Velocity Structure Along the Line of Sight

Individual absorbing clouds along the line of sight can only be distinguished if they have unique radial velocities. However, for ideal circular orbits there should be no radial component of velocity in the line directly towards the centre of the Galaxy. Sgr B2 is located very close to this line and so any observed positive or negative velocity components must be caused by deviations from circular motion. These particularly occur in the inner Galaxy where most of the emitting matter is not in circular orbits (e.g. Bally et al., 1988).

A number of different absorbing components have been observed towards Sgr B2 in the approximate range $-110$ to $+100$ km s$^{-1}$. In high resolution radio and mm observations, the observed components are narrow ($\sim$1-10 km s$^{-1}$) and occur at discrete velocities that are well separated. This indicates that they are due to individual absorbing clouds in the line of sight and these are usually attributed to the main galactic spiral arms. The clouds are illuminated by the galactic interstellar radiation field and so are very similar in structure to the PDRs found around star forming regions. UV radiation incident on the edge of the clouds causes photodissociation of molecules, creating a mainly atomic outer layer. Self shielding inside the clouds allows the core region to remain un-dissociated and this contains molecular hydrogen and other molecular species (e.g. Hollenbach & Tielens, 1999). Absorption along the line of sight to Sgr B2 has been observed due to both atomic species (e.g. HI; Garwood & Dickey, 1989) and many different molecules. These include HCN, HCO$^{+}$ (Link et al., 1981), CS, SiO, C$_{3}$H$_{2}$ (e.g. Greaves et al., 1992), NH$_{3}$, SO, H$_{2}$S (Tieftrunk et al., 1994), H$_{2}$CO (e.g. Mehringer et al., 1995), N$_{2}$H$^{+}$, CH$_{3}$OH (Greaves & Nyman, 1996) and H$_{2}$O (Neufeld et al., 2000). Velocity components observed in these spectra broadly agree with those found in the HI spectrum and they can be divided into several distinct groups that belong to different features along the line of sight. Table 2.3 shows the components observed in HI, H$_{2}$CO and CO observations. These features are described in more detail in the following sections. Figure 2.4 shows an overview of the line of sight based on a diagram from Greaves & Williams (1994). Well known features from the literature and proposed identifications for the remaining velocities are shown.

Figure 2.4: Schematic diagram (not to scale) showing the line of sight to Sgr B2 with proposed locations for foreground clouds with their associated velocities in km s$^{-1}$ (adapted from Greaves & Williams, 1994).

Table 2.3: Velocities (and FWHM) of features in HI (Garwood & Dickey, 1989), H$_{2}$CO (Mehringer et al., 1995) and CO (Vastel et al., 2002) measurements towards Sgr B2. The approximate distance of each feature from the Galactic Centre is shown in the final column. References for the various identifications are discussed in the text.

HI	H$_{2}$CO	CO	Identification	Distance from
(km s$^{-1}$)	(km s$^{-1}$)	(km s$^{-1}$)		Galactic Centre
				(kpc)
		$-113.6$ (4.3)	Inner regions of galactic bar	$>1.0$
$-107.6$ (6.6)	$-104.0$ (10)	$-104.8$ (7.5)	Inner regions of galactic bar	$>1.0$
$-81.7$ (28.0)		$-76.0$ (1.4)	Inner regions of galactic bar	$>1.0$
	$-60.0$ (3)		Inner regions of galactic bar	$>1.0$
$-51.9$ (17.2)		$-57.8$ (1.6)	?3 - 4 kpc arm	3.5
$-44.04$ (7.5)	$-46.0$ (2.3)	$46.2$ (3.0)	3 - 4 kpc arm	3.5
	$-41.1$ (2.2)	$-41.0$ (4.2)	3 - 4 kpc arm	3.5
$-24.39$ (13.7)	$-26.5$ (4.3)	$-25.8$ (2.4)	4.5 - 5 kpc arm	4.7
	$-21.2$ (3.4)	$-20.6$ (2.7)	4.5 - 5 kpc arm	4.7
1.1 (18.7)	1.8 (8.2)	1.7 (15.0)	local + Sgr B2	$\sim8.5$
15.71 (7.3)	14.5 (15)	22.7 (2.1)	probably local	$\sim8.5$
31.4 (20.7)	32.9 (7.7)	33.3 (4.5)	?Scutum arm	$\sim5$-7
52.78 (11.1)			Sgr B2	0.1
66.72 (16.2)	65.3 (11.8)		Sgr B2	0.1