3 Abundances and kinematics

The abundances determined from Voigt profile fitting for both DLAs are somewhat unusual, as we discuss in more detail below. An interesting possibility is that these absorbers may be part of a large structure, for example a proto-cluster or galaxy filament. If so, the main environmentally driven effects we may expect to witness will most likely impact upon the gas kinematics and chemical abundances of the two systems.

Figure 2: Unsaturated metal lines associated with DLA A with Voigt profile fits overlaid. The velocity scale on each panel is relative to $z_{\rm abs}$ = 1.8573.

Figure 3: Unsaturated metal lines associated with DLA B with Voigt profile fits overlaid. The velocity scale on each panel is relative to $z_{\rm abs}$ = 1.8573.

 

Table 1: Voigt profile fit parameters for DLAs A and B towards Q2314-409.

Cloud	Redshift	b	Log10 N(X)
			Fe II	Zn II	Cr II	Al II	Si II	O I	Ni II	S II
DLA A
1	1.857311	12.98	15.08	12.52	13.38	...	15.41	...	13.84	15.10
DLA B
1	1.875032	5.02	13.34	...	...	11.99	13.43	14.41	...	...
2	1.875197	7.74	13.31	...	...	12.01	13.38	14.28	...	...
3	1.875431	8.42	13.07	...	...	11.87	13.05	13.95	...	...
4	1.875661	6.82	...	...	...	...	...	13.32	...	...

The overall metallicity of both DLAs is typical of those measured at this redshift (Pettini et al. 1999), indicating that there is no particular evidence of extended periods of either highly enhanced or suppressed star formation over the span of each galaxy's star-forming life. However, we note that in documented DLAs there is a large observed spread of metallicities at a given redshift, so that to notice a marked difference in [Zn/H] or [Fe/H] would require a very pronounced effect. Next we consider the abundances of $\alpha$ elements which, when compared with Fe-peak elements, provide clues to the history of star formation in the galaxy. In DLA A, both [Si/Fe] and [S/Zn] are roughly solar (see Table 2), Zn being the usual Fe-peak element of choice because of its non-refractory nature. However, since there are very few [S/Zn] measurements, in Fig. 4 we plot [S/Fe] in order to facilitate a useful comparison with literature values. Combined with the lower limit [S/Fe] <0.15 in DLA B, Fig. 4 reveals that both absorbers have relatively low S/Fe ratios compared with other known DLAs. Note that although the fit to Fe II $\lambda$1608 in DLA B appears somewhat poor (due to fixing the b-values), allowing a completely free fit to the data results in only a 0.01 dex change in N(Fe). The abundance ratios plotted in Fig. 4 relative to Fe will all require some dust correction, which will be different from system to system (see next section). However, we nevertheless note that the [Si/Fe] ratios for both DLA A and B are lower than any in the UCSD DLA database (Prochaska et al. 2001) and the large compilation of Lu et al. (1996). Furthermore, for DLA B we determine a very low [O/Fe] = -0.40, which would be further reduced if there was some correction to be made to Fe due to dust. In reality, however, [O/Fe] = -0.40 is probably a lower limit because of mild saturation of the O line (see next section). Overall, both DLAs exhibit relatively low $\alpha$/Fe-peak abundances, although not excessively so, given the uncertainties.

Although DLA A appears to be a single component from unsaturated lines, stronger transitions such as Si II $\lambda$1526 and Al II $\lambda$1670 reveal this system to have a somewhat more complicated multi-component structure. In fact, both DLAs have absorption profiles that extend over approximately 100 km s^-1, a velocity not atypical compared with other damped systems (Prochaska & Wolfe 2001). It therefore appears that the interstellar gas has not undergone significant disruption. The few observations of other absorbers in high redshift galaxy groups provide mixed results with regards to kinematics. Q0201+1120 has a velocity spread of 300 km s^-1 consisting of many components (Ellison et al. 2001). Similarly, the DLA at $z_{\rm abs}$ = 2.38 towards B2138-4427 has components over 200 km s^-1 (C. Ledoux, private communication), but the possible LLS in the same group towards B2139-4434 has only a $\sim$60 km s^-1 spread (V. D'Odorico, private communication).

 

 

Table 2: Abundance measurements (and 3$\sigma$ upper limits) for DLAs A and B towards B2314-409.

	DLA A		DLA B
X	N(X)	[X/H]	N(X)	[X/H]
Fe	$15.08\pm0.10$	$-1.33\pm0.14$	$13.73\pm0.1$	$-1.88\pm0.24$
Si	$15.41\pm0.10$	$-1.44\pm0.14$	$13.79\pm0.10$	$-1.86\pm0.22$
Cr	$13.38\pm0.08$	$-1.20\pm0.13$	<12.17	<-1.61
Zn	$12.52\pm0.10$	$-1.02\pm0.14$	<11.56	<-1.19
Ni	$13.84\pm0.08$	$-1.31\pm0.13$	<12.70	<-1.65
S	$15.10\pm0.15$	$-1.07\pm0.18$	<13.64	<-1.73
O	...	...	$14.75\pm0.12$	$-2.28\pm0.22^a$
Al	...	...	$12.44\pm0.08$	$-2.14\pm0.22$

^a Probably a lower limit due to mild saturation of the O line.