Prospects of Newly Detecting Nearby Star-Forming Galaxies by the Cherenkov Telescope Array

Prospects of newly detecting nearby star-forming galaxies by the Cherenkov Telescope Array

Naoya Shimono (Univ. of Tokyo)

Co-researchers: Tomonori Totani Takahiro Sudoh

1 Star-forming galaxy as a gamma-ray emitter

Galactic emissivity ■ Galaxies with star-forming activity (star-forming galaxies, SFGs) are gamma-ray emitters. Cosmic rays (CRs) accelerated by supernovae collide with ISM, and produce a pion. Then a pion produces photons. p + p → �! , �! → � + � ■ This process is dominant component of galactic diffuse emission

Abdo et al. 2009 2 Detection of gamma-ray from SFGs

■ Fermi-LAT detected GeV gamma-ray from nearby galaxies: LMC, SMC, M31, NGC 253, M82, NGC 4945, NGC 1068, NGC 2146, Arp 220, Arp299, M33 (Abdo et al. 2010, Ackermann et al. 2012, Tang et al. 2014, Peng et al. 2016, Ajello et al 2002, Xi et al 2020) ■ H.E.S.S. and VERITAS detected TeV gamma-rays from NGC 253, and M82 (Acero et al. 2009, VERITAS Collaboration et al. 2009) ■ These detection proves that gamma-ray from SFGs is ubiquitous.

3 Cherenkov Telescope Array Project Differential sensitivities of CTA and other telescopes ■ More detections in TeV are expected by Cherenkov Telescope Array (CTA). ■ Large Sample of SFGs in GeV-TeV will lead to the understanding of production and propagation of CRs in diverse galaxies. ■ It is important to prepare theoretical prediction for gamma-ray luminosities and spectra of nearby SFGs.

https://www.cta-observatory.org

4 Modeling in Past Studies ■ Modeling of SFGs:

l Simply relate some of physical quantities (for many studies, one or two) of the galaxy with gamma-ray luminosity Torres 2004; Domingo-Santamaria & Torres 2005; Persic et al. 2008; Lacki et al 2010, 2011; Inoue 2010; Wang & Fields 2018 l Microphysical parameters are introduced as free parameter, and the these are fit to reproduce the observed gamma-ray luminosity of a galaxy. Yoast-Hull et al. 2013, 2014; Eichmann & Becker Tjus 2016; Peretti et al. 2019; Krumholz et al. 2020 l Simulations, though it is not convenient to predict for many galaxies Martin 2014, Pfrommer et al. 2017, Chan et al. 2019 ■ For the theoretical prediction of large sample of galaxies yet to be detected, the approach to simply relate some physical quantities of a galaxy with gamma-ray luminosity is appropriate.

5 The aim of this work

■ Model constructed by Sudoh et al. 2018 (S18) Using four input parameters of a galaxy [Star-formation rate (SFR) (�), gas mass (�gas), stellar mass (�star), and effective radius (�eff)] This model can explain the observed luminosities of nearby galaxies better than the model using only SFR and/or gas mass.

■ The aim of this study: Applying S18 model to nearby galaxies that are not yet detected in TeV, we predict their gamma-ray emission properties and discuss the relation to galactic physical parameters.

6 The overview of our model ■ Proton spectrum �� E = � �� M⊙ yr GeV � is determined by the fitting with Fermi-LAT observation

■ Probability of collision with ISM, �(�), is determined from collision timescale (�) and escape timescale (�) :i.e. � � = 1 − exp − � ■ Escape timescale is minimum of advection timescale and magnetic diffusion timescale ■ Advection timescale is estimated by gravitational equilibrium along the perpendicular direction to the disc surface. ■ Diffusion coefficient: � �� + � ≤ � � 3 � � � � = �� > � � 3

�: Larmor radius �: coherent length of turbulence �: scale height of a galaxy 7 Physical quantities for nearby GeV-detected SFGs

Objects � �� (0.1−100GeV) SFR Mgas Mstar Reff (Mpc) � � � � (erg s ) (M⊙ yr ) (�� M⊙ ) (�� M⊙ ) (kpc) MW 0.82 ± 0.27 2.6 4.9 50 6.0 LMC 0.05 0.047 ± 0.005 0.30 0.59 1.8 2.2

SMC 0.06 0.011 ± 0.003 0.043 0.46 0.30 0.7

NGC 253 3.5 12 ± 4 3.3 3.2 54.4 0.5

M82 3.3 14 ± 3 4.4 4.7 21.9 0.9

NGC 2146 17.2 51 ± 27 11.4 10.4 87.1 3.0

n Energy range of NGC 2146 is 0.2--100 GeV n SFR is Derived from Infrared luminosity and Hα luminosity assuming Salpeter IMF. n �gas is sum of neutral hydrogen and molecular hydrogen, derived from 21-cm line flux and CO(1-0) line flux respectively .

8 Fitting to nearby SFGs

■ � is the only parameter determined by the fitting. ■ Our model can explain observed luminosities of nearby galaxies better.

Our model vs. simple assumption (� ∝ � and � ∝ �M)

1042 1042

observed NGC 2146 observed 41 41 10 our model (Ginj=2.2) 10 our model (Ginj=2.2) ) ) M82 Lg µ y M82 Lg µ yMgas -1 -1 40 1.58 40 0.99 10 Lg µ y NGC 253 10 Lg µ (yMgas) NGC 253 NGC 2146

1039 1039

MW MW 1038 M33 1038 (0.1-100 GeV) (erg s (0.1-100 GeV) (erg s SMC g g L L M33 LMC LMC 1037 1037 SMC 1036 1036 10-1 100 101 10-2 10-1 100 101 102 y (M yr-1) yM (109 M 2 yr-1) sun gas sun 9 Test of our model in TeV energy range

n GeV-TeV gamma-ray spectra of NGC 253 and M82 fit well when Γinj ∼ 2.2 n Our model works well both in GeV and TeV

NGC 253 -11 M82 10 10-11

Fermi-LAT (l,b)=(120°,45°) TS=25, 0.25 dex bin ) )

-1 VERITAS 50h H.E.S.S. 50h -1

s -12 10 5s, 0.2 dex bin s -12 5s, 0.2 dex bin -2 10 -2 (erg cm (erg cm g g /dE /dE g -13 g 10 Ginj=2.0 -13 G =2.0

dF 10 inj dF g 2.1 g 2.1 E 2.2 E 2.2 2.3 2.3 2.4 2.4 Fermi-LAT Fermi-LAT H.E.S.S. VERITAS -14 -14 10 -4 -3 -2 -1 0 1 2 10 10 10 10 10 10 10 10 10-4 10-3 10-2 10-1 100 101 102 E (TeV) g Eg (TeV) 10 Galaxy Sample

■ 59 galaxies in KINGFISH (Key Insights on Nearby Galaxies: a Far-infrared Survey with Herschel) Galaxy catalog aimed to understand physical process of star-forming activity and ISM SFR, �gas, and �star of these galaxies are compiled (Remy-Ruyer et al. 2014, 2015)

We apply our model to 59/61 galaxies in KINGFISH catalog. (2 galaxies, NGC 1377 and NGC 1404 are omitted due to lack of information.) ■ Other 8 galaxies Because KINGFISH is not a complete catalog of nearby galaxies, and we might miss galaxies whose emissions can be detected by the CTA, we add 8 galaxies based on their SFRs and distances. Among these galaxies, M33 and NGC 2043 have been already detected by Fermi-LAT observations The emission of NGC 2403 perhaps originate from not diffuse emission, but SN 11 ■ Estimate of 1 TeV luminosity from SFR: � 1 TeV � Our model predictions ∼ 9.1×10 erg s M⊙ yr ■ Estimate of 3 GeV luminosity from SFR: • Our model fluxes are typically lower than simple �" ∝ � � 3 GeV � ∼ 5.8×10 estimates by a factor of 10 for 1 TeV and 6 for 3 GeV erg s M⊙ yr These coefficients are determined based on NGC 253

12 Predicted spectrum Example of derived spectrum

■ ~2-3 times lower flux than CTA NGC 5236 -11 sensitivity (per energy bin) will be 10 detectable considering the condition of detection of NGC 253 Fermi-LAT (l,b)=(120°,45°) -12 TS=25, 0.25 dex bin and M82 ) 10 -1 s

-2 CTA South 50h 5s, 0.2 dex bin

■ Galaxies with high possibilities of 10-13 (erg cm TeV detection by CTA : g /dE g NGC 5236, M33 NGC 6946, G =2.0 dF inj g

IC 342, … E 10-14 2.1 2.2 2.3 2.4 Fermi-LAT 10-15 10-4 10-3 10-2 10-1 100 101 102 Eg (TeV)

13 Hardness of galaxies

■ NGC 1482 and DDO 165 have smaller � − Γ#$% value compared with other galaxies ■ If NGC 1482 has hard spectrum, and there is a chance of detection. ■ DDO 165 has no chance of detection (too faint) NGC 1482 40 10-11 NGC 5236 10-11 35 Histgram of � − Γ

-12 30 ) 10 Fermi-LAT (l,b)=(120°,45°) �: fitting slope of -1

s -12 TS=25, 0.25 dex bin ) 10 -2 25 0.1-100 TeV spectrum -1 s

-2 CTA South 50h Γ : proton spectrum index 20 10-13 5s, 0.2 dex bin (erg cm g

Number 10-13 (erg cm /dE g 15 g /dE dF g g

10 E -14 G =2.0 dF 10 inj g

E 10-14 2.1 5 2.2 2.3 NGC 1482 DDO 165 2.4 0 10-15 Fermi-LAT 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 -4 -3 -2 -1 0 1 2 -1510 10 10 10 10 10 10 a-Ginj 10 -4 -3 -2 Eg (TeV)-1 0 1 2 10 10 10 10 10 10 1014 Eg (TeV) Relationships between gamma-ray luminosities and other physical quantities (1)

n The relationships between � 1 TeV /� and surface gas density (Σ) or �gas are investigated. n If cosmic-ray protons are completely confined in a galaxy, �(1 TeV)/� is expected to be a constant. This trend is seen for Σ region (NGC 1482 and DDO 165) n The relationship between �(1 TeV)/� and �gas has a large dispersion, which indicates a limitation of using ��gas an an indicator of gamma-ray luminosity.

NGC 253 -1 -1 ) )) NGC 253 M82 10 M82 DDO 165 10 -1 -1 yr NGC 2146 yr

sun NGC 2146 NGC 1482 sum /M /M -1 -2 -1 -2 M33 10 M33 10

erg s SMC MW erg s

39 SMC 39 MW (10

LMC (10 y y LMC 10-3 10-3 (1 TeV)/ (1 TeV)/ g g L L

10-4 10-4 10-3 10-2 10-1 100 101 10-2 10-1 100 101 -2 9 S (g cm ) Mgas (10 Msun) 15 Relationships between gamma-ray luminosities and other physical quantities (2) ■ Same figures as the previous slide, but this time gamma-ray luminosities are at 3 GeV.

■ �"(3 GeV)/� becomes flat at lower Σ than TeV case, which is expected because protons responsible for GeV gamma rays are more confined. ■ Roughly consistent with the study by Lacki et al. (2011)

100 100 NGC 253 ) NGC 2146 M82 ) M82 NGC 2146 -1 -1 NGC 253 yr yr sun sun /M /M -1 NGC 1482 -1 10-1 10-1 MW DDO 165 SMC erg s erg s MW 39 LMC SMC 39 M33 LMC M33 (10 (10 y y 10-2 10-2 (3 GeV)/ (3 GeV)/ g g L L

10-3 10-3 10-3 10-2 10-1 100 101 10-2 10-1 100 101 -2 9 16 S (g cm ) Mgas (10 Msun) The origin of GeV emission form NGC 2403 (1)

■ GeV gamma-ray detected from the direction of NGC 2403 is proposed to originate from SN 2004dj. ■ Flux decreases with time ■ TS peak coincidence with SN 2004dj

TS map for the first 5.7 years

TS map for the second 5.7 years

All figures are taken from Xi et al. 2020 17 The origin of GeV emission from NGC 2403 (2)

NGC 2403 ■ Our model spectrum cannot explain -11 10 NGC 5236 observed GeV emission of NGC 2403 10-11 ■ This result support the conclusion -12 Fermi-LAT (l,b)=(120°,45°) that GeV detection from NGC 2403 ) 10 -1 -12 TS=25, 0.25 dex bin ) 10 s originates from SN 2004dj -1 -2 s

-2 CTA South 50h 5s, 0.2 dex bin 10-13

(erg cm -13 SN model spectrum g 10 (erg cm g /dE g /dE g dF

g G =2.0 dF inj g

E -14

E 10 10-14 2.1 2.2 2.3 2.4 Fermi-LAT -15 10 -4 -3 -2 -1 0 1 2 10-4 1010-3 1010-2 1010-1 10100 10110 10210 E (TeV) Eg (TeV)g Observation data is taken from Xi et al. 2020

Xi et al. 2020 18 Summary

■ SFGs are gamma-ray emitters, and CTA is expected to detect TeV gamma rays from nearby SFGs. ■ Understanding GeV--TeV emission mechanism of SFGs leads to understanding of production and propagation of CRs in diverse galaxies. ■ We apply S18 model to nearby galaxies mainly from KINGFISH catalog. ■ NGC 5236, M33, NGC 6946, and IC 342 are expected to be detected in TeV with high possibilities, and NGC 1482 might be detected if it has hard spectrum. ■ Gamma-ray emission has significant dependence on surface gas density of a galaxy.