Guangwen Chen

Abstract:

Baryonic cycling is reflected in the spatial distribution of metallicity within galaxies; however, gas-phase metallicity distribution and its connection with other properties of dwarf galaxies are largely unexplored. We present the first systematic study of radial gradients of gas-phase metallicities for a sample of 55 normal nearby star-forming dwarf galaxies (stellar mass M ⋆ ranging from 10 7 to 10 9.5 M ⊙ ) based on MUSE wide-field spectroscopic observations. We find that the metallicity gradient has a significant negative correlation (Spearman’s rank correlation coefficient r ≃ −0.56) with M ⋆ , which is in contrast with the flat or even positive correlation observed for higher-mass galaxies. The negative correlation is accompanied by a stronger central suppression of metallicity compared to the outskirts in lower-mass galaxies. Among the other explored galaxy properties, including baryonic mass, star formation distribution, galaxy environment, regularity of gaseous velocity field, and effective yield of metals y eff , only the regularity of gaseous velocity field and y eff have residual correlation with metallicity gradient after controlling for M ⋆ , in the sense that galaxies with an irregular velocity field or lower y eff favor a less negative or more positive metallicity gradient. Particularly, a linear combination of logarithmic stellar mass and y eff significantly improves the correlation with metallicity gradients ( r ∼ −0.68) compared to using stellar mass alone. The lack of correlation with environment disproves gas accretion as a relevant factor shaping the metallicity distribution. The correlation with both gaseous velocity field regularity and y eff implies the importance of stellar feedback-driven metal redistribution within the ISM. Our finding suggests that the metal mixing and transport process, including but not limited to feedback-driven outflow, are more important than in situ metal production in shaping the metallicity distribution of dwarf galaxies.

Abstract:

Nuclear star clusters (NSCs) are commonly found in the centers of galaxies, but their dominant formation mechanisms remain elusive. We perform a consistent analysis of stellar populations of 97 nearby NSCs, based on spectroscopic data from the Very Large Telescope. The sample covers a galaxy stellar mass range of 107–1011 M⊙ and is more than 3 times larger than any previous study. We identify three galaxy stellar mass regimes with distinct NSC properties. In the low-mass regime of logMhost ≲ 8.5 (Mhost is in units of M⊙), nearly all NSCs have metallicities lower than their circum-NSC host but similar to those of typical red globular clusters (GCs), supporting the GC inspiral–merger scenario of NSC formation. In the high-mass regime of logMhost ≳ 9.5, nearly all NSCs have higher metallicities than their circum-NSC host and red GCs, suggesting significant contributions from in situ star formation. In the intermediate-mass regime, a comparable fraction of NSCs have higher or lower metallicities than their circum-NSC host and red GCs, with no clear dependence on NSC mass, suggesting intermittent in situ star formation. The majority of NSCs with higher metallicities than their host exhibit a negative age–metallicity correlation, providing clear evidence of long-term chemical enrichment. The average metallicity difference between NSC and host peaks broadly around logMhost∼9.8 and declines toward both higher and lower galaxy masses. We find that the efficiency of dynamical-friction-driven inspiral of GCs observed in present-day galaxies can explain the NSC mass at logMhost≲9.5 but falls short of observed ones at higher galaxy mass, reinforcing our conclusions based on stellar population analysis.