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Astron. Astrophys. 333, 841-863 (1998) 5. Other metal-rich and Ly
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Fig. 18.
Spectrum of PKS 0454+039 around the Mg II absorption induced by the ![]() ![]() |
The 2796 Å line appears at , just
in between the Fe II 1608 line from the DLAS and the
Si IV 1396 line at
. The latter
feature is severely blended with the second Mg II
doublet line. Absorption from the Fe II 2600 line is
also definitely present; the other major Fe II
transitions (at 2344 and 2382 Å) are expected where strong
lines from other systems occur.
In order to better probe the physical conditions in this absorber
we searched for 21 cm absorption. Observations were performed in
absentee at the ARECIBO radiotelescope by M. Lewis and his
collaborators (on the 21st and 22nd of October 1993). The velocity
range covered was 20700 - 22700 km s-1 which
corresponds to a redshift range of 0.0690 - 0.0757; the channel width
was 2.2 km s-1. To check the pointing, a test
galaxy was observed and detected. The continuum flux was measured to
be 390.7 mJy (four different calibration sources have been used which
gave concordant results). No significant absorption was detected; the
3 upper limit is 2.4 mJy. Assuming that the
velocity width of the H I gas in front of the
radiosource,
, is larger than the channel
width, we get the following upper limit on
:
cm-2.
This limit holds for a linear scale of
kpc or below since the radiosource appears unresolved with a
beam of
(Neff et al. 1989). Our result
indicates that the intervening dwarf galaxy near PKS 0454+039
does not have a very extended H I disk which is
consistent with H I distributions observed in other
nearby dwarf galaxies (Carignan & Puche 1989; Jobin & Carignan
1989).
On the other hand, the detection of Mg II and
Fe II absorption shows that, in this class of objects,
ionized gas is present well beyond the optical disk. Therefore, a
large total mass is not a necessary condition for a galaxy to display
an extended ionized gas component, at least when star formation is
occurring at a sufficiently high rate, as is the case in this galaxy.
It should be noted that the impact parameter of this low redshift
dwarf galaxy is 2.5 times smaller than the radius expected from the
size - luminosity scaling law measured for brighter metal-rich
absorbers (Guillemin & Bergeron 1997; the absolute magnitude is
(B)=-17.02). This is also the only dwarf
galaxy giving rise to Mg II absorption in the available
absorber samples although such objects could have been detected up to
redshifts of 0.2 - 0.3 (Steidel et al. 1994b; Guillemin & Bergeron
1997).
Three other galaxies close to the QSO lines of sight studied in
this paper have measured redshifts. We have already mentioned the
bright spiral near EX 0302-223 which gives rise to
Mg II and Fe II absorption at 0.118. We
note that Fe II 2600 is stronger than
Mg II 2796 which suggests a large
value. On the other hand, the relative strengths of
Fe II lines suggest that Fe II 2374 and
Fe II 2586 are nearly optically thin with
which rules out a value of
above 19 (the latter would correspond to
[Fe/H]
assuming that most of the Fe is in the
form of Fe II). There is however no contradiction
since, as discussed by Bergeron et al. (1994), the strong evolution in
redshift of the diffuse UV background implies that, at z
, large values of the Fe II
2600/Mg II 2796 equivalent width ratio can be found in
systems with
-
cm-2.
Another dwarf galaxy at low redshift and intermediate impact
parameter, = 0.199 and
kpc, is present in the field of PKS 1229-021 (G2 in
Bergeron & Boissé 1991; G4 in Steidel et al. 1994a). In the
spectrum of PKS 1229-021 several strong lines lie in the region
where the C IV doublet is expected; the latter is not
detected at a
limit of
Å for individual lines. The expected Al III
1854 line coincides with another strong absorption feature, but
Al III 1862 could have been detected and is not present
at a
level of 0.15 Å. This suggests that
this dwarf galaxy does not have an extended gaseous halo. This is not
unexpected from its luminosity,
, since it lies
just above the radius - luminosity scaling law of brighter metal-rich
absorbers (Guillemin & Bergeron 1997).
A low redshift, , bright,
, galaxy is present in the field of
PKS 0454+039 (Steidel et al. 1993). As expected from its large
impact parameter,
kpc, there is neither
associated absorption from Mg II (Steidel et al. 1993)
nor from C IV at a
upper limit
of
Å.
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998
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