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A relationship between stellar metallicity gradients and galaxy age in dwarf galaxies

Mercado, Francisco J. and Bullock, James S. and Boylan-Kolchin, Michael and Moreno, Jorge and Wetzel, Andrew and El-Badry, Kareem and Graus, Andrew S. and Fitts, Alex and Hopkins, Philip F. and Faucher-Giguère, Claude-André and Gurvich, Alexander B. (2021) A relationship between stellar metallicity gradients and galaxy age in dwarf galaxies. Monthly Notices of the Royal Astronomical Society, 501 (4). pp. 5121-5134. ISSN 0035-8711. doi:10.1093/mnras/staa3958.

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We explore the origin of stellar metallicity gradients in simulated and observed dwarf galaxies. We use FIRE-2 cosmological baryonic zoom-in simulations of 26 isolated galaxies as well as existing observational data for 10 Local Group dwarf galaxies. Our simulated galaxies have stellar masses between 10^(5.5) and 10^(8.6) M⊙. Whilst gas-phase metallicty gradients are generally weak in our simulated galaxies, we find that stellar metallicity gradients are common, with central regions tending to be more metal-rich than the outer parts. The strength of the gradient is correlated with galaxy-wide median stellar age, such that galaxies with younger stellar populations have flatter gradients. Stellar metallicty gradients are set by two competing processes: (1) the steady ‘puffing’ of old, metal-poor stars by feedback-driven potential fluctuations and (2) the accretion of extended, metal-rich gas at late times, which fuels late-time metal-rich star formation. If recent star formation dominates, then extended, metal-rich star formation washes out pre-existing gradients from the ‘puffing’ process. We use published results from ten Local Group dwarf galaxies to show that a similar relationship between age and stellar metallicity-gradient strength exists among real dwarfs. This suggests that observed stellar metallicity gradients may be driven largely by the baryon/feedback cycle rather than by external environmental effects.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper ItemGIZMO Code ItemData
Mercado, Francisco J.0000-0002-5908-737X
Bullock, James S.0000-0003-4298-5082
Boylan-Kolchin, Michael0000-0002-9604-343X
Moreno, Jorge0000-0002-3430-3232
Wetzel, Andrew0000-0003-0603-8942
El-Badry, Kareem0000-0002-6871-1752
Fitts, Alex0000-0002-8928-6011
Hopkins, Philip F.0000-0003-3729-1684
Faucher-Giguère, Claude-André0000-0002-4900-6628
Gurvich, Alexander B.0000-0002-6145-3674
Alternate Title:Totally Metal: A Relationship Between Stellar Metallicity Gradients and Galaxy Age in Dwarf Galaxies
Additional Information:© 2020 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 December 18. Received 2020 December 2; in original form 2020 September 2. Published: 24 December 2020. We dedicate this paper to Perla Maritza Mercado and José Antonio Florez Velázquez – two bright lights that had a tremendous impact on FJM’s life. Que en paz descansen. We honour the invaluable labour of the maintenance and clerical staff at our institutions, whose contributions make our scientific discoveries a reality. This research was conducted on Acjachemen and Tongva Indigenous land. The functionalities provided by the NUMPY library played a critical role in the analysis presented in this paper (Van Der Walt, Colbert & Varoquaux 2011). We used the FIRE studio (learn more here), an open source PYTHON visualization package designed with the FIRE simulations in mind, to create the images in Fig. 1. We used the WebPlotDigitizer tool ( to determine the median stellar age, t50, from the 10 observed galaxies’ star formation histories. FJM and JSB were supported by NSF Grants AST-1518291 and AST-1910965. We thank Glenn Tipton and Bill Ward for inspiration. MBK acknowledges support from NSF CAREER award AST-1752913, NSF grant AST-1910346, NASA grant NNX17AG29G, and HST-AR-15006, HST-AR-15809, HST-GO-15658, HST-GO-15901, and HST-GO-15902 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. AW received support from NASA through ATP grant 80NSSC18K1097 and HST grants GO-14734, AR-15057, AR-15809, and GO-15902 from STScI; the Heising-Simons Foundation; and a Hellman Fellowship. Support for JM is provided by the NSF (AST Award Number 1516374), and by the Harvard Institute for Theory and Computation, through their Visiting Scholars Program. ASG is supported by the Harlan J. Smith postdoctoral fellowship. CAFG was supported by NSF through grants AST-1715216 and CAREER award AST-1652522; by NASA through grant 17-ATP17-0067; and by a Cottrell Scholar Award from the Research Corporation for Science Advancement. Data Availability: The data supporting the plots within this article are available on reasonable request to the corresponding author. A public version of the GIZMO code is available at Additional data including simulation snapshots, initial conditions, and derived data products are available at
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
NASA Hubble FellowshipGO-14734
NASA Hubble FellowshipAR-15057
NASA Hubble FellowshipAR-15809
NASA Hubble FellowshipGO-15902
Heising-Simons FoundationUNSPECIFIED
Hellman FellowshipUNSPECIFIED
Harvard UniversityUNSPECIFIED
University of Texas at AustinUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Subject Keywords:galaxies: dwarf – galaxies: formation – cosmology: theory
Issue or Number:4
Record Number:CaltechAUTHORS:20210507-085546557
Persistent URL:
Official Citation:Francisco J Mercado, James S Bullock, Michael Boylan-Kolchin, Jorge Moreno, Andrew Wetzel, Kareem El-Badry, Andrew S Graus, Alex Fitts, Philip F Hopkins, Claude-André Faucher-Giguère, Alexander B Gurvich, A relationship between stellar metallicity gradients and galaxy age in dwarf galaxies, Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 4, March 2021, Pages 5121–5134,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109001
Deposited By: Tony Diaz
Deposited On:07 May 2021 17:34
Last Modified:07 May 2021 17:34

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