Hyunjin Shim

VERY STRONG EMISSION-LINE GALAXIES IN THE WFC3 INFRARED SPECTROSCOPIC PARALLEL SURVEY AND IMPLICATIONS FOR HIGH-REDSHIFT GALAXIES* , **

The WFC3 Infrared Spectroscopic Parallel Survey uses the Hubble Space Telescope (HST) infrared grism capabilities to obtain slitless spectra of thousands of galaxies over a wide redshift range including the peak of star formation history of the universe. We select a population of very strong emission-line galaxies with rest-frame equivalent widths (EWs) higher than 200 A. A total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the 180 arcmin^2 area that we have analyzed so far. This population consists of young and low-mass starbursts with high specific star formation rates (sSFR). After spectroscopic follow-up of one of these galaxies with Keck/Low Resolution Imaging Spectrometer, we report the detection at z = 0.7 of an extremely metal-poor galaxy with 12 + log(O/H) =7.47 ± 0.11. After estimating the active galactic nucleus fraction in the sample, we show that the high-EW galaxies have higher sSFR than normal star-forming galaxies at any redshift. We find that the nebular emission lines can substantially affect the total broadband flux density with a median brightening of 0.3 mag, with some examples of line contamination producing brightening of up to 1 mag. We show that the presence of strong emission lines in low-z galaxies can mimic the color-selection criteria used in the z ~ 8 dropout surveys. In order to effectively remove low-redshift interlopers, deep optical imaging is needed, at least 1 mag deeper than the bands in which the objects are detected. Without deep optical data, most of the interlopers cannot be ruled out in the wide shallow HST imaging surveys. Finally, we empirically demonstrate that strong nebular lines can lead to an overestimation of the mass and the age of galaxies derived from fitting of their spectral energy distribution (SED). Without removing emission lines, the age and the stellar mass estimates are overestimated by a factor of 2 on average and up to a factor of 10 for the high-EW galaxies. Therefore, the contribution of emission lines should be systematically taken into account in SED fitting of star-forming galaxies at all redshifts.

THE WFC3 INFRARED SPECTROSCOPIC PARALLEL (WISP) SURVEY

We present the WFC3 Infrared Spectroscopic Parallel (WISP) Survey. WISP is obtaining slitless, near-infrared grism spectroscopy of ~90 independent, high-latitude fields by observing in the pure-parallel mode with the Wide Field Camera Three on the Hubble Space Telescope for a total of ~250 orbits. Spectra are obtained with the G_(102) (λ = 0.8–1.17 μm, R ~ 210) and G_(141) grisms (λ = 1.11–1.67 μm, R ~ 130), together with direct imaging in the J and H bands (F110W and F140W, respectively). In the present paper, we present the first results from 19 WISP fields, covering approximately 63 arcmin^2. For typical exposure times (~6400 s in G_(102) and ~2700 s in G_(141)), we reach 5σ detection limits for emission lines of f ~ 5 × 10^(−17) erg s^(−1) cm^(−2) for compact objects. Typical direct imaging 5σ limits are 26.3 and 26.1 mag. (AB) in F110W and F140W, respectively. Restricting ourselves to the lines measured with the highest confidence, we present a list of 328 emission lines, in 229 objects, in a redshift range 0.3 < z < 3. The single-line emitters are likely to be a mix of Hα and [O_III]5007,4959 A, with Hα predominating. The overall surface density of high-confidence emission-line objects in our sample is approximately 4 per arcmin^2. These first fields show high equivalent width sources, active galactic nucleus, and post-starburst galaxies. The median observed star formation rate (SFR) of our Hα-selected sample is 4M_⊙ yr^(−1). At intermediate redshifts, we detect emission lines in galaxies as faint as H_(140) ~ 25, or M_R < −19, and are sensitive to SFRs down to less than 1M_⊙ yr^(−1). The slitless grisms on WFC3 provide a unique opportunity to study the spectral properties of galaxies much fainter than L^* at the peak of the galaxy assembly epoch.

Evolution of infrared luminosity functions of galaxies in the AKARI NEP-deep field - Revealing the cosmic star formation history hidden by dust

Aims. Dust-obscured star-formation increases with increasing intensity and increasing redshift. We aim to reveal the cosmic starformation history obscured by dust using deep infrared observation with AKARI. Methods. We constructed restframe 8 μm, 12 μm, and total infrared (TIR) luminosity functions (LFs) at 0.15 < z < 2.2 using 4128 infrared sources in the AKARI NEP-deep field. A continuous filter coverage in the mid-IR wavelength (2.4, 3.2, 4.1, 7, 9, 11, 15, 18, and 24 μm) by the AKARI satellite allowed us to estimate restframe 8 μm and 12 μm luminosities without using a large extrapolation based on an SED fit, which was the largest uncertainty in previous work. Results. We find that all 8 μm (0.38 < z < 2.2), 12 μm (0.15 < z < 1.16), and TIR LFs (0.2 < z < 1.6) show continuous and strong evolution toward higher redshift. Our direct estimate of 8 μm LFs is useful since previous work often had to use a large extrapolation from the Spitzer 24 μm to 8 μm, where SED modeling is more difficult because of the PAH emissions. In terms of cosmic infrared luminosity density (Ω_(IR)), which was obtained by integrating analytic fits to the LFs, we find good agreement with previous work at z < 1.2. We find the ΩIR evolves as ∝(1 + z)^(4.4±1.0). When we separate contributions to Ω_(IR) by LIRGs and ULIRGs, we found more IR luminous sources are increasingly more important at higher redshift. We find that the ULIRG (LIRG) contribution increases by a factor of 10 (1.8) from z = 0.35 to z = 1.4.

Global Star Formation Rate Density over 0.7 < z < 1.9

We determine the global star formation rate (SFR) density at 0.7 <z< 1.9 using emission-line-selected galaxies identified in Hubble Space Telescope-Near Infrared Camera and Multi-Object Spectrograph (HSTNICMOS) grism spectroscopy observations. Observing in a pure parallel mode throughout HST Cycles 12 and 13, our survey covers ∼104 arcmin 2 from which we select 80 galaxies with likely redshifted Hα emission lines. In several cases, a somewhat weaker [Oiii] doublet emission is also detected. The Hα luminosity range of the emission-line galaxy sample is 4.4 × 10 41 <L (Hα) < 1.5 × 10 43 erg s −1 . In this range, the luminosity function is well described by a Schechter function with φ ∗ = (4.24 ± 3.55) × 10 −3 Mpc −3 , L ∗ = (2.88 ± 1.58) × 10 42 erg s −1 , and α =− 1.39 ± 0.43. We derive a volume-averaged SFR density of 0

Polycyclic aromatic hydrocarbon (PAH) luminous galaxies at z 1

Aims. The NEP-deep survey, an extragalactic AKARI survey towards the north ecliptic pole (NEP), provides a comprehensive wavelength coverage from 2 to 24 μm using all 9 photometric bands of the infrared camera (IRC). It allows us to photometrically identify galaxies whose mid-IR emission is clearly dominated by PAHs. Methods. We propose a single-colour selection method to identify such galaxies, using two mid-IR flux ratios at 11-to-7 μ ma nd 15-to-9 μm (PAH-to-continuum flux ratio in the rest frame), which are useful for identifying starburst galaxies at z ∼ 0.5 and 1, respectively. We perform a fitting of the spectral energy distributions (SEDs) from optical to mid-IR wavelengths, using an evolutionary starburst model with a proper treatment of radiative transfer (SBURT), in order to investigate their nature. Results. The SBURT model reproduces observed optical-to-mid-IR SEDs of more than a half of the PAH-selected galaxies. Based on the 8 μm luminosity, we find ultra luminous infrared galaxies (ULIRGs) among PAH-selected galaxies. Their PAH luminosity is higher than local ULIRGs with a similar luminosity, and the PAH-to-total IR luminosity ratio is consistent with that of less luminous starburst galaxies. They are a unique galaxy population at high redshifts, and we call these PAH-selected ULIRGs “PAH-luminous”Aims. Using an AKARI multi-wavelength mid-infrared (IR) survey, we identify luminous starburst galaxies at z >∼ 0.5 based on the PAH luminosity, and investigate the nature of these PAH-sel ect d starbursts. Methods. An extragalactic survey with AKARI towards the north eclipt ic pole (NEP), the NEP-Deep survey, is unique in terms of a comprehensive wavelength coverage from 2 to 24 μm using all 9 photometric bands of the InfraRed Camera (IRC). This survey allows us to photometrically identify galaxies whose mid-IR emiss ion is clearly dominated by PAHs. We propose a single colour s election method to identify such galaxies, using two mid-IR flux ratio s at 11-to-7μm and 15-to-9μm (PAH-to-continuum flux ratio in the rest-frame), which are useful to identify starburst galaxi es atz ∼ 0.5 and 1, respectively. We perform a fitting of the spectral ene rgy distributions (SEDs) from optical to mid-IR wavelengths, u ing an evolutionary starburst model with a proper treatmen of radiative transfer (SBURT), in order to investigate their nature. Results. The SBURT model reproduces observed optical-to-mid-IR SED s of more than a half of PAH-selected galaxies. Based on the 8μm luminosity, we find ultra luminous infrared galaxies (ULIR Gs) among PAH-selected galaxies. Their PAH luminosity is hi gher than local ULIRGs with a similar luminosity, and the PAH-tototal IR luminosity ratio is consistent with that of less lum inous starburst galaxies. They are a unique galaxy population at high redshi fts and we call these PAH-selected ULIRGs “PAH-luminous” ga laxies. Although they are not as massive as submillimetre galaxies a t z ∼ 2, they have the stellar mass of > 3 × 1010 M⊙ and therefore moderately massive.

A Tale of Two Feedbacks: Star Formation in the Host Galaxies of Radio AGNs

Several lines of argument support the existence of a link between activity at the nuclei of galaxies, in the form of an accreting supermassive black hole, and star formation activity in these galaxies. Radio jets have long been argued to be an ideal mechanism that allows active galactic nuclei (AGNs) to interact with their host galaxies and affect star formation. We use a sample of radio sources in the North Ecliptic Pole (NEP) field to study the nature of this putative link, by means of spectral energy distribution (SED) fitting. We employ the excellent spectral coverage of the AKARI infrared space telescope and the rich ancillary data available in the NEP to build SEDs extending from UV to far-IR wavelengths. We find a significant AGN component in our sample of relatively faint radio sources (<mJy). A positive correlation is found between the luminosity of the AGN component and that of star formation in the host galaxy, independent of the radio luminosity. In contrast, for narrow redshift and AGN luminosity ranges, we find that increasing radio luminosity leads to a decrease in the specific star formation rate. The most radio-loud AGNs are found to lie on the main sequence of star formation for their respective redshifts. For the first time, we potentially see such a two-sided feedback process in the same sample. We discuss the possible suppression of star formation, but not total quenching, in systems with strong radio jets, that supports the maintenance nature of feedback from radio AGN jets.

MERGING GALAXY CLUSTER A2255 IN MID-INFRARED

We present the mid-infrared (MIR) observation of a nearby galaxy cluster, A2255, by the AKARI space telescope. Using AKARIs continuous wavelength coverage between 3 and 24 μm and the wide field of view, we investigate the properties of cluster member galaxies to see how the infall of the galaxies, the cluster substructures, and the cluster-cluster merger influence their evolution. We show that the excess of MIR (~11 μm) flux is a good indicator for discriminating galaxies at different evolutionary stages and for dividing galaxies into three classes accordingly: strong MIR-excess (N3 – S11 > 0.2) galaxies that include both unobscured and obscured star-forming galaxies; weak MIR-excess (–2.0 5 Gyr) galaxies where the MIR emission arises mainly from the circumstellar dust around AGB stars; and intermediate MIR-excess (–1.2 < N3 – S11 < 0.2) galaxies in between the two classes that are less than a few Gyr old past the prime star formation activity. With the MIR-excess diagnostics, we investigate how local and cluster-scale environments affect the individual galaxies. We derive the total star formation rate (SFR) and the specific SFR of A2255 using the strong MIR-excess galaxies. The dust-free, total SFR of A2255 is ~130 M_⊙ yr^(–1), which is consistent with the SFRs of other clusters of galaxies at similar redshifts and with similar masses. We find no strong evidence that supports enhanced star formation either inside the cluster or in the substructure region, suggesting that the infall or the cluster merging activities tend to suppress star formation. The intermediate MIR-excess galaxies, representing galaxies in transition from star-forming galaxies to quiescent galaxies, are located preferentially at the medium density region or cluster substructures with higher surface density of galaxies. Our findings suggest that galaxies are being transformed from star-forming galaxies into red, quiescent galaxies from the infall region through near the core which can be explained well by ram-pressure stripping as previous simulation results suggest. We conclude that the cluster merging and the group/galaxy infall suppress star formation and transform galaxies from star-forming galaxies into quiescent galaxies, most likely due to ram-pressure stripping.

Optical - near-infrared catalog for the AKARI north ecliptic pole Deep field

Aims. We present an 8-band (u∗, g′, r′, i′, z′, Y, J, Ks) optical to near-infrared deep photometric catalog based on the observations made with MegaCam and WIRCam at the CFHT, and compute photometric redshifts, zp in the north ecliptic pole (NEP) region. AKARI infrared satellite carried out a deep survey in the NEP region at near- to mid-infrared wavelengths. Our optical to near-infrared catalog allows us to identify the counterparts and zp for the AKARI sources. Methods. We obtained seven-band (g′, r′, i′, z′, Y, J, Ks) imaging data, and we crossmatched them with existing u∗-band data (limiting magnitude = 24.6 mag [5σ; AB]) to design the band-merged catalog. We included all z′-band sources with counterparts in at least one of the other bands in the catalog. We used a template-fitting methods to compute zp for all the cataloged sources. Results. The estimated 4σ detection limits within a 1 arcsec aperture radius are 26.7, 25.9, 25.1, and 24.1 mag [AB] for the optical g′, r′, i′, and z′-bands and 23.4, 23.0, and 22.7 mag for the near-infrared Y, J, and Ks-bands, respectively. There are a total of 85 797 sources in the band-merged catalog. An astrometric accuracy of this catalog determined by examining coordinate offsets with regard to 2MASS is 0.013 arcsec with a root mean square offset of 0.32 arcsec. We distinguish 5441 secure stars from extended sources using the u∗ − J versus g′ − Ks colours, combined with the SExtractor stellarity index of the images. Comparing with galaxy spectroscopic redshifts, we find a photometric redshift dispersion, σΔz/(1 + z), of 0.032 and catastrophic failure rate, Δz/(1 + z) > 0.15, of 5.8% at z 1. We extend the estimate of the zp uncertainty over the full magnitude/redshift space with a redshift probability distribution function and find that our redshifts are highly accurate with z′ < 22 at zp< 2.5 and for fainter sources with z′ < 24 at zp< 1. From the investigation of photometric properties of AKARI infrared sources (23 354 sources) using the g′z′Ks diagram, < 5% of AKARI sources with optical counterparts are classified as high-z (1.4

GALAXY EVOLUTION IN THE MID-INFRARED GREEN VALLEY: A CASE OF THE A2199 SUPERCLUSTER

We study the mid-infrared (MIR) properties of the galaxies in the A2199 supercluster at z = 0.03 to understand the star formation activity of galaxy groups and clusters in the supercluster environment. Using the Wide-field Infrared Survey Explorer data, we find no dependence of mass-normalized integrated SFRs of galaxy groups/clusters on their virial masses. We classify the supercluster galaxies into three classes in the MIR color-luminosity diagram: MIR blue cloud (massive, quiescent and mostly early-type), MIR star-forming sequence (mostly late-type), and MIR green valley galaxies. These MIR green valley galaxies are distinguishable from the optical green valley galaxies, in the sense that they belong to the optical red sequence. We find that the fraction of each MIR class does not depend on virial mass of each group/cluster. We compare the cumulative distributions of surface galaxy number density and cluster/group-centric distance for the three MIR classes. MIR green valley galaxies show the distribution between MIR blue cloud and MIR SF sequence galaxies. However, if we fix galaxy morphology, early- and late-type MIR green valley galaxies show different distributions. Our results suggest a possible evolutionary scenario of these galaxies: 1) Late-type MIR SF sequence galaxies -> 2) Late-type MIR green valley galaxies -> 3) Early-type MIR green valley galaxies -> 4) Early-type MIR blue cloud galaxies. In this sequence, star formation of galaxies is quenched before the galaxies enter the MIR green valley, and then morphological transformation occurs in the MIR green valley.

DISSECTION OF Hα EMITTERS : LOW-z ANALOGS OF z> 4 STAR-FORMING GALAXIES

Strong Hα emitters (HAEs) dominate the z ~ 4 Lyman-break galaxy (LBG) population. We have identified local analogs of these HAEs using the Sloan Digital Sky Survey. At z < 0.4, only 0.04% of the galaxies are classified as HAEs with Hα equivalent widths (≳500 A) comparable to that of z ~ 4 HAEs. Local HAEs have lower stellar mass and lower ultraviolet (UV) luminosity than z ~ 4 HAEs, yet the Hα-to-UV luminosity ratio, as well as their specific star formation rate, is consistent with that of z ~ 4 HAEs, indicating that they are scaled-down versions of high-z star-forming galaxies. Compared to the previously studied local analogs of LBGs selected using rest-frame UV properties, local HAEs show similar UV luminosity surface density, weaker D_n (4000) break, lower metallicity, and lower stellar mass. This implies that the local HAEs are less evolved galaxies than the traditional Lyman break analogs. In the stacked spectrum, local HAEs show a significant He II λ4686 emission line suggesting a population of hot, massive stars similar to that seen in some Wolf-Rayet galaxies. Low [N II]/[O III] line flux ratios imply that local HAEs are inconsistent with being systems that host bright active galactic nuclei. Instead, it is highly likely that local HAEs are galaxies with an elevated ionization parameter, either due to a high electron density or large escape fraction of hydrogen ionizing photons as in the case of Wolf-Rayet galaxies.