An official website of the United States government
Abstract Studying the galaxies responsible for reionization is often conducted through local reionization-era analogs; however, many of these local analogs are too massive to be representative of the low-mass star-forming galaxies that are thought to play a dominant role in reionization. The local, low-mass dwarf starburst galaxy Pox 186 is one such system with physical conditions representative of a reionization-era starburst galaxy. We present deep ultraviolet (UV) spectroscopy of Pox 186 to study its stellar population and ionization conditions and to compare these conditions to other local starburst galaxies. The new Cosmic Origins Spectrograph data are combined with archival observations to cover ∼1150–2000 Å and allow for an assessment of Pox 186's stellar population, the relative enrichment of C and O, and the escape of ionizing photons. We detect significant Lyαand low-ionization state absorption features, indicative of previously undetected neutral gas in Pox 186. The C/O relative abundance, log(C/O) = −0.62 ± 0.02, is consistent with other low-metallicity dwarf galaxies and suggests a comparable star formation history in these systems. We compare UV line ratios in Pox 186 to those of dwarf galaxies and photoionization models, and we find excellent agreement for the ratios utilizing the intense Ciii], Oiii], and double-peaked Civlines. However, the UV and optical Heiiemission is faint and distinguishes Pox 186 from other local starburst dwarf galaxies. We explore mechanisms that could produce faint Heii, which have implications for the low-mass reionization-era galaxies that may have similar ionization conditions.
more »
« less
Characterizing Extreme Emission Line Galaxies. II. A Self-consistent Model of Their Ionizing Spectrum*
Abstract Observations of high-redshift galaxies ( z > 5) have shown that these galaxies have extreme emission lines with equivalent widths much larger than their local star-forming counterparts. Extreme emission line galaxies (EELGs) in the nearby universe are likely analogs to galaxies during the Epoch of Reionization and provide nearby laboratories to understand the physical processes important to the early universe. We use Hubble Space Telescope/Cosmic Origins Spectrograph and Large Binocular Telescope/Multi-Object Double Spectrographs spectra to study two nearby EELGs, J104457 and J141851. The far-UV spectra indicate that these two galaxies contain stellar populations with ages ≲10 Myr and metallicities ≤0.15 Z ⊙ . We use photoionization modeling to compare emission lines from models of single-age bursts of star formation to observed emission lines and find that the single-age bursts do not reproduce high-ionization lines including [O iii ] or very-high-ionization lines like He ii or O iv ]. Photoionization modeling using the stellar populations fit from the UV continuum similarly is not capable of reproducing the very-high-energy emission lines. We add a blackbody to the stellar populations fit from the UV continuum to model the necessary high-energy photons to reproduce the very-high-ionization lines of He ii and O iv ]. We find that we need a blackbody of 80,000 K and ∼45%–55% of the luminosity from the blackbody and young stellar population to reproduce the very-high-ionization lines while simultaneously reproducing the low-, intermediate-, and high-ionization emission lines. Our self-consistent model of the ionizing spectra of two nearby EELGs indicates the presence of a previously unaccounted-for source of hard ionizing photons in reionization analogs.
more »
« less
- Award ID(s):
- 1909198
- PAR ID:
- 10444197
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 938
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 16
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Stellar population models produce radiation fields that ionize oxygen up to O +2 , defining the limit of standard H ii region models (<54.9 eV). Yet, some extreme emission-line galaxies, or EELGs, have surprisingly strong emission originating from much higher ionization potentials. We present UV HST/COS and optical LBT/MODS spectra of two nearby EELGs that have very high-ionization emission lines (e.g., He ii λλ 1640,4686 C iv λλ 1548,1550, [Fe v ] λ 4227, [Ar iv ] λλ 4711,4740). We define a four-zone ionization model that is augmented by a very high-ionization zone, as characterized by He +2 (>54.4 eV). The four-zone model has little to no effect on the measured total nebular abundances, but does change the interpretation of other EELG properties: we measure steeper central ionization gradients; higher volume-averaged ionization parameters; and higher central T e , n e , and log U values. Traditional three-zone estimates of the ionization parameter can underestimate the average log U by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the α /H abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are α /Fe-enriched by more than three times. However, there still is a high-energy ionizing photon production problem (HEIP 3 ). Even for such α /Fe enrichment and very high log U s, photoionization models cannot reproduce the very high-ionization emission lines observed in EELGs.more » « less
-
ABSTRACT Binary stars are abundant in nearby galaxies, but are typically unaccounted for in simulations of the high-redshift Universe. Stellar population synthesis models that include the effects of binary evolution result in greater relative abundances of ionizing photons that could significantly affect the ambient ionizing background during the epoch of hydrogen reionization, additionally leading to differences in galaxy gas content and star formation. We use hydrodynamic cosmological simulations including in situ multifrequency radiative transfer to evaluate the effects of a high binary fraction in reionization-era galaxies on traits of the early intergalactic medium and the abundance of H i and He ii ionizing photons. We further extend this to analyse the traits of enriched gas. In comparing metrics generated using a fiducial simulation assuming single stars with one incorporating a high binary fraction, we find that binary stars cause H i reionization to complete earlier and at an accelerated pace, while also increasing the abundances of high-ionization metals (C iv and Si iv) in simulated absorption spectra while reducing the abundance of low-ionization states (O i, Si ii, and C ii). However, through increased photoheating of galactic and circumgalactic gas, they simultaneously reduce the rate of star formation in low-mass galaxies, slowing the ongoing process of enrichment and suppressing their own ionizing background. This potentially contributes to a slower He ii reionization process at $$z\ge 5$$, and further indicates that self-regulation of galaxies could be underestimated when neglecting binary stellar evolution.more » « less
-
ABSTRACT We present an updated model of the cosmic ionizing background from the UV to the X-rays. Relative to our previous model, the new model provides a better match to a large number of up-to-date empirical constraints, including: (1) new galaxy and AGN luminosity functions; (2) stellar spectra including binary stars; (3) obscured and unobscured AGN; (4) a measurement of the non-ionizing UV background; (5) measurements of the intergalactic H i and He ii photoionization rates at z ∼ 0−6; (6) the local X-ray background; and (7) improved measurements of the intergalactic opacity. In this model, AGN dominate the H i ionizing background at z ≲ 3 and star-forming galaxies dominate it at higher redshifts. Combined with the steeply declining AGN luminosity function beyond z ∼ 2, the slow evolution of the H i ionization rate inferred from the high-redshift H i Ly α forest requires an escape fraction from star-forming galaxies that increases with redshift (a population-averaged escape fraction of $$\approx 1{{\ \rm per\ cent}}$$ suffices to ionize the intergalactic medium at z = 3 when including the contribution from AGN). We provide effective photoionization and photoheating rates calibrated to match the Planck 2018 reionization optical depth and recent constraints from the He ii Ly α forest in hydrodynamic simulations.more » « less
-
While the shape of the Lyα profile is viewed as one of the best tracers of ionizing-photon escape fraction (fesc) within low redshift (z~0.3) surveys of the Lyman continuum, this connection remains untested at high redshift. Here, we combine deep, rest-UV Keck/LRIS spectra of 80 objects from the Keck Lyman Continuum Spectroscopic Survey with rest-optical Keck/MOSFIRE spectroscopy in order to examine potential correlations between Lyα profile shape and the escape of ionizing radiation within z~3 star-forming galaxies. We measure the velocity separation between double-peaked Lyα emission structure (vsep), between red-side Lyα emission peaks and systemic (vLyα,red), and between red-side emission peaks and low-ionization interstellar absorption lines (vLyα−LIS). We find that the IGM-corrected ratio of ionizing to non-ionizing flux density is significantly higher in KLCS objects with lower vLyα,red. We find no significant trend between measures of ionizing-photon escape and vLyα−LIS. We compare our results to measurements of z~0.3 "Green Peas" from the literature and find that KLCS objects have larger vsep at fixed vLyα,red, larger fesc at fixed vLyα,red, and higher vLyα,red overall than z~0.3 analogs. We conclude that the Lyα profile shapes of our high-redshift sources are fundamentally different, and that measurements of profile shape such as vLyα,red map on to fesc in different ways. We caution against building reionization-era fesc diagnostics based purely on Lyα profiles of low-redshift dwarf galaxies. Tracing vsep, vLyα,red, and fesc in a larger sample of z~3 galaxies will reveal how these variables may be connected for galaxies at the epoch of reionization.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac8f2c
