We investigate the FIR properties of a sample of BCDs observed by AKARI. By utilizing the data at wavelengths of $\lambda =65 \mu$m, 90 $\mu$m, and 140 $\mu$m, we find that the FIR colours of the BCDs are located at the natural high-temperature extension of those of the Milky Way and the Magellanic Clouds. This implies that the optical properties of dust in BCDs are similar to those in the Milky Way. Indeed, we explain the FIR colours by assuming the same grain optical properties, which may be appropriate for amorphous dust grains, and the same size distribution as those adopted for the Milky Way dust. Since both interstellar radiation field and dust optical depth affect the dust temperature, it is difficult to distinguish which of these two physical properties is responsible for the change of FIR colours. Then, in order to examine if the dust optical depth plays an important role in determining the dust temperature, we investigate the correlation between FIR colour (dust temperature) and dust-to-gas ratio. We find that the dust temperature tends to be high as the dust-to-gas ratio decreases but that this trend cannot be explained by the effect of dust optical depth. Rather, it indicates a correlation between dust-to-gas ratio and interstellar radiation field. Although the metallicity may also play a role in this correlation, we suggest that the dust optical depth could regulate the star formation activities, which govern the interstellar radiation field. We also mention the importance of submillimetre data in tracing the emission from highly shielded low-temperature dust.
Deep Dive into Effects of dust abundance on the far-infrared colours of blue compact dwarf galaxies.
We investigate the FIR properties of a sample of BCDs observed by AKARI. By utilizing the data at wavelengths of $\lambda =65 \mu$m, 90 $\mu$m, and 140 $\mu$m, we find that the FIR colours of the BCDs are located at the natural high-temperature extension of those of the Milky Way and the Magellanic Clouds. This implies that the optical properties of dust in BCDs are similar to those in the Milky Way. Indeed, we explain the FIR colours by assuming the same grain optical properties, which may be appropriate for amorphous dust grains, and the same size distribution as those adopted for the Milky Way dust. Since both interstellar radiation field and dust optical depth affect the dust temperature, it is difficult to distinguish which of these two physical properties is responsible for the change of FIR colours. Then, in order to examine if the dust optical depth plays an important role in determining the dust temperature, we investigate the correlation between FIR colour (dust temperature)
The far-infrared (FIR) emission from galaxies is often used to trace the star formation activities (Kennicutt 1998;Inoue et al. 2000;Iglesias-Páramo et al. 2004). Dust grains are the source of FIR emission, and the strong connection between FIR luminosity and star formation rate can be explained if the ultraviolet (UV) light from massive stars is the dominant source of dust heating. The FIR spectral energy distribution (SED) of dust grains reflects various information on the grains themselves and on the sources of grain heating (e.g. Takagi, Vansevičius, & Arimoto 2003;Takeuchi et al. 2005;Dopita et al. 2005). Dust temperature is determined by the intensity of interstellar radiation field (ISRF) and the optical properties of dust (i.e. how it absorbs and emits light). Since dust grains absorb UV light efficiently, the FIR luminosity and the dust temperature mostly reflect the UV radiation field (Buat & Xu 1996).
Observationally, dust temperature can be estimated from FIR colour, which is defined as the flux ratio between two FIR wavelengths. If more than two ( 3) bands are available in FIR, we ⋆ E-mail: hirashita@asiaa.sinica.edu.tw can take two independent FIR colours and examine a FIR colourcolour relation. A colour-colour relation, because of increased information compared with a single FIR colour, enables us to obtain not only the dust temperature but also the wavelength dependence of dust emissivity. Nagata et al. (2002), adopting 60 µm, 100 µm, and 140 µm as three wavelengths, show that there is a tight relation in the FIR colour-colour diagram. This tightness implies a common wavelength dependence of FIR emissivity among various galaxies. Their work has been developed by Hibi et al. (2006), who show that a tight FIR colour-colour relation found for the Milky Way dust emission, called “main correlation”, is also consistent with the FIR colour-colour relation of a sample of nearby galaxies (see also Sakon et al. 2004;Onaka et al. 2007). As stated by Hibi et al. (2006), this implies that the optical properties of grains in FIR are common among the Milky Way and nearby galaxies.
Since the FIR colour-colour relation can be used to constrain the optical properties of dust grains, the FIR colourcolour relation of galaxies with various evolutionary stages provides pieces of information on the grain evolution in galactic environments. In particular, the dominant production source of dust grain could be different in early phase of galaxy evolution (Todini & Ferrara 2001;Nozawa et al. 2003;Maiolino et al. 2004;Schneider, Ferrara, & Salvaterra 2004). Moreover, the effects of interstellar processing of dust grains, especially accretion of heavy elements onto dust grains, coagulation, and shattering, depend on metallicity (or dust abundance). Thus, it is probable that the grain properties evolve as galaxies evolve.
Although it is difficult to obtain the rest-frame FIR data of distant galaxies in an early phase of galaxy evolution, there are nearby possible “templates” of primeval galaxies, blue compact dwarf galaxies (BCDs). Indeed, BCDs have on-going star formation in metal-poor and gas-rich environments (Sargent & Searle 1970;van Zee, Skillman, & Salzer 1998), which could be similar to those in high-redshift star-forming galaxies. Moreover, BCDs harbour an appreciable amount of dust (e.g. Thuan, Sauvage, & Madden 1999) and can be used to investigate the dust properties in chemically unevolved galaxies (Takeuchi et al. 2005).
It is possible to investigate the metallicity dependence of dust properties by sampling BCDs with a variety of metallicity. Recently Engelbracht et al. (2008) have examined the metallicity dependence of dust emission in various wavelengths by using Spitzer data. In FIR, they have used Multiband Imaging Photometer for Spitzer (MIPS) 70 µm and 160 µm bands, and have shown that there is a correlation between dust temperature derived from these two bands and metallicity. This indicates the importance of studies on dust emission in a wide metallicity range.
The AKARI satellite (Murakami et al. 2007) provides us with a good opportunity to study FIR colour-colour relations, since Far-Infrared Surveyor (FIS) on AKARI has four bands in FIR (65 µm, 90 µm, 140 µm, and 160 µm) (Murakami et al. 2007;Kawada et al. 2007). Indeed, seven of the eight BCDs in Hirashita et al. (2008, hereafter H08) are detected at 65 µm, 90 µm, and 140 µm. This indicates that it is possible to study the FIR colour-colour relation of BCDs by using AKARI data. In this paper, we add four more BCDs available in the AKARI archive and examine the FIR colour-colour relation of BCDs. Then, we extract information on what determines or regulates the FIR emission in metal-poor environments with the aid of theoretical models for dust emission.
This paper is organized as follows. First, in Section 2, we describe the models of dust emission with a simple radiative transfer recipe. Then, in Section 3, we explain the data analysis of th
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