SAMI hexabundles probe biases in local star formation rates
Over the past few decades, studies into the evolution of galaxies have been reliant on data that has come from observing galaxies with individual fibre optics. The light from a galaxy travels down the fibre into a spectrograph where it is split into all of its wavelengths (from blue to red like a rainbow). This spectrum contains a wealth of information about the workings of the galaxy: how far away it is, how fast it is rotating, how many stars are being born etc.
There is a problem with this method though: “local galaxies” are close enough that the single fibre optic only covers the central part of the galaxy (see Figure 1A), so the data that astronomers use from this method is incomplete. The SAMI Galaxy Survey (using the “Sydney-AAO Integral-field Spectrograph” on the Anglo-Australian Telescope) is a major part of CAASTRO’s “The Evolving Universe” research theme and instead uses a device called “hexabundle” to solve this problem and to observe the entire galaxy with many fibres instead of just one.
Before SAMI, elaborate methods (“aperture corrections”) had been developed to deal with the bias of estimating a galaxy’s star formation rate from using single fibre optics. CAASTRO PhD student Samuel Richards (University of Sydney) and colleagues have now tested these aperture correction methods using the full data of 1212 different galaxies observed with SAMI.
They scrutinised two of the most commonly used aperture corrections, by first comparing their estimated total star formation rates, against the SAMI observed total star formation rates. With the ability to actually probe the assumptions in the aperture corrections, the researchers were able to establish that biases arise when assuming that instantaneous star formation can be traced by broadband optical images (Hopkins et al. 2003, GAMA) and when the aperture correction is built only from spectra from the centre of galaxies (Brinchmann et al. 2004, SDSS). These biases can be significant depending on what types of galaxies are being observed. Understanding the sensitivities of these aperture corrections is essential for correct handling of errors in galaxy evolution studies.
Samuel Richards, Julia Bryant, Scott Croom, Andrew Hopkins, Adam Schaefer, Joss Bland-Hawthorn, James Allen et al. in MNRAS (2015) “The SAMI Galaxy Survey: Can we trust aperture corrections to predict star formation?”