arXiv subjects

Takashi Kojima

Publications and source records attributed to Takashi Kojima.

21 records · Page 2Linked to original sources

arXiv2017

First Data Release of the Hyper Suprime-Cam Subaru Strategic Program

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layered imaging survey aimed at addressing some of the most outstanding questions in astronomy today, including the nature of dark matter and dark energy. The survey has been awarded 300 nights of observing time at the Subaru Telescope and it started in March 2014. This paper presents the first public data release of HSC-SSP. This release includes data taken in the first 1.7 years of observations (61.5 nights) and each of the Wide, Deep, and UltraDeep layers covers about 108, 26, and 4 square degrees down to depths of i~26.4, ~26.5, and ~27.0 mag, respectively (5sigma for point sources). All the layers are observed in five broad bands (grizy), and the Deep and UltraDeep layers are observed in narrow bands as well. We achieve an impressive image quality of 0.6 arcsec in the i-band in the Wide layer. We show that we achieve 1-2 per cent PSF photometry (rms) both internally and externally (against Pan-STARRS1), and ~10 mas and 40 mas internal and external astrometric accuracy, respectively. Both the calibrated images and catalogs are made available to the community through dedicated user interfaces and database servers. In addition to the pipeline products, we also provide value-added products such as photometric redshifts and a collection of public spectroscopic redshifts. Detailed descriptions of all the data can be found online. The data release website is https://hsc-release.mtk.nao.ac.jp/.

astro-ph.IM↗

arXiv2017

Systematic Survey for [OII], [OIII], and H$α$ Blobs at $z=0.1-1.5$: The Implication for Evolution of Galactic-Scale Outflow

We conduct a systematic search for galaxies at $z=0.1-1.5$ with [OII]$\lambda3727$, [OIII]$\lambda5007$, or H$α\lambda6563$ emission lines extended over at least 30 kpc by using deep narrowband and broadband imaging in Subaru-XMM Deep Survey (SXDS) field. These extended emission-line galaxies are dubbed [OII], [OIII], or H$α$ blobs. Based on a new selection method that securely selects extended emission-line galaxies, we find 77 blobs at $z=0.40-1.46$ with the isophotal area of emission lines down to $1.2\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ kpc$^{-2}$. Four of them are spectroscopically confirmed to be [OIII] blobs at $z=0.83$. We identify AGN activities in 8 blobs with X-ray and radio data and find that the fraction of AGN contribution increases with increasing isophotal area of the extended emission. With the Kolmogorov-Smirnov (KS) and Anderson-Darling tests, we confirm that the stellar-mass distributions of H$α$ and [OII] blobs are not drawn from those of the emitters at the $>90$% confidence level in that H$α$ and [OII] blobs are located at the massive end of the distributions, but cannot reject null hypothesis of being the same distributions in terms of the specific star formation rates. It is suggested that galactic-scale outflows tend to be more prominent in more massive star-forming galaxies. Exploiting our sample homogeneously selected over the large area, we derive the number densities of blobs at each epoch. The number densities of blobs decrease drastically with redshifts at the rate that is larger than that of the decrease of cosmic star formation densities.

astro-ph.GA↗

arXiv2017

Evolution of N/O Abundance Ratios and Ionization Parameters from z~0 to 2 Investigated by the Direct Temperature Method

We present N/O abundance ratios, ionization parameters {$q_{\rm ion}$}, and oxygen abundance O/H for a total of 41 galaxies (11 individual galaxies and a 30-galaxy stack) including {Ly$α$} emitters and Lyman break galaxies at $z\sim 2$, and investigate galaxy evolution from $z\sim 0$ to $2$ in conjunction with 208,529 local SDSS galaxies and 9 green pea galaxies (GPs). In contrast with most of the recent studies, we obtain the N/O ratio, {$q_{\rm ion}$}, and O/H measurements by direct $T_{\rm e}$ methods with {[O\,\textsc{iii}]} $λ$4363 and {O\,\textsc{iii}]} $λ$1665 lines. Based on these reliable measurements, we find that there exist $z\sim 2$ galaxies with an excess of N/O falling beyond the local average of N/O--O/H relation, while the majority of the $z\sim 2$ galaxies have the N/O ratios nearly comparable with $z\sim 0$ galaxies in the N/O--stellar mass relation. Our galaxies place the upper limit of N/O ratio $\log ({\rm N/O})\le -1.26$ on average, suggesting that the N/O ratio evolves, if at all, by $<0.17$ dex. Exploiting our reliable measurements free from the N/O--{$q_{\rm ion}$}--O/H degeneracies, we identify, for the first time, that $z\sim 2$ galaxies with offsets in the BPT diagram indicate either 1) only an N/O excess, 2) only a {$q_{\rm ion}$} excess, or 3) both N/O and {$q_{\rm ion}$} excesses. We argue that the BPT offsets at $z\sim 2$ are not made by one of 1)--3) galaxy populations alone, but the composite of 1)--3) populations. We confirm that these 1)--3) populations also exist at $z\sim 0$, such as GPs and SDSS low-mass and high-SFR galaxies (LMHSs).

astro-ph.GA↗