Redshifted Cool Gas in the Disturbed HI Disk of the Starburst Dwarf NGC 4214

Abstract

We report redshifted cool gas toward the central star-forming region of the nearby starburst dwarf galaxy NGC 4214. HST/COS spectra show low-ionization absorbers redshifted relative to the systemic velocity, most clearly in SII ($\Delta v=19.8\pm 2.6\mathrm{km}{\mathrm{s}}^{-1}$) and supported by MgII ($\Delta v=30.3\pm 3.0\mathrm{km}{\mathrm{s}}^{-1}$), while PII remains tentative ($\Delta v=7.5\pm 3.4\mathrm{km}{\mathrm{s}}^{-1}$). The Keck/HIRES spectra also reveal a redshifted CaII absorption component ($\Delta v=25.6\pm 1.5\mathrm{km}{\mathrm{s}}^{-1}$). Because the absorption is detected against the central UV or optical continuum, the gas must lie in the foreground of the star-forming region and is moving away from the observer and toward the background UV source in projection. A VLA--THINGS HI spectrum extracted at the COS position within one VLA beam also shows a redshifted neutral component at $v_{\mathrm{hel}}=329.2\pm 0.9\mathrm{km}{\mathrm{s}}^{-1}$, offset by $38.2\pm 1.3\mathrm{km}{\mathrm{s}}^{-1}$ from systemic and by $27.6\pm 1.0\mathrm{km}{\mathrm{s}}^{-1}$ from the dominant local HI component. The FAST HI mapping further reveals an extended, kinematically disturbed neutral disk, with residual structures relative to a warped-disk model. Together, the redshifted UV absorption, the redshifted HI component, and the disturbed large-scale HI velocity field show that the central anomalous cool gas is observed in a galaxy with a disturbed extended neutral disk. This geometry suggests that the redshifted cool gas may trace inflow or recycled fallback toward the central region, although warped-disk or non-circular disk motions cannot be fully excluded.

arXiv comment14 pages, 3 figures, accepted by The Astrophysical Journal Letters