1. Observations of Near Infrared Extragalactic Background (NIREBL) ISAS/JAXAT. Matsumoto Dec.2-5, 2003 Japan/Italy seminar at Niigata Univ.

2. Infrared Extragalactic Background Light (IREBL) Cosmic Infrared Background (CIB) Integrated light of distant faint galaxies redshifted star light ⇨ near infrared thermal radiation of dust ⇨ far infrared, submillimeter Indicator of the total energy generation Constituents: unresolved distant galaxies, such as primeval galaxies complementary observation with galaxy deep counts Due to the bright atmospheric emission, space observation is essential DIRBE/COBE and NIRS/IRTS first provided significant results

3. COBE(COsmic Background Explorer) FIRAS DMR DIRBE(Diffuse Infrared Background Experiment) Absolute photometry of the sky brightness at 1,25, 2.2, 3.5, 4.9, 12. 25, 60, 100, 140, 240  m beam size ~0.7 degree COBE was launched on 1989 and attained all sky survey. As for the CIB, detection at far infrared bands upper limits for other bands

4. Near Infrared Sky Main emission source zodiacal light scattered sunlight by interplanetary dust -> physical model of IPD star light unresolved stars (K>3mag!)

5. IRTS(Infrared Telescope in Space) NIRS(Near Infrared Spectrometer) One of 4 focal plane instruments of IRTS wavelength coverage 1.4-4.0  m spectral resolution 0.13  m beam size 8 arcmin. x 8 arcmin. Compared with COBE/DIRBE smaller beam capability of the spectroscopy smaller spatial coverage ~7% of the sky One of mission instruments of small space platform, SFU launched on March 15, 1995 15cm cold telescope Optimized for diffuse Extended sources Mission life ~ 1 month

6. Observation 7% of the sky was surveyed during IRTS observation period (4 weeks) The data for 5 days before liq. He ran out were used to avoid contamination The data at high galactic latitudes are sampled 40<b<58 degree, 10<  <70 degree

7. Selection of the data Orbits without SAA passage were selected. Only the data with no distinguishable stars, no cosmic ray hits, no obvious noise, in 5 sec. for all wavelength bands were taken. ⇨ Full spectra for 1010 data points Data integrated for 5sec. were taken, and linear fitted ⇨ effective beam size 8arcmin. x 20 arcmin.

8. Subtraction of foreground emission main emission component is zodiacal light! Integrated light of faint stars constructed logN/logS model based on the NIRS observation (M.Cohen) obtained magnitudes of stars that correspond to the noises → cut off magnitudes for all wavelength bands cf. 10.4 mag. at 2.24  m calculated integrated light of stars fainter than cut off magnitudes for 40<b<44, 44<b<48, 48<b Zodiacal light Apply physical model by Kelsall et al. (ApJ, 508, 44 1998) to NIRS bands. Calculate the brightness of zodiacal light/emission for all points.

9. After subtracting the star light and zodiacal light/emission Significant isotropic emission was detected for all bands !

10. Breakdown to emission components Observed sky brightness at high ecliptic latitude Zodiacal light/emission Isotropic emission ~20 % of dark sky Integrated light of faint stars

11. COBE/DIRBE and star counts Comparison with other observation J-band K-bandL-band Dwek & Arendt (1998) 9.9 ±2.9 Gorjian et al. (2000) 22.4±611.0 ±3.3 30.7±6 15.4 ±3.3 Wright and Reese (2000) 23.1±5.916.8 ± 3.2 31.4±5.9 Wright (2001) 28.9 ±16.3 20.2±6.3 61.9 ±16.3 28.5 ±6.3 Kiso star counts 60.1±15 IRTS/NIRS 27±5 （ 2.24  m) In unit of nW.m -2.sr -1 Red numbers are based on "very strong no-zodi principle" (VSNZP) All observations are consistent if same zodi model is used!

12. Spectrum of the observed isotropic emission Stellar like spectrum was found. Main error is uncertainty of the zodiacal light model Consistent with COBE/DIRBE Significantly brighter than the integrated light of galaxies ! Spectral gap around 1  m In-band energy flux is ~ 30 nW.m -2.sr -1 ● IRTS/NIRS ■ COBE data □ Optical EBL ◆ Integrated light of galaxies

13. Spectrum of the EBL at NIR and FIR Energy flux NIR ~30 nW.m -2.sr -1 FIR 20~50 Submillimeter EBL can be explained by SCUBA sources

14. Energetics total energy flux detected, ~30 nW.m -2.sr -1 Recent WMAP result : optical depth for CMB  ~0.17 ± 0.04 re-ionization of the universez~17 Origin of NIREBL could be the first generation stars that ionized universe at 6<z<17 while, expected EBL, assuming single star burst at the redshift, z f, is 〜 30(h 2  B /0.024)(  X/0.05)(10/1+z f ) nW.m -2.sr -1  B : Baryon density  X : ratio of burned hydrogen to total hydrogen Produced metal must be in black holes!

15. Fluctuation of the sky -1 rms fluctuation ● Observed sky fluctuation ● Residual fluctuation after subtracting fluctuation due to stars and read out noise ■ Fluctuation, COBE/DIRBE (Kashlinsky) Fluctuation of zodiacal emission at 12  m is less than 1% (IRAS, COBE, ISO)! ⇨ Zodiacal light can not explain observed sky fluctuation! Stellar fluctuation is estimated by using the model Observed rms fluctuation: ~5% of the sky brightness, ~6% of the zodiacal light, ~20% of the isotropic emission

16. Fluctuation of the sky -2 Correlation between wavelength bands Clear correlation between wavelength bands was detected. Spectrum (color) of fluctuation component is similar to that of isotropic emission ⇨ Isotropic emission is fluctuating keeping the same spectrum! If origin of observed sky fluctuation is large scale structure at high redshift, epoch of energy generation must be very narrow redshift range!?

17. Fluctuation of the sky -3 2-point correlation function for integrated brightness of short wavelength bands Clear spatial structure at 100~200 arcmin. scale was found. 10 Mpc at present day corresponds to ~ 1 degree at z ~ 10. This could be an evidence that the large scale structure already existed at high redshft.

18. Summary 1. IRTS/NIRS detected significant isotropic emission that can be attributed to the extragalactic origin. Observed sky brightness is consistent with DIRBE/COBE data but is significantly brighter than the integrated light of known galaxies. 2. Observed spectrum is stellar like and energy flux amounts to 30 nW.m -2.sr -1. Possible origin is first generation stars at high redshift suggested by recent result of WMAP observation. 3. Significant sky fluctuation with a characteristic scale of 100 ~ 200 arcmin. Was detected. This could be an evidence of the large scale structure at high redshift. Observation of NIREBL provides a new mean to investigate the first generation stars.