Ultraviolet absorption observations of intermediate-velocity cloud gas detected towards the M 15 globular cluster

Abstract

Aims. We present medium resolution ultraviolet interstellar absorption measurements recorded with the HST-COS and FUSE spectrographs towards two post-AGB stars (K 559 & K 648) located within the M 15 globular cluster (l ∼ 65 ◦ , b ∼ –27 ◦ ). By sampling interstellar gas over the 10.4 kpc sight-line towards M 15 we wish to reveal spectral features that are associated with absorption due to the foreground g1 intermediate velocity cloud (IVC), whose distance has previously been constrained to lie between 1.8–3.8 kpc. Methods. Inspection of the UV line profiles recorded towards both stars have revealed measurable IVC absorption at Vlsr ∼ +61.5 ± 5k m s −1 in the profiles of the CI, CII, CII*, CIV, NI, NII, OI, OVI, AlII, SiII, SiIII, SiIV, PII, SII, FeII, FeIII and NiII ions. Line-profile fitting to the IVC absorption features has resulted in column density determinations for the low and high ions. The best-fit column density values for the various ions have near identical values along both sight-lines, such that our UV data does not reveal the significant small-scale structure that has been reported previously for the g1 cloud. Results. The observed column density ratios of the CIV, SiIV, OVI and NV IV components are consistent with what may be expected from turbulent mixing layers, shock ionization or halo SNR models, although no one model can predict all of the observed ratios. We derive sub-solar metallicity values of [O/H] = −1.22 ± 0.44 and [N/H] = −1.21 ± 0.38 for the neutral IVC gas, and also the IV components of C, Si, Al, Ni and Fe mainly possess less than solar abundance ratios relative to that of N. A possible origin for the g1 IVC is that of a low metallicity cloud that has been accreted towards our galaxy, has passed through the disk and is now moving away from the galactic plane.