Abstract
We present a star formation rate function (SFRF) at
z
∼ 6 based on star formation rates (SFRs) derived by spectral energy distribution fitting on data from rest-frame UV to optical ...wavelengths of galaxies in the CANDELS GOODS-South and North fields. The resulting SFRF shows an excess compared to the previous estimations by using rest-frame UV luminosity functions (LFs) corrected for the dust attenuation and is comparable to that estimated from a far-infrared LF. This suggests that the number density of dust-obscured intensively star-forming galaxies at
z
∼ 6 has been underestimated in the previous approach based only on rest-frame UV observations. We parameterize the SFRF using the Schechter function and obtain the best-fit parameter of the characteristic SFR (SFR*) when the faint-end slope and characteristic number density are fixed. The best-fit SFR* at
z
∼ 6 is comparable to that at
z
∼ 2, when the cosmic star formation activity reaches its peak. Together with SFRF estimations with a similar approach using rest-frame UV to optical data, the SFR* is roughly constant from
z
∼ 2 to ∼6 and may decrease above
z
∼ 6. Since the SFR* is sensitive to the high-SFR end of the SFRF, this evolution of SFR* suggests that the high-SFR end of the SFRF grows rapidly during the epoch of reionization and reaches a similar level observed at
z
∼ 2.
Abstract
The dependence of the star formation efficiency (SFE) on galactic structures—especially whether the SFE in the bar region is lower than those in other regions—has recently been debated. We ...report the SFEs of 18 nearby gas-rich massive star-forming barred galaxies with large apparent bar major axes (≧75″). We statistically measure the SFE by distinguishing the center, the bar end, and the bar regions for the first time. The molecular gas surface density is derived from archival CO(1–0) and/or CO(2–1) data by assuming a constant CO-to-H
2
conversion factor (
α
CO
), and the star formation rate surface density is derived from a linear combination of far-UV and mid-IR intensities. The angular resolution is 15″, which corresponds to 0.3–1.8 kpc. We find that the ratio of the SFE in the bar to that in the disk was systematically lower than unity (typically 0.6–0.8), which means that the star formation in the bar is systematically suppressed. Our results are inconsistent with similar recent statistical studies, which have reported that the SFE tends to be independent of galactic structures. This inconsistency can be attributed to the differences in the definitions of the bar region, the spatial resolutions, the
α
CO
, and the sample galaxies. Furthermore, we find a negative correlation between the SFE and the velocity width of the CO spectrum, which is consistent with the idea that the large dynamical effects—such as strong shocks, large shears, and fast cloud–cloud collisions caused by the noncircular motion of the bar—result in a low SFE.
ABSTRACT
Cloud-cloud collision (CCC) has been suggested as a mechanism to induce massive star formation. Recent simulations suggest that a CCC speed is different among galactic-scale environments, ...which is responsible for observed differences in star formation activity. In particular, a high-speed CCC is proposed as a cause of star formation suppression in the bar regions in barred spiral galaxies. Focusing on the strongly barred galaxy NGC 1300, we investigate the CCC speed. We find the CCC speed in the bar and bar-end tend to be higher than that in the arm. The estimated CCC speed is ${\sim}20$, ${\sim}16$, and ${\sim}11~\rm km~s^{-1}$ in the bar, bar-end, and arm, respectively. Although the star formation activity is different in the bar and bar-end, the CCC speed and the number density of high-speed CCC with ${\gt}20~\rm km~s^{-1}$ are high in both regions, implying the existence of other parameters that control the star formation. The difference in molecular gas mass (average density) of the giant molecular clouds (GMCs) between the bar (lower mass and lower density) and bar-end (higher mass and higher density) may be cause for the different star formation activity. Combining with our previous study, the leading candidates of causes for the star formation suppression in the bar in NGC 1300 are the presence of a large amount of diffuse molecular gases and high-speed CCCs between low-mass GMCs.
Abstract
We present a star formation rate function (SFRF) at
z
∼ 4.5 based on photometric data from the rest UV to optical of galaxies in the CANDELS GOODS-South field using spectral energy ...distribution (SED) fitting. We evaluate the incompleteness of our sample and correct for it to properly compare the SFRF in this study with those estimated based on other probes. The SFRF is obtained down to ∼10
M
⊙
yr
−1
, and it shows a significant excess compared to that estimated from the UV luminosity function and dust correction based on the UV spectral slope. As compared with the UV-based SFRF, the number density is larger by ∼1 dex at a fixed SFR, or the best-fit Schechter parameter of the characteristic SFR, SFR*, is larger by ∼1 dex. We extensively examine several assumptions on SED fitting to see the robustness of our result and find that the excess still exists even if the assumptions change such as star formation histories, dust extinction laws, and a one- or two-component model. By integrating our SFRF to 0.22
M
⊙
yr
−1
, the cosmic star formation rate density (CSFRD) at this epoch is calculated to be
4.53
−
0.87
+
0.94
×
10
−
2
M
⊙
yr
−
1
Mpc
−
3
, which is ∼0.25 dex larger than the previous measurement based on UV observations. We also find that galaxies with intensive star formation (>10
M
⊙
yr
−1
) occupy most of the CSFRD (∼80%), suggesting that star formation activity at this epoch is dominated by intensively star-forming galaxies.
ABSTRACT
Recent galaxy observations show that star formation activity changes depending on galactic environments. In order to understand the diversity of galactic-scale star formation, it is crucial ...to understand the formation and evolution of giant molecular clouds in an extreme environment. We focus on observational evidence that bars in strongly barred galaxies lack massive stars even though quantities of molecular gas are sufficient to form stars. In this paper, we present a hydrodynamical simulation of a strongly barred galaxy, using a stellar potential which is taken from observational results of NGC 1300, and we compare cloud properties between different galactic environments: bar, bar-end, and spiral arms. We find that the mean of cloud’s virial parameter is αvir ∼ 1 and that there is no environmental dependence, indicating that the gravitationally bound state of a cloud is not behind the observational evidence of the lack of massive stars in strong bars. Instead, we focus on cloud–cloud collisions, which have been proposed as a triggering mechanism for massive star formation. We find that the collision speed in the bar is faster than those in the other regions. We examine the collision frequency using clouds’ kinematics and conclude that the fast collisions in the bar could originate from random-like motion of clouds due to elliptical gas orbits shifted by the bar potential. These results suggest that the observed regions of lack of active star formation in the strong bar originate from the fast cloud–cloud collisions, which are inefficient in forming massive stars, due to the galactic-scale violent gas motion.
We measure the redshift-space correlation function from a spectroscopic sample of 2783 emission line galaxies from the FastSound survey. The survey, which uses the Subaru Telescope and covers a ...redshift range of 1.19 < z < 1.55, is the first cosmological study at such high redshifts. We detect clear anisotropy due to redshift-space distortions (RSD) both in the correlation function as a function of separations parallel and perpendicular to the line of sight and its quadrupole moment. RSD has been extensively used to test general relativity on cosmological scales at z < 1. Adopting a ΛCDM cosmology with the fixed expansion history and no velocity dispersion (σv = 0), and using the RSD measurements on scales above 8 h−1 Mpc, we obtain the first constraint on the growth rate at the redshift, f (z)σ8(z) = 0.482 ± 0.116 at z ∼ 1.4 after marginalizing over the galaxy bias parameter b(z)σ8(z). This corresponds to 4.2 σ detection of RSD. Our constraint is consistent with the prediction of general relativity fσ8 ∼ 0.392 within the 1 σ confidence level. When we allow σv to vary and marginalize over it, the growth rate constraint becomes $f\sigma _8=0.494^{+0.126}_{-0.120}$. We also demonstrate that by combining with the low-z constraints on fσ8, high-z galaxy surveys like the FastSound can be useful to distinguish modified gravity models without relying on CMB anisotropy experiments.
The aim of this study is to clarify the deflection, splaying, and abrasion of single tufts of polybutylene terephthalate (PBT) toothbrushes after use. A single-center randomized controlled trial is ...performed. The changes in deflection, bristle splaying, and abrasion are investigated for the middle single tuft of the top line (top–middle tuft) and the middle single tuft of the bottom line (bottom–middle tuft) of PBT toothbrushes with medium stiffness after 1 month, 2 months, and 3 months of use by 34 participants. A soft-material bending-resistance tester is used to assess the deflection of the single tufts. The deflection value of the top–middle tuft significantly increased after 1 month of use compared with the baseline. In contrast, the deflection of the bottom–middle tuft significantly increased after 3 months of use compared with the baseline and after 1 month and 2 months of use. Importantly, the change in deflection was distinctly different between the top– and bottom–middle tufts. The bristle splaying of both tufts significantly increased after use, but a significant change in bristle abrasion was not found. The bending stiffness of the top tuft of a PBT toothbrush may decrease more rapidly than that of the bottom tuft with use.
We present the results of CO observations toward 14 host galaxies of long-duration gamma-ray bursts (GRBs) at z = 0.1-2.5 by using the Atacama Large Millimeter/submillimeter Array. We successfully ...detected CO (3-2) or CO (4-3) emission in eight hosts (z = 0.3-2), which more than doubles the sample size of GRB hosts with CO detection. The derived molecular gas mass is Mgas = (0.2-6) × 1010 M assuming metallicity-dependent CO-to-H2 conversion factors. By using the largest sample of GRB hosts with molecular gas estimates (25 in total, of which 14 are CO detected), including results from the literature, we compared molecular gas properties with those of other star-forming galaxies (SFGs). The GRB hosts tend to have a higher molecular gas mass fraction ( gas) and a shorter gas depletion timescale (tdepl) as compared with other SFGs at similar redshifts, especially at z 1. This could be a common property of GRB hosts or an effect introduced by the selection of targets that are typically above the main-sequence line. To eliminate the effect of selection bias, we analyzed gas and tdepl as a function of the distance from the main-sequence line (δMS). We find that the GRB hosts follow the same scaling relations as other SFGs, where gas increases and tdepl decreases with increasing δMS. No molecular gas deficit is observed when compared to other SFGs of similar star formation rate and stellar mass. These findings suggest that the same star formation mechanism is expected to be happening in GRB hosts as in other SFGs.
ABSTRACT
An overview of the Seimei telescope, a 3.8 m optical infrared telescope located on Mt. Chikurinji in the Okayama prefecture of Japan, is presented. Seimei is a segmented-mirror telescope ...whose primary mirror consists of 18 petal-shaped segments. The telescope tube supporting the thin segmented mirrors is structurally incorporated within large arc-rails providing the elevation axis. The tube has a light-weight homologous structure designed with a genetic algorithm. The total weight of the telescope tube, including 1.4-ton optics, is only 8 tons. By virtue of its light weight, the telescope is able to point at an object anywhere in the observable sky within one minute. The telescope is operated by Kyoto University in collaboration with the National Astronomical Observatory of Japan (NAOJ). Half of the telescope time is used by Kyoto University. The remaining time is open to the Japanese astronomical community. NAOJ is responsible for the management of the open-use time, including handling of the observation proposals. The telescope is now regularly performing scientific observations on the basis of a variety of proposals.
Abstract
CO(2–1) emission is often used as a tracer of giant molecular clouds (GMCs) as an alternative to CO(1–0) emission in recent years. Therefore, understanding the environmental dependence of ...the line ratio of CO(2–1)/CO(1–0),
R
21
, on the GMC scale is important to accurately estimate the mass of GMCs. We thus measured
R
21
in the strongly barred galaxy NGC 1300, where star formation activity strongly depends on galactic structure, on a ∼100 pc scale. CO images were obtained from the Atacama Large Millimeter/submillimeter Array and the Nobeyama 45 m telescope. The resultant typical
R
21
in NGC 1300 is 0.57 ± 0.06. We find environmental variations in
R
21
: it is the highest in the bar-end region (0.72 ± 0.08), followed by arm (0.60 ± 0.07) and bar regions (0.50 ± 0.06). GMCs with H
α
emission show a systematically higher ratio (0.67 ± 0.07) than those without H
α
(0.47 ± 0.05). In the bar region, where massive star formation is suppressed, H
α
emission is not associated with most GMCs, resulting in the lowest
R
21
. These results raise a possibility that properties of GMCs derived from CO(2–1) observations with the assumption of a constant
R
21
are different from those derived from CO(1–0) observations. Furthermore, we find the
R
21
measured on the kiloparsec scale tends to be lower than that of the GMCs, probably due to the presence of an extended diffuse molecular gas in NGC 1300.
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