Search for Hyper Infrared-Luminous Dust Obscured Galaxies selected with WISE and SDSS

We aim to search for hyperliminous infrared (IR) galaxies (HyLIRGs) with IR luminosity $L_{{\rm IR}}$ $>$ 10$^{13}$ $L_{\odot}$ by applying the selection method of Dust Obscured Galaxies (DOGs). They are spatially rare but could correspond to a maximum phase of cosmic star formation and/or active galactic nucleus (AGN) activity, hence they are a crucial population for understanding the star formation and mass assembly history of galaxies. Combining the optical and IR catalogs obtained from Sloan Digital Sky Survey (SDSS) and Wide-field Infrared Survey Explorer (WISE), we performed the extensive HyLIRGs survey; we selected 5,311 IR-bright DOGs with $i$ -- [22] $>$ 7.0 and flux at 22 $\mu$m $>$ 3.8 mJy in 14,555 deg$^2$, where $i$ and [22] are $i$-band and 22 $\mu$m AB magnitudes, respectively. Among them, 67 DOGs have reliable spectroscopic redshifts that enable us to estimate their total IR luminosity based on the SED fitting. Consequently, we successfully discovered 24 HyLIRGs among the 67 spectroscopically-confirmed DOGs. We found that (i) $i$ - [22] color of IR-bright DOGs correlates with the total IR luminosity and (ii) surface number density of HyLIRGs is $>$ 0.17 deg$^{-2}$. A high fraction ($\sim$ 73%) of IR-bright DOGs with $i$ - [22] $>$ 7.5 shows $L_{{\rm IR}}$ $>$ 10$^{13}$ $L_{\odot}$, and the DOGs criterion we adopted could be independently-effective against "W1W2-dropout method" based on four WISE bands, for searching hyper IR luminous populations of galaxies.Comment: 10 pages and 14 figures, accepted for publication in Ap

Seyfert-Type Dependences of Narrow Emission-Line Ratios and Physical Properties of High-Ionization Nuclear Emission-Line Regions in Seyfert Galaxies

In order to examine how narrow emission-line flux ratios depend on the Seyfert type, we compiled various narrow emission-line flux ratios of 355 Seyfert galaxies from the literature. We present in this paper that the intensity of the high-ionization emission lines, [Fe VII]6087, [Fe X]6374 and [Ne V]3426, tend to be stronger in Seyfert 1 galaxies than in Seyfert 2 galaxies. In addition to these lines, [O III]4363 and [Ne III]3869, whose ionization potentials are not high (< 100 eV), but whose critical densities are significantly high (> 10^7 cm^-3), also exhibit the same tendency. On the other hand, the emission-line flux ratios among low-ionization emission lines do not show such a tendency. We point out that the most plausible interpretation of these results is that the high-ionization emission lines arise mainly from highly-ionized, dense gas clouds, which are located very close to nuclei, and thus can be hidden by dusty tori. To examine the physical properties of these highly-ionized dense gas clouds, photoionization model calculations were performed. As a result, we find that the hydrogen density and the ionization parameter of these highly-ionized dense gas clouds are constrained to be n_H > 10^6 cm^-3 and U > 10^-2, respectively. These lower limits are almost independent both from the metallicity of gas clouds and from the spectral energy distribution of the nuclear ionizing radiation.Comment: 32 pages, to appear in Publications of the Astronomical Society of Japa

Gas Metallicity in the Narrow-Line Regions of High-Redshift Active Galactic Nuclei

We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas metallicity of the narrow-line regions and their evolution in this redshift range. Such spectra are also compared with UV spectra of local Seyfert 2 galaxies. The observational data are inconsistent with the predictions of shock models, suggesting that the narrow-line regions are mainly photoionized. The photoionization models with dust grains predict line flux ratios which are also in disagreement with most of the observed values, suggesting that the high-ionization part of the narrow-line regions (which is sampled by the available spectra) is dust-free. The photoionization dust-free models provide two possible scenarios which are consistent with the observed data: low-density gas clouds (n < 10^3 cm^-3) with a sub-solar metallicity (0.2 < Z/Z_sun < 1.0), or high-density gas clouds (n ~ 10^5 cm^-3) with a wide range of gas metallicity (0.2 < Z/Z_sun < 5.0). Regardless of the specific interpretation, the observational data do not show any evidence for a significant evolution of the gas metallicity in the narrow-line regions within the redshift range 1.2 < z < 3.8. Instead, we find a trend for more luminous active galactic nuclei to have more metal-rich gas clouds (luminosity-metallicity relation), which is in agreement with the same finding in the studies of the broad-line regions. The lack of evolution for the gas metallicity of the narrow-line regions implies that the major epoch of star formation in the host galaxies of these active galactic nuclei is at z > 4.Comment: 16 pages, 12 figures, submitted to Astronomy and Astrophysic

The Metal Abundances across Cosmic Time (MACT\mathcal{MACT}) Survey. II. Evolution of the Mass-Metallicity Relation over 8 Billion Years, using [OIII]λ\lambda4363\AA-based Metallicities

We present the first results from MMT and Keck spectroscopy for a large sample of $0.1\leq z\leq1$ emission-line galaxies selected from our narrow-band imaging in the Subaru Deep Field. We measured the weak [OIII]$\lambda$4363 emission line for 164 galaxies (66 with at least 3$\sigma$ detections, and 98 with significant upper limits). The strength of this line is set by the electron temperature for the ionized gas. Because the gas temperature is regulated by the metal content, the gas-phase oxygen abundance is inversely correlated with [OIII]$\lambda$4363 line strength. Our temperature-based metallicity study is the first to span $\approx$8 Gyr of cosmic time and $\approx$3 dex in stellar mass for low-mass galaxies, $\log{\left(M_{\rm star}/M_{\rm sun}\right)}\approx6.0-9.0$. Using extensive multi-wavelength photometry, we measure the evolution of the stellar mass--gas metallicity relation and its dependence on dust-corrected star formation rate (SFR). The latter is obtained from high signal-to-noise Balmer emission-line measurements. Our mass-metallicity relation is consistent with Andrews & Martini at $z\leq0.3$, and evolves toward lower abundances at a given stellar mass, $\log{({\rm O/H})}\propto(1+z)^{-2.32^{+0.52}_{-0.26}}$. We find that galaxies with lower metallicities have higher SFRs at a given stellar mass and redshift, although the scatter is large ($\approx$0.3 dex), and the trend is weaker than seen in local studies. We also compare our mass--metallicity relation against predictions from high-resolution galaxy formation simulations, and find good agreement with models that adopt energy- and momentum-driven stellar feedback. We have identified 16 extremely metal-poor galaxies with abundances less than a tenth of solar; our most metal-poor galaxy at $z\approx0.84$ is similar to I Zw 18.Comment: 18 pages, 11 figures, 2 tables. Updated to match published version in the Astrophysical Journa

How Do We See the Nuclear Region (r < 0.1 pc) of Narrow-Line Seyfert 1 Galaxies?

We propose two statistical tests to investigate how we see the nuclear region (r < 0.1 pc) of Narrow-Line Seyfert 1 galaxies (NLS1s). 1) The high-ionization nuclear emission-line region (HINER) test: Seyfert 1 galaxies (S1s) have systematically higher flux ratios of [Fe VII] lambda 6087 to [O III] lambda 5007 than Seyfert 2 galaxies (S2s). This is interpreted in that a significant part of the [Fe VII] lambda 6087 emission arises from the inner walls of dusty tori that cannot be seen in S2s (Murayama & Taniguchi 1998a,b). 2) The mid-infrared test: S1s have systematically higher flux ratios of the L band (3.5 micrometer) to the IRAS 25 micrometer band than S2s. This is also interpreted in that a significant part of the L band emission arises from the inner walls of dusty tori, because the tori are optically thick enough to absorb the L band emission if the tori are viewed nearly edge on (Murayama et al. 2000). Applying these tests to a sample of NLS1s, we have found that the NLS1s possibly have nearly the same properties as S1s.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho

"Direct" Gas-phase Metallicities, Stellar Properties, and Local Environments of Emission-line Galaxies at Redshift below 0.90

Using deep narrow-band (NB) imaging and optical spectroscopy from the Keck telescope and MMT, we identify a sample of 20 emission-line galaxies (ELGs) at z=0.065-0.90 where the weak auroral emission line, [OIII]4363, is detected at >3\sigma. These detections allow us to determine the gas-phase metallicity using the "direct'' method. With electron temperature measurements and dust attenuation corrections from Balmer decrements, we find that 4 of these low-mass galaxies are extremely metal-poor with 12+log(O/H) <= 7.65 or one-tenth solar. Our most metal-deficient galaxy has 12+log(O/H) = 7.24^{+0.45}_{-0.30} (95% confidence), similar to some of the lowest metallicity galaxies identified in the local universe. We find that our galaxies are all undergoing significant star formation with average specific star formation rate (SFR) of (100 Myr)^{-1}, and that they have high central SFR surface densities (average of 0.5 Msun/yr/kpc^2. In addition, more than two-thirds of our galaxies have between one and four nearby companions within a projected radius of 100 kpc, which we find is an excess among star-forming galaxies at z=0.4-0.85. We also find that the gas-phase metallicities for a given stellar mass and SFR lie systematically below the local M-Z-(SFR) relation by \approx0.2 dex (2\sigma\ significance). These results are partly due to selection effects, since galaxies with strong star formation and low metallicity are more likely to yield [OIII]4363 detections. Finally, the observed higher ionization parameter and electron density suggest that they are lower redshift analogs to typical z>1 galaxies.Comment: Accepted for publication in the Astrophysical Journal (15 November 2013). 31 pages in emulateapj format with 16 figures and 7 tables. Revised to address referee's comments, which include discussion on selection effects, similarities to green pea galaxies, and nebular continuum contribution. Modifications were made for some electron temperature and metallicity measurement