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Stellar Populations with MaNGA: Iron Kink and Nitrogen Fuzz

Guy Worthey, Tathagata Pal

Abstract

Recent analysis of 2968 MaNGA early type galaxies has yielded two notable trends with velocity dispersion ($σ$) not previously discussed in the literature. First, Fe abundance rises with $σ$, but only until $σ\approx100$ km s$^{-1}$, after which it falls. This kink is reproduced by TNG100 simulations, implying that hierarchical merger processes might explain it. Second, astrophysical scatter in N is high for galaxies with $σ< 100$ km s$^{-1}$. Due to the restricted list of nucleosynthetic sources for N, it is likely that asymptotic giant branch stars provide most of this N. A varied star formation history (compared to that of massive galaxies) along with variable retention and recycling of N-enriched gas might explain the fuzz of N abundance in low-$σ$ galaxies. Because a timescale argument seems necessary to explain the nitrogen fuzz, and an initial mass function argument is ruled out, similar timescale arguments for the [Mg/Fe] trend as a function of velocity dispersion are supported.

Stellar Populations with MaNGA: Iron Kink and Nitrogen Fuzz

Abstract

Recent analysis of 2968 MaNGA early type galaxies has yielded two notable trends with velocity dispersion () not previously discussed in the literature. First, Fe abundance rises with , but only until km s, after which it falls. This kink is reproduced by TNG100 simulations, implying that hierarchical merger processes might explain it. Second, astrophysical scatter in N is high for galaxies with km s. Due to the restricted list of nucleosynthetic sources for N, it is likely that asymptotic giant branch stars provide most of this N. A varied star formation history (compared to that of massive galaxies) along with variable retention and recycling of N-enriched gas might explain the fuzz of N abundance in low- galaxies. Because a timescale argument seems necessary to explain the nitrogen fuzz, and an initial mass function argument is ruled out, similar timescale arguments for the [Mg/Fe] trend as a function of velocity dispersion are supported.

Paper Structure

This paper contains 5 sections, 5 figures.

Table of Contents

  1. Introduction
  2. Data, Analysis, and Results
  3. Nitrogen Fuzz
  4. Iron Kink
  5. Discussion

Figures (5)

  • Figure 1: Light element abundances relative to Fe as a function of velocity dispersion. Errorbars are Monte Carlo estimates based on random observational noise within a 0.7 Gaussian $\sigma$ envelope. Galaxies with $\sigma < 100$ km s$^{-1}$ show pronounced scatter in [N/Fe] that is not seen in [C/Fe] or [Mg/Fe].
  • Figure 2: Astrophysical scatter in logarithmic abundances binned by galaxy velocity dispersion. The smallest galaxies have the largest scatter in all quantities, but nitrogen's scatter has the greatest magnitude.
  • Figure 3: Log age and abundances for N and Fe as a function of $\sigma$ with errorbars as in Fig. \ref{['fig:light']}. One-error regressions for a line and a broken line are overplotted in the Fe panel. The top of the Fe panel is set to [Fe/H] = 0.4, the limit imposed by the model grid.
  • Figure 4: Nitrogen yield relative to iron as a function of initial stellar mass for asymptotic giant branch (AGB) stars 2011ApJS..197...17C at low masses, super-AGB stars 2014MNRAS.437..195D at intermediate masses, and core-collapse supernovae 2013ARAA..51..457N at high masses. Colors indicate initial progenitor heavy element mass fractions ($Z$) as marked.
  • Figure 5: [C/Fe], [N/Fe], and [Mg/Fe] versus velocity dispersion. This version of Fig. \ref{['fig:light']} suppresses error bars and zooms to a narrower velocity dispersion range. A line fit over data in the range 110 km s$^{-1} < \sigma > 340$ km s$^{-1}$ is superimposed (solid line). Note that for galaxies $< 100$ km s$^{-1}$ (to the left of the dotted line), [C/Fe] and [Mg/Fe] lie low, as if these galaxies were enhanced in Fe, but [N/Fe] follows the trend set by high-$\sigma$ galaxies.