Hitoshi Hanami

Detection of the Cosmic Far-infrared Background in AKARI Deep Field South

We report new limits on the absolute brightness and spatial fluctuations of the cosmic infrared background (CIB) via the AKARI satellite. We carried out observations at 65, 90, 140, and 160 μm as a cosmological survey in AKARI Deep Field South, which is one of the lowest cirrus regions with a contiguous area of the sky. After removing bright galaxies and subtracting zodiacal and Galactic foregrounds from the measured sky brightness, we successfully measured the CIB brightness and its fluctuations across a wide range of angular scales, from arcminutes to degrees. The measured CIB brightness is consistent with previous results reported from COBE data, but significantly higher than the lower limits at 70 and 160 μm obtained via Spitzer from the stacking analysis of selected 24 μm sources. The discrepancy with the Spitzer result is possibly due to a new galaxy population at high redshift obscured by hot dust or unknown diffuse emission. From a power spectrum analysis at 90 μm, two components were identified: the CIB fluctuations with shot noise due to individual galaxies in a small angular scale from the beam size up to 10 arcminutes, and Galactic cirrus emission dominating at the largest angular scales of a few degrees. The overall shape of the power spectrum at 90 μm is very similar to that at longer wavelengths, as observed by Spitzer and the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST). Our power spectrum, with an intermediate angular scale of 10-30 arcminutes, gives a firm upper limit for galaxy clustering, which was found by Spitzer and BLAST. Moreover, the color of the CIB fluctuations, which is obtained by combining our data with the previous results, is as red as ultra-luminous infrared galaxies at high redshift. These galaxies are not likely to provide the majority of the CIB emission at 90 μm, but are responsible for the fluctuations. Our results provide new constraints on the evolution and clustering properties of distant infrared galaxies and any diffuse emission from the early universe.

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.

Evolutionary spectral energy distribution diagnostics of starburst galaxies: signature of bimodality

We construct an evolutionary spectral energy distribution (SED) model of a starburst region, from the ultraviolet to submillimetre wavelengths. This model allows us to derive the star formation rate, optical depth by dust and apparent effective radius of starburst regions at various wavelengths; as a result, the intrinsic surface brightness of starburst regions can be derived. Using this SED model, we analyse 16 ultraviolet-selected starburst galaxies and 10 ultraluminous infrared galaxies. The derived star formation rates and optical depths are compared with emission-line measurements and are found to be consistent. The derived apparent effective radii are also consistent with observations. From the SED analysis, we find a bimodal property of the star formation rate with the optical depth and the compactness of stellar distributions. While mild starbursts have a limiting intrinsic surface brightness L b o l r e - 2 ≃ 10 1 2 L⊙ kpc - 2 , intense starbursts tend to be more heavily obscured and concentrated within a characteristic scale of r e ≃ 0.3 kpc. We suggest that the mild starbursts can be triggered by a self-gravitating disc instability in which feedback is effective in the shallow gravitational potential. On the other hand, the intense starbursts can be induced via an external dynamical perturbation such as galaxy merging, in which feedback is less effective owing to the deep gravitational potential attained by the large gas concentration within the central starburst region.

Deep Extragalactic Surveys around the Ecliptic Poles with AKARI (ASTRO-F)

AKARI (formerly ASTRO-F) is an infrared space telescope designed for an all-sky survey at 10-180 µm, and deep pointed surveys of selected areas at 2-180 µm. The deep pointed surveys with AKARI will significantly advance our understanding of galaxy evolution, the structure formation of the Universe, the nature of the buried AGNs, and the cosmic infrared background. Here we describe the important characteristics of the AKARI mission: the orbit, and the attitude control system, and investigate the optimum survey area based on the updated pre-flight sensitivities of

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.

The AKARI NEP-Deep survey: a mid-infrared source catalogue

We present a new catalogue of mid-IR sources using the AKARI NEP-Deep survey. The InfraRed Camera (IRC) onboard AKARI has a comprehensive mid-IR wavelength coverage with 9 photometric bands at 2–24 μm. We utilized all of these bands to cover a nearly circular area adjacent to the north ecliptic pole (NEP). We designed the catalogue to include most of sources detected in 7, 9, 11, 15 and 18 μm bands, and found 7284 sources in a 0.67 deg 2 area. From our simulations, we estimate that the catalogue is ~80 per cent complete to 200 μJy at 15–18 μm, and ~10 per cent of sources are missed, owing to source blending. Star-galaxy separation is conducted using only AKARI photometry, as a result of which 10 per cent of catalogued sources are found to be stars. The number counts at 11, 15, 18, and 24 μm are presented for both stars and galaxies. A drastic increase in the source density is found in between 11 and 15 μm at the flux level of ~ 300 μJy. This is likely due to the redshifted PAH emission at 8 μm, given our rough estimate of redshifts from an AKARI colour–colour plot. Along with the mid-IR source catalogue, we present optical-NIR photometry for sources falling inside a Subaru/Sprime-cam image covering part of the AKARI NEP-Deep field, which is deep enough to detect most of AKARI mid-IR sources, and useful to study optical characteristics of a complete mid-IR source sample.

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.

J- and Ks-Band Galaxy Counts and Color Distributions in the AKARI North Ecliptic Pole Field

We present the J- and Ks-band galaxy counts and galaxy colors covering 750 arcmin2 in the deep AKARI north ecliptic pole (NEP) field, using the Florida Multi-object Imaging Near-IR Grism Observational Spectrometer on the Kitt Peak National Observatory 2.1 m telescope. The limiting magnitudes with a signal-to-noise ratio of 3 in the deepest regions are 21.85 and 20.15 in the J and Ks bands, respectively, in the Vega magnitude system. The J- and Ks-band galaxy counts in the AKARI NEP field are broadly in good agreement with those of other results in the literature; however, we find some indication of a change in the galaxy number count slope at J ~ 19.5 and over the magnitude range 18.0 < Ks < 19.5. We interpret this feature as a change in the dominant population at these magnitudes because we also find an associated change in the B - Ks color distribution at these magnitudes, where the number of blue samples in the magnitude range 18.5 < Ks < 19.5 is significantly larger than that of Ks < 17.5.

Present‐day scaling relations for submillimetre galaxies: the origin of spheroidal systems

We analyse the spectral energy distributions (SEDs) of 23 submillimetre galaxies and three ISO-detected extremely red objects, all of which have the spectroscopic redshifts, using an evolutionary SED model of starbursts. This SED model allows us to investigate the intrinsic properties of starbursts, such as the starburst age and the mean stellar metallicity, as it consistently takes chemical evolution into account. Also, the intrinsic size of the starburst region is estimated from the observed SEDs. Using this SED model, we predict colours, magnitudes and sizes of present-day descendants of submillimetre galaxies to derive the scaling relations such as the colour‐magnitude and size‐magnitude relations. We argue that submillimetre galaxies are the progenitors of present-day elliptical galaxies, provided that the initial mass function (IMF) of submillimetre galaxies is slightly flatter than the Salpeter IMF. In this case, we find that: (1) the mean present-day magnitude of submillimetre galaxies is similar to that of L ∗ elliptical galaxies; (2) the present-day colour‐magnitude relation is consistent with that of nearby elliptical galaxies; and (3) the present-day size‐magnitude relation of elliptical galaxies can be reproduced if massive submillimetre galaxies consist of multiple starburst regions. Also, we find that starburst regions in submillimetre galaxies are likely to be self-regulated, i.e. the effect of feedback is nearly balanced by the self-gravity of starburst regions. Ke yw ords: dust, extinction ‐ galaxies: starburst ‐ infrared: galaxies ‐ submillimetre.

Star Formation and AGN Activity in Galaxies Classified Using the 1.6 μm Bump and PAH Features at z = 0.4–2

We studied the star-formation and AGN activity of massive galaxies in the redshift range z = 0.4-2, which were detected in a deep survey field using the AKARI InfraRed (IR) astronomical satellite and Subaru telescope toward the North Ecliptic Pole (NEP). The AKARI/IRC Mid-InfraRed (MIR) multiband photometry was used to trace the star-forming activities with Polycyclic Aromatic Hydrocarbon (PAH) emission, which is effective not only to distinguish between star-forming and AGN galaxies, but also to estimate the Star Formation Rate (SFR) with converting its flux to the total emitting IR (TIR) luminosity. In combination with the analyses of the stellar components, we studied the MIR SED features of star-forming and AGN-harboring galaxies, which we summarize below: (1) The rest-frame 7.7-μm and 5-μm luminosities are good tracers of star-forming and AGN activities from their PAH and dusty tori emissions, respectively. (2) For dusty star-forming galaxies without AGN, their SFR shows a correlation that is nearly proportional to their stellar mass, and their specific SFR (sSFR) per unit stellar mass increases with redshift. Extinctions estimated from their TIR luminosities are larger than those from their optical SED fittings, which may be caused by geometric variations of dust in them. (3) Even for dusty star-forming galaxies with AGN, SFRs can be derived from their TIR luminosities with subtraction of the obscured AGN contribution, which indicates that their SFRs were possibly quenched around z ≃ 0.8 compared with those without AGN. (4) The AGN activity from their rest-frame 5-μm luminosity suggests that their Super Massive Black Holes (SMBHs) could already have grown to ≃ 3 × 10 8 M ⊙ in most massive galaxies with 10 11 M ⊙ at z > 1.2, and the mass relation between SMBHs and their host galaxies has already become established by z ≃ 1-2.