Decomposing cool stellar populations with H-band spectral fluctuations: Long-period variable stars in NGC 5128 and carbon stars in NGC 5102
We analyse new H-band integral-field unit observations of two galaxies at ~4 Mpc, using a principal components analysis of pixel spectra to probe their giant star content. In both galaxies, the signals arise in near-resolved point-like sources without large-scale variation, consistent with each pixel sampling stars randomly from a common underlying population. In the (mostly) old bulge of NGC 5128, the observed pixel-to-pixel variation is dominated by a component with a mid-M giant spectrum with prominent CO bandheads. We also recover a smoother second spectral component, apparently driven by contributions from later spectral types. This component is not present in predictions from Poisson-sampled models of old stellar populations; we suggest that it arises from the cool phases of long-period variable stars. (An appendix provides direct evidence for such variables in complementary two-epoch MUSE observations.) In the contrasting galaxy NGC 5102, where a post-starburst stellar population is known to be present, we again find two distinct components. As before, the first component carries the CO bands typical of M-giants. The second eigenspectrum in this younger galaxy shows a strong 1.77 micron C2 bandhead, a feature which is characteristic of carbon stars. Our results highlight the ability of integral field data to access information beyond the total spectrum, even when individual stars cannot be classically resolved.