1.3 What is in the torus?
1.3.2 Basic properties of Interstellar Dust Grain
Dust grains are solid and macroscopic particles, composed of dielectric and refractory materials. The composition of dust reflects the abundance of cer-tain elements in the universe, which include hydrogen, oxygen, carbon, ni-trogen, and silicon. Those common elements make up water (H2O), methane
(CH4), carbon dioxide (CO2), ammonia (NH3), and silicates (SiO−44 ), With a solid-state temperatures low (about 100K), that they are under form of ice.
A model has been developed for describing the dust grain properties (see Fig.1.5). It assumes the core of the dust grain is very small (about 0.05µm) and may consist of silicates, iron, or graphite, silicate being in majority.
Outside of the inner core, there is a about 0.5µm which mantle made of icy materials (Zeilik, 2002). If the grain enter a hot environment, the mantles will be evaporated, and then the grain consist of a bare core only.
Infrared wavelengths can be used to investigate interstellar dust and get more information about the emissivity of dust grain, especially, from the sil-icate and ices (Zeilik, 2002). Because we know the temperature of formation dust (or non-evaporate) is below 2000K (more details at table.1.1 ), it is easy to estimate the wavelength emitted from dust via the Wien displacement law (Eq.1.2):
Table 1.1: The condensation temperature of some species (Evans, 1993).
λmaxTW ien = 2890µmK, (1.2)
which λmax means the wavelength of maximum intensity and TW ien is temperature. Therefore, we could estimate the wavelength emitted from dust is ≥1.5µm, which is in the infrared. Although in the real case, the materials do not emit as perfect blackbodies, it is absorbed or emitted by some matters.
Therefore, the infrared observation can give us the much useful information for the dust, and we could get the observed infrared flux distribution with different range of grain temperatures and sizes (Evans, 1993).
There are a lot of gas and dust in the universe, and the gas-to-dust ratio could be estimated from the interstellar extinction (see Table.1.2). Deducing from the density of the interstellar gas and dust grains, we get:
ρ(dust particles)
ρ(gas) ∼ 10−2. (1.3)
The gas-to-dust ratio is about 100:1 in galaxies. The ISM, contains about 10% of the baryonic mass of the galaxy, including the 0.1% dust grain (Evans, 1993). This implies a large amount of heavy elements (heavier than hydrogen and helium), are linked up with dust grain, and the elemental abundances will decrease, with increasing atomic number.
Ingredient Extinction law Cross-section Density (kgm(− 3))
Free electrons λ0 σT 4.1 × 10−22
Molecules λ−4 ∼ πa2 4.4 × 10−26
Small(a λ) particles λ−4 πa2 1.5 × 10−27
Large(a λ) particles λ0 πa2 1.5 × 10−27
Particles with a ∼ λ λ−1 Qextπa2 1.2 × 10−23 Table 1.2: Interstellar extinction (Evans, 1993).
The dust grain is mostly composed of silicon atoms, which in fact is a
major component of interstellar medium in the galaxies (Hao et al., 2005).
Since the atoms of silicon do not subsist alone, they always form networks with oxygen and other abundant elements, such as Fe and Mg, and sometimes link up with Ca and Al. In fact, oxygen and silicon are the most common non-gaseous and non-metallic elements in the dust composition. The silicon also tie with other abundant elements,
In the laboratory, the spectrum of silicate dust has been measured. It has two spectral features: one is Si-O stretching mode, and the other is O-Si-O bending mode. The emission feature at the first one is 10 µm and the second one is 18µm (Knacke and Thomson, 1973). In the observation, we can indeed find the two spectral features are in the infrared region. The silicate emission was detected by Hao et al. (2005); Sturm et al. (2005); Siebenmorgen et al.
(2005).
Figure 1.4: This is dust formation window, which is given by the temperature gradient (T∝R−0.4) that is determined by the radiative transfer (Ivezic and Elitzur, 1997). It shows the range of the appropriate condition (the temper-ature and pressure) for forming dust. Below the thin solid lines, the dust precursor will be formed in the AGB case (adapted from Lodders and Fegley (1999)), and the thick solid lines show the range for forming the dust in the expanding BELCs. Top: Phase transition lines for O-rich; Bottom: Phase transition lines for C-rich environments.
Figure 1.5: Simplified model for an interstellar dust grain.(Zeilik, 2002)
Chapter 2
Target and Sample
2.1 Target selection
Based on the model developed by Elvis et al. (2002) (see section 1.3.2), out-flowing winds of quasars may be suitable environments for forming dust. We selected some quasars which display different degree signature of outflow-ing winds for analyzoutflow-ing their dust composition. We chose five radio-quiet quasars, including two BALs quasars with winds, two non BAL quasars (one with very strong outflows and the other without noticeable outflow), and a mini-BAL quasar with weak outflows.
The spectrum of type 1 quasars show broad and blueshifted absorption resonance transitions in ultraviolet, indicating exposed gas from the center, and generating an observed spectrum with P Cygni-type features. Those features could be evidences for existence of outflowing winds in the type 1 quasar, and are more obvious in BAL quasars, which represent 20% (Hewett and Foltz, 2003) of optically selected type 1 quasars.
Silicate is a major element in the composition of galactic dust, so it can be used as a racer of presence of dust for selecting our sources. The silicate lines will be in emission for the type 1 AGN (Laor and Draine, 1993), therefore, we choose some spectra with high-luminosity, high signal-to-noise ratios, and obvious silicate emission feature both at 10 µm and 18 µm to be included in our sample. Two of them, PG 2112+059 (Gallagher et al., 2004; Barvainis et al., 1996) and PG 0043+039 (HTS archive), are two of the MIR-brightest BAL quasars. In addition, we also select two non-BAL quasars, PG 0050+124 and PG 1211+143, to be antithesis. Although PG 1211+143 (Pounds et al., 2003) is not a BAL, it presents very strong outflows and PG 0050+124 (I Zw 1) Hao et al. (2005); Barvainis et al. (1996); Dhanda et al. (2007) is a non-BAL without noticeable outflows. Finally, the last one, PG 1351+640 Hao et al. (2005); Zheng et al. (2001), is a mini-BAL with weak outflow.
These quasars are all radio-quiet; it support us focusing on comparing their components and figuring out the relationship between components and outflows winds. (more details in table. 2.1)
Although these quasars have dissimilar redshifts (three of them with local redshifts, PG 1351+640 is 0.0881, PG 0050+124 is 0.0808 and PG 1211+143 is 0.0611; two higher redshifts, PG 2112+059 is 0.466 and PG 0043+039 is 0.385), we assume the physical processes for forming dust owe to be the same in the galaxy at any redshift.
NamezTypeReference PG2112+0590.466shallowBAL,broad,blueGallagheretal.(2004);Barvainisetal.(1996) PG0043+0390.385smoothBAL,broad,redHSTarchive PG1351+6400.0881miniBAL,weakoutflowHaoetal.(2005);Zhengetal.(2001) PG0050+1240.0808nonBAL,weakoutflowHaoetal.(2005);Barvainisetal.(1996);Dhandaetal.(2007) PG1211+1430.0611nonBAL,strongoutflowPoundsetal.(2003) Table2.1:SampleCharacteristics.Weselectionsomedifferenttypequasarsforanalyzing,buttheyallareradio-quiet Type1QSOs