ALBERT

Description: Using star-forming galaxies sample in the nearby Universe (0.02 〈  z  〈 0.10) selected from the Sloan Digital Sky Survey (DR7) and Galaxy Evolution Explorer all-sky survey (GR5), we present a new empirical calibration for predicting dust extinction of galaxies from the Hα-to-FUV flux ratio. We find that the Hα dust extinction ( A Hα ) derived with Hα/Hβ ratio (Balmer decrement) increases with increasing Hα/UV ratio as expected, but there remains a considerable scatter around the relation, which is largely dependent on stellar mass and/or Hα equivalent width (EW Hα ). At fixed Hα/UV ratio, galaxies with higher stellar mass (or galaxies with lower EW Hα ) tend to be more highly obscured by dust. We quantify this trend and establish an empirical calibration for predicting A Hα with a combination of Hα/UV ratio, stellar mass, and EW Hα , with which we can successfully reduce the systematic uncertainties accompanying the simple Hα/UV approach by ~15–30 per cent. The new recipes proposed in this study will provide a convenient tool for predicting dust extinction level of galaxies particularly when Balmer decrement is not available. By comparing A Hα (derived with Balmer decrement) and A UV (derived with IR/UV luminosity ratio) for a subsample of galaxies for which AKARI far-infrared photometry is available, we demonstrate that more massive galaxies tend to have higher extra extinction towards the nebular regions compared to the stellar continuum light. Considering recent studies reporting smaller extra extinction towards nebular regions for high-redshift galaxies, we argue that the dust geometry within high-redshift galaxies resembles low-mass galaxies in the nearby Universe.

Description: We investigate the properties of z  = 2.23 Hα and [O iii ] 5007 emitters using the narrow-band-selected samples obtained from the High- z Emission Line Survey. We construct two samples of the Hα and [O iii ] emitters and compare their integrated physical properties. We find that the distribution of stellar masses, dust extinction, star formation rates (SFRs), and specific SFRs (sSFRs) is not statistically different between the two samples. When we separate the full galaxy sample into three subsamples according to the detections of the Hα and/or [O iii ] emission lines, most of the sources detected with both Hα and [O iii ] show log(sSFR UV ) –9.5. The comparison of the three subsamples suggests that sources with strong [O iii ] line emission tend to have the highest star-forming activity out all galaxies that we study. We argue that the [O iii ] emission line can be used as a tracer of star-forming galaxies at high redshift, and that it is especially useful to investigate star-forming galaxies at z  〉 3, for which Hα emission is no longer observable from the ground.

Description: In the redshift interval of 2 〈 z 〈 3, the physical conditions of the interstellar 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 α-selected star-forming galaxies at z = 2.5 taken with Multi-Object Spectrometer for InfraRed Exploration on Keck-1 telescope, we measure electron densities ( n e ) using the oxygen line ratio ([O ii ] 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 ( SFR ) are correlated with the electron density at z = 2.5 for the first time. The SFR – n e relation is likely to be linked to the star formation law in H ii 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 SFR suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation ( SFR ) and thus higher n e values as well.

Description: This work presents the results from our near-infrared spectroscopy of narrow-band-selected Hα emitters (HAEs) in two rich overdensities (PKS 1138–262 at z  = 2.2 and USS 1558–003 at z  = 2.5) with the Multi-Object Infrared Camera and Spectrograph on the Subaru telescope. These protoclusters are promising candidates for the most massive class of galaxy clusters seen today (Paper I). The confirmed HAEs in the protoclusters at z  〉 2 show high excitation levels as characterized by much higher [O iii ]/Hβ or [O iii ]/Hα line ratios than those of general galaxies at low- z . Such a high excitation level may not only be driven by high specific star formation rates and lower gaseous metallicities, but also be contributed by some other effects. We investigate the environmental dependence of gaseous metallicities by comparing the HAEs in the protoclustrers with those in the general field at similar redshifts. We find that the gaseous metallicities of protocluster galaxies are more chemically enriched than those of field galaxies at a given stellar mass in the range of M *   10 11 M . This can be attributed to many processes, such as intrinsic (or nature) effects, external (or nurture) effects, and/or some systematic sampling effects. The intrinsic (nature) effect leads to the advanced stage of ‘downsizing’ galaxy evolution in protoclusters. On the other hand, the external (nurture) effects include the recycling of chemically enriched gas due to the higher pressure of intergalactic medium and/or stripping of outer gas in the reservoir in protoclusters. We also find that the offset of the mass–metallicity relation in dense environment becomes larger at higher redshifts. This can be naturally understood by the fact that the inflow/outflow rates in star-forming galaxies are much higher at higher redshifts. Therefore, the environmental dependence of such ‘feeding’ and ‘feedback’ mechanisms in galaxy formation is probably playing major roles in producing the offset of the mass–metallicity relation for the protocluster galaxies at z  〉 2.

Description: In the redshift interval of 2 〈 z 〈 3, the physical conditions of the interstellar 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 α-selected star-forming galaxies at z = 2.5 taken with Multi-Object Spectrometer for InfraRed Exploration on Keck-1 telescope, we measure electron densities ( n e ) using the oxygen line ratio ([O ii ] 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 ( SFR ) are correlated with the electron density at z = 2.5 for the first time. The SFR – n e relation is likely to be linked to the star formation law in H ii 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 SFR suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation ( SFR ) and thus higher n e values as well.

Description: The galaxy cluster CLG0218.3-0510 at z  = 1.62 is one of the most distant galaxy clusters known, with a rich multiwavelength data set that confirms a mature galaxy population already in place. Using very deep, wide-area (20 Mpc 20 Mpc) imaging by Spitzer MIPS at 24 μm, in conjunction with Herschel five-band imaging from 100 to 500 μm, we investigate the dust-obscured, star formation properties in the cluster and its associated large-scale environment. Our galaxy sample of 693 galaxies at z  ~ 1.62 detected at 24 μm (10 spectroscopic and 683 photo- z ) includes both cluster galaxies (i.e. within r  〈 1 Mpc projected cluster-centric radius) and field galaxies, defined as the region beyond a radius of 3 Mpc. The star formation rates (SFRs) derived from the measured infrared luminosity range from 18 to 2500 M  yr –1 , with a median of 55 M  yr –1 , over the entire radial range (10 Mpc). The cluster's brightest far-infrared galaxy, taken as the centre of the galaxy system, is vigorously forming stars at a rate of 256 ± 70 M  yr –1 , and the total cluster SFR enclosed in a circle of 1 Mpc is 1161 ± 96 M  yr –1 . We estimate a dust extinction of ~3 mag by comparing the SFRs derived from [O ii ] luminosity with the ones computed from the 24 μm fluxes. We find that the in-falling region (1–3 Mpc) is special: there is a significant decrement (3.5 x ) of passive relative to star-forming galaxies in this region, and the total SFR of the galaxies located in this region is lower (~130 M  yr –1 Mpc –2 ) than anywhere in the cluster or field, regardless of their stellar mass. In a complementary approach, we compute the local galaxy density, 5 , and find no trend between SFR and 5 . However, we measure an excess of star-forming galaxies in the cluster relative to the field by a factor of 1.7, that lends support to a reversal of SF–density relation in CLG0218.

Description: We present the results of near-infrared spectroscopy of Hα emitters (HAEs) associated with two protoclusters around radio galaxies (PKS 1138–262 at z = 2.2 and USS 1558–003 at z = 2.5) with the Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope. Among the HAE candidates constructed from our narrow-band imaging, we have confirmed membership of 27 and 36 HAEs for the respective protoclusters, with a success rate of 70 per cent of our observed targets. The large number of spectroscopically confirmed members per cluster has enabled us for the first time to reveal the detailed kinematical structures of the protoclusters at z  〉 2. The clusters show prominent substructures such as clumps, filaments and velocity gradients, suggesting that they are still in the midst of rapid construction to grow to rich clusters at later times. We also estimate the dynamical masses of the clusters and substructures, assuming their local virialization. The inferred masses (~10 14  M ) of the protocluster cores are consistent with their being typical progenitors of the present-day most massive class of galaxy clusters (~10 15  M ) if we take into account the typical mass growth history of clusters. We then calculate the integrated star formation rates of the protocluster cores normalized by the dynamical masses and compare these with lower redshift descendants. We see a marked increase of star-forming activities in the cluster cores, by almost three orders of magnitude, as we go back in time to 11 billion years ago; this scales as (1 + z ) 6 .

Description: We present a novel method to estimate accurate redshifts of star-forming galaxies by measuring the flux ratio of the same emission line observed through two adjacent narrow-band filters. We apply this method to our NB 912 and new NB 921 data taken with Suprime-Cam on the Subaru Telescope of a galaxy cluster, XMMXCS J2215.9–1738, at z  = 1.46 and its surrounding structures. We obtain redshifts for 170 [O ii ] emission line galaxies at z ~ 1.46, among which 41 galaxies are spectroscopically confirmed with Multi-Object Infrared Camera and Spectrograph and Fibre Multi Object Spectrograph on the Subaru mainly, showing an accuracy of (( z  –  z spec )/(1 +  z spec )) = 0.002. This allows us to reveal filamentary structures that penetrate towards the centre of the galaxy cluster and intersect with other structures, consistent with the picture of hierarchical cluster formation. We also find that the projected celestial distribution does not precisely trace the real distribution of galaxies, indicating the importance of the three-dimensional view of structures to properly identify and quantify galaxy environments. We investigate the environmental dependence of galaxy properties with local density, confirming that the median colour of galaxies becomes redder in higher density region, while the star formation rate of star-forming galaxies does not depend strongly on local environment in this structure. This implies that the star-forming activity in galaxies is truncated on a relatively short time-scale in the cluster centre.

Description: This paper discusses the evolution of the correlation between galaxy star formation rates (SFRs) and stellar mass ( M * ) over the last ~10 Gyr, particularly focusing on its environmental dependence. We first present the mid-infrared (MIR) properties of the Hα-selected galaxies in a rich cluster Cl 0939+4713 at z  = 0.4. We use wide-field Spitzer /MIPS 24 μm data to show that the optically red Hα emitters, which are most prevalent in group-scale environments, tend to have higher SFRs and higher dust extinction than the majority population of blue Hα sources. With an MIR stacking analysis, we find that the median SFR of Hα emitters is higher in higher density environment at z  = 0.4. We also find that star-forming galaxies in high-density environment tend to have higher specific SFR (SSFR), although the trend is much less significant compared to that of SFR. This increase of SSFR in high-density environment is not visible when we consider the SFR derived from Hα alone, suggesting that the dust attenuation in galaxies depends on environment; galaxies in high-density environment tend to be dustier (by up to ~0.5 mag), probably reflecting a higher fraction of nucleated, dusty starbursts in higher density environments at z  = 0.4. We then discuss the environmental dependence of the SFR– M * relation for star-forming galaxies since z  ~ 2, by compiling our comparable, narrow-band-selected, large Hα emitter samples in both distant cluster environments and field environments. We find that the SSFR of Hα-selected galaxies (at the fixed mass of log ( M * /M ) = 10) rapidly evolves as (1 + z ) 3 , but the SFR– M * relation is independent of the environment since z  ~ 2, as far as we rely on the Hα-based SFRs (with M * -dependent extinction correction). Even if we consider the possible environmental variation in the dust attenuation, we conclude that the difference in the SFR– M * relation between cluster and field star-forming galaxies is always small (0.2 dex level) at any time in the history of the Universe since z  ~ 2.