Photometric identification and MMT spectroscopy of new extremely metal-poor galaxies: Towards a better understanding of young stellar populations at low metallicity

Peter Senchyna, Daniel P Stark

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

Extremely metal-poor star-forming galaxies (XMPs) represent one of our only laboratories for study of the low-metallicity stars we expect to encounter at early epochs. But as our understanding of the z > 6 universe has improved, it has become clear that the majority of known XMPs within 100 Mpc host significantly less prominent massive star populations than their reionization-era counterparts, severely limiting their utility as testbeds for interpreting spectral features found at the highest redshifts. Here we present a new photometric selection technique designed to identify nearby XMPs dominated by young stellar populations comparable to those expected in the reionization era. We apply our technique to uncover candidate XMPs in SDSS imaging at magnitudes 16 < i < 23, extending significantly below the completeness limits of the SDSS spectroscopic survey. Spectroscopic observations with the MMT confirm that 32 of the 53 uniformly metal-poor and high specific star formation rate targets we observed have gas-phase oxygen abundances 12 logO/H < 7.7 (Z/Zo. < 0.1), including two in the range of the lowest metallicity galaxies known, Z/Zo. < 0.05. Our observations shed new light on to the long-standing mystery of He II emission in star-forming galaxies:we find that the equivalent width of the He II λ4686 high-ionization emission line does not scale with that of Hβ in our sample, suggesting that binary evolution or other processes on >10 Myr time-scales contribute substantially to the He+ionizing photon budget in this metallicity regime. Applying such selection techniques coupled with deep spectroscopy to next-generation photometric surveys like Large Synoptic Survey Telescope may eventually provide a basis for an empirical understanding of metal-poor massive stars.

Original languageEnglish (US)
Pages (from-to)1270-1284
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Volume484
Issue number1
DOIs
StatePublished - Jan 1 2019

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young population
massive stars
metallicity
spectroscopy
galaxies
stars
metal
encounters
budgets
metals
universe
time measurement
telescopes
timescale
photons
laboratory
budget

Keywords

  • galaxies: Dwarf
  • galaxies: Evolution
  • galaxies: Stellar content.
  • stars: Massive

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

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title = "Photometric identification and MMT spectroscopy of new extremely metal-poor galaxies: Towards a better understanding of young stellar populations at low metallicity",
abstract = "Extremely metal-poor star-forming galaxies (XMPs) represent one of our only laboratories for study of the low-metallicity stars we expect to encounter at early epochs. But as our understanding of the z > 6 universe has improved, it has become clear that the majority of known XMPs within 100 Mpc host significantly less prominent massive star populations than their reionization-era counterparts, severely limiting their utility as testbeds for interpreting spectral features found at the highest redshifts. Here we present a new photometric selection technique designed to identify nearby XMPs dominated by young stellar populations comparable to those expected in the reionization era. We apply our technique to uncover candidate XMPs in SDSS imaging at magnitudes 16 < i < 23, extending significantly below the completeness limits of the SDSS spectroscopic survey. Spectroscopic observations with the MMT confirm that 32 of the 53 uniformly metal-poor and high specific star formation rate targets we observed have gas-phase oxygen abundances 12 logO/H < 7.7 (Z/Zo. < 0.1), including two in the range of the lowest metallicity galaxies known, Z/Zo. < 0.05. Our observations shed new light on to the long-standing mystery of He II emission in star-forming galaxies:we find that the equivalent width of the He II λ4686 high-ionization emission line does not scale with that of Hβ in our sample, suggesting that binary evolution or other processes on >10 Myr time-scales contribute substantially to the He+ionizing photon budget in this metallicity regime. Applying such selection techniques coupled with deep spectroscopy to next-generation photometric surveys like Large Synoptic Survey Telescope may eventually provide a basis for an empirical understanding of metal-poor massive stars.",
keywords = "galaxies: Dwarf, galaxies: Evolution, galaxies: Stellar content., stars: Massive",
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TY - JOUR

T1 - Photometric identification and MMT spectroscopy of new extremely metal-poor galaxies

T2 - Towards a better understanding of young stellar populations at low metallicity

AU - Senchyna, Peter

AU - Stark, Daniel P

PY - 2019/1/1

Y1 - 2019/1/1

N2 - Extremely metal-poor star-forming galaxies (XMPs) represent one of our only laboratories for study of the low-metallicity stars we expect to encounter at early epochs. But as our understanding of the z > 6 universe has improved, it has become clear that the majority of known XMPs within 100 Mpc host significantly less prominent massive star populations than their reionization-era counterparts, severely limiting their utility as testbeds for interpreting spectral features found at the highest redshifts. Here we present a new photometric selection technique designed to identify nearby XMPs dominated by young stellar populations comparable to those expected in the reionization era. We apply our technique to uncover candidate XMPs in SDSS imaging at magnitudes 16 < i < 23, extending significantly below the completeness limits of the SDSS spectroscopic survey. Spectroscopic observations with the MMT confirm that 32 of the 53 uniformly metal-poor and high specific star formation rate targets we observed have gas-phase oxygen abundances 12 logO/H < 7.7 (Z/Zo. < 0.1), including two in the range of the lowest metallicity galaxies known, Z/Zo. < 0.05. Our observations shed new light on to the long-standing mystery of He II emission in star-forming galaxies:we find that the equivalent width of the He II λ4686 high-ionization emission line does not scale with that of Hβ in our sample, suggesting that binary evolution or other processes on >10 Myr time-scales contribute substantially to the He+ionizing photon budget in this metallicity regime. Applying such selection techniques coupled with deep spectroscopy to next-generation photometric surveys like Large Synoptic Survey Telescope may eventually provide a basis for an empirical understanding of metal-poor massive stars.

AB - Extremely metal-poor star-forming galaxies (XMPs) represent one of our only laboratories for study of the low-metallicity stars we expect to encounter at early epochs. But as our understanding of the z > 6 universe has improved, it has become clear that the majority of known XMPs within 100 Mpc host significantly less prominent massive star populations than their reionization-era counterparts, severely limiting their utility as testbeds for interpreting spectral features found at the highest redshifts. Here we present a new photometric selection technique designed to identify nearby XMPs dominated by young stellar populations comparable to those expected in the reionization era. We apply our technique to uncover candidate XMPs in SDSS imaging at magnitudes 16 < i < 23, extending significantly below the completeness limits of the SDSS spectroscopic survey. Spectroscopic observations with the MMT confirm that 32 of the 53 uniformly metal-poor and high specific star formation rate targets we observed have gas-phase oxygen abundances 12 logO/H < 7.7 (Z/Zo. < 0.1), including two in the range of the lowest metallicity galaxies known, Z/Zo. < 0.05. Our observations shed new light on to the long-standing mystery of He II emission in star-forming galaxies:we find that the equivalent width of the He II λ4686 high-ionization emission line does not scale with that of Hβ in our sample, suggesting that binary evolution or other processes on >10 Myr time-scales contribute substantially to the He+ionizing photon budget in this metallicity regime. Applying such selection techniques coupled with deep spectroscopy to next-generation photometric surveys like Large Synoptic Survey Telescope may eventually provide a basis for an empirical understanding of metal-poor massive stars.

KW - galaxies: Dwarf

KW - galaxies: Evolution

KW - galaxies: Stellar content.

KW - stars: Massive

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