Statistical properties of substructures around MW-sized halos
and their implications for the formation of stellar streams

Yu Morinaga (Chiba University, M2)
Collaborators: Tomoaki Ishiyama, Takanobu Kirihara, Kazuki Kinjo (Chiba University)

Based on:Yu Morinaga et al, 2019 (accepted, MNRAS, arXiv 1901.04748)
6th Galaxy evolution workshop 2019 @ KAVLI IPMU - June 5thBackground – Substructures around galaxies

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国立天文台4次元デジタル宇宙プロジェクト

 Galaxies are formed via hierarchical mergers
 Tidally disrupted remnants including stellar streams are formed
around galaxies

Background – Substructures around galaxies

3Recent surveys have been discovering new faint stellar streams.

Tidally disrupted remnants including stellar streams inform us theiraccretion history.
Stellar streams are important clues to galaxy formation history
Milky Way

Andromeda
NW stream

Giant stream

Belokurov+2008

G.Lewis+2013

Motivation

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Many early studies…

Hozumi et al. 2014 Fig.1

N-body simulations have been extensively used to reproduce
the observed properties and the dynamical evolution of them.
However in a cosmological context,
their dynamical evolution is affected by complicated physics…
For example✦dynamical friction✦multiple interaction✦evolution history of host galaxies

We need to investigate the relationship between structuralproperties of substructures and orbits of their progenitors within

Cosmological context

This work – Cosmological N-body simulation
We use a high-resolution
cosmological simulation

5over 24,000 samples(Ishiyama et al. 2016)

 Particles: 𝟐𝟎𝟒𝟖𝟑
 Boxsize: 8 Mpc/h
 𝒎𝐩 : 5.13 × 𝟏𝟎𝟑 Msun/h Softening: 120 pc/h
 Nine MW-sized halos

We investigate the relationship between structural properties of substructures at z =0
and orbits of their progenitors

Semi-analytic model

6Particle Tagging

A model to assign stellar mass

(De Lucia et al. 2008)(Koposov et al. 2009)

Dark matter halo
Dark matter halo𝒛𝐚𝐜𝐜

𝑴∗
𝒛=0?

=𝑓∗ × (𝑀acc − 𝑀rei )+ 𝑓∗ × 𝑀rei
(1 + 0.26(𝑉crit /𝑉circ (𝑧acc ))3 )3for 𝑉circ,r < 10 km s−1𝑴∗=𝑓∗ × 𝑀acc
(1 + 0.26(𝑉crit /𝑉circ (𝑧acc ))3 )3Stellar component,where 𝑉crit = 30 km s −1 , 𝑧rei = 11𝒛𝐚𝐜𝐜 ⇒ The redshift when the progenitor ofsubstructure was accreted into the host halo

We reproduce the observed stellar mass
We treat 10 % of the most-bound DM
functions in the MW and M31.
particles in a progenitor halo at 𝒛𝐚𝐜𝐜 as theWe analyze the substructures withstellar component and trace it down to z=0.
𝑴∗ > 𝟏𝟎𝟒 𝑴⊙ .
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Structural properties of substructures
𝒛=𝟎
Quantifying structuralproperties with

5Stellar particle

thin & shortthin & longthick & shortthick & longhigh(thin)➊ LengthHigh⇒ longLow ⇒ shortthinness❷ Thinness
High ⇒ thin
Low ⇒ thicklow(thick)low(short)lengthhigh(long)

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Resultthin

〇 Stream× Other substructureshigh-z𝒛𝐚𝐜𝐜 ≲ 𝟎.5

𝒛𝒂𝒄𝒄

Thinness

Short & thick

2.5≲ 𝒛𝐚𝐜𝐜

long & thick

0.5≲ 𝒛𝐚𝐜𝐜 ≲2.5

thick
low-z

Length[kpc] by their hosts within
A large part of streams (~𝟗𝟎%)
is accreted
0.5≲ 𝒛𝐚𝐜𝐜 ≲2.5

long9 & thinOrbits - Structures

11thin10 kpc ≲ peri ≲ 100 kpc〇 Stream× Other substructures

50 kpc ≲ apo ≲ 300 kpc

apocenter [kpc] shortthinness

length [kpc]

pericenter [kpc]longapocenter [kpc] thick✦It is clear that substructures observed like streams are concentrated in the rather narrow
region of the pericenter (10-100 kpc) and apocenter (50-300 kpc) plane.
✦small pericenter and apocenter (< 10 kpc) ⇒ largely disrupted because of strong perturbation✦large pericenter (> 100 kpc) ⇒ less affected by tidal force and keep gravitationally boundedstructureSummary

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We can infer the evolution of properties of substructures in terms of accretion
redshift and orbital parameters by using cosmological simulation.

✦A large part of streams (~90%) is accreted by their host within

0.5≲ 𝒛𝐚𝐜𝐜 ≲2.5.
✦ Streams are concentrated in the narrow region ofpericenter (10-100 kpc) and apocenter (50 – 300 kpc) plane✦ Substructures with high-𝑧acc (> 2.5) suffer from strong tidal forces⇒ They are entirely disrupted and their stellar components are well-mixed✦ Substructures with low-𝑧acc (< 0.5) are less affected by the tidal forces and keep
larger pericenter and apocenter⇒ They also keep their gravitational bound structures※𝒛𝐚𝐜𝐜 ⇒ The redshift when the progenitor of substructure is accreted into their host