arXiv subjects

Tomotsugu Goto

Publications and source records attributed to Tomotsugu Goto.

At least 163 records · Page 9Linked to original sources

arXiv2011

Star Formation and AGN activity in Galaxies classified using the 1.6 μm Bump and PAH features at $z = 0.4-2$

We have studied the star-formation and AGN activity of massive galaxies in the redshift range $z=0.4-2$, which are detected in a deep survey field using the AKARI InfraRed (IR) astronomical satellite and {\em Subaru} telescope toward the North Ecliptic Pole (NEP). The AKARI/IRC Mid-InfraRed (MIR) multiband photometry is used to trace their star-forming activities with the Polycyclic-Aromatic Hydrocarbon (PAH) emissions, which is also used to distinguish star-forming populations from AGN dominated ones and to estimate the Star Formation Rate (SFR) derived from their total emitting IR (TIR) luminosities. In combination with analyses of their stellar components, we have studied the MIR SED features of star-forming and AGN-harboring galaxies.

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arXiv2011

Spectroscopy of the spatially-extended Lya emission around a QSO at z=6.4

We have taken a deep, moderate-resolution Keck/Deimos spectra of QSO, CFHQS2329, at z=6.4. At the wavelength of Lya, the spectrum shows a spatially-extended component, which is significantly more extended than a stellar spectrum, and also a continuum part of the spectrum. The restframe line width of the extended component is 21+-7 A, and thus smaller than that of QSO (52+-4 A), where they should be identical if the light is incomplete subtraction of the QSO component. Therefore, these comparisons argue for the detection of a spatially extended Lya nebulae around this QSO. This is the first z>6 QSO that an extended Lya halo has been observed around. Careful subtraction of the central QSO spectrum reveals a lower limit to the Lya luminosity of (1.7+-0.1)x 10^43 erg s^-1. This emission may be from the theoretically predicted infalling gas in the process of forming a primordial galaxy that is ionized by a central QSO. On the other hand, if it is photoionized by the host galaxy, an estimated star-formation rate of >3.0 Msun yr^-1 is required. If we assume the gas is virialized, we obtain dynamical mass estimate of Mdyn=1.2x10^12 Msun. The derived MBH/Mhost is 2.1x10^-4, which is two orders smaller than those from more massive z~6 QSOs, and places this galaxy in accordance with the local M-sigma relation, in contrast to a previous claim on the evolution of M-sigma relation at z~6. We do not claim evolution or non-evolution of the M-sigma relation based on a single object, but our result highlights the importance of investigating fainter QSOs at z~6.

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arXiv2011

Far Infrared Luminosity Function of Local Star-forming Galaxies in the AKARI Deep Field South

We present a far-infrared galaxy luminosity function for the local universe. We have obtained 389 spectroscopic redshifts for galaxies observed at 90 microns in the AKARI Deep Field South, using the AAOmega fibre spectrograph via optical identifications in the digitized sky survey and 4m-class optical imaging. For the luminosity function presented in this paper, we have used those galaxies which have redshifts 0<z<0.25, have optical magnitudes and are not part of a newly discovered cluster of galaxies (giving a total of 130 sources). Infrared and optical completeness functions were estimated using earlier Spitzer data and APM B-band optical data respectively, and the luminosity function has been prepared using the 1/Vmax method. We also separate the luminosity function between galaxies which show evidence of predominantly star-forming activity and predominantly active galactic nucleus (AGN) activity in their optical spectra. Our luminosity function is in good agreement with the previous 90 micron luminosity function from the European Large Area ISO Survey, and we also present a luminosity function with combined AKARI and ISO data. The result is in reasonable agreement with predictions based on the earlier IRAS 60 micron PSC-z catalogue, and with a recent backward evolution model.

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arXiv2011

A Gunn-Peterson test with a QSO at z=6.4

Understanding the cosmic re-ionization is one of the key goals of the modern observational cosmology. High redshift QSO spectra can be used as background light sources to measure absorption by intervening neutral hydrogen. We investigate neutral hydrogen absorption in a deep, moderate-resolution Keck/Deimos spectrum of QSO CFHQSJ2329-0301 at z=6.4. This QSO is one of the highest redshift QSOs presently known at z=6.4 but is 2.5 mag fainter than a previously well-studied QSO SDSSJ1148+5251 at z=6.4. Therefore, it has a smaller Stromgren sphere, and allows us to probe the highest redshift hydrogen absorption to date. The average transmitted flux at 5.915 4.9. These results are consistent with strong evolution of the optical depth at z>5.7.

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arXiv2011

Infrared Luminosity Functions of AKARI-SDSS Galaxies

By cross-correlating AKARI infrared (IR) sources with the SDSS galaxies, we identified 2357 infrared galaxies with a spectroscopic redshift. This is not just one of the largest samples of local IR galaxies, but AKARI provides crucial FIR bands (9, 18, 65, 90, 140, and 160um) in accurately measuring galaxy SED across the peak of the dust emission at ~100um. By fitting modern IR SED models to the AKARI photometry, we measured the total infrared luminosity (L_IR) of individual galaxies more accurately. Using this L_IR, we constructed luminosity functions of infrared galaxies at a median redshift of z=0.031, with 4 times larger sample than previous work. The LF agrees well with that at z=0.0082 (RBGS), showing smooth and continuous evolution toward higher redshift LFs measured in the AKARI NEP deep field. The derived local cosmic IR luminosity density is Omega_IR=3.8x10^8 LsunMpc^-3. We separate galaxies into AGN, star-forming, and composite by using the [NII]/Ha vs [OIII]/Hb line ratios. The fraction of AGN shows a continuous increase with increasing L_IR from 25% to 90% at 9 10^11Lsun, coinciding the break of the both SFG and AGN IR LFs. At L_IR<10^11Lsun, SFG dominates IR Lfs. Only 1.1% of Omega_IR is produced by LIRG, and only 0.03% is by ULIRG in the local Universe. This work also provides the most accurate infrared luminosity density of the local Universe to date. Compared with high redshift results from the AKARI NEP deep survey, we observed a strong evolution of Omega_IR^SFG ~(1+z)^4.1+-0.4 and Omega_IR^AGN ~(1+z)^4.1+-0.5 (abridged).

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arXiv2011

Spatially Resolved Spectroscopic Observations of a Possible E+A Progenitor SDSS J160241.00+521426.9

In order to investigate the evolution of E+A galaxies, we observed a galaxy SDSS J160241.00+521426.9, a possible E+A progenitor which shows both emission and strong Balmer absorptions, and its neighbor galaxy. We used the integral field spectroscopic mode of the Kyoto Tridimensional Spectrograph (Kyoto3DII), mounted on the University of Hawaii 88-inch telescope located on Mauna Kea, and the slit-spectroscopic mode of the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope. We found a strong Balmer absorption region in the center of the galaxy and an emission-line region located 2 kpc from the center, in the direction of its neighbor galaxy. The recession velocities of the galaxy and its neighbor galaxy differ only by 100 km s^-1, which suggests that they are a physical pair and would have been interacting. Comparing observed Lick indices of Balmer lines and color indices with those predicted from stellar population synthesis models, we find that a suddenly quenched star-formation scenario is plausible for the star-formation history of the central region. We consider that star formation started in the galaxy due to galaxy interactions and was quenched in the central region, whereas star formation in a region offset from the center still continues or has begun recently. This work is the first study of a possible E+A progenitor using spatially resolved spectroscopy.

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arXiv2010

Merging Galaxy Cluster Abell 2255 in Mid-Infrared

We present the mid-infrared (MIR) observation of a nearby galaxy cluster, Abell 2255 by the AKARI space telescope. Using the AKARI's continuous wavelength coverage between 3-24 micron 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 micron) flux is a good indicator to discriminate galaxies at different evolutionary stages, and divide 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 Gyrs 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 of ~130 Msun/yr for A2255 using the strong MIR-excess galaxies, which is consistent with other clusters of galaxies at similar redshifts and with similar masses. We find no strong evidence that supports enhanced star formation neither inside the cluster nor in the substructure region. 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. 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 well-explained by the ram-pressure stripping as previous simulation suggests.

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arXiv2010

A Large Number of z > 6 Galaxies around a QSO at z = 6.43: Evidence for a Protocluster?

QSOs have been thought to be important for tracing highly biased regions in the early universe, from which the present-day massive galaxies and galaxy clusters formed. While overdensities of star-forming galaxies have been found around QSOs at 2 6 is less clear. Previous studies with HST have reported the detection of small excesses of faint dropout galaxies in some QSO fields, but these surveys probed a relatively small region surrounding the QSOs. To overcome this problem, we have observed the most distant QSO at z=6.4 using the large field of view of the Suprime-Cam (34' x 27'). Newly-installed CCDs allowed us to select Lyman break galaxies (LBG) at z~6.4 more efficiently. We found seven LBGs in the QSO field, whereas only one exists in a comparison field. The significance of this apparent excess is difficult to quantify without spectroscopic confirmation and additional control fields. The Poisson probability to find seven objects when one expects four is ~10%, while the probability to find seven objects in one field and only one in the other is less than 0.4%, suggesting that the QSO field is significantly overdense relative to the control field. We find some evidence that the LBGs are distributed in a ring-like shape centered on the QSO with a radius of ~3 Mpc. There are no candidate LBGs within 2 Mpc from the QSO, i.e., galaxies are clustered around the QSO but appear to avoid the very center. These results suggest that the QSO is embedded in an overdense region when defined on a sufficiently large scale. This suggests that the QSO was indeed born in a massive halo. The central deficit of galaxies may indicate that (1) the strong UV radiation from the QSO suppressed galaxy formation in its vicinity, or (2) that star-formation closest to the QSO occurs mostly in an obscured mode that is missed by our UV selection.

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arXiv2010

Luminosity Functions of Local Infrared Galaxies with AKARI: Implications to the Cosmic Star Formation History and AGN Evolution

Infrared (IR) luminosity is fundamental to understanding the cosmic star formation history and AGN evolution. The AKARI IR space telescope performed all sky survey in 6 IR bands (9, 18, 65, 90, 140, and 160um) with 3-10 times better sensitivity than IRAS, covering the crucial far-IR wavelengths across the peak of the dust emission. Combined with a better spatial resolution, AKARI can much more precisely measure the total infrared luminosity (L_TIR) of individual galaxies, and thus, the total infrared luminosity density in the local Universe. By fitting IR SED models, we have re-measured L_TIR of the IRAS Revised Bright Galaxy Sample. We present mid-IR monochromatic luminosity to L_TIR conversions for Spitzer 8,24um, AKARI 9,18um, IRAS 12um, WISE 12,22um, and ISO 15um filters, with scatter ranging 13-44%. The resulting AKARI IR luminosity function (LF) agrees well with that from the IRAS. We integrate the LF weighted by L_TIR to obtain a cosmic IR luminosity density of Omega_TIR= (8.5^{+1.5}_{-2.3})x 10^7 L Mpc^-3, of which 7+-1% is produced by LIRGs, and only 0.4+-0.1% is from ULIRGs in the local Universe. Once IR contributions from AGN and star-forming galaxies (SFG) are separated, SFG IR LF shows a steep decline at the bright-end. Compared with high-redshift results from the AKARI NEP deep survey, these data show a strong evolution of Omega_TIRSF propto (1+z)^4.0+-0.5, and Omega_TIRAGN propto (1+z)^4.4+-0.4. For Omega_TIRAGN, the ULIRG contribution exceeds that from LIRG already by z~1. A rapid evolution in both Omega_TIRAGN and Omega_TIRSFG suggests the correlation between star formation and black hole accretion rate continues up to higher redshifts. We compare the evolution of Omega_TIRAGN to that of X-ray luminosity density. The Omega_TIRAGN/Omega_X-rayAGN ratio shows a possible increase at z>1, suggesting an increase of obscured AGN at z>1.

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arXiv2010

A deep radio survey of the AKARI North Ecliptic Pole Field - WSRT 20 cm Radio survey description, observations and data reduction

The Westerbork Radio Synthesis Telescope, WSRT, has been used to make a deep radio survey of an ~ 1.7 sq degree field coinciding with the AKARI North Ecliptic Pole Deep Field. The observations, data reduction and source count analysis are presented, along with a description of the overall scientific objectives. The survey consisted of 10 pointings, mosaiced with enough overlap to maintain a similar sensitivity across the central region that reached as low as 21 microJy per beam at 1.4 GHz. A catalogue containing 462 sources detected with a resolution of 17"x15" is presented. The differential source counts calculated from the WSRT data have been compared with those from the shallow VLA-NEP survey of Kollgaard et al 1994, and show a pronounced excess for sources fainter than ~ 1 mJy, consistent with the presence of a population of star forming galaxies at sub-mJy flux levels. The AKARI North Ecliptic Pole Deep field is the focus of a major observing campaign conducted across the entire spectral region. The combination of these data sets, along with the deep nature of the radio observations will allow unique studies of a large range of topics including the redshift evolution of the luminosity function of radio sources, the clustering environment of radio galaxies, the nature of obscured radio-loud active galactic nuclei, and the radio/far-infrared correlation for distant galaxies. This catalogue provides the basic data set for a future series of paper dealing with source identifications, morphologies, and the associated properties of the identified radio sources.

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arXiv2010

Evolution of Infrared Luminosity functions of Galaxies in the AKARI NEP-Deep field: Revealing the cosmic star formation history hidden by dust

Dust-obscured star-formation becomes much more important with increasing intensity, and increasing redshift. We aim to reveal cosmic star-formation history obscured by dust using deep infrared observation with the AKARI. We construct restframe 8um, 12um, 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 24um) by the AKARI satellite allows us to estimate restframe 8um and 12um luminosities without using a large extrapolation based on a SED fit, which was the largest uncertainty in previous work. We have found that all 8um (0.38<z<2.2), 12um (0.15<z<1.16), and TIR LFs (0.2<z<1.6), show a continuous and strong evolution toward higher redshift. In terms of cosmic infrared luminosity density (Omega_IR), which was obtained by integrating analytic fits to the LFs, we found a good agreement with previous work at z<1.2, and that the Omega_IR evolves as propto (1+z)^4.4+-1.0. When we separate contributions to Omega_IR by LIRGs and ULIRGs, we found more IR luminous sources are increasingly more important at higher redshift. We found that the ULIRG (LIRG) contribution increases by a factor of 10 (1.8) from z=0.35 to z=1.4.

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arXiv2009

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. AKARI's wide field of view (10'x10') is suitable to investigate wide range of galaxy environments. AKARI's 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.

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arXiv2009

Cosmic star formation history revealed by the AKARI, & Spatially-resolved spectroscopy of an E+A (Post-starburst) system

We reveal cosmic star-formation history obscured by dust using deep infrared observation with the AKARI. A continuous filter coverage in the mid-IR wavelength (2.4, 3.2, 4.1, 7, 9, 11, 15, 18, and 24um) by the AKARI satellite allows us to estimate restframe 8um and 12um luminosities without using a large extrapolation based on a SED fit, which was the largest uncertainty in previous work. We found that restframe 8um (0.38<z<2.2), 12um (0.15<z<1.16), and total infrared (TIR) luminosity functions (LFs) (0.2<z<1.6) constructed from the AKARI NEP deep data, show a continuous and strong evolution toward higher redshift. In terms of cosmic infrared luminosity density (Omega_IR), which was obtained by integrating analytic fits to the LFs, we found a good agreement with previous work at z<1.2, with Omega_IR propto (1+z)^4.4+-1.0. When we separate contributions to Omega_IR by LIRGs and ULIRGs, we found more IR luminous sources are increasingly more important at higher redshift. We found that the ULIRG (LIRG) contribution increases by a factor of 10 (1.8) from z=0.35 to z=1.4.

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arXiv2009

A QSO host galaxy and its Lyalpha emission at z=6.43

We report an optical detection of an extended structure around a QSO at z=6.43 (CFHQSJ2329-0301, the highest redshift QSO currently known) in deep z' and z_r-band images of the Subaru/Suprime-Cam. After a careful PSF (QSO) subtraction, a structure in the z'-band extends more than 4'' on the sky (R_e=11 kpc), and thus, is well-resolved (16sigma detection). The PSF-subtracted z_r-band structure is in a similar shape to that in the z'-band, but less significant with a 3 sigma detection. In the z'-band, a radial profile of the QSO+host shows a clear excess over that of the averaged PSF in 0.8-3'' radius. Since the z'-band includes a Lya emission at z=6.43, the z' flux is perhaps a mixture of the host (continuum light) and its Lya emission, whereas the z_r-band flux is from the host. Through a SED modeling, we estimate 40% of the PSF-subtracted z'-band light is from the host (continuum) and 60% is from Lya emission. The absolute magnitude of the host is M_{1450}=-23.9 (c.f. M_{1450}=-26.4 for the QSO). A lower limit of the SFR(Lya) is 1.6 Msun yr^{-1} with stellar mass ranging 6.2 x 10^8 to 1.1 x 10^10 Msun when 100 Myrs of age is assumed. The detection shows that a luminous QSO is already harbored by a large, star-forming galaxy in the early Universe only after ~840 Myr after the big bang. The host may be a forming giant galaxy, co-evolving with a super massive black hole.

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arXiv2009

The Fundamental Planes of E+A galaxies and GALEX UV-excess early-type galaxies: Revealing their intimate connection

Strong Balmer absorption lines and the lack of Ha and [OII] emission lines signify that E+As are post-starburst systems. Recent studies suggest that E+As may undergo the transition from the `blue cloud' to the `red sequence' and eventually migrate to red sequence ETGs. An observational validation of this scenario is to identify the intervening galaxy population between E+As and the red-sequence. Motivated by recent findings with GALEX that a large fraction of ETGs exhibit UV-excess as a sign of RSF, we investigate the possible connection of the UV-excess galaxies to E+As. In particular, we examine the FP scaling relations of the largest sample of ~1,000 E+As selected from the SDSS and ~20,000 morphologically-selected SDSS ETGs with GALEX UV data. The FP parameters, combined with stellar population indicators, reveal a certain group of UV-excess ETGs that bridges between E+As and quiescent red galaxies. The newly identified galaxies are the post-starburst systems characterized by UV-excess but no Ha emission. This is a conceptual generalisation of "E+A", in that the Balmer absorption line in the "E+A" definition is replaced with UV-optical colours that are far more sensitive to RSF than the Balmer lines. We refer to these UV-excess galaxies as "E+a" galaxies, which stands for elliptical ("E") galaxies with a minority of A-type ("a") young stars. The species are either (1) galaxies that experienced starbursts weaker than those observed in E+As (1~10% of E+As, "mild E+As") or (2) the products of passively evolved E+As after quenching star formation quite a while ago (~1 Gyr, "old E+As"). We suggest that the latter type of E+a galaxies represents the most recent arrival to the red sequence in the final phase of the "E+A" to "red early-type" transition. (Abridged)

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arXiv2009

The Mid-infrared View of Red Sequence Galaxies in Abell 2218 with AKARI

We present the {\it AKARI} InfraRed Camera (IRC) imaging observation of early-type galaxies in A2218 at z $\simeq$ 0.175. Mid-infrared (MIR) emission from early-type galaxies traces circumstellar dust emission from AGB stars or/and residual star formation. Including the unique imaging capability at 11 and 15 $μ$m, our {\it AKARI} data provide an effective way to investigate MIR properties of early-type galaxies in the cluster environment. Among our flux-limited sample of 22 red sequence early-type galaxies with precise dynamical and line strength measurements ($<$ 18 mag at 3 $μm$), we find that at least 41% have MIR-excess emission. The $N3-S11$ versus $N3$ (3 and 11 $μ$m) color-magnitude relation shows the expected blue sequence, but the MIR-excess galaxies add a red wing to the relation especially at the fainter end. A SED analysis reveals that the dust emission from AGB stars is the most likely cause for the MIR-excess, with low level of star formation being the next possible explanation. The MIR-excess galaxies show a wide spread of $N3-S11$ colors, implying a significant spread (2--11 Gyr) in the estimated mean ages of stellar populations. We study the environmental dependence of MIR-excess early-type galaxies over an area out to a half virial radius ($\sim$1 Mpc). We find that the MIR-excess early-type galaxies are preferentially located in the outer region. From these evidences, we suggest that the fainter, MIR-excess early-type galaxies have just joined the red sequence, possibly due to the infall and subsequent morphological/spectral transformation induced by the cluster environment.

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arXiv2008

Galaxy Clusters at 0.9<z<1.7 in the AKARI NEP deep field

There is a huge gap between properties of red-sequence selected massive galaxy clusters at z<1 and Lyman-break selected proto-clusters at z>3. It is important to understand when and how the z>3 proto-clusters evolve into passive clusters at z<1. We aim to fill this cluster desert by using the space-based N4(4um) imaging with the AKARI. The z'-N4 color is a powerful separator of cluster galaxies at z>1, taking advantage of the 4000A break and the 1.6um bump. We carefully selected 16 promising cluster candidates at 0.9<z<1.7, which all show obvious over-density of galaxies and a prominent red-sequence. At this redshift range, the mid-infrared S15um/S9um flux ratio is an extinction-free indicator of galaxy star formation activity due to the redshifted PAH emission lines (6.2,7.7 and 8.6um). We show statistically that the cluster galaxies have a lower S15um/S9um flux ratio than field galaxies, i.e., cluster galaxies already have lower star-formation activity at 0.9<z<1.7, pushing the formation epoch of these galaxy clusters to a higher redshift.

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arXiv2008

E+A and Companion Galaxies - I : A Catalogue and Statistics

Based on our intensive spectroscopic campaign with the GoldCam spectrograph on the Kitt Peak National Observatory (KPNO) 2.1-m telescope, we have constructed the first catalogue of E+A galaxies with spectroscopic companion galaxies, and investigated a probability that an E+A galaxy have close companion galaxies. We selected 660 E+A galaxies with $4.0{\rm Å} < {\rm H}δ{\rm EW}$ at a redshift of $<0.167$ from the Data Release 5 of the Sloan Digital Sky Survey (SDSS). We selected their companion candidates from the SDSS imaging data, and classified them into true companions, fore/background galaxies and companion candidates using the SDSS and our KPNO spectra. We observed 26 companion candidates of E+A galaxies at the KPNO to measure their redshifts. Their spectra showed that 17 targets are true companion galaxies. The number of spectroscopically-confirmed E+A's companions are now 34. This becomes the first catalogue of E+A galaxies with spectroscopic companion systems. We found that E+A galaxies have an 54% larger probability of having companion galaxies (7.88%) compared to the comparison sample of normal galaxies (5.12%). A statistical test shows the probabilities are different with 99.7% significance. Our results based on spectroscopy tightens the connection between the dynamical merger/interaction and the origin of E+A galaxies.

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