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

During the course of our optical deep survey program on L$\alpha$ emitters at $z \approx 5.7$ in the sky area surrounding the quasar SDSSp J104433.04$-$012502.2 at $z=5.74$, we found that a faint galaxy with $m_B$(AB) $\approx 25$ is located at \timeform{1".9} southwest of the quasar. Its broad-band color properties from $B$ to $z^\prime$ suggest that the galaxy is located at a redshift of $z \sim 1.5$ -- 2.5. This is consistent with no strong emission line in our optical spectroscopy. Since the counter image of the quasar cannot be seen in our deep optical images, the magnification factor seems not to be very high. Our modest estimate is that this quasar is gravitationally magnified by a factor of 2.Comment: 11 pages, 5 figures, PASJ, in pres

The Discovery of a Very Narrow-Line Star Forming Obat a Redshift of 5.66ject

We report on the discovery of a very narrow-line star forming object beyond 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 $\lambda_{\rm c}$ = 8150 \AA with passband of $\Delta\lambda$ = 120 \AA, 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 broad-band 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 $z=5.655\pm0.002$, forming stars at a rate $\sim 13 ~ h_{0.7}^{-2} ~ M_\odot$ yr$^{-1}$. Remarkably, the velocity dispersion of Ly$\alpha$-emitting gas is only 22 km s$^{-1}$. Since a blue half of the Ly$\alpha$ emission could be absorbed by neutral hydrogen gas, perhaps in the system, a modest estimate of the velocity dispersion may be $\gtrsim$ 44 km s$^{-1}$. Together with a linear size of 7.7 $h_{0.7}^{-1}$ kpc, we estimate a lower limit of the dynamical mass of this object to be $\sim 2 \times 10^9 M_\odot$. It is thus suggested that LAE J1044$-$0123 is a star-forming dwarf galaxy (i.e., a subgalactic object or a building block) beyond redshift 5 although we cannot exclude a possibility that most Ly$\alpha$ emission is absorbed by the red damping wing of neutral intergalactic matter.Comment: 6 pages, 2 figures. ApJ Letters, in pres

Down-Sizing in Galaxy Formation at z~1 in the Subaru/XMM-Newton Deep Survey (SXDS)

We use the deep wide-field optical imaging data of the Subaru/XMM-Newton Deep Survey (SXDS) to discuss the luminosity (mass) dependent galaxy colours down to z'=25.0 (5 x 10^9 h_{70}^{-2} Msun) for z~1 galaxies in colour-selected high density regions. We find an apparent absence of galaxies on the red colour-magnitude sequence below z'~24.2, corresponding to ~M*+2 (~10^{10} Msun) with respect to passively evolving galaxies at z~1. Galaxies brighter than M*-0.5 (8 x 10^{10} Msun), however, are predominantly red passively evolving systems, with few blue star forming galaxies at these magnitudes. This apparent age gradient, where massive galaxies are dominated by old stellar populations while less massive galaxies have more extended star formation histories, supports the `down-sizing' idea where the mass of galaxies hosting star formation decreases as the Universe ages. Combined with the lack of evolution in the shape of the stellar mass function for massive galaxies since at least z~1, it appears that galaxy formation processes (both star formation and mass assembly) should have occurred in an accelerated way in massive systems in high density regions, while these processes should have been slower in smaller systems. This result provides an interesting challenge for modern CDM-based galaxy formation theories which predict later formation epochs of massive systems, commonly referred to as ``bottom-up''.Comment: proof corrected version (MNRAS in press), 10 pages, 12 figures (of which 3 are in jpg format