THE CURRENT STATUS OF THE AKARI MID-INFRARED ALL-SKY DIFFUSE MAPS

Publications of the Korean Astronomical Society pISSN 1225-1534

32: 025 ∼ 027, 2017 March eISSN 2287-6936

⃝2017. The Korean Astronomical Society. All rights reserved. c https://doi.org/10.5303/PKAS.2017.32.1.025

THE CURRENT STATUS OF

THE AKARI MID-INFRARED ALL-SKY DIFFUSE MAPS

Tomoya Amatsutsu

, Daisuke Ishihara

, Toru Kondo

, Hidehiro Kaneda

, Shinki Oyabu

, Mitsuyoshi Yamagishi

, Keichiro Nakamichi

, Hidetoshi Sano

, and Takashi Onaka

1

Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan

2

Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan E-mail: [email protected]

(Received September 3, 2015; Revised October 21, 2016; Accepted October 21, 2016)

ABSTRACT

We are creating all-sky diffuse maps from the AKARI mid-infrared survey data with the two photometric bands centered at wavelengths of 9 and 18 µm. The AKARI mid-infrared diffuse maps achieve higher spatial resolution and higher sensitivity than the IRAS maps. In particular, the 9 µm data are unique resources as an all-sky tracer of the emission of polycyclic aromatic hydrocarbons (PAHs). However, the original data suffer many artifacts. Thus, we have been developing correction methods. Among them, we have recently improved correction methods for the non-linearity and the reset anomaly of the detector response. These corrections successfully reduce the artifact level down to 0.1 MJy sr

on average, which is essential for discussion on faint extended emission (e.g., the Galactic PAH emission). We have also made progress in the subtraction of the scattered light caused in the camera optics. We plan to release the improved diffuse maps to the public within a year.

Key words: methods: data analysis; technique: image processing; galaxies: ISM; infrared; survey

1. INTRODUCTION

AKARI carried out all-sky surveys in mid- and far- infrared photometric bands. We are creating diffuse maps from the data of the wavelengths of 9 and 18 µm.

These maps achieve higher spatial resolution ( ∼ 6

) and higher sensitivity than the IRAS 12 and 25 µm all- sky maps (Ishihara et al., 2010). The 9 µm data are unique resources which represent the all-sky distribu- tion of polycyclic aromatic hydrocarbons (PAHs) cov- ering their 6.2, 7.7, 8.2 and 11.3 µm spectral features.

The 18 µm data are also useful as hot dust tracers. By combining with other mid-IR survey maps of different band profiles such as WISE, we can discuss the spatial distribution of the physical properties of the interstellar dust grains.

Even after the basic calibration, the data still suffer many artifacts. Thus, we have been developing data corrections, such as the corrections of the ionizing ra-

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diation effect (Mouri et al., 2011) and the removal of the scattered light from the moon. As a result, the artifact level becomes sufficiently lower than Galactic diffuse emission of a ∼ 1 MJy sr

level (Kondo et al., 2014). However, for studies of faint diffuse emission of a

∼ 0.1 MJy sr

level, such as the Galactic high-latitude clouds, MBM 53, 54 and 55, the artifact level should be reduced to a comparable level (Figure 1a). In this paper, we report the current status of the AKARI mid- infrared all-sky diffuse maps with the recent significant improvements in the data reduction process: the non- linearity correction, the reset anomaly correction, and the correction of the scattered light caused in the cam- era optics.

2. DATA ANALYSIS

The non-linearity of the photoresponse is caused by de-

crease in the effective detector bias with increase in the

number of stored photoelectrons in the charge integrated

25

26 T. AMATSUTSU ET AL.

Figure 1. The AKARI 9 µm images of a 12

× 14

region around the high-latitude clouds MBM 53, 54, and 55. The zodiacal light and the uniform background are subtracted. (a) The image corrected by the previous method. (b) The image corrected by the new methods described in Section 2. The contour indicates the Planck dust opacity at 353 GHz with τ = 8 × 10

. The scan direction is indicated by the arrows. The blank sky regions used for evaluating the RMS are indicated by blue boxes.

Time

Storedelectrons

Time

Intensity

Reset Reset

(a)

(b)

Reset anomaly effect

Detection of a point source

Non-linearity effect

0 5 10 [s]

Figure 2. Schematic images of the non-linearity and reset anomaly effects. (a) Output of the detector with the CIA versus time. (b) The output is differentiated with respect to time. The solid lines in both images indicate observational data affected by the non-linearity and the reset anomaly, while the dotted line indicates the ideal data. The timing of resets are indicated by the yellow arrows. In this example, a point source is detected while the sky brightness is constant.

amplifier (CIA). The reset anomaly is unstable behavior of signals after resets of stored photoelectrons. The re- set current temporarily warms up the cryogenic output amplifier, which causes the drift of the offset level of the output signals until the amplifier is cooled down again.

Figure 2 illustrates these effects. The aslant stripe arti- facts seen in Figure 1a is caused by periodic anomalous signals along the satellite’s scan direction due to the non-linearity and the reset anomaly effects. To model these effects, we should investigate the detector response against incident light with various intensity levels. We develop the correction method for these effects by ana- lyzing the photoresponse against the zodiacal light emis- sion. The zodiacal light is a good calibrator because it provides us a data set of spatially smooth signals of var-

Figure 3. The AKARI 18 µm images of a 0.4

× 0.4

region around the starburst galaxy, M 82. (a) The original image.

(b) The image from which the scattered light caused in the camera optics is subtracted.

ious levels.

In addition to the non-linearity and the reset anomaly, artificial image patterns appear around bright sources in the 18 µm data. Figure 3a shows an example of the starburst galaxy, M 82 (170 Jy in the 18 µm band). The patterns are due to scattered light caused in the camera optics. The energy of the scattered light amounts to 10

% of the total incident light in the 18 µm band. We analyze the intensity and the shape of this component using 89 scan images of 26 bright sources. Then we subtract it from each image.

3. RESULTS

Figure 1 demonstrates the result of the non-linearity and the reset anomaly corrections for MBM 53, 54 and 55.

In this image, the aslant striped artifacts are reduced.

The background RMS measured on the sky regions in-

dicated in boxes after masking the point sources is

changed from 0.2 to 0.1 MJy sr

. We can clearly iden-

THE CURRENT STATUS OF THE AKARI MID-INFRARED ALL-SKY DIFFUSE MAPS 27

tify very faint diffuse emission which is spatially associ- ated with the cold dust distribution revealed by Planck.

Figure 3b shows the result of the scattered light cor- rection for M 82. The residual level is ∼ 4 % of the scatted light level. It provides us with real images on faint diffuse emission around the galaxy.

4. SUMMARY

We are creating the all-sky diffuse maps from the AKARI mid-infrared all-sky survey data. Recently, we have improved non-linearity and reset anomaly correc- tions. The improved data enable us to study faint ex- tended emission of < 0.5 MJy sr

level, such as the high-latitude clouds, MBM 53, 54, and 55. We have also established a subtraction method for the scattered light caused in the camera optics. It provides us with real images of faint diffuse emission around bright sources, such as the starburst galaxy, M 82.

ACKNOWLEDGMENTS

This research is based on the observations with AKARI, a JAXA project with the participation of ESA.

This work is supported by the Grant-in-Aid for the Sci- entific Research Fund (Nos. 24740122, 26707008, and 50377925) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

REFERENCES

Ishihara, D., Onaka, T., Kataza, H., et al., 2010, The AKARI/IRC mid-infrared all-sky survey., A&A,, 514, A1 Mouri, A., Kaneda, H., Ishihara, D., et al., 2011, Effects of high-energy ionizing particles on the Si:As mid-IR detector array on board the AKARI satellite, PASP, 123, 561 Kondo, T., Ishihara, D., Kaneda, H., et al. 2014, Modeling

of the zodiacal light for the AKARI mid-IR all-sky diffuse

maps, in this volume