Families and Clusters of Diffuse Interstellar Bands: a Data-Driven
  Correlation Analysis
  More than 500 diffuse interstellar bands (DIBs) have been observed in
astronomical spectra, and their signatures and correlations in different
environments have been studied over the past decades to reveal clues about the
nature of the carriers. We compare the equivalent widths of the DIBs,
normalized to the amount of reddening, E_B-V, to search for anti-correlated DIB
pairs using a data sample containing 54 DIBs measured in 25 sight lines. This
data sample covers most of the strong and commonly detected DIBs in the optical
region, and the sight lines probe a variety of ISM conditions. We find that
12.9% of the DIB pairs are anti-correlated, and the lowest Pearson correlation
coefficient is r_norm ~ -0.7. We revisit correlation-based DIB families and are
able to reproduce the assignments of such families for the well-studied DIBs by
applying hierarchical agglomerative and k-means clustering algorithms. We
visualize the dissimilarities between DIBs, represented by 1 - r_norm, using
multi-dimensional scaling (MDS). With this representation, we find that the
DIBs form a rather continuous sequence, which implies that some properties of
the DIB carriers are changing gradually following this sequence. We also find
at that least two factors are needed to properly explain the dissimilarities
between DIBs. While the first factor may be interpreted as related to the
ionization properties of the DIB carriers, a physical interpretation of the
second factor is less clear and may be related to how DIB carriers interact
with surrounding interstellar material.
