Planetary nebula luminosity function
Encyclopedia
Planetary nebula luminosity function (PNLF) is a secondary distance indicator used in astronomy
. It makes use of the [O III] λ5007 forbidden line found in all planetary nebula
which are members of the old stellar populations (Population II).
It works well for both spiral
and elliptical
galaxies despite their completely different stellar populations and is part of the Extragalactic Distance Scale.
s, and Lyα galaxies. After the PNe are determined, to estimate a distance one must measure their monochromatic[O III] λ5007 fluxes. With this one then has a statistical sample of
PNe. One then fits the observed luminosity function to some standard law.
Finally, one must estimate the foreground interstellar extinction. There are two sources of this, from within the Milkyway
and internal extinction of the target galaxy. The first is well known and can be taken from sources such as reddening maps computed from H I
measurements and galaxy counts or from IRAS
and DIRBE
satellite experiments. The later, only occurs in target galaxies which are either late type spiral
or irregular
. However, this extinction is difficult to measure. In the Milkyway, the scale height of PNe is much bigger than that of the dust. Observational data and models support that this holds true for other galaxies, that the bright edge of the PNLF is primarily due to PNe in front of the dust layer. The data and models support a less than 0.05 magnitude
internal extinction of a galaxy's PNe.
. This is because oxygen
is a primary nebular coolant; any drop in its concentration raises the plasma’s electron temperature and raises the amount of collisional excitation
s per ion. This compensates for having a smaller number of emitting ions in the PNe. Consequently, a reduction in oxygen density only lowers the emergent[O III] λ5007 emission line flux by approximately the square root of the difference in abundance. At the same time, the PNe’s core responds to metallicity the opposite way. In the case where the metallicity of the progenitor star is smaller, then the PNe’s central star
will be a bit more massive and its emergent ultraviolet flux will be a bit larger. This added energy almost precisely accounts for the decreased emissions of the PNe. Consequently, the total[O III] λ5007 flux that is produced by a PNe is practically
uncorrelated to metallicity. This beneficial negation is in agreement with more precise models of PNe evolution. Only in extremely metal-poor PNe does the brightness of the PNLF cutoff dim 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] 5007 flux of a PNe directly correlates to the brightness of its central star. Further, the brightness of its central star directly correlates to its mass. In a PNe, the central star's mass directly varies in relation to its progenitor's mass. However, by observation, it is demonstrated that reduced brightness does not happen.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
. It makes use of the [O III] λ5007 forbidden line found in all planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
which are members of the old stellar populations (Population II).
It works well for both spiral
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
and elliptical
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
galaxies despite their completely different stellar populations and is part of the Extragalactic Distance Scale.
Procedure
To estimate the distance to a galaxy using the PNLF one must first locate point sources within the galaxy that are visible at λ5007 but not when the entire spectrum is considered. These points are candidate PNe, however, there are three other types of objects that would also exhibit such an emission line that must be filtered out: HII regions, supernova remnantSupernova remnant
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.There are two...
s, and Lyα galaxies. After the PNe are determined, to estimate a distance one must measure their monochromatic
PNe. One then fits the observed luminosity function to some standard law.
Finally, one must estimate the foreground interstellar extinction. There are two sources of this, from within the Milkyway
MilkyWay
is a Hello! Project trio consisting of Morning Musume member and soloist Koharu Kusumi and Hello Pro Egg members Sayaka Kitahara and Yuu Kikkawa. The unit was made mainly for the anime Kirarin Revolution as all girls lend voices to characters in the series...
and internal extinction of the target galaxy. The first is well known and can be taken from sources such as reddening maps computed from H I
H I region
An H I region is an interstellar cloud composed of neutral atomic hydrogen , in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm region spectral line. This line has a very low transition probability, so requires large...
measurements and galaxy counts or from IRAS
IRAS
The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....
and DIRBE
Diffuse Infrared Background Experiment
Diffuse Infrared Background Experiment was an experiment on NASA's COBE mission, to survey the diffuse infrared sky. The DIRBE instrument was an absolute radiometer with an off-axis folded-Gregorian reflecting telescope, with 19 cm diameter aperture...
satellite experiments. The later, only occurs in target galaxies which are either late type spiral
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
or irregular
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
. However, this extinction is difficult to measure. In the Milkyway, the scale height of PNe is much bigger than that of the dust. Observational data and models support that this holds true for other galaxies, that the bright edge of the PNLF is primarily due to PNe in front of the dust layer. The data and models support a less than 0.05 magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
internal extinction of a galaxy's PNe.
Physics behind process
The PNLF method is unbiased by metallicityMetallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
. This is because oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
is a primary nebular coolant; any drop in its concentration raises the plasma’s electron temperature and raises the amount of collisional excitation
Collisional excitation
Collisional excitation is a process in which the translational energy of a collision partner is converted into the internal energy of a reactant species.-Astronomy:...
s per ion. This compensates for having a smaller number of emitting ions in the PNe. Consequently, a reduction in oxygen density only lowers the emergent
Central Star
Central Star consists of four residential skyscrapers under construction in Busan, South Korea. The tallest landmark tower measures tall and has 58 floors. Ground was broken by POSCO E&C in 2007 and is completing in 2011....
will be a bit more massive and its emergent ultraviolet flux will be a bit larger. This added energy almost precisely accounts for the decreased emissions of the PNe. Consequently, the total
uncorrelated to metallicity. This beneficial negation is in agreement with more precise models of PNe evolution. Only in extremely metal-poor PNe does the brightness of the PNLF cutoff dim 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] 5007 flux of a PNe directly correlates to the brightness of its central star. Further, the brightness of its central star directly correlates to its mass. In a PNe, the central star's mass directly varies in relation to its progenitor's mass. However, by observation, it is demonstrated that reduced brightness does not happen.