Far-Infrared Surveyor Research Articles

Widely Extended [O III] 88μm Line Emission around the 30 Doradus Region Revealed with AKARI FIS-FTS

Abstract We present a distribution map of the far-infrared [O III] 88 $\mu$m line emission around the 30 Doradus (30 Dor) region in the Large Magellanic Cloud obtained with the Fourier Transform Spectrometer of the Far-Infrared Surveyor on board AKARI. The map reveals that the [O III] emission is widely distributed by more than 10$'$ around the super star cluster R 136, implying that the 30 Dor region is affluent with interstellar radiation field that is hard enough to ionize O$^{2+}$. The observed [O III] line intensities are as high as (1–2) $\times$ 10$^{-6}$ W m$^{-2}$ sr$^{-1}$ on the peripheral regions 4$'$–5$'$ away from the center of 30 Dor, which requires gas densities of 60–100 cm$^{-3}$. However, the observed size of the distribution of the [O III] emission is too large to be explained by massive stars in the 30 Dor region enshrouded by clouds with a constant gas density of 10$^{2}$ cm$^{-3}$. Therefore, the surrounding structure is likely to be highly clumpy. We also find a global correlation between the [O III] and the far-infrared continuum emission, suggesting that the gas and dust are well mixed in the highly ionized region where the dust survives in clumpy dense clouds shielded from energetic photons.

FAR-INFRARED EMISSION FROM THE INTERGALACTIC MEDIUM IN STEPHAN’S QUINTET REVEALED BY AKARI

The Stephan's Quintet (SQ, HCG92) was observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared (IR) bands at 65, 90, 140, and 160 um. The AKARI four-band images of the SQ show far-IR emission in the intergalactic medium (IGM) of the SQ. In particular, the 160 um band image shows single peak emission in addition to the structure extending in the North-South direction along the shock ridge as seen in the 140 um band, H2 emission and X-ray emission. Whereas most of the far-IR emission in the shocked region comes from the cold dust component, shock-powered [CII]158um emission can significantly contribute to the emission in the 160 um band that shows a single peak at the shocked region. In the shocked region, the observed gas-to-dust mass ratio is in agreement with the Galactic one. The color temperature of the cold dust component (~20 K) is lower than that in surrounding galaxies (~30 K). We discuss a possible origin of the intergalactic dust emission.

FAR-INFRARED IMAGING OF POST-ASYMPTOTIC GIANT BRANCH STARS AND (PROTO)-PLANETARY NEBULAE WITH THE AKARI FAR-INFRARED SURVEYOR

By tracing the distribution of cool dust in the extended envelopes of post-AGB stars and (proto)-planetary nebulae ((P)PNe) we aim to recover, or constrain, the mass loss history experienced by these stars in their recent past. The Far-Infrared Surveyor (FIS) instrument on board the AKARI satellite was used to obtain far-infrared maps for a selected sample of post-AGB stars and (P)PNe. We derived flux densities (aperture photometry) for 13 post-AGB stars and (P)PNe at four far-infrared wavelengths (60, 90, 140, and 160 um). Radial (azimuthally averaged) profiles are used to investigate the presence of extended emission from cool dust. No (detached) extended emission is detected for any target in our sample at levels significant with respect to background and cirrus emission. Only IRAS 21046+4739 reveals tentative excess emission between 30 and 130". Estimates of the total dust and gas mass from the obtained maps indicate that the envelope masses of these stars should be large in order to be detected with the AKARI FIS. Imaging with higher sensitivity and higher spatial resolution is needed to detect and resolve, if present, any cool compact or extended emission associated with these evolved stars.

Extended dust shell of the carbon star U Hydrae observed with AKARI

Context. Low- to intermediate-mass stars lose a significant fraction of their mass while they are on the asymptotic giant branch (AGB). This mass loss is considered to determine the final stages of their evolution. The material ejected from the stellar photosphere forms a circumstellar envelope in its surroundings. Layers of circumstellar envelope constitute the footprint of mass-loss history. Aims. Our aim is to probe the mass-loss history in the carbon star U Hya in the last ∼10 4 years by investigating the distribution of dust in the circumstellar envelope with high spatial resolution. Methods. We observed U Hya in the far-infrared (FIR) at 65, 90, 140, and 160 μm simultaneously, using the slow scan observing mode of the far-infrared surveyor (FIS) aboard the infrared astronomical satellite AKARI. It produced a map of ∼10 � × 40 � in size in each band. Results. The FIS maps reveal remarkably circular, ring-like emission structure almost centered on the star, showing the presence of a detached, spherical dust shell. A hollow dust shell model gives the inner radius Rin of 101–107 �� [(2.5–2.6) × 10 17 cm], thickness that covers a half of the total dust mass ΔRhm of 16–23 �� [(3.8–5.6) × 10 16 cm], which gives ΔRhm/Rin ∼ 0.2, and the power-law index of the dust opacity distribution of 1.10–1.15. The dust mass in the shell is well-constrained to be (0.9–1.4) × 10 −4 (κ100/25) −1 M� , where κ100 is the dust absorptivity at 100 μm in units of cm 2 g −1 . The dust mass-loss rate at Rin is found to be (1.8–

EFFECT OF FIR FLUXES ON CONSTRAINING PROPERTIES OF YSOS

Young Stellar Objects (YSOs) in the early evolutionary stages are very embedded, and thus they emit most of their energy at long wavelengths such as far-infrared (FIR) and submillimeter (Submm). Therefore, the FIR observational data are very important to classify the accurate evolutionary stages of these embedded YSOs, and to better constrain their physical parameters in the dust continuum modeling. We selected 28 YSOs, which were detected in the AKARI Far-Infrared Surveyor (FIS), from the Spitzer c2d legacy YSO catalogs to test the effect of FIR fluxes on the classification of their evolutionary stages and on the constraining of envelope properties, internal luminosity, and UV strength of the Interstellar Radiation Field (ISRF). According to our test, one can mis-classify the evolutionary stages of YSOs, especially the very embedded ones if the FIR fluxes are not included. In addition, the total amount of heating of YSOs can be underestimated without the FIR observational data.

Brightness map of the zodiacal emission from the AKARI IRC All-Sky Survey

The first Japanese infrared space mission AKARI successfully scanned the whole sky with its two main instruments, the Infrared Camera (IRC) and the Far-Infrared Surveyor (FIS). The AKARI All-Sky Survey provides us with an invaluable opportunity to examine the zodiacal emission (ZE) over the entire sky in the leading as well as the trailing direction of the Earth’s motion. We describe our efforts to reduce the ZE brightness map from the AKARI’s survey in the 9 μm waveband. Compared with the interplanetary dust cloud model of Kelsall et al. (1998), the map requires an increase of the contribution of the resonance ring component to the ZE brightness by about 20%. We paid special attention to the north and south ecliptic pole brightnesses. The symmetry plane’s inclination and longitude of ascending node need to be modified from those in Kelsall et al. (1998) to reach a best fit to the observed pole brightness difference.

Calibration of the AKARI Far-Infrared Imaging Fourier-Transform Spectrometer

The Far-Infrared Surveyor (FIS) onboard the AKARI satellite has a spectroscopic capability provided by a Fourier transform spectrometer (FIS–FTS). FIS–FTS is the first space-borne imaging FTS dedicated to far-infrared astronomical observations. We describe the calibration process of the FIS–FTS, and discuss its accuracy and reliability. The calibration is based on observational data of bright astronomical sources as well as two instrumental sources. We have compared the FIS–FTS spectra with the spectra obtained from the Long Wavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO), having a similar spectral coverage. The present calibration method accurately reproduces the spectra of several solar system objects having a reliable spectral model. Under this condition the relative uncertainty of the calibration of the continuum is estimated to be $\pm$ 15% for SW, $\pm$ 10% for 70–85 cm$^{-1}$ of LW, and $\pm$ 20% for 60–70 cm$^{-1}$ of LW; and the absolute uncertainty is estimated to be $+$35%/$-$55% for SW, $+$35%/$-$55% for 70–85 cm$^{-1}$ of LW, and $+$40%/$-$60% for 60–70 cm$^{-1}$ of LW. These values have been confirmed by comparisons with theoretical models and previous observations by the ISO / LWS.

Kiloparsec-scale star formation law in M 81 and M 101 based on AKARI far-infrared observations

We assess the relationships between the surface densities of the gas and star formation rate (SFR) within spiral arms of the nearby late-type spiral galaxies M81 and M101. By analyzing these relationships locally, we derive empirically a kiloparsec scale Kennicutt-Schmidt Law. Both M81 and M101 were observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared bands at 65, 90, 140, and 160 um. The spectral energy distributions of the whole galaxies show the presence of the cold dust component (Tc~20 K) in addition to the warm dust component (Tw~60 K). We deconvolved the cold and warm dust emission components spatially by making the best use of the multi-band photometric capability of the FIS. The cold and warm dust components show power-law correlations in various regions, which can be converted into the gas mass and the SFR, respectively. We find a power-law correlation between the gas and SFR surface densities with significant differences in the power law index N between giant HII regions (N=1.0) and spiral arms (N=2.2) in M101. The power-law index for spiral arms in M81 is similar (N=1.9) to that of spiral arms in M101. Conclusions: The power-law index is not always constant within a galaxy. The difference in the power-law index can be attributed to the difference in the star formation processes on a kiloparsec scale. N~2 seen in the spiral arms in M81 and M101 supports the scenario of star formation triggered by cloud-cloud collisions enhanced by spiral density wave, while N~1 derived in giant HII regions in M101 suggests the star formation induced by the Parker instability triggered by high velocity HI gas infall. The present method can be applied to a large galaxy sample for which the AKARI All Sky Survey provides the same 4 far-infrared band data.

Origin of the dust emission from Tycho's SNR

Aims: We investigate the spatial distribution of dust emission around Tycho's SNR to understand its origin. We distinguish the dust associated with the SNR from that of the surrounding ISM. Methods: We performed mid- to far-infrared imaging observations of the remnant at wavelengths of 9, 15, 18, 24, 65, 90, 140, and 160um using the Infrared Camera and the Far-Infrared Surveyor onboard AKARI. We compared the AKARI images with the Suzaku X-ray image and the 12CO image of Tycho's SNR. Results: All the AKARI images except the 9, 140, and 160um band images show a shell-like emission structure with brightness peaks at the north east (NE) and north west (NW) boundaries, sharply outlining part of the X-ray shell. The 140 and 160um bands are dominated by cold dust emission from the surrounding ISM near the NE boundary. Conclusion: We conclude that the dust emission at the NE boundary comes from the ambient cloud interacting with the shock front, while the origin of the dust emission at the NW boundary is rather unclear because of the absence of prominent interstellar clouds near the corresponding region. We cannot rule out the possibility that the latter is mostly of an SN ejecta origin.

Enhanced dust heating in the bulges of early-type spiral galaxies

Stellar density and bar strength should affect the temperatures of the cool (<i>T<i/> ~ 20–30 K) dust component in the inner regions of galaxies, which implies that the ratio of temperatures in the circumnuclear regions to the disk should depend on Hubble type. We investigate the differences between cool dust temperatures in the central 3 kpc and disk of 13 nearby galaxies by fitting models to measurements between 70 and 500 <i>μ<i/>m. We attempt to quantify temperature trends in nearby disk galaxies, with archival data from <i>Spitzer<i/>/MIPS and new observations with <i>Herschel<i/>/SPIRE, which were acquired during the first phases of the <i>Herschel<i/> observations for the KINGFISH (Key Insights on Nearby Galaxies: a Far-Infrared Survey with <i>Herschel<i/>) sample. We fit single-temperature modified blackbodies to far-infrared and submillimeter measurements of the central and disk regions of galaxies to determine the temperature of the component(s) emitting at those wavelengths. We present the ratio of central-region-to-disk-temperatures of the cool dust component of 13 nearby galaxies as a function of morphological type. We find a significant temperature gradient in the cool dust component in all galaxies, with a mean center-to-disk temperature ratio of 1.15 <i>±<i/> 0.03. The cool dust temperatures in the central ~3 kpc of nearby galaxies are 23 (<i>±<i/>3)% hotter for morphological types earlier than Sc, and only 9 (<i>±<i/>3)% hotter for later types. The temperature ratio is also correlated with bar strength, with only strongly barred galaxies having a ratio over 1.2. The strong radiation field in the high stellar density of a galactic bulge tends to heat the cool dust component to higher temperatures, at least in early-type spirals with relatively large bulges, especially when paired with a strong bar.

Large-scale distributions of mid- and far-infrared emission from the center to the halo of M 82 revealed with AKARI

The edge-on starburst galaxy M82 exhibits complicated distributions of gaseous materials in its halo, which include ionized superwinds driven by nuclear starbursts, neutral materials entrained by the superwinds, and large-scale neutral streamers probably caused by a past tidal interaction with M81. We investigate detailed distributions of dust grains and polycyclic aromatic hydrocarbons (PAHs) around M82 to understand their interplay with the gaseous components. We performed mid- (MIR) and far-infrared (FIR) observations of M82 with the Infrared Camera and Far-Infrared Surveyor on board AKARI. We obtain new MIR and FIR images of M82, which reveal both faint extended emission in the halo and very bright emission in the center with signal dynamic ranges as large as five and three orders of magnitude for the MIR and FIR, respectively. We detect MIR and FIR emission in the regions far away from the disk of the galaxy, reflecting the presence of dust and PAHs in the halo of M82. We find that the dust and PAHs are contained in both ionized and neutral gas components, implying that they have been expelled into the halo of M82 by both starbursts and galaxy interaction. In particular, we obtain a tight correlation between the PAH and H$\alpha$ emission, which provides evidence that the PAHs are well mixed in the ionized superwind gas and outflowing from the disk.

Preliminary analysis of the AKARI FIS BSC

We present a preliminary analysis of AKARI Far-Infrared Surveyor Bright Source Catalog. The AKARI mission collected data on 63370 bright IR sources. Galactic spatial distribution of different flux quality types was studied. The surface density shows correlation with already known places of IR bright galactic regions. Difference between the spatial distribution of different flux quality objects is not notable. This research is part of the AKARI Mission Program "Star Formation".

AKARI results on the Taurus-Auriga star forming region

We present an analysis of AKARI Far-Infrared Surveyor, Spitzer Space Telescope Infrared Array Camera and Mid-Infrared Photometer for Spitzer data as well as various other photometric data of the Taurus-Auriga region. Our aim was to locate and describe young stellar objects (YSO), and to test the potential of AKARI FIS data in the classification of YSOs. We studied 32 YSOs, 7 of those had not been observed by the SST. We succesfully modeled 21 YSOs with the SED Fitting Tool of Robitaille. We confirmed the previous models in 20 cases, and corrected the physical parameters for 5 YSOs. This research is part of the AKARI Mission Program "Star Formation".

A far-infrared survey at the North Galactic Pole - I. Nearby star-forming galaxies and effect of confused sources on source counts

We present follow-up observations of the far-infrared (FIR) sources at 90, 150 and 180 mu detected as part of the ISOPHOT EBL project, which has recently measured the absolute surface brightness of the cosmic infrared background radiation (CIRB) for the first time independently from COBE data. We have observed the fields at the North Galactic Pole region in the optical and near-IR, and complement these data with SDSS photometry, and spectroscopy where available, and present identifications of the 25 FIR sources which reach down to ~150 mJy in all three ISOPHOT bands. Identifications are done by means of full spectral energy density fitting to all sources in the FIR error circle areas. Approximately 80 per cent are identified as star-forming or star-bursting galaxies at z<0.3. We also find that more than half of the counterparts have disturbed morphologies, with signs of past or present interactions. However, only 20 per cent of all the sources are uniquely matched with a single galaxy -- 40 per cent are blends of two or more of these nearby star-forming galaxies, while another 20 per cent are likely blends of nearby and fainter galaxies. The final 20 per cent are likely to be more luminous IR galaxies at higher redshifts. The blended sources have an effect on the FIR source counts. In particular, taking into account realistic confusion or blending of sources, the differential FIR counts move down by a factor of ~1.5 and steepen in the 100 to 400 mJy range.

AKARI Far-Infrared View of Nearby Galaxies

AbstractWe have observed 57 nearby galaxies in the far-infrared with the Far-Infrared Surveyor on AKARI to study the properties of dust in various environments.

AKARIINFRARED OBSERVATIONS OF THE SUPERNOVA REMNANT G292.0+1.8: UNVEILING CIRCUMSTELLAR MEDIUM AND SUPERNOVA EJECTA

We present the results of AKARI observations of the O-rich supernova remnant (SNR) G292.0+1.8 using six Infrared Camera (IRC) and four Far-Infrared Surveyor bands covering 2.7–26.5 μm and 50–180 μm, respectively. The AKARI images show two prominent structures; a bright equatorial ring (ER) structure along the east–west direction and an outer elliptical shell structure surrounding the remnant. The ER structure is clumpy and incomplete with its western end opened. The outer shell is almost complete and slightly squeezed along the north–south direction. The central position of the outer shell is ∼1' northwest from the embedded pulsar and coincides with the center of the ER structure. In the northern and southwestern regions, there is also faint emission with a sharp boundary beyond the bright shell structure. The ER and the elliptical shell structures were partly visible in optical and/or X-rays, but they are much more clearly revealed in our AKARI images. There is no evident difference in infrared colors of the two prominent structures, which is consistent with the previous proposition that both structures are of circumstellar origin. However, we have detected faint infrared emission of a considerably high 15/24 μm ratio associated with the supernova (SN) ejecta in the southeastern and northwestern areas. Our IRC spectra show that the high ratio is at least partly due to the emission lines from Ne ions in the SN ejecta material. In addition, we detect a narrow, elongated feature outside the SNR shell. We derive the physical parameters of the infrared-emitting dust grains in the shocked circumstellar medium (CSM) and compare the result with model calculations of dust destruction by an SN shock. The AKARI results suggest that the progenitor was at the center of the infrared circumstellar shell in the red supergiant stage and that the observed asymmetry in the SN ejecta could be a result of either a dense CSM in the equatorial plane and/or an asymmetric explosion.

Calibration and Performance of the AKARI Far-Infrared Surveyor (FIS) — Slow-Scan Observation Mode for Point-Sources

Abstract We present the characterization and calibration of the Slow-Scan observation mode of the Far-Infrared Surveyor (FIS) onboard the AKARI satellite. The FIS, one of the two focal-plane instruments on AKARI, has four photometric bands between 50–180$\mu$m with two types of Ge:Ga array detectors. In addition to the All-Sky Survey, FIS has also taken detailed far-infrared images of selected targets by using the Slow-Scan mode. The sensitivity of the Slow-Scan mode is one to two orders of magnitude better than that of the All-Sky Survey, because the exposure time on a targeted source is much longer. The point spread functions (PSFs) were obtained by observing several bright point-like objects, such as asteroids, stars, and galaxies. The derived full widths at the half maximum (FWHMs) are $\sim$30$^{\prime\prime}$ for the two shorter wavelength bands and $\sim$40$^{\prime\prime}$ for the two longer wavelength bands, being consistent with those expected by optical simulation, although a certain amount of excess is seen in the tails of the PSFs. A flux calibration was performed by observations of well-established photometric calibration standards (asteroids and stars) over a wide range of fluxes. After establishing the method of aperture photometry, the photometric accuracy for point-sources is better than $\pm$15% in all of the bands, expect for the longest wavelength.

AKARI Far-Infrared Spectroscopic Observations of the Galactic Center Region

Abstract We have observed the Quintuplet- and Arches-cluster regions with the Fourier Transform Spectrometer (FTS) of the Far-Infrared Surveyor (FIS) aboard AKARI to investigate the physical conditions of interstellar matter around the clusters. The FIS-FTS mapping data reveal differences in the spatial distribution among the far-infrared [O iii], [N ii], and [C ii] line emissions near the Arches cluster; the emission of an ionic line with a higher ionization potential is distributed closer to the Arches cluster. This clearly indicates that UV photons from the Arches cluster are ionizing the surface of nearby molecular clouds, and penetrating deeper to dissociate the cloud. We have estimated the effective temperature of the Arches cluster to be about 34000 K from the ratio of the [O iii]$/$[N ii] lines.

AKARI DETECTION OF FAR-INFRARED DUST EMISSION IN THE HALO OF NGC 253

We present new far-infrared (FIR) images of the edge-on starburst galaxy NGC253 obtained with the Far-Infrared Surveyor (FIS) onboard AKARI at wavelengths of 90 um and 140 um. We have clearly detected FIR dust emission extended in the halo of the galaxy; there are two filamentary emission structures extending from the galactic disk up to 9 kpc in the northern and 6 kpc in the northwestern direction. From its spatial coincidence with the X-ray plasma outflow, the extended FIR emission is very likely to represent outflowing dust entrained by superwinds. The ratios of surface brightness at 90 um to that at 140 um suggest that the temperatures of the dust in the halo are getting higher in the regions far from the disk, implying that there exist extra dust heating sources in the halo of the galaxy.

AKARI: space infrared cooled telescope

AKARI, formerly known as ASTRO-F, is the second Japanese space mission to perform infrared astronomical observations. AKARI was launched on 21 February 2006 (UT) and brought into a sun-synchronous polar orbit at an altitude of 700 km by a JAXA M-V rocket. AKARI has a telescope with a primary-mirror aperture size of 685 mm together with two focal-plane instruments on board: the Infrared Camera (IRC), which covers the spectral range 2–26 μm and the Far-Infrared Surveyor (FIS), which operates in the range 50–180 μm. The telescope mirrors are made of sandwich-type silicon carbide, specially developed for AKARI. The focal-plane instruments and the telescope are cooled by a unique cryogenic system that kept the telescope at 6K for 550 days with 180 l super-fluid liquid Helium (LHe) with the help of mechanical coolers on board. Despite the small telescope size, the cold environment and the state-of-the-art detectors enable very sensitive observations at infrared wavelengths. To take advantage of the characteristics of the sun-synchronous polar orbit, AKARI performed an all-sky survey during the LHe holding period in four far-infrared bands with FIS and two mid-infrared bands with IRC, which surpasses the IRAS survey made in 1983 in sensitivity, spatial resolution, and spectral coverage. AKARI also made over 5,000 pointing observations at given targets in the sky for approximately 10 min each, for deep imaging and spectroscopy from 2 to 180 μm during the LHe holding period. The LHe ran out on 26 August 2007, since which date the telescope and instrument are still kept around 40K by the mechanical cooler on board, and near-infrared imaging and spectroscopic observations with IRC are now being continued in pointing mode.