MY STUDIES OF DWARF GALAXIES ARE HELPING ESTABLISH THE LOCAL PARADIGM FOR GALAXY EVOLUTION
probe the most massive systems. In Berg+12 I used direct metallicities to extend the MZR to lower masses by more than two orders of magnitude, showing the same processes dominate galaxy evolution down to the smallest building blocks and instituting the principal benchmark comparison for the low-mass populations common at higher redshifts.
DIRECT ABUNDANCES FOR LOW-LUMINOSITY LVL GALAXIES
A fundamental probe of galaxy evolution is the relationship between stellar mass and gas-phase metallicity (MZR). The MZR is shaped by the cumulative galaxy evolutionary processes, where star formation builds up nucleosynthetic metal products, but enriched galactic-scale outflows and pristine gas infall dilute them (e.g., Tremonti+04; Dalcanton+07; Peeples+11; Zahid+14). Accurate MZR trends require direct abundance measurements, typically via faint, temperature-sensitive auroral lines. Yet, most MZR studies derive abundances from empirical correlations with strong emission lines and only ____
Leoncino is a local dwarf galaxy of important because it is an extremely metal-poor (XMP) galaxy. XMPs are low-mass, star-forming galaxies with gas-phase oxygen abundances of 12+log(O/H) < 7.35 (1/20 Z☉) that provide boundary conditions and essential constraints on chemical-enrichment pathways. Galaxy evolution scenarios suggest three possible pathways to form an XMP: (1) secular evolution at low galaxy masses, (2) slow evolution in voids, or (3) dilution of abundances from infall of pristine gas. We recently compiled the known XMPs in the local universe to explore these different evolution scenarios, as shown in the fig. below. First, we find that when only direct abundances are considered, void and field galaxies agree with one another within the uncertainties. Second, we examined individual galaxies. Leo P (Skillman+13) is the only known XMP that is consistent with both the MZ and LZ relations, following typical secular evolution. Leoncino (Hirschauer+16), on the other hand, is offset from the LZ relation, likely due to an increase in recent star formation triggered by a minor interaction. In contrast, many other XMPs are outliers in both the LZ and MZ relations; in such cases, the low oxygen abundances are best explained by dilution due to the infall of pristine gas.
THE LEONCINO DWARF GALAXY: EXPLORING THE LOW-METALLICITY END OF THE LUMINOISTY-METALLICITY AND MASS-METALLICITY RELATIONS
THE CHEMICAL EVOLUTION OF C, N, & O IN METAL-POOR DWARF GALAXIES
Diagnosing the physical and chemical conditions within star forming galaxiess is of paramount importance to understanding the stellar and baryon cycles driving galaxy evolution. Studying carbon, nitrogen, and oxygen abundances are particularly informative owing to the fact that
they primarily originate from stars of different mass ranges; O is synthesized mostly in massive stars (M > 10 M ), while C and N are produced in both massive and intermediate-mass stars. Theoretically, only primary (metallicity independent) nucleosynthetic processes are known to produce C such that we expect the C/O ratio to be constant under the assumption of a universal initial mass function. Observationally, we showed in Berg+16 and Berg+19a that the relationship between C/O and O/H is nominally consistent with an increasing trend, suggesting that secondary (metallicity--dependent) C production is prominent and the large observed scatter in C/O could be due to the delayed C released from intermediate-mass stars. By modeling the chemical evolution of C, N, and O of individual targets (see figure below), we find that the C/O ratio is very sensitive to both the detailed star formation history and to supernova feedback. Longer burst durations and lower star formation efficiencies correspond to low C/O ratios, while the escape of oxygen atoms in supernovae winds produces decreased effective oxygen yields and larger C/O ratios.
This work implies that measuring the UV C and O emission lines alone does not provide a reliable indicator of the O/H abundance. Unfortunately, the well-established optical emission line diagnostics used to discern properties of local star forming galaxies will be observationally inaccessible for large telescopes, such as the GMT, emphasizing the need for a robust, reliable framework of ISM diagnostics at FUV wavelengths. I have been leading observational and photoionization modeling efforts to address this problem, using nebular UV lines to constrain radiation fields, ionization sources, and metallicity (Berg+16,18,19a; Byler+18,19). My studies of C/O abundances have demonstrated the powerful, but complex nature of the UV emission lines, revealing their sensitivity to stellar age, feedback, and metallicity (Berg+16,19a). This has galvanized broad interest in the diagnostic utility of UV diagnostics; however, they remain poorly calibrated. By design, the unrivaled parameter space of the upcoming CLASSY project is well matched to properties of high-z galaxies to facilitate proper calibration.
Our ability to study the properties of the ISM in the earliest galaxies will rely on emission line diagnostics at rest-frame UV wavelengths. In these work, we identify metallicity-sensitive diagnostics using UV nebular emission lines and stellar features. As a goal of the astronomy community is to calibrate these UV features to the well-established optical diagnostics, we compare UV-derived metallicities with standard optical metallicities in the figure below. We find that the He2--O3C3 diagnostic (HeII λ1640/CIII] λλ1906,1909 vs. OIII] λ1666/CIII] λλ1906,9) is currently our most reliable metallicity tracer, particularly at low metallicity (12+log10(O/H) ≤ 8.0), where stellar contributions to HeII emission (e.g., Wolfe Rayet emission) are minimal. We find that the Si3--O3C3 diagnostic (SiIII] λλ1883,92/CIII] λλ1906,9 vs. [OIII] λ1666 / CIII] λλ1906,9) can also be a useful metallicity tracer, though with large scatter (0.2--0.3 dex), which is likely driven by variations in gas--phase abundances. Caution should also be noted with SiIII], which heavily dust depletes. The C4--O3C3 diagnostic (CIV λλ1548,50/OIII] λ1666 vs. OIII] λ1666/CIII] λλ1906,9), on the other hand, correlates poorly with optically--derived metallicities. Hardness of the ionizing spectrum, contributions from CIV stellar wind emission, and non-solar-scaled gas-phase abundances (e.g., C/O and N/O) may also play a significant role in the UV--to--optical metallicity discrepancies.
A COMPARISON OF UV AND OPTICAL METALLICITIES IN STAR-FORMING GALAXIES
STELLAR AND NEBULAR DIAGNOSTICS IN THE ULTRAVIOLET FOR STAR-FORMING GALAXIES
CONSTRAINING THE METALLICITIES, AGES, STAR FORMATION HISTORIES, AND IONIZING CONTINUA OF EXTRAGALACTIC MASSIVE STAR POPULATIONS
indices that are unique to very young (< 7 Myr) stellar populations: (1) strong and broad CIV and NV P-Cygni features, (2) broad (> 300 km/s) HeII emission, and (3) weak SiIII 1299Å photospheric absorption features (equivalent widths < 0.1Å). In the figure to the right, we show these features, as well as the OV 1371Å wind feature, which traces the hottest outflowing phase from the most massive, young stars (< 3 Myr). The sensitivity of the NV, SiIII, OV, and CIV features to stellar population age is show (gold line; 0.2 Z⊙ population), where as other features such as SiIV (and also CIV) are more sensitive diagnostics of the stellar metallicity (blue line; 5 Myr population).
Young stellar populations produce the most ionizing photons, and as such, the UV is a powerful spectral regime because it contains several stellar spectral features that characterize the properties of the ionizing stellar population. In particular, there are spectral indices
Using these UV diagnostic stellar features, we fit the stellar continuum of 61 star-forming galaxies (42 at z~0 and 19 at z∼2) with a linear combination of single age+metallicity starburst99 models. From these fits, we derived light-weighted ages and metallicities spanning 0.05–1.5 Z⊙. Comparing the stellar and nebular metallicities in the figure to the left demonstrates that either single burst or mixed aged combined models produce consistent and better fit results than continuous star-formation models. This has profound implications for interpreting the ionizing continua produce by and escaping from galaxies, and is discussed in detail in the paper.