Novel trick draws fundamental scaling relation to high redshifts
30 September 2015
Galactic scaling relations are an important part of extragalactic research as they are tracers of galaxy evolution processes and are also quite often useful observational tools. The Tully-Fisher Relation (TFR) is one such scaling relation linking the mass (using luminosity as a proxy) and rotation velocity (using the HI emission line width as a proxy) of spiral galaxies. The TFR not only serves as an important cosmological distance tool but also describes the baryonic and dark matter components of galaxies at a given distance, or redshift (z). Measuring the TFR over a range of redshifts therefore allows a glimpse into galaxy evolution.
Probing the rotation velocity of galaxies is a difficult job though: using the optical light, galaxies appear limited to their stellar component. Measuring rotation velocities from optical data is a bit like trying to measure how fast a ballerina’s arms are spinning by measuring how fast their torso is spinning: you get a very different answer depending on if their arms are tucked in or extended. A more favourable method is to measure the rotation of the neutral Hydrogen gas (HI) in galaxies as it also extends out to the edge of galaxies.
This gas has a spectral line feature at 1420.4 MHz (about 21cm), and its width is a direct measurement of the maximum velocity that the gas is rotating at in the galaxy. This 21cm emission is a very faint signal though and quite difficult to measure at a distance, and hence the TFR is poorly constrained beyond the very nearby Universe
(z < 0.1).
CAASTRO PhD student Scott Meyer (ICRAR – UWA) and his team of all-CAASTRO colleagues have now used a technique called HI stacking that involves co-adding the signal from multiple galaxies to create a statistical signal with lower noise properties. HI stacking had previously been used to extend the redshift range accessible for astronomers to study the neutral gas content of the Universe. The researchers applied this technique by stacking galaxies with similar absolute magnitudes and measuring the width of the resulting stacked HI emission lines. They could use the measured widths from these signals to determine average rotation velocities for the absolute magnitude ranges these galaxies come from.
For calibration purposes, the team first used simulated galaxies from the S3-SAX simulation, then galaxies detected with the HI Parkes All-Sky Survey (HIPASS), and they also demonstrate its utility with noisy spectra. Using the stacking technique in this novel way, the spectral line features of the galaxies do not need to be detectable above the noise, such that more galaxies – of lower mass and much more distant than currently possible – can be studied. Even when including galaxy spectra that had not been corrected for inclination or dispersion, the researchers found that the width of the resulting stacked spectrum traces the average width of the corrected galaxies with only a small (~7%) systematic error.