91̽»¨

Abstract:

JWST has recently discovered a subset of reionization era galaxies with ionized gas that is metal-poor in oxygen and carbon but heavily enriched in nitrogen. This abundance pattern is almost never seen in lower-redshift galaxies but is commonly observed in globular cluster stars. We have recently demonstrated that this peculiar abundance pattern appears in a compact (≃20 pc) metal-poor galaxy undergoing a strong burst of star formation. This galaxy was originally selected based on strong C iv emission, indicating a hard radiation field rarely seen locally. In this paper, we present JWST/NIRSpec observations of another reionization-era galaxy known to power strong C iv emission, the z = 7.04 gravitationally lensed galaxy A1703-zd6. The emission-line spectrum reveals this is a metal-poor galaxy ( 12+log(O/H)=7.47±0.19 ) dominated by a young stellar population ( 1.6−0.4+0.5 Myr) that powers a very hard ionizing spectrum (C iv equivalent width, EW = 19.4 Å, He ii EW = 2.2 Å). The interstellar medium is highly enriched in nitrogen ( log(N/O)=−0.6 ) with very high electron densities (8–19 × 104 cm−3) and extreme ionization conditions rarely seen at lower redshift. We also find intense CIV emission (EW ≳ 20 Å) in two new z ≳ 6 metal-poor galaxies. To put these results in context, we search for UV line emission in a sample of 737 z ≳ 4 galaxies with NIRSpec spectra, establishing that 40%(30%) of systems with [O iii]+Hβ EW > 2000 Å have N iv] (C iv) detections with EW > 5 Å(> 10 Å). These results suggest high N/O ratios, and hard ionizing sources appear in a brief phase following a burst of star formation in compact high-density stellar complexes.

Abstract:

The PHANGS project is assembling a comprehensive, multiwavelength data set of nearby (∼5–20 Mpc), massive star-forming galaxies to enable multiphase, multiscale investigations into the processes that drive star formation and galaxy evolution. To date, large survey programs have provided molecular gas (CO) cubes with the Atacama Large Millimeter/submillimeter Array, optical integral field unit (IFU) spectroscopy with the Very Large Telescope/Multi-Unit Spectroscopic Explorer (MUSE), high-resolution near-ultraviolet–optical imaging in five broadband filters with Hubble Space Telescope (HST), and infrared imaging in NIRCAM+MIRI filters with JWST. Here we present PHANGS-HST-Hα, which has obtained high-resolution (∼2–10 pc), narrowband imaging in the F658N or F657N filters with the HST/WFC3 camera of the warm ionized gas in the first 19 nearby galaxies observed in common by all four of the PHANGS large programs. We summarize our data reduction process, with a detailed discussion of the production of flux-calibrated, Milky Way extinction-corrected, continuum-subtracted Hα maps. PHANGS-MUSE IFU spectroscopy data are used to background-subtract the HST-Hα maps and to determine the [N ii] correction factors for each galaxy. We describe our public data products (the data released as part of this work include the reduced drizzled narrowband images and the flux-calibrated, continuum-subtracted Hα maps for each galaxy; these images are available for download via MAST at https://archive.stsci.edu/hlsp/phangs.html, as well as at the Canadian Astronomy Data Centre as part of the PHANGS archive at https://www.canfar.net/storage/vault/list/phangs/RELEASES) and highlight a few key science cases enabled by the PHANGS-HST-Hα observations.