Yuko Kakazu

Mapping Extremely Low Metallicity Galaxies to Redshift One

We describe the results of a narrowband search for ultrastrong emission line galaxies (USELs) with EW(Hβ) ≥ 30 A. A total of 542 candidate galaxies are found in a one-half square degree survey using two ~120 A filters centered at 8150 and 9140 A with Subaru SuprimeCam. Follow-up spectroscopy for randomly selected objects in the candidate sample with Keck II DEIMOS shows that they consist of [O III] λ5007-, [O II] λ3727-, and Hα-selected strong emission line galaxies at intermediate redshift (z < 1) and Lyα-emitting galaxies at high redshift (z 5). We determine the Hβ luminosity functions and the star formation density of the USELs, which is 5%-10% of the value found from ultraviolet continuum objects at z = 0-1, suggesting that they correspond to a major epoch in the galaxy formation process at these redshifts. Many of the USELs show the temperature-sensitive [O III] λ4363 auroral lines, and about a dozen have oxygen abundances satisfying the criteria of extremely metal-poor galaxies (XMPGs). These XMPGs are the most distant known today. Our high yield rate of XMPGs suggests that the narrowband method is a powerful way to find such populations. Moreover, the lowest metallicity measured in our sample is close to the minimum metallicity found in local galaxies [12 + log(O/H) ~ 7.1], although we need deeper spectra to minimize the errors. HST ACS images of several USELs exhibit widespread morphologies from relatively compact high surface brightness objects to very diffuse low surface brightness ones. The luminosities, metallicities, and star formation rates of USELs are consistent with the strong emitters being start-up intermediate-mass galaxies that will evolve into more normal galaxies and suggest that galaxies are still forming in relatively chemically pristine sites at z < 1.

A New High-Redshift Lyα Emitter: Possible Superwind Galaxy at z = 5.69*

During the course of our deep optical imaging survey for Ly alpha emitters at z approximately 5.7 in the field around the z=5.74 quasar SDSSp J104433.04-012502.2, we have found a candidate strong emission-line source. Follow-up optical spectroscopy shows that the emission line profile of this object is asymmetric, showing excess red-wing emission. These properties are consistent with an identification of Ly alpha emission at a redshift of z=5.687 +/- 0.002. The observed broad line width, Delta V_{FWHM} ~= 340 km s^{-1} and excess red-wing emission also suggest that this object hosts a galactic superwind.

Deep Spectroscopy of Ultra-Strong Emission-Line Galaxies

Ultra-strong emission-line galaxies (USELs) with extremely high equivalent widths (EW(H?) 30 ?) can be used to pick out galaxies of extremely low metallicity in the z = 0-1 redshift range. Large numbers of these objects are easily detected in deep narrow band searches. Since most have detectable [O III]?4363, their metallicities can be determined using the direct method. These large samples hold out the possibility for determining whether there is a metallicity floor for the galaxy population. In this, the second of our papers on the topic, we describe the results of an extensive spectroscopic follow-up of the Kakazu et?al. catalog of 542 USELs carried out with the DEIMOS spectrograph on Keck. We have obtained high S/N spectra of 348 galaxies. The two lowest metallicity galaxies in our sample have 12 + log(O/H) = 6.97 ? 0.17 and 7.25 ? 0.03?values comparable to the lowest metallicity galaxies found to date. We determine an empirical relation between metallicity and the R23 parameter for our sample, and we compare this to the relationship for low-redshift galaxies. The determined metallicity-luminosity relation for this sample is compared with that of magnitude selected samples in the same redshift range. The emission-line-selected galaxies show a metal-luminosity relation where the metallicity decreases with luminosity, and they appear to define the lower bound of the galaxy metallicity distribution at a given continuum luminosity. We also compute the H? luminosity function of the USELs as a function of redshift and use this to compute an upper bound on the Ly? emitter luminosity function over the z = 0-1 redshift range.

On the Origin of L[CLC]y[/CLC]α Blobs at High Redshift: Submillimetric Evidence for a Hyperwind Galaxy at [ITAL][CLC]z[/CLC][/ITAL] = 3.1

The most remarkable class of high-redshift objects observed so far is made up of extended Lyα emission-line blobs found in an overdensity region at a redshift of 3.1. They may be either a dust-enshrouded extreme starburst galaxy with a large-scale galactic outflow (superwind) or cooling radiation from dark matter halos. Recently one of these Lyα blobs has been detected at submillimeter wavelengths (450 and 850 μm). Here we show that its rest-frame spectral energy distribution between optical and far-infrared is quite similar to that of Arp 220, which is a typical ultraluminous starburst/superwind galaxy in the local universe. This suggests strongly that the superwind model proposed by Taniguchi & Shioya is applicable to this Lyα blob. Since the blob is more luminous in the infrared by a factor of 30 than Arp 220, it comprises a new population of hyperwind galaxies at high redshift.

The Discovery of a Very Narrow Line Star-forming Object at a Redshift of 5.66

We report on the discovery of a very narrow line star-forming object beyond a redshift of 5. Using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru Telescope together with a narrow-passband filter centered at λc = 8150 A with a passband of Δλ = 120 A, we have obtained a very deep image of the field surrounding the quasar SDSSp J104433.04-012502.2 at a redshift of 5.74. Comparing this image with optical broadband images, we have found an object with a very strong emission line. Our follow-up optical spectroscopy has revealed that this source is at a redshift of z = 5.655 ± 0.002, forming stars at a rate ~13 h M☉ yr-1. Remarkably, the velocity dispersion of Lyα-emitting gas is only 22 km s-1. Since the blue half of the Lyα emission could be absorbed by neutral hydrogen gas, perhaps in the system, a modest estimate of the velocity dispersion may be 44 km s-1. Together with a linear size of 7.7 h kpc, we estimate a lower limit of the dynamical mass of this object to be ~2 × 109 M☉. It is thus suggested that LAE J1044-0123 is a star-forming dwarf galaxy (i.e., a subgalactic object or a building block) beyond redshift 5, although we cannot exclude the possibility that most Lyα emission is absorbed by the red damping wing of neutral intergalactic matter.

A Shock-induced Pair of Superbubbles in the High-Redshift Powerful Radio Galaxy MRC 0406–244

We present new optical spectroscopy of the high-redshift powerful radio galaxy MRC 0406-244 at redshift of 2.429. We find that the two extensions toward the northwest and southeast probed in the rest-frame ultraviolet image are heated mainly by the nonthermal continuum of the active galactic nucleus. However, each extension shows a shell-like morphology, suggesting that they are a pair of superbubbles induced by the superwind activity rather than by the interaction between the radio jet and the ambient gas clouds. If this is the case, the intense starburst responsible for the formation of superbubbles could occur ~1 × 109 yr ago. On the other hand, the age of the radio jets may be on the order of ~106 yr, being much shorter than the starburst age. Therefore, the two events, i.e., the starburst and the radio jet activities, are independent phenomena. However, their directions of the expanding motions could be governed by the rotational motion of the gaseous component in the host galaxy. This idea appears to explain the alignment effect of MRC 0406-244.

Effects of a Burst of Formation of First-Generation Stars on the Evolution of Galaxies

First-generation (Population III) stars in the universe play an important role in the early enrichment of heavy elements in galaxies and intergalactic medium and thus affect the history of galaxies. The physical and chemical properties of primordial gas clouds are significantly different from those of present-day gas clouds observed in the nearby universe because the primordial gas clouds do not contain any heavy elements, which are important coolants in gas. Previous theoretical considerations have suggested that typical masses of the first-generation stars are between several solar masses and ≈10 M☉, although it has been argued that the formation of very massive stars (e.g., >100 M☉) is also likely. If stars with several solar masses are the most popular ones at the epoch of galaxy formation, most stars will evolve to hot (e.g., 105 K), luminous (~104 L☉) stars with gaseous and dusty envelopes prior to going on to die as white dwarf stars. Although the duration of this phase is short (e.g., ~105 yr), such evolved stars could contribute both to the ionization of gas in galaxies and to the production of a lot of dust grains if the formation of intermediate-mass stars is highly enhanced. We compare gaseous emission-line properties of such nebulae with some interesting high-redshift galaxies such as IRAS F10214+4724 and powerful radio galaxies.

SDSSp J104433.04−012502.2 at z=5.74 is Gravitationally Magnified by an Intervening Galaxy

During the course of our optical deep survey program on Lα emitters at z ≈ 5.7 in the sky area surrounding the quasar SDSSp J104433.04−012502.2 at z =5 .74, we found that a faint galaxy with mB(AB) ≈ 25 is located at 1. 9

Spectroscopic studies of

The recent development of large, complete samples which identify high-redshift galaxies at

z \sim5.7

z\sim5.7