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Astron. Astrophys. 351, 47-58 (1999)

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4. Results

4.1. The spectra

The spatially integrated spectra of our objects are shown in Fig. 1. We extracted each spectrum from an aperture inside which the weakest lines were detected with higher S/N: MRC2025-218 (1.9 arc sec, centered at the spatial continuum centroid); MRC2104-242 (5.9 arc sec centered at the intersection between the two bright clumps); MRC1558-003 (2.7 arc sec centered at the spatial position of centroid of the brightest Ly[FORMULA] component). The two spectra of SMM J02399-0136 correspond to a) the whole system (L1+L2) (the spectrum was extracted form a 5.4 arc sec aperture covering the brightest emission of the L1+L2 components) and b) he AGN component, L1. (2.2 arc sec aperture centered at the centroid of Ly[FORMULA] emission in L1)

[FIGURE] Fig. 1. Integrated spectra of the four objects in the sample discussed in this paper. The last spectrum corresponds to component L1 (adopting IV98 nomenclature) in the system SMM J02399-0136 (the active galaxy). Flux is given in units of 10-17erg s-1 cm-2 Å-1, except for MRC2104-242, which is given in units of 10-16erg s-1 cm- 2 Å-1.

The spectra (except for SMM J02399-0136) are typical of HzRG. Ly[FORMULA] is the strongest line and weaker CIV[FORMULA]1550, HeII[FORMULA]1640 and CIII][FORMULA]1909 are also detected. NV[FORMULA]1240 is detected only in MRC2025-218 and SMM J02399-0136. This line is often not detected in HzRG (Röttgering et al. 1997). We compare in Fig. 2 the spectrum of L1 with an average spectrum of HzRG (Vernet et al. 1999) and the hyperluminous (also gravitational lensed) Seyfert 2 galaxy FSC10214+4724 (Goodrich et al. 1996). The differences are striking. L1 presents very weak HeII, strong NV and weak Ly[FORMULA] compared to typical HzRG spectra. It is similar to FSC10214+4724 in the sense that Ly[FORMULA] is weak and NV strong, but HeII is relatively much weaker in L1.

[FIGURE] Fig. 2. Comparison between the spectrum of L1 (the AGN in SMM J02399-0136), an average HzRG spectrum (Vernet et al. 1999) and the hyperluminous Seyfert 2 galaxy FSC10214+4724 (Goodrich et al. 1996). Notice the relative weakness of Ly[FORMULA] and HeII and the strength of NV in L1 compared to a typical spectrum of HzRG.

We present in Table 1 some parameters characterizing the main UV emission lines, resulting from 1-D Gaussian fits to the line profiles.


Table 1. CIV flux (in units of 10-16 erg s-1 cm-2). The fluxes of the strongest lines are given relative
to CIV. The FWHM of the lines are also shown

4.2. The spatial distribution of the continuum and the emission lines

We described in VMFB99 the spatial properties of the Ly[FORMULA] emitting gas derived from the 2-D spectra. Here we present 1-D spatial profiles for Ly[FORMULA], the continuum and the strongest UV lines. We created the emission line spatial profiles by adding those pixels (in the dispersion direction) where a given line is detected and subtracting the underlying continuum from a window of identical size (in Å) close to the line. The continuum spatial profile was created using a much larger window to increase the S/N ratio (typically 100-150 pixels in the dispersion direction, i.e. [FORMULA] 65-90 Å). We present in Fig. 3 the spatial profiles for the strongest lines and the continuum.

  • SMM J02399-0136: The two optical components (L1 and L2) identified by IV98 are clearly seen both in continuum (dominated by L1) and Ly[FORMULA] (similar strength in both components) separated by [FORMULA] 3 arc sec. L1 is detected in the other UV lines as well, but not L2 (the spatially integrated spectrum of L2 shows weak CIV). Both the Ly[FORMULA] and the continuum profiles reveal a region beyond L2 (named L3 in the figures) up to [FORMULA]10 arc sec beyond the continuum centroid of L1. VMBF99 showed that Ly[FORMULA] presents large velocity widths ([FORMULA]1000 km s-1) in L3. No other lines are detected in this region except possibly CIV. L1 is rather compact, but it appears to be marginally resolved (1.45 arc sec, while the average seeing was 1.20 arc sec). It contains an unresolved component (WF/PC1 F702W and WFPC2 F336W images show an unresolved source at the L1 position with a FWHM of [FORMULA]0.1-0.2 arc sec [IV98]) and some extended emission.

  • MRC2025-218: The spatial profiles of Ly[FORMULA] and the continuum reveal very different distributions. The continuum emission is extended, but the dominant component is just marginally resolved (1.3 arc sec while the average seeing was 1.1). Ly[FORMULA] is more extended and the flux does not peak at the position of the continuum centroid, but it presents a plateau. CIV (maybe CIII] as well) are also extended. The 2-D spectrum (see VMBF99) reveals a bimodal distribution for Ly[FORMULA] which was also mentioned by McCarthy et al. 1990

  • MRC1558-003: The Ly[FORMULA] profile reveals the presence of two main components separated by [FORMULA]9 arc sec: the main optical component and a region named A in the figure. VMBF99 showed that this A region emits lines of large widths (FWHM[FORMULA]1500 km s-1). Both components emit also CIV, CIII], HeII and continuum. The continuum and the emission lines (at least CIV and Ly[FORMULA]) are spatially resolved in both regions.

  • MRC2104-242: Ly[FORMULA] presents a bimodal distribution, clearly seen in CIV as well. Both components are spatially resolved. The centroids are separated by [FORMULA]7 arc sec. We do not present the spatial profile of the continuum in Fig. 3 because it is too noisy, but very weak continuum is detected associated with the two Ly[FORMULA] components.

[FIGURE] Fig. 3. Spatial profiles of the continuum and brightest UV lines. Left panels: Continuum in solid lines and Ly[FORMULA] in dashed lines (when shown). Right panels: Emission lines only. Bottom: Emission line spatial profiles in MRC2104-242. In all panels a constant has been added to some of the profiles to make the plots clearer. The spatial zero for each object has been selected as the spatial position of the continuum centroid, except for MRC2104-214 that has a very weak continuum. For this object we selected the spatial position which appears to best define the separation between the two main optical components.

Therefore, all objects present extended continuum over several arc sec (up to [FORMULA]15 arc sec in SMM J02399-0136). The continuum in MRC2025-218, MRC1558-003 and SMM J02399-0136 is dominated by a bright component which is rather compact. It is probably unresolved in SMM J02399-0136 and marginally resolved in MRC2025-218. All objects show extended emission lines which present rather different spatial profiles compared to the continuum (compare, for instance, the Ly[FORMULA] and continuum profiles in SMM J02399-0136 and MRC2025-218). Both lines and continuum reveal the presence of several spatially distinct regions.

4.3. Absorption features in the spectrum of MRC2025-218

The 2-D spectrum of MRC2025-218 shows a clear absorption feature blueshifted with respect the CIV emission (see Fig. 4). In order to search for other absorption features, we have extracted a 1-D spectrum from the continuum emitting region (8 pixels or 2.2 arc sec aperture). We fitted the profiles of all possible absorption detections. We assumed Gaussian profiles (although it does not necessarily have to be the case). Some absorption features are detected. We show in Fig. 5 (bottom) the spectrum in the range 1180-1700 Å, with the expected position of some absorption features commonly found in nearby starburst galaxies. We present for comparison the spectrum of the B1 star forming knot in NGC1741 (Fig. 5 top) (Conti et al. 1996).

[FIGURE] Fig. 4. 2-D spectrum of MRC2025-218 in the CIV region. There are some residuals of a night sky line on the blue side clearly seen in the figure. Spectral dispersion runs in the horizontal coordinates and spatial direction in vertical coordinates.

[FIGURE] Fig. 5. Top: Spectrum of the star forming knot B1 in the nearby starburst galaxy NGC1741 (Conti et al. 1996). Bottom: Spectrum in the same spectral region of MRC2025-218. We indicate in this diagram the possible absorption features detected in MRC2025-218.

We rejected those features such that: a) the spectral profile was (taking errors into account) narrower than the instrumental profile (IP) (2.98 Å in the rest frame) and/or b) the absorbed flux was lower than the detection limit. This was the case of SiII[FORMULA]1260.4, OV[FORMULA]1371, SiIII[FORMULA]1417, SV[FORMULA]1502. In order to calculate the detection limit for an absorption feature at a given position, we created Gaussians with the expected FWHM (IP in all cases) and varied the amplitude (the profiles could be broader, but this just means that a larger flux would be needed for detection). The Gaussians were added to the continuum near the expected position. The upper limit was chosen by eye, as the flux of that Gaussian that we considered detectable.

We present in Fig. 6 the fits to those features that we accepted as real. Except for CIV (for which the original spectrum is shown), we present smoothed spectra to make the figures clearer. The fits were done to the original (non-smoothed) spectra. There are some sky emission residuals on the blue side of the CIV absorption feature, but they do not affect the fit (see Fig. 4). We present in Table 2 some parameters obtained from the fits: wavelength, line identification, EW (rest frame) and FWHM. The values measured in the radio galaxy 4C41.17 ([FORMULA]3.80) (Dey et al. 1997) and the associated absorption system of the quasar 3C191 ([FORMULA]1.95) (Bahcall et al. 1967) are also shown.

[FIGURE] Fig. 6. Absorption lines in the spectrum of MRC2025-218. The fit and the data are presented for all lines. Note the P Cygni profile in CIV and SiIV[FORMULA]1393.8,1402.8. The panel on the right-bottom side shows the individual components fitted to the SiIV lines. 


Table 2. Absorption lines measured in the spectrum of MRC2025-218. The EWs are given in the rest frame. Parameters of the emission line are given for those cases where a PCygni profile is detected. We present also the values measured for 4C41.17 (Dey et al. 1997) and 3C191 (Bahcall et al. 1967).

The fitting to SiIV[FORMULA]1393,1402 is difficult due to the low S/N ratio, however, the presence of two P-Cygni profiles for the SiIV[FORMULA]1393.8 and SiIV[FORMULA]1402.8 lines is suggested by the data. The best fit is obtained by considering two absorption and two emission features. We constrain the fit so that the two absorption features on one hand, and the two emission features on the other, have the same FWHM and are separated by 9 Å (as is expected from the doublet components [FORMULA]1393,1402). The results of the fit are presented in Table 2. The FWHM of the emission line features are consistent with the value measured for the CIV and Ly[FORMULA] emission. The absorption feature presents a much larger FWHM than measured for CIV absorption. The value is consistent with measurements in galactic outflows.

The absorption features are narrower than the emission lines (also narrower than in 4C41.17) except for the SiIV[FORMULA]1393,1402 lines, that have FWHMabs [FORMULA] 1400[FORMULA]500. The absorption features in the P Cygni profiles are blueshifted by 1100[FORMULA]200 km s-1 (CIV) and 1200[FORMULA]200 (SiIV[FORMULA]1393.8 and SiIV[FORMULA]1402.8) with respect to the emission.

We confirm the detection in absorption of CIV[FORMULA]1550, CII[FORMULA]1334.5, SiIV[FORMULA]1393.8 +1402.8 and, maybe, OI[FORMULA]1302.2 +SiII[FORMULA]1304.4 (the absorbed flux is slightly higher than the detection limit). There is no clear evidence in our data for Ly[FORMULA] absorption, although the asymmetry of the profile (steeper on the blue side) is probably due to absorption. The presence of P Cygni profiles is confirmed in the case of CIV, SiIV[FORMULA]1393.8 and SiIV[FORMULA]1402.8.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999