Astron. Astrophys. 333, 841-863 (1998)

5. Other metal-rich and Ly forest absorbers

5.1. The dwarf galaxy near PKS 0454+039

This galaxy, discovered by Steidel et al. (1993) has a redshift , and shows signs of star formation activity. Steidel et al. (1993) found no Ca II absorption induced by this object in the spectrum of PKS 0454+039 although the impact parameter,  kpc, is relatively small. However, Ca II does not probe efficiently the outer regions of galaxies where Ca is expected to be mostly in the form of Ca III. In Fig. 18, we display an enlarged portion of the G270H spectrum which shows unambiguously the presence of Mg II absorption from this dwarf galaxy.

Fig. 18. Spectrum of PKS 0454+039 around the Mg II absorption induced by the dwarf galaxy (full line). A fit to the Mg II doublet is also shown (dashed line)

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).

5.2. Other cases

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

helpdesk.link@springer.de