Enrichment of Heavy Elements
in Local Group Galaxies
Yutaka Hirai
RIKEN Center for Computational ScienceFirstStars

Formation and Evolution of Galaxies

e.g., SupernovaeElements

Image credits: NASA

Where are the astrophysicalsites of heavy elements?

How abundances of heavyelements are related to galaxyevolution?

α-element abundances in the Local Group galaxies

Tolstoy et al. 2009, ARAA, 47, 371-2[Fe/H]-10α-element abundances can be an indicator forgalactic chemical evolution

Observations of the neutron-capture elements[Sr/Fe][Ba/Fe][Eu/Fe]SAGA database (Suda et al. 2008; 2017)[Fe/H][Fe/H][Fe/H]

Scatters of [Sr, Ba, Eu/Fe] at [Fe/H] < −2.5
Astrophysical sites of neutron-capture
elements are not well understood.

Neutron starmergers canexplain Eu
abundances

Observation (SAGA database, Suda et al. 2008; 2017)
dSphs
Milky Way

Hirai et al. 2015, ApJ, 814, 41;
Hirai et al. 2017, MNRAS, 466, 2474;
Hirai 2019, Springer Theses

Ratios of light to heavy neutron-capture elements

Astrophysical sitesof light neutroncapture elements(e.g., Sr, Y, Zr) andheavy neutroncapture elements(e.g., Ba, Eu) may
be different

SAGA database (Suda et al. 2008; 2017)

Possible sites for light neutron-capture elements
Electron-capture supernovae (ECSNe)(Supernovae with low mass progenitors)©NASA

Neutron star mergers (NSMs)

This study

Rotating massive stars
e.g.,
Chiappini et al. (2011), Nature, 472, 454;
Cescutti & Chiappini (2014), A&A, 565, A51;
Prantzos et al. (2018), MNRAS, 476, 3432

The role of each site is not clarified
http://swift.gsfc.nasa.gov/

Purpose of this study
Clarify the astrophysical sites ofheavy elements
Put heavy elements as anindicator for galaxy evolution

Method
N-body/SPH code ASURA (Saitoh et al. 2008; 2009)
・Star Formation
・Cooling and Heating Function (Cloudy, Ferland et al. 2013)
・Supernova Feedback
・Chemical Evolution (CELib, Saitoh 2017)- Core-Collapse Supernovae, Hypernovae(yield: Nomoto et al. 2013, mass range: 8-100 Msun)-Electron-Capture Supernovae (yield: Wanajo et al.
2018, mass range: Doherty et al. 2015)

Neutron Star Mergers (yield: Wanajo et al. 2014)- Type Ia Supernovae (yield: Seitenzahl et al. 2013)-Isolated dwarf spheroidal galaxy modelGas and dark matter density profile：pseudo-isothermal profile(e.g., Revaz & Jablonka 2012, A&A, 538, A82)

Total mass of the halo:

7×108 M
Final stellar mass of the galaxy:3×106 M
Total number of particles:

3×105
Gravitational softening length:

7.8 pcHirai et al. 2015, ApJ, 814, 41

Enrichment of Sr
Electron-Capture
Supernovae +Neutron Star Mergers

s-process
ECSNe

NSM

Hirai et al. (2019) ApJ submitted• Main contributors on
the enrichment of Sr at[Fe/H] < −2.5
• ~ 20 % of total mass of
Sr is come fromECSNe and NSMs

[Sr/Ba] as a function of [Fe/H]
Ejecta fromECSNe produce
high [Sr/Ba] ratios

ECSNeNSMs
BH-NS orrotating massive stars

Hirai et al. (2019) ApJ submitted~ 2 % of stars has[Sr/Ba] > 1

Purpose of this study
Clarify the astrophysical sites ofheavy elements
Put heavy elements as anindicator for galaxy evolution

©Takayuki Saitoh, Takaaki Takeda

https://youtu.be/1qU̲I9R6CU8

[α/Fe] vs. [Fe/H]
Hirai et al. in prep

Mass resolution of oursimulation (~103 Msun)
is enough to resolve
satellite dwarf galaxies[Mg/Fe]20−2−4−2[Fe/H]

0Direct comparison withSubaru PFS data

Star-by-star simulation
Boötes I UFD

CELib
Saitoh 2017, AJ, 153, 85

Stellar mass, lifetime,yields, energyStar formation

ASURA

Pos, Vel, Acc

Direct N-body

Hydrodynamics+N-body (Tree)
Saitoh et al. 2008, PASJ, 60, 667;
Saitoh et al. 2009, PASJ, 61, 481

BRIDGE
Fujii et al. 2007, PASJ, 59, 1095

Pos

Simulations of galaxy formation
resolving individual starsConclusions• Electron-capture supernovae (ECSNe) canbe the main site of Sr at [Fe/H] < −3
• Neutron star mergers (NSMs) contribute to

the enrichment of both light (e.g., Sr) andheavy (e.g., Eu) neutron-capture elements• High-resolution cosmological zoom-in

simulations will connect the abundancesof heavy elements and evolutionary
histories of galaxies