Predicting dust extinction properties of star-forming galaxies from H-alpha/UV ratio

Using star-forming galaxies sample in the nearby Universe (0.02<z<0.10) selected from the SDSS (DR7) and GALEX all-sky survey (GR5), we present a new empirical calibration for predicting dust extinction of galaxies from H-alpha-to-FUV flux ratio. We find that the H-alpha dust extinction (A(Ha)) derived with H-alpha/H-beta ratio (Balmer decrement) increases with increasing H-alpha/UV ratio as expected, but there remains a considerable scatter around the relation, which is largely dependent on stellar mass and/or H-alpha equivalent width (EW(Ha)). At fixed H-alpha/UV ratio, galaxies with higher stellar mass (or galaxies with lower EW(Ha)) tend to be more highly obscured by dust. We quantify this trend and establish an empirical calibration for predicting A(Ha) with a combination of H-alpha/UV ratio, stellar mass and EW(Ha), with which we can successfully reduce the systematic uncertainties accompanying the simple H-alpha/UV approach by ~15-30%. 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(Ha) (derived with Balmer decrement) and A(UV) (derived with IR/UV luminosity ratio) for a subsample of galaxies for which AKARI FIR 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 resemble more like low-mass galaxies in the nearby Universe.

Environmental dependence of polycyclic aromatic hydrocarbon emission at z~0.8. Investigation by observing the RX J0152.7-1357 with AKARI

We study the environmental dependence of the strength of polycyclic aromatic hydrocarbon (PAH) emission by AKARI observations of RX J0152.7-1357, a galaxy cluster at z=0.84. PAH emission reflects the physical conditions of galaxies and dominates 8 um luminosity (L8), which can directly be measured with the L15 band of AKARI. L8 to infrared luminosity (LIR) ratio is used as a tracer of the PAH strength. Both photometric and spectroscopic redshifts are applied to identify the cluster members. The L15-band-detected galaxies tend to reside in the outskirt of the cluster and have optically green colour, R-z~ 1.2. We find no clear difference of the L8/LIR behaviour of galaxies in field and cluster environment. The L8/LIR of cluster galaxies decreases with specific-star-formation rate divided by that of main-sequence galaxies, and with LIR, consistent with the results for field galaxies. The relation between L8/LIR and LIR is between those at z=0 and z=2 in the literature. Our data also shows that starburst galaxies, which have lower L8/LIR than main-sequence, are located only in the outskirt of the cluster. All these findings extend previous studies, indicating that environment affects only the fraction of galaxy types and does not affect the L8/LIR behaviour of star-forming galaxies.

Large-scale environment of z~5.7 CIV absorption systems I: projected distribution of galaxies

Metal absorption systems are products of star formation. They are believed to be associated with massive star forming galaxies, which have significantly enriched their surroundings. To test this idea with high column density CIV absorption systems at z~5.7, we study the projected distribution of galaxies and characterise the environment of CIV systems in two independent quasar lines-of-sight: J103027.01+052455.0 and J113717.73+354956.9. Using wide field photometry (~80x60h$^{-1}$ comoving Mpc), we select bright (Muv(1350AA)<-21.0 mag.) Lyman break galaxies (LBGs) at z~5.7 in a redshift slice Delta z~0.2 and we compare their projected distribution with z~5.7 narrow-band selected Lyman alpha emitters (LAEs, Delta z~0.08). We find that the CIV systems are located more than 10h$^{-1}$ projected comoving Mpc from the main concentrations of LBGs and no candidate is closer than ~5h$^{-1}$ projected comoving Mpc. In contrast, an excess of LAEs -lower mass galaxies- is found on scales of ~10h$^{-1}$ comoving Mpc, suggesting that LAEs are the primary candidates for the source of the CIV systems. Furthermore, the closest object to the system in the field J1030+0524 is a faint LAE at a projected distance of 212h$^{-1}$ physical kpc. However, this work cannot rule out undiscovered lower mass galaxies as the origin of these absorption systems. We conclude that, in contrast with lower redshift examples (z<3.5), strong CIV absorption systems at z~5.7 trace low-to-intermediate density environments dominated by low-mass galaxies. Moreover, the excess of LAEs associated with high levels of ionizing flux agrees with the idea that faint galaxies dominate the ionizing photon budget at this redshift.

Mapping the large scale structure around a z=1.46 galaxy cluster in 3-D using two adjacent narrow-band filters

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 NB912 and new NB921 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 [OII] emission line galaxies at z~1.46, among which 41 galaxies are spectroscopically confirmed with MOIRCS and FMOS on the Subaru mainly, showing an accuracy of $sigma$((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.

Environmental dependence of 8um luminosity functions of galaxies at z~0.8: Comparison between RXJ1716.4+6708 and the AKARI NEP deep field

We aim to reveal environmental dependence of infrared luminosity functions (IR LFs) of galaxies at z~0.8 using the AKARI satellite. We construct restframe 8um IR LFs in the cluster region RXJ1716.4+6708 at z=0.81, and compare them with a blank field using the AKARI North Ecliptic Pole deep field data at the same redshift. AKARIs wide field of view (10x10) is suitable to investigate wide range of galaxy environments. AKARIs 15um filter is advantageous here since it directly probes restframe 8um at z~0.8, without relying on a large extrapolation based on a SED fit, which was the largest uncertainty in previous work. We have found that cluster IR LFs at restframe 8um have a factor of 2.4 smaller L^* and a steeper faint-end slope than that of the field. Confirming this trend, we also found that faint-end slopes of the cluster LFs becomes flatter and flatter with decreasing local galaxy density. These changes in LFs cannot be explained by a simple infall of field galaxy population into a cluster. Physics that can preferentially suppress IR luminous galaxies in high density regions is required to explain the observed results.

High star formation activity in the central region of a distant cluster at z=1.46

We present an unbiased deep [OII] emission survey of a cluster XMMXCS J2215.9-1738 at z=1.46, the most distant cluster to date with a detection of extended X-ray emission. With wide-field optical and near-infrared cameras (Suprime-Cam and MOIRCS, respectively) on Subaru telescope, we performed deep imaging with a narrow-band filter NB912 (lambda_c=9139A, Delta_lambda=134A) as well as broad-band filters (B, z, J and Ks). From the photometric catalogues, we have identified 44 [OII] emitters in the cluster central region of 6x6 down to a dust-free star formation rate of 2.6 Msun/yr (3 sigma). Interestingly, it is found that there are many [OII] emitters even in the central high density region. In fact, the fraction of [OII] emitters to the cluster members as well as their star formation rates and equivalent widths stay almost constant with decreasing cluster-centric distance up to the cluster core. Unlike clusters at lower redshifts (z<1) where star formation activity is mostly quenched in their central regions, this higher redshift 2215 cluster shows its high star formation activity even at its centre, suggesting that we are beginning to enter the formation epoch of some galaxies in the cluster core eventually. Moreover, we find a deficit of galaxies on the red sequence at magnitudes fainter than ~M*+0.5 on the colour-magnitude diagram. This break magnitude is brighter than that of lower redshift clusters, and it is likely that we are seeing the formation phase of more massive red galaxies in the cluster core at z~1. These results may indicate inside-out and down-sizing propagation of star formation activity in the course of cluster evolution.

Dependence of the Build-up of the Colour-Magnitude Relation on Cluster Richness at z ~ 0.8

We present environmental dependence of the build-up of the colour-magnitude relation (CMR) at z ~ 0.8. It is well established that massive early-type galaxies exhibit a tight CMR in clusters up to at least z ~ 1. The faint end of the relation, however, has been much less explored especially at high redshifts primarily due to limited depths of the data. Some recent papers have reported a deficit of the faint red galaxies on the CMR at 0.8 < z < 1, but this has not been well confirmed yet and is still controversial. Using a deep, multi-colour, panoramic imaging data set of the distant cluster RXJ1716.4+6708 at z=0.81, newly taken with the Prime Focus Camera (Suprime-Cam) on the Subaru Telescope, we carry out an analysis of faint red galaxies with a care for incompleteness. We find that there is a sharp decline in the number of red galaxies toward the faint end of the CMR below M*+2. We compare our result with those for other clusters at z ~ 0.8 taken from the literature, which show or do not show the deficit. We suggest that the deficit of faint red galaxies is dependent on the richness or mass of the clusters, in the sense that poorer systems show stronger deficits. This indicates that the evolutionary stage of less massive galaxies depends critically on environment.