The optical spectrum of 1ES 1927+654 was taken in June 2001 using the Carelec spectrograph with the 1.93 m telescope at the Haute-Provence observatory. The spectral resolution is 6 Å (3 pixels) and the signal-to-noise ratio is about 30.
The optical spectrum in Fig. 10 shows narrow emission lines
and several galaxy absorption line features of Ca H+K
,
CH G
,
Mg I
,
and Na I
indicating the
presence of a host galaxy continuum.
The observed FWHM of most emission lines is about 330 km s-1, which
corresponds to the instrumental resolution.
The lower FHWM value of km s-1 for the
H
line is probably due to the presence of an underlying absorption
component, which can barely
be seen when the spectrum is displayed at full resolution.
The flux ratios of
[O III]5007/H
,
[O I]6300/H
,
[N II]6584/H
,
and [S II]/H
point to a Seyfert 2 classification using
the standard diagnostics of Veilleux & Osterbrock (1987).
The rest frame equivalent widths of the absorption lines CH G
and
Mg I
of
Å and
Å, respectively, have been used to decompose
the spectrum into the
power-law featureless continuum and the host galaxy contribution. Assuming an optical
spectral index of
for AGN and typical rest frame equivalent widths of
3.3 to 5.8 Å for CH G
and of 2.4 to 4.8 Å for Mg I
(see Goodrich & Osterbrock 1983) we have
determined the fraction of the power-law featureless continuum to be between 92 and 97% at 4800 Å.
The optical continuum emission of 1 ES 1927+654 seems therefore to be dominated by the AGN.
From the H
to H
flux ratio we have determined the optical
extinction AV=2.0 in the narrow line region, assuming Case B recombination (see Veilleux et al. 1997).
The actual visible extinction may in fact be even lower by a few percent, baring
in mind that the H
emission line is affected by an underlying absorption,
which can not be precisely measured here. While the the optical spectrum does not clearly indicate the presence of a significant
broad H
component,
we have nevertheless tried to fit the H
region with an additional component (see Fig. 10, lower panel),
to estimate its possible strength.
The addition of the broad component does not significantly improve the fit,
indicating that the presence of a broad
component is not required by the data. The line flux of any possible broad component,
has to be lower than 5 percent of the narrow H
component. The equivalent width of
the narrow H
line
is 2.4 Å.
The central wavelength of this possible
broad H
emission component is also slightly shifted to the red.
We note that the lower signal-to-noise spectrum of Perlman et al. (1996)
also did not show a broad
component.
Following Goodrich et al. (1994),
the non-detection of a significant broad H
emission line suggests an extinction
value of the Broad Line Region (BLR) of at least AV = 3.7. Near-infrared spectra
of 1ES 1927+654 are required to estimate the upper limit of the extinction of the BLR by the detection
of broad emission line components at longer wavelength, e.g. at Br
or Br
.
The AV values discussed above refer to the sum of the Galactic and intrinsic extinction.
The Galactic AV value in the direction of 1ES 1927+654 is 0.291 as found in NED. Therefore,
the corrected AV for the Narrow Line Region in the distant AGN is at most 1.71 (or slightly lower if we could correct for the underlying absorption line at H).
Copyright ESO 2003