The Malmquist bias is an effect in observational astronomy which leads to the preferential detection of intrinsically bright objects. It was first described in 1922 by Swedish astronomer Gunnar Malmquist (1893–1982), who then greatly elaborated upon this work in 1925. In statistics, this bias is referred to as a selection bias or data censoring. It affects the results in a brightness-limited survey, where stars below a certain apparent brightness cannot be included. Since observed stars and galaxies appear dimmer when farther away, the brightness that is measured will fall off with distance until their brightness falls below the observational threshold. Objects which are more luminous, or intrinsically brighter, can be observed at a greater distance, creating a false trend of increasing intrinsic brightness, and other related quantities, with distance. This effect has led to many spurious claims in the field of astronomy. Properly correcting for these effects has become an area of great focus.
Understanding the bias
Magnitudes and brightness
In everyday life it is easy to see that light dims as it gets farther away. This can be seen with car headlights, candles, flashlights, and many other lit objects. This dimming follows the inverse square law, which states that the brightness of an object decreases as , where is the distance between the observer and the object.
Starlight also follows the inverse square law. Light rays leave the star in equal amounts in all directions. The light rays create a sphere of light surrounding the star. As time progresses, the sphere grows as the light rays travel through space away from the star. While the sphere of light grows, the number of light rays stays the same. So, the amount of light per unit of surface area of the sphere (called flux in astronomy) decreases with distance and therefore time. When observing a star, only the light rays that are in the given area being viewed can be detected. This is why a star appears dimmer the farther away it is.
If there are two stars with the same intrinsic brightness (called luminosity in astronomy), each at a different distance, the closer star will appear brighter while the further will appear dimmer. In astronomy, the apparent brightness of a star, or any other luminous object, is called the apparent magnitude. The apparent magnitude depends on the intrinsic brightness (also called absolute magnitude) of the object and its distance.
If all stars had the same luminosity, the distance from Earth to a particular star could be easily determined. However, stars have a wide range in luminosities. Therefore, it can be difficult to distinguish a very luminous star that is very far away from a less luminous star that is closer. This is why it is so hard to calculate the distance to astronomical objects.
Source of the Malmquist bias
Typically, when looking at an area of sky filled with stars, only stars that are brighter than a limiting apparent magnitude can be seen. As discussed above, the very luminous stars that are farther away will be seen, as well as luminous and faint stars that are closer. There will appear to be more luminous objects within a certain distance from Earth than faint objects. However, there are many more faint stars: they simply cannot be seen because they are so dim. The bias towards luminous stars when observing a patch of sky affects calculations of the average absolute magnitude and average distance to a group of stars. Because of the luminous stars that are at a further distance, it will appear as if our sample of stars is farther away than it actually is, and that each star is intrinsically brighter than it actually is. This effect is known as the Malmquist bias.
When studying a sample of luminous objects, whether they be stars or galaxies, it is important to correct for the bias towards the more luminous objects. There are many different methods that can be used to correct for the Malmquist bias as discussed below.
The Malmquist bias is not limited to luminosities. It affects any observational quantity whose detectability diminishes with distance.
Correction methods The ideal situation is to avoid this bias from entering a data survey. However, magnitude limited surveys are the simplest to perform, and other methods are difficult to put together, with their own uncertainties involved, and may be impossible for first observations of objects. As such, many different methods exist to attempt to correct the data, removing the bias and allowing the survey to be usable. The methods are presented in order of increasing difficulty, but also increasing accuracy and effectiveness.
Limiting the sample The simplest method of correction is to only use the non-biased portions of the data set, if any, and throw away the rest of the data. Depending on the limiting magnitude selected, there may be a range of distances in the data set over which all objects of any possible absolute magnitude could be seen. As such, this small subset of data should be free of the Malmquist bias. This is easily accomplished by cutting off the data at the edge of where the lowest absolute magnitude objects would be hitting the limiting magnitude. Unfortunately, this method would waste a great deal of good data, and would limit the analysis to nearby objects only, making it less than desirable. (Looking at the figure to the right, only the first fifth of the data in distance could be kept before a data point is lost to the bias.) Of course, this method assumes that distances are known with relatively good accuracy, which as mentioned before, is a difficult process in astronomy.
Traditional correction The first solution, proposed by Malmquist in his 1922 work, was to correct the calculated average absolute magnitude () of the sample back to the true average absolute magnitude (M0). The correction would be
To calculate the bias correction, Malmquist and others following this method follow six main assumptions: There exists no interstellar absorption, or that the stuff in space between stars (like gas and dust) is not affecting the light and absorbing parts of it. This assumes that the brightness is simply following the inverse square law, mentioned above. The luminosity function (Φ) is independent of the distance (r). This basically just means that the universe is the same everywhere, and that stars will be similarly distributed somewhere else as they are here. For a given area on the sky, or more specifically the celestial sphere, the spatial density of stars (ρ) depends only on distance. This assumes that there are the same number of stars in each direction, on average. There is completeness, meaning the sample is complete and nothing is missed, to an apparent magnitude limit (mlim). The luminosity function can be approximated as a Gaussian function, centered on an intrinsic mean absolute magnitude M0. Stars are of the same spectral type, with intrinsic mean absolute magnitude M0 and dispersion σ.
Obviously, this is a very ideal situation, with the final assumption being particularly troubling, but allows for an approximate correction of simple form. By integrating the luminosity function over all distances and all magnitudes brighter than mlim,
where A(mlim) is the total number of stars brighter than mlim. If the spatial distribution of stars can be assumed to be homogeneous, this relation is simplified even further, to the generally accepted form of
Multiple-band observation corrections The traditional method assumes that the measurements of apparent magnitude and the measurements from which distance is determined are from the same band, or predefined range, of wavelengths (e.g. the H band, a range of infrared wavelengths from roughly about 1300–2000 nanometers), and this leads to the correction form of cσ2, where c is some constant. Unfortunately, this is rarely the case, as many samples of objects are selected from one wavelength band but the distance is calculated from another. For example, astronomers frequently select galaxies from B-band catalogs, which are the most complete, and use these B band magnitudes, but the distances for the galaxies are calculated using the Tully–Fisher relation and the H band. When this happens, the variance is replaced by the covariance between the scatter in the distance measurements and in the galaxy selection property (e.g. magnitude).
Volume weighting Another fairly straightforward correction method is to use a weighted mean to properly account for the relative contributions at each magnitude. Since the objects at different absolute magnitudes can be seen out to different distances, each point's contribution to the average absolute magnitude or to the luminosity function can be weighted by 1/Vmax, where Vmax is the maximum volume over which the objects could have been seen. Brighter objects (that is, objects with smaller absolute magnitudes) will have a larger volume over which they could have been detected, before falling under the threshold, and thus will be given less weight through this method since these bright objects will be more fully sampled. The maximum volume can be approximated as a sphere with radius found from the distance modulus, using the object's absolute magnitude and the limiting apparent magnitude.
However, there are two major complications to calculating Vmax. First is the completeness of the area covered in the sky, which is the percentage of the sky that the objects were taken from. A full sky survey would collect objects from the entire sphere, 4π steradians, of sky but this is usually impractical, both from time constraints and geographical limitations (ground-based telescopes can only see a limited amount of sky due to the Earth being in the way). Instead, astronomers will generally look at a small patch or area of sky and then infer universal distributions by assuming that space is either isotropic, that it is generally the same in every direction, or is following a known distribution, such as that one will see more stars by looking toward the center of a galaxy than by looking directly away. Generally, the volume can be simply scaled down by the percentage actually viewed, giving the correct number of objects to volume relation. This effect could potentially be ignored in a single sample, all from the same survey, as the objects will basically all be altered by the same numerical factor, but it is incredibly important to account for in order to be able to compare between different surveys with different sky coverage.
The second complication is cosmological concerns of redshift and the expanding universe, which must be considered when looking at distant objects. In these cases, the quantity of interest is the comoving distance, which is a constant distance between two objects assuming that they are moving away from each other solely with the expansion of the universe, known as the Hubble flow. In effect, this comoving distance is the object's separation if the universe's expansion were neglected, and it can be easily related to the actual distance by accounting for how it would have expanded. The comoving distance can be used to calculate the respective comoving volume as usual, or a relation between the actual and comoving volumes can also be easily established. If z is the object's redshift, relating to how far emitted light is shifted toward longer wavelengths as a result of the object moving away from us with the universal expansion, DA and VA are the actual distance and volume (or what would be measured today) and DC and VC are the comoving distance and volumes of interest, then