NGC 147 Corroborates the Break in the Stellar Mass–Stellar Metallicity Relation for Galaxies

Abstract

The stellar mass–stellar metallicity relation (MZR) is an essential approach to probing the chemical evolution of galaxies. It reflects the balance between galactic feedback and gravitational potential as a function of stellar mass. However, the current MZR of local dwarf satellite galaxies (M* ≲ 10⁸ M_⊙; measured from resolved stellar spectroscopy) may not be reconcilable with that of more massive galaxies (M* ≳ 10^(9.5) M_⊙; measured from integrated-light spectroscopy). Such a discrepancy may result from a systematic difference between the two methods, or it may indicate a break in the MZR around 10⁹ M_⊙. To address this question, we measured the stellar metallicity of NGC 147 from integrated light using the Palomar Cosmic Web Imager. We compared the stellar metallicity estimates from integrated light with measurements from resolved stellar spectroscopy and found them to be consistent within 0.1 dex. On the other hand, the high-mass MZR overpredicts the metallicity by 0.6 dex at the mass of NGC 147. Therefore, our results tentatively suggest that the discrepancy between the low-mass MZR and high-mass MZR should not be attributed to a systematic difference in techniques. Instead, real physical processes cause the transition in the MZR. In addition, we discovered a positive age gradient in the innermost region and a negative metallicity gradient from the resolved stars at larger radii, suggesting a possible outside-in formation of NGC 147.