The Low-Mass Galaxy Frontier

Low-mass (M<109 Msun), low-metallicity (1–50% Zsolar) galaxies dominate the hot, metal-poor early universe, power cosmic reionization, and constitute the majority of galaxies in the local universe. Despite their importance, they remain one of the least explored galaxy frontiers due to their intrinsic faintness and spread across the sky. UVEX will provide a comprehensive census of local, low-mass, low-metallicity (LMLZ) galaxies, which is crucial for understanding how their environments affect their properties. These local LMLZ galaxies are key for understanding the processes of galaxy formation, stellar evolution and demise, and the formation of compact objects in metal-poor environments. UVEX will identify low-mass, low-metallicity galaxies in the nearby Universe, diagnose the nebular emission of analogs to high-redshift galaxies, and study hot and stripped stars in the Large and Small Magellanic Clouds, our neighboring low-metallicity laboratories.

UVEX: leading the discovery of low-mass, low-metallicity galaxies in the local universe

Our current census of nearby LMLZ galaxies is highly incomplete. We only know of ~20,000 LMLZ galaxies within 100 Mpc, while theoretical matching of stellar and dark matter halo masses predicts a population of ~10–200 million LMLZ galaxies within this volume. Finding and mapping these local LMLZ galaxies requires a wide-area, sensitive UV imaging survey, since these systems cannot be distinguished from more massive, higher redshift systems at optical and IR wavelengths alone. 

With its all-sky ultraviolet survey, UVEX will identify millions of LMLZ galaxies within 100 Mpc down to masses of about a million times the mass of the sun. This large sample of LMLZ galaxies is essential for anchoring the stellar-halo mass relation, providing 3D maps of the low-mass, low-density universe, enabling the first large-scale study of how the lowest-mass halos evolve as a function of environment, and finding the most extreme systems for follow-up.

UVEX discovers low-mass, low metallicity galaxies

UVEX imaging picks out low-mass galaxies in the local universe by providing the crucial UV photometry needed to differentiate between low metallicity systems (blue) and higher redshift galaxies (orange) that may look similar at optical and infrared wavelengths.

Probing nebular emission in the lowest metallicity galaxies with UVEX

Integrated nebular emission lines from the earliest galaxies provide powerful diagnostics of the baryonic processes that shape galaxy evolution, including the history of star formation, supernova feedback, and ionizing radiation from massive stars. Current and future missions including ALMA, JWST, and the Extremely Large Telescopes will give us an unprecedented view into nebular emission from the first galaxies and their stars. However, without a solid understanding of local analogs, interpreting these emission lines will be challenging. 

With targeted R>1000 spectroscopy of local LMLZ galaxies, UVEX will provide crucial templates for understanding galaxies in the early universe. The Hubble Space Telescope opened this field by obtaining UV spectra of several dozen local, star-forming, modestly low-metallicity galaxies. UVEX will not only be able to observe known LMLZ galaxies that are too faint to observe with HST, it will expand discovery space and observe currently unknown extreme LMLZ galaxies.

Probing nebular emission with UVEX

The UVEX spectrograph is optimized for observing nebular emission lines over the crucial wavelength range. Left: A gri image of a local extremely metal-poor galaxy showing the HST/COS and UVEX spectroscopic apertures. Right: A simulated UVEX spectrum of a ~1% Zsolar low-mass galaxy at 100 Mpc compared to a spectrum from HST/COS for similar integration times.

The Magellanic Clouds: a laboratory for low-metallicity stars

The Small and Large Magellanic Clouds are the nearest low-metallicity galaxies to the Milky Way, presenting a laboratory where individual low-metallicity, massive stars can be resolved. Massive stellar evolution is key for understanding galaxy evolution, and mass loss is key for understanding massive stars. Mass loss via stellar winds has a strong metallicity dependence, and additional variation in stellar wind properties is now believed to be a consequence of binary interactions. Most massive stars are in binary systems, and this fraction increases at lower metallicities. Binary interactions including mass transfer and common envelope ejection can strip stars of their hydrogen-rich envelopes, leaving hot compact helium cores, which are prolific sources of ionizing radiation that contributed to cosmic reionization. These binary stripped stars can also be the progenitors of merging compact objects, which may be sources of gravitational wave radiation detectable with LIGO. 

UVEX will provide a near-complete census of hot and stripped stars in the Magellanic Cloud and obtain UV spectra of >100 stripped stars and >1000 hot (O and B type) single and binary stars, allowing for the study of their terminal wind velocities. It will also image the entirety of the Small and Large Magellanic Clouds on a weekly cadence, providing a wealth of variability information on hot binaries, pulsating stars, and eclipsing systems.

UVEX versus HST images

A comparison of the resolution of UVEX (right) compared to the previous UV mission GALEX (left). A 1 arcminute NUV image from the Hubble Space Telescope image in 30 Doradus, a large HII region in the Large Magellanic Cloud (left) smoothed with the UVEX/GALEX PSF, binned at the relevant pixel size. Outside the most crowded regions (<10% of the dense cluster) photometry can be accurately extracted. Star densities at FUV wavelengths will be lower than those pictured here.