Cosmological formation of stellar streams
宇宙論的シミュレーションに基づいたストリームの構造形成
Morinaga et al. 2019, MNRAS, 487, 2718

Yu Morinaga (Chiba University)
Collaborator: Tomoaki Ishiyama, Takanobu Kirihara
初代星・初代銀河研究会2019@名古屋

2BackgroundØRecent surveys have been discovering new faint stellar streams.
ØStructural properties of stellar streams depend on their progenitor
orbits and the environments of their host galaxiesØStellar streams are important clues to the formation histories andenvironments of galaxies
Milky Way

Andromeda

reamNW streamMonoceros stSsuirtatigamaerststreamOrph
an stream

GD-1

Belokurov+2008

Giant stream

G.Lewis+2013

Motivation

3

Many early studies…
ØN-body simulations have been extensively used
to reproduce the observed properties and thedynamical evolution of streams.

Hozumi et al. 2014 Fig.1

ØHowever in a cosmological context,
their dynamical evolution is affected bycomplicated physics…
For example✦dynamical friction✦multiple interaction✦evolution history of host galaxies

We need to investigate the formation histories of streams within

Cosmological context

This work – Cosmological N-body simulation
MW-sized halo4We use high-resolution and large cosmological simulations (Ishiyama et al)
Numerical
parametersofof cosmologicalcosmological simulations
Numerical
parameterssimulations

𝑵𝒑𝑳 [𝒉%𝟏 Mpc]

𝒎𝒑 [𝒉%𝟏𝑴⨀]

MW-sized halo

2048.

8.0

5.13×10.

94096.

16.0

5.13×10.

26

Various substructures in cosmological simulations

Substructures
Stream

We investigate the relationship between structural properties of substructuresat z =0 and orbits of their progenitors around MW-sized halos in a cosmologicalcontext
We also analyzed the relationship between the structural properties of streams(orientation of tidal tails) and environments of their host halos such as shapeof underlying gravitational potential.

Semi-analytic model
Particle Tagging

5A model to assign stellar mass
(Koposov et al. 2009)(De Lucia et al. 2008)

Dark matter halo
Dark matter halo𝒛𝐚𝐜𝐜

𝑴∗
𝒛=0?

=𝑓∗ ×(𝑀BCC − 𝑀EFG )+ 𝑓∗ ×𝑀EFG(1 + 0.26(𝑉CEGK /𝑉CGEC (𝑧BCC )).).

for 𝑉CGEC,E < 10 km s %S𝑴∗=𝑓∗ ×𝑀BCC(1 + 0.26(𝑉CEGK /𝑉CGEC (𝑧BCC )).).

Stellar component,where 𝑉CEGK = 30 km s %S , 𝑧EFG = 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⊙ .
5

Structural properties of substructures
Quantifying structuralproperties(Morinaga et al. 2019)

thin & short

𝒛=𝟎

6
thin & longhigh(thin)➊ LengthHigh⇒ longLow ⇒ shortthinness❷ Thinness
High ⇒ thin
Low ⇒ thick

thick & longthick & shortlow(thick)low(short)lengthhigh6(long)Orbits - Structures
length [kpc]longthinness

thin

7

pericenter [kpc]

10 kpc ≲ peri ≲ 100 kpc

50 kpc ≲ apo ≲ 300 kpc

apocenter [kpc] short

apocenter [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 percenter (> 100 kpc) ⇒ less affected by tidal force and keep gravitationally boundedstructurethin𝒛𝐚𝐜𝐜 ≲ 𝟎.5

high-z𝒛𝒂𝒄𝒄

Thinness

Short & thick

2.5≲ 𝒛𝐚𝐜𝐜

long & thick

thick

low-z

0.5≲ 𝒛𝐚𝐜𝐜 ≲2.5

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

long8 & thin

0.5≲ 𝒛𝐚𝐜𝐜 ≲2.5

𝒛𝐚𝐜𝐜 ≲ 0.5

2.5 ≲ 𝒛𝐚𝐜𝐜

Morinaga et al. 2019, MNRAS, 487, 2718

Morinaga et al. 2019, MNRAS, 487, 2718

Relationship between the major axis of host halos
11and tidal tails of streamsl Identifying the major axis of MW-sized haloes at z=0
l Picking up only streams from various substructures and Identifying the orientationof tidal tails by using the stellar particles of the streams at z=0l Quantifying the angle between the major axis of host and tidal tails of streams
Major axis of a host halo

z=0

Blue: tidal tails Red: major axis

Orientation oftidal tails

Structural evolution

Orbital evolution

Tidal tails

MW-sized halo1240% of all streams are aligned with the major axis of their host halosalignment

misalignment

Sample size:
282 streams around
26 MW-sized haloes
in a cosmological simulation
at z=040%
The orientation of the tidal tails
tends to be slightly aligned withthe tidal field of host haloes
The angle between tidal tails and major axis of host haloes

MW-sized halos

Streams around each MW-sized halo

13

The effect of eccentricity on stream alignmentalignment

14

misalignment

50％ of streams withhigh-eccentricity tend
to be aligned with
their host halos

Tidal tails of streams on
high-eccentricity orbits are
more strongly aligned withmajor axis of host halos

ー eccentricity > 0.55 (median)ー eccentricity < 0.55
ー all streams
The angle between tidal tails and major axis of host haloes

The effect of mass and accretion redshift on stream alignment 15
Progenitor Mass at 𝑧BCC
alignment
misalignment

ー 𝑀[GE >1.3×10\ 𝑀⨀
ー 𝑀[GE <1.3×10\ 𝑀⨀
ー all streams
Alignment angle

Accretion redshiftalignment
misalignment

ー 𝑧BCC < 1.5
ー 𝑧BCC > 1.5
ー all streams
Alignment angle

Streams with high-mass or low-𝒛𝐚𝐜𝐜 are
more strongly aligned with major axis

The effect of cosmic filament on stream alignment

16

Some properties of subhalos accreted intoa host halo along cosmic filaments● Their entry points tend to be aligned withthe major axis of their host halos● Their orbits tend to be more eccentric● Accretion redshift tends to be low● Masses tend to be high(Libeskind+2014,Gonzalez+2016, Veena+2018…)

In this work, streams originating in progenitorswith such properties are preferentially alignedwith major axis of their host haloes.
⇨ Cosmic filaments may affect the streamalignment with shape of their host haloes
The 𝜈bGC Simulation, performed by Ishiyama + 2015

Summary17

We can infer the evolution of properties of stellar streams in terms ofaccretion redshift, orbital parameters and the relationship withenvironments of their host haloes in cosmological simulations.
✦ A large part of streams (~𝟗𝟎%) is accreted by their hostwithin 0.5≲ 𝒛𝐚𝐜𝐜 ≲2.5.
✦ Streams are concentrated in the narrow region ofpericenter (10-100 kpc) and apocenter (50 – 300 kpc) plane
Morinaga et al. 2019, MNRAS, 487, 2718

✦ 40% of all streams are aligned with the major axis of their host halos✦ Streams with high-eccentricity, high-mass or low- 𝒛𝐚𝐜𝐜 are
preferentially aligned with the major axis of their host halos※𝒛𝐚𝐜𝐜 ⇒ The redshift when the progenitor of substructure is accreted into their host