The Star Formation Propertiesof Merging Galaxies
at 0.3<z<2.5
Andrea Silva (NAOJ)
andrea.silva@nao.ac.jp

Silva et al. 2018, ApJ 868 46S
Danilo Marchesini (Tufts Univ.), John Silverman (Kavli IPMU),
Ros Skelton (SAAO), Daisuke Iono (NAOJ), Nick Martis (Tufts Univ.), CemileMarsan (York Univ.), Ken-Ichi Tadaki (NAOJ), Gabriel Brammer (Cosmic DawnCenter), Jeyhan Kartaltepe (Rochester IT)

Why study high-z galaxy mergers?
❖ The merging of two galaxies with similar mass(major mergers) can make profound changes inthe morphology and the properties of galaxies

c) Interactiond) Coalescencee) Blowoutg) Decay
SFR [Msun/yr]b) Early-stagef) Quasara) Isolated Disk-2-1012h) DeadElliptical

Time Relative to Merger [Gyr]Hopkins+08

Why study high-z galaxy mergers?
❖ The merging of two galaxies with similar mass(major mergers) can make profound changes inthe morphology and the properties of galaxies❖ Their fraction seem to increase with redshift

Most of studies of major mergers at z>1 have focus
on global statistics (e.g. merger rate) and not
impact on properties

Major mergers have been studied in
depth in local Universe:
Local mergers show higher Star Formation activity
than non-merging galaxies (Darg+10, Jogee+09,Robaina+09, Ellison+13, Kampczyk+13)

Is the same at high redshift?

Method to Select Mergers
Lackner et al. 2014
A. Early-Stage

B. Pre-coalescence

C. Post-Merger

Selection of galaxy
pairs withseparations <100kpc

We select galaxies with
two intact nuclei
separated by few kpc(just beforecoalescence).

Selection of galaxies
with disturbed
morphologiesBright regions in an image

Restrictions onproperties toselect mergers

H160

Mstar>1010Msun

Apply method to near-IR
HST/F160W images❖ Selection of mergers in rest-frame opticalout to z=2.5❖ C e n t e re d o n ~ 5 7 0 0 g a l a x i e s w i t h
Mstar>1010Msun and 0.3<zbest<2.5 in CANDELS(COSMOS, AEGIS, GOODS-N, GOODS-S,UDS) fields

Match selected regionswith 3D-HST catalogs tofind properties

Separate mergers
from line ofsight contaminantsFinal Sample of Mergers❖ Sample: 130 merging systems❖ Major Mergers constructed using acut in stellar mass ratio

3-15kpc❖Projected distance 3-15 kpc❖ 0.3<z<2.5

Use UVJ colors to separate galaxies by type

[U-V]

Quiescent
Dusty
SF

UnobscSF[V-J]

Use UVJ colors to separate galaxies by type
Separate Galaxies[U-V]

Quiescent
Dusty
SF

UnobscSF

Quiescent
(36%)

[V-J]

Star-forming
dusty (42%) or
unobscured(22%)

Separate Mergers
Wet (53%)

Mixed (22%)

Dry (25%)

Examples of Merger Types

Examples of Merger Types

Examples of Merger TypesDefinition of Starburst

SFR(Msun/yr)

5.
0at
Srurbniastscneueq

foe

FSalgaSMehT

Mass (Msun)

sexixedStarbursts
Star-forming
merginggalaxies—— MS fit Whitaker+14

Comparison SF activity in Mergers & non mergers

At masses log(M★/M⦿)>10

KS test: No
difference
12% of the merger
sample are
starbursts
All the starbursts
are dusty SF
galaxies and are
in wet mergers
High fgas?

At masses log(M★/M⦿)<10

KS test: Different
populationHigher fraction ofstarbursts (20%) inmerging galaxies
Higher enhancement
in SF activity in low
mass merginggalaxies

Major mergers have been studied in
depth in local Universe:
Local mergers show higher Star Formation activity
than non-merging galaxies (Darg+10, Jogee+09,Robaina+09, Ellison+13, Kampczyk+13)

Is the same at high redshift?
Depends

Conclusions
Star formation enhancement might depend on propertiesof merging galaxies

Galaxies with higher gas/dust content andthose with lower masses might increase
their SF activity even before coalescence

Silva et al. 2018, ApJ 868