Planetary nebula luminosity function
A secondary distance indicator using [O III] λ5007 emission.
The planetary nebula luminosity function (PNLF) serves as a secondary distance indicator on the Extragalactic Distance Scale. It relies on the [O III] λ5007 forbidden line, which is present in every planetary nebula (PN) belonging to old stellar populations (Population II). This method works for both spiral and elliptical galaxies, even though their stellar populations are entirely different.
To estimate a galaxy’s distance using the PNLF, astronomers first identify objects in the target galaxy that are visible at λ5007 but not in the full spectrum. These are candidate PNe, but they must be distinguished from three other sources that also produce this emission: HII regions, supernova remnants, and Lyman-alpha galaxies. Once the true PNe are isolated, their monochromatic [O III] λ5007 luminosity is measured, creating a statistical sample. The observed luminosity function is then fitted to a standard law. Finally, foreground interstellar extinction must be accounted for. Extinction comes from two sources: the Milky Way (well known from reddening maps based on H I measurements, galaxy counts, or IRAS and DIRBE data) and internal extinction within the target galaxy. Internal extinction only occurs in late-type spiral or irregular galaxies and is difficult to measure. However, in the Milky Way, the scale height of PNe is much larger than that of dust, and observations and models suggest the same holds for other galaxies: the bright edge of the PNLF is dominated by PNe in front of the dust layer. Data and models indicate that internal extinction of a galaxy’s PNe is less than 0.05 apparent magnitudes.
The PNLF method is unbiased by metallicity. Oxygen is a primary nebular coolant; if its concentration drops, the plasma’s electron temperature rises, increasing collisional excitations per ion. This compensates for fewer emitting ions, so a reduction in oxygen density lowers the [O III] λ5007 emission by only about the square root of the abundance difference. Meanwhile, the PN’s central star responds in the opposite way: lower metallicity in the progenitor star makes the central star slightly more massive, boosting its ultraviolet flux. This added energy nearly exactly offsets the decreased nebular emission. As a result, the total [O III] λ5007 luminosity of a PN is practically uncorrelated with metallicity.
- Emission line
- [O III] λ5007
- Stellar population
- Population II
- Galaxy types applicable
- spiral and elliptical
- Internal extinction upper limit
- less than 0.05 apparent magnitude
Lore & Background
The distance estimate to a galaxy using the PNLF requires discovery of objects in the target galaxy that are visible at λ5007 but not when the entire spectrum is considered. These points are candidate PNe, but three other types of objects—HII regions, supernova remnants, and Lyα galaxies—must be filtered out. After the PNe are determined, one measures their monochromatic [O III] λ5007 luminosity, and the observed luminosity function is fitted to some standard law. Foreground interstellar extinction must then be estimated, with two sources: extinction from within the Milky Way, which is well known from reddening maps, and internal extinction of the target galaxy, which occurs only in late-type spiral or irregular galaxies and is difficult to measure. In the Milky Way, the scale height of PNe is much bigger than that of dust, and observational data and models support that this holds for other galaxies, so the bright edge of the PNLF is primarily due to PNe in front of the dust layer, with less than 0.05 apparent magnitude internal extinction.
Reader's Guide
The PNLF method is unbiased by metallicity because oxygen is a primary nebular coolant; any drop in its concentration raises the plasma's electron temperature and raises collisional excitations per ion, compensating for fewer emitting ions. A reduction in oxygen density lowers the emergent [O III] λ5007 emission line intensity by approximately the square root of the difference in abundance. Meanwhile, the PNe's core responds oppositely: with lower progenitor metallicity, the central star is slightly more massive and its ultraviolet flux greater, almost precisely accounting for decreased emissions. Thus total [O III] λ5007 luminosity is practically uncorrelated with metallicity, except in extremely metal-poor PNe where the cutoff dims by more than a small percentage. The relative independence of the PNLF cutoff with respect to population age is harder to understand: the [O III] λ5007 flux correlates with central star brightness, which correlates with mass, which varies with progenitor mass, yet observation shows reduced brightness does not happen.
Did You Know?
- The PNLF uses the [O III] λ5007 forbidden line found in all planetary nebulae of old stellar populations.
- Candidate PNe must be filtered to exclude HII regions, supernova remnants, and Lyα galaxies.
- The PNLF method is unbiased by metallicity due to a compensating effect in the nebula and its central star.
More in Planetary Nebulae, Part 2 1-24
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