Astron. Astrophys. 356, 23-32 (2000)

2. Observations of C IV (and Ly ) in absorption in 0943-242

2.1. Earlier observations of 0943-242 at

The low resolution spectrum of 0943-242 shown in RO95 and discussed also in van Ojik et al. (1996) displays the characteristic emission lines of a distant radio galaxy: strong Ly , weaker C IV , and possibly C III ] . This object was also observed at intermediate resolution (1.5Å) by RO95 in the region of Ly with the slit positioned along the radio axis. The initial discovery of extended absorption troughs was based on this latter spectrum which we reproduce in Fig. 1.

Fig. 1. An expanded plot of the Ly spectral region obtained by RO95. The H II emission gas redshift is and the main absorber of column lies at .

2.2. New observations of and at intermediate resolution

With the objective of providing constraints on the abundances and kinematics of the gas in 0943-242, sensitive high-resolution spectroscopic observations centered at the C IV and lines were performed at the Anglo Australian Telescope (AAT) on 1995 March 31 and April 1 under photometric conditions and with a seeing which varied from 1" to 2". The RGO spectrograph was used with a 1200 grooves mm^-1 grating and a Tektronix 1024² thinned CCD, yielding projected pixel sizes of Å. The projected slit width was 1.3", resulting in a resolution as measured from the copper-argon calibration spectrum of 1.5Å FWHM; the slit was oriented at a position angle of 74^o, i.e. along the radio axis (as in RO95).

The total integration time of 25000s was split into 22000s and 73000s exposures in order to facilitate removal of cosmic rays. Exposure times were chosen to ensure that the background was dominated by shot noise from the sky rather than CCD readout noise. Between observations the telescope was moved, shifting the object slit by about 3 spatial pixels, so that for each exposure the spectrum was recorded on a different region of the detector. The individual spectra were flat-fielded and sky-subtracted in a standard way using the long-slit package in the NOAO reduction system IRAF. The precise offsets along the slit were determined using the position of the peak of the spatial profile of the C IV and lines. Using these offsets, the images were registered using linear interpolation and summed to obtain the two-dimensional spectrum. The resultant seeing in the final two-dimensional spectrum, measured from two stars on the slit, was 1.5" FWHM. The corresponding FWHM of C IV emission along the slit was 2.2", giving a deconvolved (Gaussian) width of 1.6" or 12 kpc. Within the errors, this is the same as that found for Ly emission by RO95.

The two-dimensional spectrum was weighted summed over a 7 pixel (5") aperture to obtain a one-dimensional spectrum. In Fig. 2 we show the AAT data in the form of a full-resolution spectrum.

Fig. 2. The full-resolution AAT spectrum showing the C IV 1548, 1551 and lines.

2.3. Profile fitting of the emission and absorption Ly and lines

One deep trough is observed in the Ly emission line (Fig. 1) which was interpreted as a large scale H I absorber by RO95. In addition there are a number of weaker troughs, presumably due to weak H I absorption. Fitting the emission line by a Gaussian and the H I absorption by Voigt profiles, RO95 infer a column density of for the deep trough, a redshift and a Doppler parameter b of km s^-1. For the three shallow troughs, they find ranging from to and b ranging from 7 to 100 km s^-1. The redshift difference of the absorbers relative to systemic velocity when converted into inflow/outflow velocities indicate values not exceeding 800 km s^-1. Because at the bottom of the main trough no emission is observed, the covering factor of the absorbing gas must be equal or larger than unity over the complete area subtended by the Ly emission, indicating that the spatial scale of the absorber exceeds 13 kpc. This work will concern only the deep absorption trough.

To parameterize the C IV profile we have assumed that the underlying emission line is Gaussian, with Voigt profiles due to the C IV doublet absorption superimposed. We used an iterative scheme that minimizes the sum of the squares of the difference between the model and the observed spectrum, thereby solving for the parameters of the model (e.g. Webb 1987, v097). Initial values were assumed for the shape of the Gaussian profile and the redshift of the absorber.

In Fig. 3 we show a portion of the spectrum with the model fits superimposed. The Gaussian fitted to the C IV emission line peaks at and has a FWHM of Å. We have corrected all wavelengths to the vacuum heliocentric system (+1.13 Å) before computing the redshifts. The two troughs in this figure correspond to the C IV 1548, 1551 doublet produced by the same absorption system. Therefore, within the fitting procedure, the wavelength separation and the ratio of the two profiles' depths are fixed by atomic physics while the two values for b are set to be equal. The fit gives for the location of the bottoms of the two troughs 6068.2 and 6078.3Å resulting in a redshift of 2.9202 0.0002. Within the errors this redshift is equivalent to that of the main H I absorber and in the subsequent analysis we will assume that the Ly and C IV absorption gas belongs to the same absorber. We derive a Doppler parameter b for the doublet of km s^-1 and a column density of as summarized in Table 1.

Fig. 3. An expanded plot of the full-resolution spectrum with the fit superimposed (solid line).

Table 1. Parameters for the Gaussian and Voigt profile fits

As expected, appears only in emission without any absorption since it is not a resonance line. Parameters for the emission profile were obtained by fitting a Gaussian using the same iterative scheme (see Fig. 1 in Röttgering & Miley 1997). The peak is positioned at and has a FWHM of Å. The fitted parameters of the emission and absorption profiles are presented in Table 1. We recall that the FWHM of the Ly emission profile is km s^-1 (v097), significantly larger than that of (see Table 1). Inspection of the various profiles in Fig. 1 and Fig. 2 (or Fig. 3) suggests the presence of an excess flux on the blue wings of all the emission profiles. Combining information from all the emission lines, our best estimate of the emission gas redshift is .

2.4. Velocity shear and subcomponents

To investigate whether there is any velocity shear in the C IV emission profile we fitted spatial Gaussian profiles to the emission line as a function of wavelength. In Fig. 4 we show the wavelength maxima of these spatial profiles and a line fitted through these points. The spatial profile of the C IV emission spectrum is displaced by 0.2", corresponding to a displacement of 1.5 kpc, over a wavelength range of 50Å. RO95 measured a comparable shift for Ly of 1.8 kpc ² although it appears that the latter displacement is due to a far more pronounced and abrupt difference in locations of the Ly peak on both sides of the absorption trough. As Fig. 4 shows, the peaks of C IV emission form a wavy line. We believe the velocity shear in the CIV profile to be less significant than the shear in the Ly profile. We cannot rule out that the small velocity shear might be masking a possible break up of the absorption regions into a few saturated absorption components of smaller b.

Fig. 4. The relative location of the peak of the C IV emission at constant wavelength as a function of wavelength. The line is a weighted fit to these peaks. The zero offset is arbitrary.

A concern about the determination of is the possibility that that there exist subcomponents in the absorption systems that have high column densities but low b values and are, therefore, not acounted for whenever individual velocity subcomponents are not resolved. Although we cannot strictly exclude this possibility, we adopt the stand of Jenkins (1986) and Steidel (1990a) who, using extensive absorption line studies, argue that this is unlikely to be the case, at least for C IV , and that a single-component curve-of-growth analysis can be used to infer total columns although the inferred effective b value has no physival meaning in terms of temperature. It is interesting to note that the physical conditions inferred from the C IV fit are fully consistent with the observed ratio of the doublet (since both troughs are equally well fitted). If the underlying continuum was flat, the column and the b value we infer would imply a theoretical ratio of equivalent widths of , which is where the curve of growth just begins to leave the linear part (Steidel 1990a). Clearly the column might be susceptible to a larger error since Ly is saturated. With these caveats in mind, we will assume in the following analysis that the adopted columns do not lie far off from reality.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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