Interstellar Absorption Lines in the Spectrum of the Starburst Galaxy NGC 1705

Abstract

A Goddard High Resolution Spectrograph archival study of the interstellar absorption lines in the line of sight to the H I–rich, starburst dwarf galaxy NGC 1705 in the 1170 to 1740 A range at ~120 km s-1 resolution is presented. The absorption features arising because of photospheric lines are distinctly different from the interstellar lines: the photospheric lines are weak, broad (equivalent widths >1 A), asymmetric, and centered around the systemic LSR velocity of NGC 1705 (~610 km s-1). The interstellar lines consist of three relatively narrow components at LSR velocities of -20, 260, and 540 km s-1, and include absorption by neutral atoms (N I λ1200 triplet and O I λ1302), singly ionized atoms (Si II λλ1190, 1193, 1260, 1304, and 1526, S II λ1253, C II λ1334, C II* λ1336, Fe II λ1608, and Al II λ1670), and atoms in higher ionization states (Si III λ1206, Si IV λλ1393, 1402, and C IV λλ1548, 1550). The Si IV and C IV absorption features have both interstellar and photospheric contributions. In an earlier study, Sahu & Blades identified the absorption system at -20 km s-1 with Milky Way disk/halo gas, and the 260 km s-1 system with a small, isolated high-velocity cloud HVC 487, which is probably associated with Magellanic Stream gas. The 540 km s-1 absorption system is associated with a kiloparsec-scale expanding, ionized supershell centered on the super–star cluster NGC 1705-1. The analysis presented in this paper consists of (1) a list of all interstellar absorption features with greater than 3 σ significance and their measured equivalent widths, (2) plots of the lines in the various atomic species together with the results of nonlinear least-squares fit profiles to the observed data, and (3) unpublished 21 cm maps from the Wakker & van Woerden survey showing the large-scale H I distribution in the region near the NGC 1705 sight line and HVC 487. Furthermore, weak N I λ1200 triplet absorption for the supershell component is reported, which in the absence of dust depletion and ionization corrections implies a low N abundance. A low N abundance for the supershell is consistent with an interpretation of nucleosynthetic enrichment by time-delayed primary nitrogen production, the age estimate of (10–20) × 106 yr for the central super–star cluster NGC 1705-1 of Heckman & Leitherer, and the underabundance of Fe reported by Sahu & Blades. However, using the N I λ1200 triplet alone to estimate the total N abundance could result in a severe underestimation of this quantity: although N does not deplete onto interstellar dust grains, photoionization and collisional ionization effects could increase the fraction of N found in higher ionization stages. Uncertainties in the total N abundance caused by photoionization and collisional ionization effects can only be addressed by future observations of the higher ionization lines, namely, N II λ1084 and N III λ989.