We reexamine the effect of Lyman continuum (λ ≤ 912Å) extinction (LCE) by dust in H ii regions in detail and discuss how it affects the estimation of the global star formation rate (SFR) of galaxies. To clarify the first issue, we establish two independent methods for estimating a parameter of LCE (f), which is defined as the fraction of Lyman continuum photons contributing to hydrogen ionization in an H ii region. One of those methods determines f from the set of Lyman continuum flux, electron density and metallicity. In the framework of this method, as the metallicity and/or the Lyman photon flux increase, f is found to decrease. The other method determines f from the ratio of infrared flux to Lyman continuum flux. Importantly, we show that f 0.5 via both methods in many H ii regions of the Galaxy. Thus, it establishes that dust in such H ii regions absorbs significant amount of Lyman continuum photons directly. To examine the second issue, we approximate f to a function of only the dust-togas mass ratio (i.e., metallicity), assuming a parameter fit for the Galactic H ii regions. We find that a characteristicf , which is defined as f averaged over a galaxy-wide scale, is 0.3 for the nearby spiral galaxies. This relatively smallf indicates that a typical increment factor due to LCE for estimating the global SFR (1/f) is large (∼ 3) for the nearby spiral galaxies. Therefore, we conclude that the effect of LCE is not negligible relative to other uncertainties of estimating the SFR of galaxies.
Abstract. This paper investigates the origin of the observed large variety in dust-to-gas ratio, D, among blue compact dwarf galaxies (BCDs). By applying our chemical evolution model, we find that the dust destruction can largely suppress the dust-to-gas ratio when the metallicity of a BCD reaches 12 + log (O/H) ∼ 8, i.e., a typical metallicity level of BCDs. We also show that dust-to-gas ratio is largely varied owing to the change of dust destruction efficiency that has two effects: (i) a significant contribution of Type Ia supernovae to total supernova rate; (ii) variation of gas mass contained in a star-forming region. While mass loss from BCDs was previously thought to be the major cause for the variance of D, we suggest that the other two effects are also important. We finally discuss the intermittent star formation history, which naturally explains the large dispersion of dust-to-gas ratio among BCDs.
We construct a new algorithm for estimating the star formation rate (SFR) of galaxies from their infrared (IR) luminosity by developing the theory of the IR emission from a dusty H II region. The derivedwhere f is the fraction of ionizing photons absorbed by hydrogen, ǫ is the efficiency of dust absorption for nonionizing photons from OB stars, and η is the cirrus fraction of observed IR luminosity. The previous conversion formulae of SFR from the IR luminosity is applicable to only the case where the observed IR luminosity is nearly equal to the bolometric luminosity (starburst galaxies etc.), except for some empirical formulae. On the other hand, our theoretical SFR is applicable to galaxies even with a moderate star formation activity. That is, our simple and convenient formula is significantly useful for estimating the SFR of various morphologies and types of galaxies -from early elliptical to late spiral and irregular galaxies, or from active starburst to quiescent galaxies -as far as they have neither an abnormal dust-to-gas ratio nor an evident active galactic nucleus.
In this paper we examine the allowed amount of intergalactic (IG) dust, which is constrained by extinction and reddening of distant Type Ia supernovae (SNe Ia) and the thermal history of the intergalactic medium (IGM) affected by dust photoelectric heating. Based on the observational cosmic star formation history, we find an upper bound of χ, the mass ratio of the IG dust to the total metal in the Universe, as χ≲ 0.1 for 10 Å ≲a≲ 0.1 μm and χ≲ 0.1(a/0.1 μm) for 0.1 ≲a≲ 1 μm, where a is a characteristic grain size of the IG dust. This upper bound of χ∼ 0.1 suggests that the dust‐to‐metal ratio in the IGM is smaller than the current Galactic value. The corresponding allowed density of the IG dust increases from ∼10−34 g cm−3 at z= 0 to ∼10−33 g cm−3 at z∼ 1, and keeps almost the value toward higher redshift. This causes IG extinction of ≲0.2 mag at the observer's B band for z∼ 1 sources and that of ≲1 mag for higher redshift sources. Furthermore, if E(B–V) ∼ 0.1 mag at the observer's frame against z≳ 1 sources is detected, we can conclude that a typical size of the IG dust is ≲100Å. The signature of the 2175‐Å feature of small graphite may be found as a local minimum at z∼ 2.5 in a plot of the observed E(B–V) as a function of the source redshift. Finally, the IGM mean temperature at z≲ 1 can be still higher than 104 K, provided the size of the IG dust is ≲100 Å.
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