Enhanced Star Formation of Less Massive Galaxies in a Proto-Cluster at z=2.5

We investigate a correlation between star-formation rate (SFR) and stellar mass for Halpha emission line galaxies (HAEs) in one of the richest proto-clusters ever known at z~2.5, USS 1558-003 proto-cluster. This study is based on a 9.7-hour narrow-band imaging data with MOIRCS on the Subaru telescope. We are able to construct a sample, in combination with additional H-band data taken with WFC3 on Hubble Space Telescope (HST), of 100 HAEs reaching the dust-corrected SFRs down to 3 Msun/yr and the stellar masses down to $10^{8.0}$ Msun. We find that while the star-forming galaxies with >$10^{9.3}$ Msun are located on the universal SFR-mass main sequence irrespective of the environment, less massive star-forming galaxies with <$10^{9.3}$ Msun show a significant upward scatter from the main sequence in this proto-cluster. This suggests that some less massive galaxies are in a starburst phase, although we do not know yet if this is due to environmental effects.Comment: 5 pages, 3 figures, 1 table, accepted for publication in the ApJ Letter

Similarities and uniqueness of Lyα\alpha emitters among star-forming galaxies at z=2.5

We conducted a deep narrow-band imaging survey with the Subaru Prime Focus Camera on the Subaru Telescope and constructed a sample of Ly$\alpha$ emitters (LAEs) at z=2.53 in the UDS-CANDELS field where a sample of H$\alpha$ emitters (HAEs) at the same redshift is already obtained from our previous narrow-band observation at NIR. The deep narrow-band and multi broadband data allow us to find LAEs of stellar masses and star-formation rates (SFRs) down to $\gtrsim$$10^8$ M$_\odot$ and $\gtrsim$0.2 M$_\odot$/yr, respectively. We show that the LAEs are located along the same mass-SFR sequence traced by normal star-forming galaxies such as HAEs, but towards a significantly lower mass regime. Likewise, LAEs seem to share the same mass--size relation with typical star-forming galaxies, except for the massive LAEs, which tend to show significantly compact sizes. We identify a vigorous mass growth in the central part of LAEs: the stellar mass density in the central region of LAEs increases as their total galaxy mass grows. On the other hand, we see no Ly$\alpha$ line in emission for most of the HAEs. Rather, we find that the Ly$\alpha$ feature is either absent or in absorption (Ly$\alpha$ absorbers; LAAs), and its absorption strength may increase with reddening of the UV continuum slope. We demonstrate that a deep Ly$\alpha$ narrow-band imaging like this study is able to search for not only LAEs but also LAAs in a certain redshift slice. This work suggests that LAEs trace normal star-forming galaxies in the low-mass regime, while they remain as a unique population because the majority of HAEs are not LAEs.Comment: 20 pages, 18 figures, 3 tables, accepted for publication in MNRA

Correlation between star formation activity and electron density of ionized gas at z=2.5

In the redshift interval of $2<z<3$, the physical conditions of the inter-stellar medium (ISM) in star-forming galaxies are likely to be different from those in the local Universe because of lower gaseous metallicities, higher gas fractions, and higher star formation activities. In fact, observations suggest that higher electron densities, higher ionization parameters, and harder UV radiation fields are common. In this paper, based on the spectra of H$\alpha$-selected star-forming galaxies at $z=2.5$ taken with Multi-Object Spectrometer for InfraRed Exploration (MOSFIRE) on Keck-1 telescope, we measure electron densities ($n_e$) using the oxygen line ratio ( [OII]$\lambda\lambda$3726,3729), and investigate the relationships between the electron density of ionized gas and other physical properties. As a result, we find that the specific star formation rate (sSFR) and the surface density of SFR ($\Sigma_\mathrm{SFR}$) are correlated with the electron density at $z=2.5$ for the first time. The $\Sigma_\mathrm{SFR}-n_e$ relation is likely to be linked to the star formation law in HII regions (where star formation activity is regulated by interstellar pressure). Moreover, we discuss the mode of star formation in those galaxies. The correlation between sSFR and $\Sigma_\mathrm{SFR}$ suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation ($\Sigma_\mathrm{SFR}$) and thus higher $n_e$ values as well.Comment: 7 pages, 1 table, 4 figures, title is changed, accepted to MNRA