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Astron. Astrophys. 324, 51-64 (1997)

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3. Results

3.1. The spectra

The absorption spectra are shown in Fig. 1, with the continuum normalized to unity. The [FORMULA] (1-0) line is not detected, despite the presence of an apparent line seen in Fig. 1. None of the 7 hyperfine components of this transition correspond to this line, moreover, its appearance differs from the lines detected in the other molecules. The [FORMULA] 'line' is an artefact caused by the receiver at this particular frequency. For [FORMULA], HCN, HNC and CS we can identify two major absorption complexes.

  • The strongest absorption feature is situated close to the systemic velocity of 552 km s-1, with a few relatively strong blue shifted absorption lines. We will henceforth call this complex of absorption lines the Low Velocity complex, or the LV complex. This complex consists of at least 4 different absorption components, extending between 540-556 km s-1. The high quality [FORMULA] spectrum shows evidence for 7 components.
  • Extended absorption at velocities redshifted relative to the systemic velocity is most evident in the [FORMULA] line, but can be seen in the CS, HCN and HNC lines as well. We will refer to this complex as the High Velocity complex or the HV complex. The HV complex consists of a broad 'diffuse' absorption, extending from about 560 km s-1 to approximately 640 km s-1. The [FORMULA] spectrum also shows several narrow absorption lines superposed on the broad absorption. These lines are absent from the other lines. The HV complex is not clearly evident in the [FORMULA] spectrum shown in Fig. 1, but when binning the spectrum the HV complex is present at significant level of several sigma.
[FIGURE] Fig. 1. Spectra of the observed transitions: [FORMULA] (1-0), [FORMULA] (1-0), HCN(1-0), HNC(1-0), CS(2-1) and [FORMULA] (1-0). Only [FORMULA] remains undetected. The spectra have been normalized to a continuum level of unity. The velocity scale is heliocentric and the frequency definition is relativistic (see text). The velocity resolution is 0.2 km s-1 for [FORMULA], 0.4 km s-1 for HCN, HNC and CS, and 0.6 km s-1 for [FORMULA] and [FORMULA].

For reference we identify 5 main absorption lines, visible in all but the [FORMULA] and [FORMULA] spectrum. No. 1-4 defines the LV complex and no. 5 is the broad diffuse HV complex. In Fig. 2 we show the LV and HV complexes in more detail. Superposed on the LV complex are the results of a 5-component gauss fit; four components for the LV complex and one for the HV complex (not shown). The fit parameters are given in Table 2. The residuals are similar to the rms noise level of the spectra, except for [FORMULA], where the high signal-to-noise reveals the several additional unresolved components in the LV complex (of course, the HV complex of [FORMULA] is poorly fitted by the fifth gauss component due to the presence of several narrow absorption lines). We will return to the [FORMULA] spectrum in Sect. 3.3.

[FIGURE] Fig. 2. The four main lines in the Low Velocity complex is shown to the left for CS(2-1), HNC(1-0) and [FORMULA] (1-0). Also shown are the results from a 5-component gaussfit. The residuals shown above the lines, are spectral values minus the fitted values. The fift component corresponds to the High Velocity complex, shown to the right. The HV complex has been binned to a velocity resolution of 3 km s-1. Notice the different scales. These five components define the main absorption lines (see Table 2).

[TABLE]

Table 2. Main gaussian components


3.2. The hyperfine components of HCN

HCN is not part of Table 2 since each absorption system here consists of three hyperfine transitions. The high quality HCN(1-0) spectrum allows us to make a decomposition of the hyperfine lines by keeping the center velocities for each absorption system fixed at the value derived from the other molecules and only varying the intensity and width of the lines. The fitting is complicated by a near overlap of the F [FORMULA] 0-1 hyperfine line of component 1 with the F [FORMULA] 2-1 line of component 2. The fitting is not perfect and the results actually suggests the presence of at least a fifth component (adding three more hyperfine lines). However, with the present signal-to-noise ratio and overlapping components, we use 4 absorption lines and smooth the HCN spectrum before fitting. The resulting decomposition is shown in Fig. 3 and the hyperfine components of the 4 main absorption lines in the LV complex is given in Table 3. The results are robust for all the F [FORMULA] 1-0 lines and for all the F [FORMULA] 2-1 lines, except for absorption line no. 2 which is situated within 0.3 km s-1 from the F [FORMULA] 0-1 line of absorption line no. 1. The F [FORMULA] 0-1 lines of line no. 3 and 4 are too weak to give completely reliable results. Nevertheless, the hyperfine line ratios, [FORMULA] and [FORMULA], as derived from the gaussian decomposition are given in Table 4.

[FIGURE] Fig. 3. Decomposition of the hyperfine components of HCN(1-0) in the LV complex. Four main absorption systems are assumed, each associated with three hyperfine lines of HCN. The center velocities of the components have been kept fixed in the fitting procedure.

[TABLE]

Table 3. HCN(1-0) hyperfine components for the LV-complex [FORMULA]



[TABLE]

Table 4. Ratios of HCN(1-0) hyperfine components for the LV-complex [FORMULA]


3.3. Parametrization of the [FORMULA] (1-0) spectrum

In order to facilitate future studies of the small scale structure of the molecular ISM along the line of sight towards the radio core in Cen A, we have parametrized the high quality [FORMULA] (1-0) absorption line shown in Fig. 1. The parametrization was done by fitting a number of gauss components to the spectrum. By starting out with a relatively small number of components and increasing the number until the residual does not show any large deviations from the rms noise of the spectrum, we found that a minimum of 17 components were needed. The reality of some of thes components should not be taken too seriously, since some absorption features may not be resolved and therefore do not show real gaussian profiles. The gaussian components can, however, be used for a precise representation of the absorption spectrum obtained by us and used for comparison with spectra obtained with different telescopes, spectrometers and spectral resolutions.

The gaussian components are given in Table 5 as the peak depth (measured from the normalized continuum), the full width at half intensity and the center velocity. The identification of these components are given from 1 to 17, and should not be confused by the identification of the 5 main absorption lines as given in Table 2. The [FORMULA] (1-0) spectrum with the fitted curve overlaid is shown in Fig. 4, together with a residual spectrum.

[FIGURE] Fig. 4. Parametrization of the [FORMULA] (1-0) spectrum through a fit of 17 gaussian components. Data for the gaussian components are given in Table. 5. The residual is spectral value minus fitted value and has an rms similar to that of the original spectrum away from the absorption complexes. The velocity resolution is 0.2 km s-1.

[TABLE]

Table 5. [FORMULA] (1-0) gaussian components


In addition to the HV and LV complexes there is an extended blueshifted absorption 'wing' in the [FORMULA] spectrum. This is evident already in Fig. 4, but is more clearly seen in a smoothed spectrum. In Fig. 5 we show the [FORMULA] spectrum binned to a velocity resolution of 2.1 km s-1. The blueshifted wing extends to about 500 km s-1. On the redshifted part of the spectrum, the broad absorption extends out to about 640 km s-1. In Fig. 6 we show the original data before baseline subtraction. The second order baseline which is subsequently used in the subtraction is shown as a dashed line. The peak of the baseline falls at [FORMULA] 550 km s-1, as it should, despite the small and uneven regions where it was fitted. The depression at 500-540 km s-1 can be seen quite clearly. It is also present in the low resolution data presented by Israel (1992). Hence, the molecular absorption along the line of sight to the radio core in Cen A occurs continuously over 140 km s-1. It is not possible to determine if the absorption in the range 500-540 km s-1 is a single blueshifted feature or if it is associated with the red 'wing' at [FORMULA] 615-640 km s-1.

[FIGURE] Fig. 5. The full extent of the [FORMULA] (1-0) spectrum. The data has been binned to give a velocity resolution of 2.1 km s-1. The blueshifted absorption 'wing' extends to [FORMULA] 500 km s-1, while the absorption extends to [FORMULA] 640 km s-1 on the redshifted side (see also Fig. 4).
[FIGURE] Fig. 6. The [FORMULA] (1-0) spectrum at a velocity resolution of 0.7 km s-1 before baseline removal. A second order baseline fit is shown as a dashed line. The blue 'wing' at 500-540 km s-1 is clearly visible.

3.4. Variations in the absorption lines?

The radio source in Cen A has a classical steep-spectrum double lobe structure, a steep-spectrum inner jet and a flat-spectrum core. VLBI observations at 8.4 GHz indicate that the core has a very small extent [FORMULA] 2 milliarcseconds (mas) (Jones et al. 1996). The core is self absorbed at 2.3 GHz and brightens up to frequencies of [FORMULA] 20 GHz. This small core is the likely source of the continuum emission seen at millimeter wavelengths. At a distance of 3 Mpc the extent of the core is thus only [FORMULA] 0.03 pc, or 6000 AU. The core may even be considerably smaller; the variability at millimeter wavelengths observed by Kellerman (1974) suggests a size of the order a light day, or 175 AU. In addition to the core, a narrow steep-spectrum jet is conspicuous at 8.4 GHz (Jones et al. 1996); its intensity is already much weaker than the core, and will be completely negligible at mm wavelengths.

Small scale structure in the molecular ISM has been directly observed down to [FORMULA] 2000 AU (e.g. Wilson & Walmsley 1989, Falgarone et al. 1992), while VLBI techniques allow detection of structures of 25 AU in the HI absorption medium in front of 3C radio sources (Diamond et al. 1989). Multi-epoch observations of 21cm absorption against high velocity pulsars also allowed detection of opacity variations of the ISM on a range of scales from 5 AU to 100 AU (Frail et al. 1994). In the molecular ISM, scales of the order of 10 AU have been inferred, through the time variations of H2 CO absorption lines (Marscher et al. 1993). Moore & Marscher (1995) confirmed these H2 CO absorption variations over a few years, in front of several point radio sources (3C111, NRAO150 and BL Lac). Through numerical simulations Marscher & Stone (1994) were able to derive constraints on the fractal structure of molecular clouds, from the time variability detections. The mean number of small clumps along the line of sight should be larger than previously thought, i.e. the size spectrum of clumps should be a steeper power-law, constraining the fractal dimension.

The surprising fact in these time variations of absorption features is that the involved gas appears to be diffuse. In the case of the 21cm absorption against pulsars, where small-scale opacity structures are detected towards all line of sights, the mean opacities are between 0.1 and 2.5, corresponding to N(HI) as low as 10 [FORMULA] cm-2 (Frail et al. 1994). Also in the case of molecular 6cm H2 CO absorptions, the mean optical depth is very low (Moore & Marscher 1995). Of course variations of opacities are much more easy to detect when the opacity is low, since saturated profiles ([FORMULA]) are acutely sensitive to small variations in the noise level. But it was not expected to find contrasted small scale structures, and clumps, in such a diffuse gas. The opacity variations dectected are quite high (half of them have [FORMULA], Frail et al. 1994), so they cannot be accounted for by mild density fluctuations. Clumps with density larger than 10 [FORMULA] cm-3 are implied (Moore & Marscher 1995). These clumps could represent 10-20% of the total column density.

The same physical conditions appear to be true for the molecular absorbing gas in front of the Cen A radio source. The opacities and column densities must be low in average, since the isotopic [FORMULA] line is observed with about the expected abundance ratio with respect to the [FORMULA] line. In these conditions, opacity variations should be easy to detect. The gaseous disk in Cen A has a rotational velocity of more than 250 km s-1. The value is somewhat uncertain due to the strong warp of the disk and could be higher (cf. van Gorkom et al. 1990, Rydbeck et al. 1993). A rotational velocity of 250 km s-1 corresponds to a shift of the gas along the line of sight towards the radio core in Cen A by [FORMULA] 50 AU per year.

The first molecular absorption line observations in Cen A was obtained in 1976 (Gardner & Whiteoak 1976), of formaldehyde. However, it wasn't until 1989 that the first high resolution molecular absorption line data was obtained, which can be used for a detailed comparison with the present data (cf. Eckart et al. 1990). Hence, the time span is about 7 years and a comparison with this data is thus sensitive to spatial scales in the ISM of 300-500 AU.

The [FORMULA] (1-0) spectrum published by Eckart et al. (1990) does not have as good signal-to-noise ratio as the current one and any comparison is limited by the rms noise in the 1989 data. In Fig. 7 we show our [FORMULA] (1-0) spectrum and the one obtained by Eckart et al. (1990) in 1989. Both spectra have been binned to a velocity resolution of 0.6 km s-1. The lower panel in Fig. 7 shows the difference between the two spectra; [FORMULA] with equal weighting. A small residual feature can be seen in the difference spectrum at a velocity of 552 km s-1, with a depth [FORMULA] 10% of the normalized continuum level. This feature corresponds to the deepest absorption line. It could, however, be an effect of a slightly incorrect continuum flux in the 1989 data. If the continuum flux has been overestimated, the line-to-continuum ratio decreases (and the difference will show up for the strongest absorption feature). Likewise, an incorrect continuum level for the 1995/96 data could mimic the residual feature. The latter is not likely, since the observing conditions for the 1995/96 data were very good. Also, a comparison between the [FORMULA] data obtained in December 1995 and July 1996 do not show any residual (Fig. 8). Although the time span is only six months, the considerably higher quality of the data makes a comparison sensitive to very small changes in both the location of the absorption lines and their depths. The sensitivity for shifts in velocity is very good and the limits are less than -0.2 to [FORMULA] km s-1 ; The difference depends on the shape of the absorption profiles. This allows us to put an upper limit of 10% to changes in the [FORMULA] (1-0) absorption line in Cen A. If the molecular ISM in Cen A have small scale structures similar to those found in our Galaxy, this negative result implies that the background continuum source at millimeter wavelengths has an extent [FORMULA] 500 AU. The limit given by VLBI is 6000 AU. However, it should be kept in mind that if the absorbing gas is made up of a large number of small clouds ([FORMULA] 200 per absorption feature) with angular extent smaller than that of the continuum source, changes will be difficult to detect since the clumps would tend to average out over time (e.g. Marscher & Stone 1994). Only future high-quality data can solve the question regarding variations in the absorbing gas in Cen A

[FIGURE] Fig. 7. [FORMULA] (1-0) spectra obtained in 1995/96 (present paper) and in 1989 (Eckart et al. 1990). The difference ([FORMULA]) is shown in the third panel. The spectra have been binned to a velocity resolution of 0.6 km s-1 before subtraction. No significant differences can be found.

[FIGURE] Fig. 8. [FORMULA] (1-0) spectra obtained in December 1995 and in July 1996. The difference is shown in the third panel. The spectra have been binned to a velocity resolution of 0.6 km s-1 before subtraction. No differences can be found.
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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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