arXiv subjects

Naoyuki Tamura

Publications and source records attributed to Naoyuki Tamura.

61 records · Page 4Linked to original sources

arXiv2004

Lyman Break Galaxies at $z\sim5$: Rest-Frame UV Spectra

We report initial results for spectroscopic observations of candidates of Lyman Break Galaxies (LBGs) at $z\sim5$ in a region centered on the Hubble Deep Field-North by using the Faint Object Camera and Spectrograph attached to the Subaru Telescope. Eight objects with $I_C\leq25.0$ mag, including one AGN, are confirmed to be at $4.5<z<5.2$. The rest-frame UV spectra of seven LBGs commonly show no or weak Lyalpha emission line (rest-frame equivalent width of 0-10Å) and relatively strong low-ionization interstellar metal absorption lines of SiII $λ$1260, OI+SiII $λ$1303, and CII $λ$1334 (mean rest-frame equivalent widths of them are $-1.2 \sim -5.1 $Å). These properties are significantly different from those of the mean rest-frame UV spectrum of LBGs at $z\sim3$, but are quite similar to those of subgroups of LBGs at $z\sim3$ with no or weak Lyalpha emission. The weakness of Lyalpha emission and strong low-ionization interstellar metal absorption lines may indicate that these LBGs at $z\sim5$ are chemically evolved to some degree and have a dusty environment. Since the fraction of such LBGs at $z\sim5$ in our sample is larger than that at $z\sim3$, we may witness some sign of evolution of LBGs from $z\sim5$ to $z\sim3$, though the present sample size is very small. It is also possible, however, that the brighter LBGs tend to show no or weak Lyalpha emission, because our spectroscopic sample is bright (brighter than $L^{\ast}$) among LBGs at $z\sim5$. More observations are required to establish spectroscopic nature of LBGs at $z\sim5$.

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arXiv2003

Cryogenic Tests of Volume-Phase Holographic Gratings: I. Results at 200 K

We present results from cryogenic tests of a Volume-Phase Holographic (VPH) grating at 200 K measured at near-infrared wavelengths. The aims of these tests were to see whether the diffraction efficiency and angular dispersion of a VPH grating are significantly different at a low temperature from those at a room temperature, and to see how many cooling and heating cycles the grating can withstand. We have completed 5 cycles between room temperature and 200 K, and find that the performance is nearly independent of temperature, at least over the temperature range which we are investigating. In future, we will not only try more cycles between these temperatures but also perform measurements at a much lower temperature (e.g., 80 K).

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arXiv2003

Color Gradients in Early-Type Galaxies in ABELL 2199

We performed B and R band surface photometry for E/S0 galaxies in a nearby rich cluster ABELL 2199 to investigate their B-R color gradients (d(B-R)/dlogr). Our aims are to study statistical properties of the color gradients and, by comparing them with those in less dense environments, to examine environmental dependence of color gradients in elliptical galaxies. We studied the distribution of the B-R color gradients in the cluster ellipticals and found that the mean value of the color gradients is -0.09 +- 0.04 mag/dex, which can be converted to a metallicity gradient (dlogZ/dlogr) of ~ -0.3 +- 0.1 assuming an old stellar population. We further studied the relations between the B-R color gradients and global properties of the galaxies. Our data suggest that for the galaxies brighter than L*, more luminous and larger galaxies tend to have steeper color gradients. The typical value of the color gradients seems to be consistent with a recent monolithic collapse model and the correlation could also appear if elliptical galaxies formed through the monolithic collapse. On the contrary, it is found based on data from the literature that any such trend is clearly weaker for ellipticals in less dense environments, while the distribution of the color gradients is quite similar to that found in the rich cluster. Based on the results from our data and the published data, we discuss formation process of elliptical galaxy and its environmental dependence.

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arXiv2001

Mass-Metallicity Relation for the Local Group Dwarf Spheroidal Galaxies: A New Picture for the Chemical Enrichment of Galaxies in the Lowest Mass Range

The virial mass ($M_{\rm vir}$)-metallicity relation among the Local Group dwarf spheroidal galaxies (dSphs) is examined. Hirashita, Takeuchi, & Tamura showed that the dSphs can be divided into two distinct classes with respect to the relation between their virial masses and luminosities: low-mass ($M_{\rm vir} \la 10^8 M_\odot$) and high-mass ($M_{\rm vir} \ga 10^8 M_\odot$) groups. We see that both the mass-metallicity and the mass-luminosity relations of the high-mass dSphs are understood as a low-mass extension of giant ellipticals. On the contrary, we find that the classical galactic-wind model is problematic to apply to the low-mass dSphs, whose low binding energy is comparable to that released by several supernova explosions. A strongly regulated star formation in their formation phase is required to reproduce their observed metallicity. Such regulation is naturally expected in a gas cloud with the primordial elemental abundance according to Nishi & Tashiro. A significant scatter in the mass-metallicity relation for the low-mass dSphs is also successfully explained along with the scenario of Hirashita and coworkers. We not only propose a new picture for a chemical enrichment of the dSphs, but also suggest that the mass-metallicity and the mass-luminosity relations be understood in a consistent context.

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arXiv2000

Color Gradients in Early-Type Galaxies in Clusters at the Redshift from 0.37 to 0.56

Color gradients in elliptical galaxies in distant clusters ($z=0.37-0.56$) are examined by using the archival deep imaging data of Wide Field Planetary Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). Obtained color gradients are compared with the two model gradients to examine the origin of the color gradients. In one model, a color gradient is assumed to be caused by a metallicity gradient of stellar populations, while in the other one, it is caused by an age gradient. Both of these model color gradients reproduce the average color gradient seen in nearby ellipticals, but predict significantly different gradients at a redshift larger than $\sim$0.3. Comparison between the observed gradients and the model gradients reveals that the metallicity gradient is much more favorable as the primary origin of color gradients in elliptical galaxies in clusters. The same conclusion has been obtained for field ellipticals by using those at the redshift from 0.1 to 1.0 in the Hubble Deep Field-North by Tamura et al. (2000). Thus, it is also suggested that the primary origin of the color gradients in elliptical galaxies does not depend on galaxy environment.

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arXiv2000

Origin of Color Gradients in Elliptical Galaxies

The origin of color gradients in elliptical galaxies is examined by comparing model gradients with those observed in the Hubble Deep Field. The models are constructed so as to reproduce color gradients in local elliptical galaxies either by a metallicity gradient or by an age gradient. By looking back a sequence of color gradient as a function of redshift, the age - metallicity degeneracy is solved. The observed color gradients in elliptical galaxies at $z=0.1-1.0$ agree excellently with those predicted by the metallicity gradient, while they deviates significantly from those predicted by the age gradient even at $z \sim 0.3$ and the deviation is getting larger with increasing redshift. This result does not depend on cosmological parameters and parameters for an evolutionary model of galaxy within a reasonable range. Thus our results clearly indicate that the origin of color gradients is not the age but the stellar metallicity.

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arXiv1999

Observational Test of Environmental Effects on The Local Group Dwarf Spheroidal Galaxies

In this paper, we examine whether tidal forces exerted by the Galaxy or M31 have an influence on the Local Group dwarf spheroidal galaxies (dSphs) which are their companions. We focus on the surface brightness profiles of the dSphs, especially their core radii because it is suggested based on the numerical simulations that tidal disturbance can make core radii extended. We examine the correlation for the dSphs between the distances from their parent galaxy (the Galaxy or M31) and the compactnesses of their surface brightness profiles by using a parameter ``C'' defined newly in this paper. Consequently, we find no significant correlation. We make some remarks on the origin of this result by considering three possible scenarios; tidal picture, dark matter picture, and heterogeneity of the group of dSphs, each of which has been often discussed to understand fundamental properties and formation processes of dSphs.

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