A second-generation star in a relic dwarf galaxy

Abstract

Stars that contain only trace amounts of elements heavier than helium, referred to as having low "metallicity", preserve the chemical fingerprints of the first generation of stars and supernovae. In the Milky Way, the lowest metallicity stars show an extreme over-abundance of carbon relative to other elements, which has been hypothesized to be a unique result of the first low-energy supernovae. However, the origin of this signature has remained a mystery, since no such stars have been discovered in the ancient dwarf galaxies where they are thought to have formed. Here, we present observations of a star in the >10 billion year old ultra-faint dwarf galaxy Pictor II, that shows the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with a factor of >3000x relative carbon enhancement. As the first unambiguous second-generation star in a relic dwarf galaxy, this object demonstrates that carbon-enhanced second-generation stars can originate in primordial small-scale systems. This star supports the hypothesis that carbon-enhancement is produced by low-energy-supernovae, since the yields of energetic supernovae are harder to retain in small-scale environments. This key local signature of chemical enrichment by the first stars traces a regime inaccessible to current high-redshift observations, which cannot detect the early enrichment of the smallest galaxies.

Figures (6)

• Figure 1: Identification of the distant Pictor II member star, PicII-503. a. Spatial distribution of Pictor II candidate member stars identified in this study in blue with the observed star PicII-503 circled in orange, confirmed members from refpace+25 in red, and foreground stars in grey. Dashed ellipses correspond to 1, 3, and 5 times the half-light radius of Pictor IIpace+25. b. A color-magnitude diagram of the stars, with a track ("isochrone") corresponding to a 12 Gyr, [Fe/H] = $-2.5$ stellar populationdcj+08 overlaid (solid black line) at the distance of Pictor II (45.7 kiloparsecs)pace+25. Some members from refpace+25 are missing due to lacking $i$ band photometry. c. A plot of the proper motions of the stars, with the systemic motion of Pictor II marked as a grey crossbtt+22. d. CaHK$-$g$-0.9\times$(g$-$i) vs. g$-$i colour–colour diagram, used to estimate photometric metallicities. Contours correspond to model predictions at surface gravity logg = 2.0 (see Methods). Candidate members are selected with [Fe/H] $< -2.5$, separating them from the more metal-rich Milky Way foreground. PicII-503 (orange) lies at the lowest-metallicity end of the distribution.
• Figure 2: Spectroscopic confirmation of PicII-503 as an ultra metal-poor, carbon-enhanced star. a. A plot of the wavelength region around the calcium II K (Ca II K) absorption feature, where the observed X-Shooter spectrum is in magenta, and synthetic spectra at [Ca/H] = $-5.2$ and [Ca/H] = $-4.6$ ($\sim2\sigma$ statistical upper limit) are overplotted. The Ca II K feature is typically the strongest metal line in stars, and is barely detected for PicII-503. The adjacent absorption feature is an otherwise weak carbon line that appears due to the high carbon abundance of the star. b. The region around the molecular CH G band region in the X-Shooter data is plotted. An extremely prominent molecular carbon feature is detected, demonstrating the high abundance of carbon relative to other metals in the star. A synthetic spectrum with negligible carbon abundance (red line) is shown for reference, as is a carbon synthesis at $\pm0.3$ dex (orange band). c. The region around the strongest Fe line at 3859 Å in the X-Shooter data is shown. A joint analysis of this feature and other weak features nearby provides a statistical upper limit of [Fe/H] $< -4.63$. Synthetic spectra at [Fe/H] = $-4.0$ and [Fe/H] = $-5.5$ are shown for illustrative purposes. d. The region around the H$\alpha$ line in the MagE data is shown, with template spectra for a dwarf star ($\log\,g$ = 4.7) and a red giant star ($\log\,g$ = 2.5) overplotted. The observed H$\alpha$ line morphologically agrees with the giant template, confirming that PicII-503 has a surface gravity that is consistent with membership in Pictor II (see methods).
• Figure 3: A plot of the carbon-to-calcium ratio versus calcium abundance, including our observed star in Pictor II (red star), along with Milky Way halo stars (black circles), other stars in UFDs (red circles), and stars in larger dwarf galaxies (dSphs; blue) and the LMC (green). Carbon-enhancement in the Milky Way is defined to occur when [C/Fe] $> 0.7$. Given that calcium behaves as an $\alpha$-element, and [$\alpha$/Fe] $\approx$ 0.4 at low metallicities due to dominant core collapse supernova enrichmentww+95, we indicate the regime where [C/Ca] $> 0.3$ as a proxy for the carbon-enhanced regime. The dramatic carbon-enhancement in the observed Pictor II star is evident, placing it among the most carbon-enhanced and metal-deficient stars known, demonstrating that these second generation stars could have originated in relic dwarf galaxies.
• Figure 4: Analysis of various absorption features in the spectrum of PicII-503. a. -- f. Cutouts from the X-Shooter spectrum of PicII-503 around the wavelength regions of prominent element absorption features. Magenta shaded regions correspond to the uncertainty inferred from the S/N of the data (see methods). Dashed lines correspond to synthetic spectra at various element abundances. Panels a through d correspond to the abundance upper limits in Table \ref{['tab:abundances']} for barium, strontium, magnesium, and nitrogen. In panel d, a spectrum with [N/H] = $-9.0$ is also shown to illustrate the regions that have nitrogen absorption features. Panels e and f show regions around other prominent iron absorption features with weaker upper limits to corroborate the exceptionally low iron abundance of PicII-503.
• Figure 5: Confirming that PicII-503 is a member of the Pictor II UFD. a. A plot of the membership score versus radial velocity, for stars in the recent spectroscopic study of Pictor IIpace+25 and PicII-503 overplotted (purple star). Non-members of Pictor II are shown in grey, and members are blue squares. Note that the systemic velocity of Pictor II is well-separated from the radial velocities of foreground stars. The red region corresponds to $Z < 16.3$ and three times the velocity dispersion (3$\times$3.5 km s$^{-1}$) around the systemic velocity (326.9 km s$^{-1}$) of Pictor IIpace+25. The former are the thresholds for velocity, proper motion, and parallax consistency with Pictor II. b. Radial velocities of stars in the left panel versus their distance from the center of Pictor II. One and two half-light radii are denoted by vertical dotted lines. PicII-503 appears as the most distant likely member. The red region corresponds to the same velocity selection as in panel a.