![]() | Annu. Rev. Astron. Astrophys. 1998. 36:
189-231 Copyright © 1998 by Annual Reviews. All rights reserved |
2.4. Forbidden Lines
The H emission line is
redshifted out of the visible window beyond z ~ 0.5, so there is
considerable interest in calibrating bluer
emission lines as quantitative SFR tracers. Unfortunately, the integrated
strengths of H
and the higher order Balmer emission lines are poor
SFR diagnostics because the lines are weak and stellar absorption more
strongly influences the emission-line fluxes. These lines, in fact, are
rarely seen in emission at all in the integrated spectra of galaxies
earlier than Sc
(Kennicutt 1992a;
also see Figure 1).
The strongest emission feature in the blue is the
[OII]3727
forbidden-line doublet. The luminosities of forbidden lines are not
directly coupled to the ionizing luminosity, and their excitation is
sensitive to abundance and the ionization state of the gas. However,
the excitation of [OII] is sufficiently well behaved that it can be
calibrated empirically (through
H
) as a quantitative SFR
tracer. Even this indirect calibration is extremely useful for
lookback studies of distant galaxies because [OII] can be observed
in the visible out to redshifts z ~ 1.6,
and it has been measured in several large samples of faint galaxies
(Cowie et al 1996,
1997;,
Ellis 1997,
and references therein).
Calibrations of SFRs in terms of [OII] luminosity have been published
by Gallagher et al
(1989),
based on large-aperture spectrophotometry of 75 blue irregular galaxies,
and by
Kennicutt (1992a),
using integrated spectrophotometry of 90 normal and peculiar
galaxies. When converted to the same IMF and
H calibration, the
resulting SFR
scales differ by a factor of 1.57, reflecting excitation differences in the
two samples. Adopting the average of these calibrations yields
![]() |
(3) |
where the uncertainty indicates the range between blue emission-line
galaxies (lower limit) and samples of more luminous spiral and irregular
galaxies (upper limit). As with Equations 1 and 2, the observed
luminosities must be corrected for extinction, in this case the
extinction at H, because
of the manner in which the [OII] fluxes were calibrated.
The SFRs derived from [OII] are less precise than those from
H because the mean [OII] /
H
ratios in individual
galaxies vary considerably, over 0.5-1.0 dex in
Gallagher et al's
(1989),
Kennicutt's (1992a)
samples, respectively. The [OII]-derived SFRs may also be prone to
systematic errors from extinction and variations in the diffuse gas
fraction. The excitation of [OII] is especially high in the diffuse ionized
gas in starburst galaxies
(Hunter & Gallagher
1990,
Hunter 1994,
Martin 1997),
enough to more than double the L[OII]/SFR ratio in the integrated spectrum
(Kennicutt 1992a).
On the other hand, metal
abundance has a relatively small effect on the [OII] calibration, over
most of the abundance range of interest (0.05
Z
Z
1
Z
).
Overall the [OII] lines provide a very useful estimate of the systematics
of SFRs in samples of distant galaxies, and they are especially useful as
a consistency check on SFRs derived in other ways.