[Paper Review] The Star Formation Rates of Elliptical Galaxies from Core-Collapse Supernovae

Thomas M. Sedgwick, I. K. Baldry+3 more|arXiv (Cornell University)|Jun 25, 2021

Gamma-ray bursts and supernovae3 references4 citations

TL;DR

This study uses core-collapse supernovae (CCSNe) as unambiguous tracers of recent star formation in elliptical galaxies, applying a Bayesian method to classify galaxies via color and morphology in the SDSS Stripe 82 region. It finds that ellipticals contribute 11.2±3.1% (stat) to the present-day cosmic star formation rate density, with a mean specific star formation rate of 9.2×10⁻¹² yr⁻¹ for massive ellipticals, consistent with SED-fitting estimates and indicating ongoing low-level star formation.

ABSTRACT

The level of star formation in elliptical galaxies is poorly constrained, due to difficulties in quantifying the contamination of flux-based estimates of star formation from unrelated phenomena, such as AGN and old stellar populations. We here utilise core-collapse supernovae (CCSNe) as unambiguous tracers of recent star formation in ellipticals within a cosmic volume. We firstly isolate a sample of 421 z < 0.2, r < 21.8 mag CCSNe from the SDSS-II Supernova Survey. We then introduce a Bayesian method of identifying ellipticals via their colours and morphologies in a manner unbiased by redshift and yet consistent with manual classification from Galaxy Zoo 1. We find ~ 25 % of z < 0.2 r < 20 mag galaxies in the Stripe 82 region are ellipticals (~ 28000 galaxies). In total, 36 CCSNe are found to reside in ellipticals. We demonstrate that such early-types contribute a non-negligible fraction of star formation to the present-day cosmic budget, at 11.2 $\pm$ 3.1 (stat) $^{+3.0}_{-4.2}$ (sys) %. Coupling this result with the galaxy stellar mass function of ellipticals, the mean specific star formation rate (SSFR; $\overline{S}$) of these systems is derived. The best-fit slope is given by log ($\overline{S}(M)$/yr) = - (0.80 $\pm$ 0.59) log ($M/10^{10.5} m{M}_{\odot}$) - 10.83 $\pm$ 0.18. The mean SSFR for all log ($M/ m{M}_{\odot}$) > 10.0 ellipticals is found to be $\overline{S} = 9.2 \pm 2.4$ (stat) $^{+2.7}_{-2.3}$ (sys) $ imes 10^{-12}$ yr$^{-1}$, which is consistent with recent estimates via SED-fitting, and is 11.8 $\pm$ 3.7 (stat) $^{+3.5}_{-2.9}$ (sys) % of the mean SSFR level on the main sequence as also derived from CCSNe. We find the median optical spectrum of elliptical CCSN hosts is statistically consistent with that of a control sample of ellipticals that do not host CCSNe, implying that these SN-derived results are well-representative of the total low-z elliptical population.

Motivation & Objective

• To resolve uncertainties in star formation rate (SFR) measurements for elliptical galaxies, which are contaminated by AGN, old stellar populations, and post-AGB stars in traditional flux-based diagnostics.
• To overcome limitations of UV, IR, and Hα indicators by using core-collapse supernovae (CCSNe) as unambiguous tracers of recent star formation.
• To develop a Bayesian classification method that identifies ellipticals based on color and morphology, unbiased by redshift and consistent with Galaxy Zoo manual classifications.
• To quantify the contribution of elliptical galaxies to the present-day cosmic star formation rate density (SFRD) and derive their mean specific star formation rate (SSFR).
• To assess whether SN-hosting ellipticals are representative of the broader low-redshift elliptical population using spectral comparisons.

Proposed method

• A sample of 421 core-collapse supernovae (CCSNe) at z<0.2 and r<21.8 was extracted from the SDSS-II Supernova Survey.
• A Bayesian classification framework was developed to identify elliptical galaxies using u-g, g-r, r-i, and i-z colors and de Vaucouleurs' concentration index, trained to match Galaxy Zoo 1 visual classifications.
• The method was applied to 28,000 galaxies in the SDSS Stripe 82 region with z<0.2 and r<20 mag, yielding a 25% elliptical fraction.
• 36 CCSNe were found to reside in the final sample of classified ellipticals.
• The cosmic star formation rate density (SFRD) contribution from these ellipticals was calculated by scaling the observed CCSN rate to SFR using the Salpeter initial mass function and a 10 Myr timescale for massive star formation.
• The specific star formation rate (SSFR) was derived by combining the SFRD result with the galaxy stellar mass function of ellipticals, fitting a power-law relation to the SSFR–mass relation.

Experimental results

Research questions

• RQ1 What fraction of the present-day cosmic star formation rate density (SFRD) is contributed by elliptical galaxies, as traced by core-collapse supernovae?
• RQ2 How does the specific star formation rate (SSFR) of massive elliptical galaxies evolve with stellar mass, and what is its mean value for log(M/M☉) > 10.0?
• RQ3 Are elliptical galaxies hosting core-collapse supernovae representative of the broader low-redshift elliptical population in terms of their optical spectra?
• RQ4 To what extent do traditional SFR indicators (e.g., UV, Hα) fail to accurately measure star formation in ellipticals due to contamination from non-star-formation processes?
• RQ5 Can a Bayesian classification method based on photometric colors and morphology reliably identify ellipticals without redshift bias, and how does it compare to visual classification?

Key findings

• Elliptical galaxies contribute 11.2±3.1% (statistical) and +3.0/-4.2% (systematic) to the present-day cosmic star formation rate density (SFRD).
• The mean specific star formation rate (SSFR) for massive ellipticals (log(M/M☉) > 10.0) is 9.2±2.4 (statistical) and +2.7/-2.3 (systematic) ×10⁻¹² yr⁻¹.
• The SSFR–mass relation for ellipticals is best fit by the power law: log(SSFR/yr⁻¹) = –(0.80±0.59)log(M/10¹⁰.⁵M☉) – 10.83±0.18.
• The median optical spectrum of elliptical galaxies hosting CCSNe is statistically indistinguishable from that of a control sample of non-SN ellipticals, indicating the SN hosts are representative of the broader population.
• The results are consistent with recent SED-fitting estimates and show that ellipticals contribute 11.8±3.7% (statistical) and +3.5/-2.9% (systematic) of the main-sequence SSFR level.
• The study confirms that core-collapse supernovae provide a robust, contamination-free tracer of recent star formation in early-type galaxies, resolving long-standing ambiguities in SFR measurements.