The Galah Survey: Classification and diagnostics with t-SNE reduction of
  spectral information
  Galah is an ongoing high-resolution spectroscopic survey with the goal of
disentangling the formation history of the Milky Way, using the fossil remnants
of disrupted star formation sites which are now dispersed around the Galaxy. It
is targeting a randomly selected, magnitude limited ($V \leq 14$) sample of
stars, with the goal of observing one million objects. To date, 300,000 spectra
have been obtained. Not all of them are correctly processed by parameter
estimation pipelines and we need to know about them. We present a
semi-automated classification scheme which identifies different types of
peculiar spectral morphologies, in an effort to discover and flag potentially
problematic spectra and thus help to preserve the integrity of the survey's
results. To this end we employ a recently developed dimensionality reduction
technique t-SNE (t-distributed Stochastic Neighbour Embedding), which enables
us to represent the complex spectral morphology in a two-dimensional projection
map while still preserving the properties of the local neighbourhoods of
spectra. We find that the majority (178,483) of the 209,533 Galah spectra
considered in this study represents normal single stars, whereas 31,050
peculiar and problematic spectra with very diverse spectral features pertaining
to 28,579 stars are distributed into 10 classification categories: Hot stars,
Cool metal-poor giants, Molecular absorption bands, Binary stars,
H$\alpha$/H$\beta$ emission, H$\alpha$/H$\beta$ emission superimposed on
absorption, H$\alpha$/H$\beta$ P-Cygni, H$\alpha$/H$\beta$ inverted P-Cygni,
Lithium absorption, and Problematic. Classified spectra with supplementary
information are presented in the catalogue, indicating candidates for follow-up
observations and population studies of the short-lived phases of stellar
evolution.
