurements show that fibers fulfilling
the basic KAGRA heat conductivity requirement of 5000W/m/K at 20K
are technologically feasible.Comment: 11 pages, 4 figure
MOIRCS Deep Survey. VII: NIR Morphologies of Star-forming Galaxies at Redshift z~1
We investigate rest-frame near-infrared (NIR) morphologies of a sample of 139
galaxies with M_{s} >= 1 x 10^{10} M_{sun} at z=0.8-1.2 in the GOODS-North
field using our deep NIR imaging data (MOIRCS Deep Survey, MODS). We focus on
Luminous Infrared Galaxies (LIRGs), which dominate high star formation rate
(SFR) density at z~1, in the sample identified by cross-correlating with the
Spitzer/MIPS 24um source catalog. We perform two-dimensional light profile
fitting of the z~1 galaxies in the Ks-band (rest-frame J-band) with a single
component Sersic model. We find that at z~1, ~90% of LIRGs have low Sersic
indices (n<2.5, similar to disk-like galaxies) in the Ks-band, and those
disk-like LIRGs consist of ~60% of the whole disk-like sample above M_{s} >= 3
x 10^{10} M_{sun}. The z~1 disk-like LIRGs are comparable or ~20% small at a
maximum in size compared to local disk-like galaxies in the same stellar mass
range. If we examine rest-frame UV-optical morphologies using the HST/ACS
images, the rest-frame B-band sizes of the z~1 disk-like galaxies are
comparable to those of the local disk-like galaxies as reported by previous
studies on size evolution of disk-like galaxies in the rest-frame optical band.
Measuring color gradients (galaxy sizes as a function of wavelength) of the z~1
and local disk-like galaxies, we find that the z~1 disk-like galaxies have 3-5
times steeper color gradient than the local ones. Our results indicate that (i)
more than a half of relatively massive disk-like galaxies at z~1 are in violent
star formation epochs observed as LIRGs, and also (ii) most of those LIRGs are
constructing their fundamental disk structure vigorously. The high SFR density
in the universe at z~1 may be dominated by such star formation in disk region
in massive galaxies.Comment: 16 pages, 15 figures, accepted for publication in PASJ. Catalog data
will be available at http://astr.tohoku.ac.jp/MODS/wiki/index.php soo
Assembly of Massive Galaxies in a High-z Protocluster
We present the results of wide-field deep JHK imaging of the SSA22 field
using MOIRCS instrument equipped with Subaru telescope. The observed field is
112 arcmin^2 in area, which covers the z=3.1 protocluster characterized by the
overdensities of Ly Alpha emitters (LAEs) and Ly Alpha Blobs (LABs). The 5
sigma limiting magnitude is K_{AB} = 24.3. We extract the potential
protocluster members from the K-selected sample by using the multi-band
photometric-redshift selection as well as the simple color cut for distant red
galaxies (DRGs; J-K_{AB}>1.4). The surface number density of DRGs in our
observed fields shows clear excess compared with those in the blank fields, and
the location of the densest area whose projected overdensity is twice the
average coincides with the large-scale density peak of LAEs. We also found that
K-band counterparts with z_{phot} = 3.1 are detected for 75% (15/20) of the
LABs within their Ly Alpha halo, and the 40 % (8/20) of LABs have multiple
components, which gives a direct evidence of the hierarchical multiple merging
in galaxy formation. The stellar mass ofLABs correlates with their luminosity,
isophotal area, and the Ly Alpha velocity widths, implying that the physical
scale and the dynamical motion of Ly Alpha emission are closely related to
their previous star-formation activities. Highly dust-obscured galaxies such as
hyper extremely red objects (HEROs; J-K_{AB}>2.1) and plausible K-band
counterparts of submillimeter sources are also populated in the high density
region.Comment: 21pages, accepted for publication in Astrophysical Journa
MOIRCS Deep Survey. I: DRG Number Counts
We use very deep near-infrared imaging data taken with Multi-Object InfraRed
Camera and Spectrograph (MOIRCS) on the Subaru Telescope to investigate the
number counts of Distant Red Galaxies (DRGs). We have observed a 4x7 arcmin^2
field in the Great Observatories Origins Deep Survey North (GOODS-N), and our
data reach J=24.6 and K=23.2 (5sigma, Vega magnitude). The surface density of
DRGs selected by J-K>2.3 is 2.35+-0.31 arcmin^-2 at K<22 and 3.54+-0.38
arcmin^-2 at K<23, respectively. These values are consistent with those in the
GOODS-South and FIRES. Our deep and wide data suggest that the number counts of
DRGs turn over at K~22, and the surface density of the faint DRGs with K>22 is
smaller than that expected from the number counts at the brighter magnitude.
The result indicates that while there are many bright galaxies at 2<z<4 with
the relatively old stellar population and/or heavy dust extinction, the number
of the faint galaxies with the similar red color is relatively small. Different
behaviors of the number counts of the DRGs and bluer galaxies with 2<z_phot<4
at K>22 suggest that the mass-dependent color distribution, where most of
low-mass galaxies are blue while more massive galaxies tend to have redder
colors, had already been established at that epoch.Comment: 6 pages, 4 figures, accepted for publication in PAS
Subaru/MOIRCS Near-Infrared Imaging in the Proto-Cluster Region at z=3.1
We present the results of deep near-infrared imaging observations of the
z=3.1 proto-cluster region in the SSA22a field taken by MOIRCS mounted on the
Subaru Telescope. We observed a 21.7 arcmin^2 field to the depths of J=24.5,
H=24.3, and K=23.9 (5 sigma). We examine the distribution of the K-selected
galaxies at z~3 by using the simple color cut for distant red galaxies (DRGs)
as well as the photometric-redshift selection technique. The marginal density
excess of DRGs and the photo-z selected objects are found around the two most
luminous Ly alpha blobs (LABs). We investigate the correlation between the
K-selected objects and the LABs, and find that several galaxies with stellar
mass M_* = 10^9-10^11 M_solar exist in vicinity of LABs, especially around the
two most luminous ones. We also find that 7 of the 8 LABs in the field have
plausible K_s-band counterparts and the sum of the stellar mass possibly
associated with LABs correlates with the luminosity and surface brightness of
them, which implies that the origin of Ly alpha emission may be closely
correlated with their stellar mass or their previous star formation phenomena.Comment: 15 pages, 9 figures, accepted for publication in PASJ Vol.60, No.
The Lifetime of Protoplanetary Disks in a Low-Metallicity Environment
The extreme outer Galaxy (EOG), the region with a Galactic radius of more
than 18 kpc, is known to have very low metallicity, about one-tenth that of the
solar neighborhood. We obtained deep near-infrared (NIR) images of two very
young (0.5 Myr) star-forming clusters that are one of the most distant
embedded clusters in the EOG. We find that in both clusters the fraction of
stars with NIR excess, which originates from the circumstellar dust disk at
radii of 0.1 AU, is significantly lower than those in the solar
neighborhood. Our results suggest that most stars forming in the
low-metallicity environment experience disk dispersal at an earlier stage (1
Myr) than those forming in the solar metallicity environment (as much as
5--6 Myr). Such rapid disk dispersal may make the formation of planets
difficult, and the shorter disk lifetime with lower metallicity could
contribute to the strong metallicity dependence of the well-known
"planet-metallicity correlation", which states the probability of a star
hosting a planet increases steeply with stellar metallicity. The reason for the
rapid disk dispersal could be increase of the mass accretion rate and/or the
effective far-ultraviolet photoevaporation due to the low extinction; however,
another unknown mechanism for the EOG environment could be contributing
significantly.Comment: 27 pages, 7 figures, Accepted for publication in The Astrophysical
Journa