Diﬃculties of Star-forming
Galaxies as the Source ofIceCube Neutrinos
Takahiro Sudoh (Univ. of Tokyo)
Tomonori Totani (UTokyo), Norita Kawanaka (Kyoto Univ.)

accepted for publication in PASJ
マルチメッセンジャー天文学研究会@千葉大学 26 March 2018

This Work•A new model of gamma-ray and neutrino from a starforming galaxy from SFR, Mstar, Mgas, radius•This model well reproduces gamma-ray luminosities ofnearby galaxies detected by Fermi•

This model is combined with a cosmological galaxyformation model to predict background radiation•Majority of the IceCube neutrinos cannot be explainedby star-forming galaxies

1/17

Introduction

IceCube neutrinos from star-forming galaxies•

Star-forming galaxies emit neutrinos via pion decay•

Many previous studies (Loeb & Waxman 06, Thompson+06, Stecker 07,
Lacki+11, Murase+13, He+13, Tamborra+14, Anchordoqui+14, Liu+14, Emig+15,
Chang+15, Giacinti+15, Senno+15, Maharani & Razzaque 16, Chakrabortty &Izaguirre 16, Xiao+16, Bechtol+17)•lack of consensus (less than 10% ? more than 50% ?)•mainly rely on empirical relations•argument sensitive to assumed spectral index

2/17

GeV gamma-rays from star-forming galaxies•Star-forming galaxies emit gamma-rays via pion decay•

Many previous studies (Strong+76, LichtiL78, Dar & Shaviv 95, Pavlidou& Fields 02, Thompson+07, Ando & Pavlidou 09, Fields+10, Makiya+11, Stecker &Venters 11, Ackermann+12, Chakraborty & Fields 13, Lacki+14, Lamastra+17)•

4-23% of Isotropic Gamma-ray Background (estimateby Fermi team)•mainly rely on empirical relations, which may
introduce signiﬁcant bias (Komis+17)

3/17

High-energy emissions from star-forming galaxies•What we need for better prediction is…
•

Physically-motivated model of gamma-ray & neutrinoemission from a star-forming galaxy•

Gamma-ray constraints from nearby galaxies•

Cosmological evolution of galactic properties such as
SFR, mass, size from realistic galaxy formation theory.

•How much contribution galaxies can make to diﬀuse
gamma-ray (Fermi unresolved) and neutrino (IceCube)
background ?

4/17

Modelling Gamma-rayand Neutrino Emissionfrom Galaxies

Modelling gamma-ray and neutrino from a galaxy•Input : SFR, Mgas, Mstar, Reﬀ, H (Reﬀ)•CR production rate : SFR•

CR spectrum at injection :•pp interaction rate : target ISM gas density (•escape time from galaxy : advection or diﬀusion)•

outﬂow velocity ( ) : escape velocity from galactic disk•

diﬀusion coeﬃcient : from Larmor radius () andﬂuctuation pattern of turbulent magnetic ﬁeld•

magnetic ﬁeld : equipartition with energy injected by SNe5/17

Cosmic ray diﬀusion in galaxy•

Diﬀusion coeﬃcient : from Larmor radius and ﬂuctuation
pattern of turbulent magnetic ﬁeld•

Kolmogorov-type turbulence is assumed•

Coherent length l0 : set to 30 pc from MW observation
6/17

Estimate of magnetic ﬁeld in galaxy•

magnetic ﬁeld : equipartition with energy density injectedby supernovae
SN energy~10^51 erg••

SN rateadvection time

Salpeter IMF assumed
to reproduce B = 6μG in the Milky Way
7/17

Modelling gamma-ray and neutrino from a galaxy•

Input : SFR, Mgas, Mstar, Reﬀ, H (Reﬀ)•

CR production rate : SFR•

CR spectrum at injection :•pp interaction rate : target ISM gas density (•escape time from galaxy : advection or diﬀusion)•outﬂow velocity ( ) : escape velocity from galactic disk•

diﬀusion coeﬃcient : from Larmor radius () andﬂuctuation pattern of turbulent magnetic ﬁeld•

magnetic ﬁeld : equipartition with energy injected by SNe
8/17

Calibration of model•

Nearby galaxies
detected in gammaray•

Note: NGC4935,NGC1068, Arp220
and M31 are not usedin this work

Model prediction Lγ(C)Fix the free parameter C•

The value of C is converted to ECR ~ 0.2 ESN
9/17

Comparison with data of nearby galaxies•

Model agrees data well (better than simple powar-laws)•

Only one free parameter is included in the model
10/17

Modelling gamma-ray and neutrino from a galaxy•

Now we can calculate gamma-ray & neutrino spectrum
from a galaxy using SFR, Mstar, Mgas, Reﬀ•

Combine this model with cosmological galaxy formation
model to predict background radiation•

Before that, we constrain Γinj from emission from theGalactic disk

11/17

Emission fromGalactic Disk

Emissions from the Milky Way disk•

Observation: MW disk is not dominant (no anisotropy)•Γinj <~ 2.2 or cutoﬀ below E~10^14 eV is necessary

12/17Cosmological
Background Radiation

Semi-analytic galaxy formation model

Cosmological parameter🌟🌟🌟gas cooling• Starburst
merger (5%dark halooccurs at galaxystar
ガス冷却・星形成
formation
ダークハローの形成
formation to CSFRD
gas
heatingby SNecontribution
at z=0)超新星でのガス加熱

🌟

銀河合体
mergers
galaxyevolution
化学進化

Properties of galaxies(SFR, Mstar, Mgas, size etc.)
13/17

Semi-analytic galaxy formation model🌟
🌟

🌟
🌟

•

Mitaka model (Nagashima & Yoshii 2004)•

reproduce observations of local and high-z galaxies(luminosity functions, luminosity-density, etc.)•

starburst occurs at galaxy merger (5% contribution to
CSFRD at z=0)
14/17

Gamma-ray background•including cascade emission•

Star-forming galaxies makeabout 20% contribution toisotropic gamma-ray
background (0.1-100 GeV)•

consistent with estimate byFermi-LAT (4-23%,Ackermann+12)

15/17

Neutrino background•Starburst galaxies dominate
at TeV-PeV•

only 0.5% for Γinj = 2.3•

22% even if Γinj = 2.0(extremely optimistic)•

majority of IceCube datacannot be explained by starforming galaxies

16/17

Summary•

A new model of gamma-ray and neutrino from SFG fromfour physical quantities: SFR, Mstar, Mgas, Reﬀ•

This model well reproduces Lγ of nearby galaxies & canbe tested by future TeV observation (CTA!)•This model is combined with a cosmological galaxy
formation model to predict background radiation•SFGs make ~20% of isotropic γ-ray background and 22%of the IceCube ﬂux for most optimistic case (Γinj = 2.0)
based on gamma-ray constraints of nearby galaxies

17/17