arXiv subjects

Tetsuya Hashimoto

Publications and source records attributed to Tetsuya Hashimoto.

95 records · Page 6Linked to original sources

arXiv2012

Constraints on Obscured Star Formation in Host Galaxies of Gamma-ray Bursts

We present the results of the 16-cm-waveband continuum observations of four host galaxies of gamma-ray bursts (GRBs) 990705, 021211, 041006, and 051022 using the Australia Telescope Compact Array. Radio emission was not detected in any of the host galaxies. The 2sigma upper limits on star-formation rates derived from the radio observations of the host galaxies are 23, 45, 27, and 26 Msun/yr, respectively, which are less than about 10 times those derived from UV/optical observations, suggesting that they have no significant dust-obscured star formation. GRBs 021211 and 051022 are known as the so-called "dark GRBs" and our results imply that dark GRBs do not always occur in galaxies enshrouded by dust. Because large dust extinction was not observed in the afterglow of GRB021211, our result {\bf suggests the possibility} that the cause of the dark GRB is the intrinsic faintness of the optical afterglow. On the other hand, by considering the high column density observed in the afterglow of GRB051022, the likely cause of the dark GRB is the dust extinction in the line of sight of the GRB.

astro-ph.CO↗

arXiv2011

Spatially Extended [P II]1.188um and [Fe II]1.257um Emission Lines in a Nearby Seyfert Galaxy NGC 1068 Observed with OAO/ISLE

We present J-band long-slit spectroscopic observation of NGC 1068 classified as a Seyfert 2 galaxy. J-band observations with OAO/ISLE provide clear detection of spatially extended [Fe II]1.257um and [P II]1.188um lines. We found that [Fe II]1.257um/[P II]1.188um increases with distance from a central continuum peak. Observed line ratios around the nucleus (continuum peak) are consistent with a typical value expected from photoionization models, while the ratios at 3" - 4" (210-280 pc) east and west of the nucleus are slightly higher than this. In the off nucleus region of NGC 1068 we also found a possible association between [Fe II]1.257um/[P II]1.188um and the radio continuum. This suggests a mild contribution of shock ionization induced by a radio jet outside nucleus while photoionization by the central energy source is dominant near the nucleus.

astro-ph.CO↗

arXiv2008

The Reionization and Galaxy Evolution Probed by z=7 Lyman Alpha Emitters

We made a narrowband NB973 (bandwidth of 200A at 9755A) imaging of the Subaru Deep Field (SDF) and found two z=7 Lyman alpha emitter (LAE) candidates down to NB973=24.9. Carrying out deep follow-up spectroscopy, we identified one of them as a real z=6.96 LAE. This has shown that galaxy formation was in progress just 750 Myr after the Big Bang. Meanwhile, the Lyman alpha line luminosity function of LAE is known to decline from z=5.7 to 6.6 in the SDF. L* at z=6.6 is 40-60% of that at z=5.7. We also confirm that the number density of z=7 LAE is only 17% of the density at z=6.6 comparing the latest SDF LAE samples. This series of significant decreases in LAE density with increasing redshift can be the result of galaxy evolution during these epochs. However, using the UV continuum luminosity functions of LAEs, those of Lyman break galaxies, and a LAE evolution model based on the hierarchical clustering, we find that galaxy evolution alone cannot explain all the decrease in density. This extra density deficit can be interpreted as the attenuation of the Lyman alpha photons from LAEs due to a rapid evolution of neutral hydrogen fraction during the ongoing cosmic reionization at z~6.6-7.

astro-ph↗

arXiv2006

A galaxy at a redshift z = 6.96

When galaxy formation started in the history of the Universe remains unclear. Studies of the cosmic microwave background indicate that the Universe, after initial cooling (following the Big Bang), was reheated and reionized by hot stars in newborn galaxies at a redshift in the range 6 < z < 14 (ref. 1). Though several candidate galaxies at redshift z > 7 have been identified photometrically (refs 2,3), galaxies with spectroscopically confirmed redshifts have been confined to z < 6.6 (refs. 4-8). Here we report a spectroscopic redshift of z = 6.96 (corresponding to just 750 Myr after the Big Bang) for a galaxy whose spectrum clearly shows Lyman-alpha emission at 9,682 A, indicating active star formation at a rate of about 10 M_sun/yr, where M_sun us the mass of the Sun. This demonstrates that galaxy formation was under way when the Universe was only about 6 per cent of its present age. The number density of galaxies at z = 7 seems to be only 18-36 per cent of the density at z = 6.6.

astro-ph↗

arXiv2005

Near-Infrared search for C IV absorption counterparts along the line-of-sights to pair quasars

We carried out a Subaru and UKIRT near infrared imaging survey for H-alpha emitting galaxies around two pair quasar systems (Q0301-005/Q0302-003 and Q2343+125/Q2344+125), and a triple quasar system (KP76/KP77/KP78). Narrow band near infrared filters covering the H-alpha emission expected for galaxies at the confirmed C IV absorption redshift toward the quasar systems were used for this survey. These quasar pairs or triplet are separated at most by 17 arcmins (~5 h^-1 Mpc in proper distance) from each other on the sky, and have common C IV absorption lines at almost identical redshifts at z=2.24-2.43, which suggests there could be a Mpc-scale absorbing systems such as a cluster, or a group, of galaxies that cover all the line-of-sights to the pair/triple quasars. Using narrow-band deep images, we detected five candidates for H-alpha emitting galaxies around two of the six fields, Q2343+125 and Q2344+125, whose apparent star formation rates are, extremely high, 20-466 M_solar/year. However, all or most of them are not likely to be galaxies at the absorption redshift but galaxies at lower redshift, because of their extreme brightness. In the fields of the other quasars, we detected no star-forming galaxies, nor did we find any number excess of galaxy counts around them. This no-detection results could be because the luminosities and star formation rates of galaxies are lower than the detection limits of our observations (K' > 21 and SFR < 1.8-240 h^-2 M_solar/year). They could be located outside of the observed fields by chance. Otherwise, most C IV absorption lines could be ascribed not to cluster of galaxies, but to isolated star forming pockets far from bright galaxies and could be analogous objects to weak Mg II absorbers.

astro-ph↗