Extragalactic masers��Paola Castangia�INAF-Osservatorio Astronomico di Cagliari�Extragalactic Radioastronomy Group�https://www.oa-cagliari.inaf.it/ricerca/ricerca-scientifica/scienza/radioastronomia-extragalattica/

Special thanks to:

A. Tarchi and G. Surcis

Outline

Introduction

• Astronomical masers
• Galactic and extragalactic masing molecules
• Masers as astronomical tools

H₂O masers

• Maser in star forming region
• Maser in AGN (disk-masers, jet/outflow-masers)
• mm/sub-mm masers

OH masers

Take home messages and Future perspectives

Astronomical Masers

Microwave Amplification by Stimulated Emission of Radiation

Astronomical Masers�Conditions for maser emission

• Non-equilibrium state

Low density, N < N_C (= critical density)

N_C = 10¹¹ cm^-3 for the H₂O molecules (very low on Earth)

The interstellar medium is not in equilibrium

N << N_C 🡪 population inversion n₂ > n₁

🡪 MASER effect is a natural phenomenon!

• Pump mechanism

Creates and maintains the population inversion (n₂ > n₁)

The pump mechanism can be collisional or radiative

(for H₂O masers: collisions with H₂ molecules)

Reid & Moran 1988, Elitzur 1992, Gray 2012

Astronomical Masers�Conditions for maser emission

• Input source

Backgound radiation or

Spontaneous emission inside the maser cloud

• Large dimension of the path length

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Dimensions of the clouds ~10¹³ cm

• Velocity coherence

Reid & Moran 1988, Elitzur 1992, Gray 2012

Astronomical Masers�History

Astronomical Masers�History

The first astronomical was discovered in 1965 toward a star forming region in the Milky Way by H. Weaver

(Weaver et al 1965)

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• Peculiar line ratios
• Very small linewidths
• High T_B
• Highly polarized

OH MASER!

Astronomical Masers�Masing molecules

Common species

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OH (Hydroxyl)

ν = 1612, 1665, 1667, 1720 MHz 🡪 λ ~ 18 cm

Milky way: high mass star forming regions, supernova renmants, evolved stars

External galaxies: LIRGs or ULIRGs, nuclear starburst regions or AGN

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H₂O (Water)

ν = 22.235 GHz 🡪 λ ~ 1.3 cm

Milky way: star forming regions, evolved stars

External galaxies: starburst galaxies and AGN

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Astronomical Masers�Masing molecules

Common species

SiO (Silicon monoxide)

ν = 43 and 86 GHz 🡪 λ ~ 0.7 cm and 3.5 mm

CH₃OH (Methanol)

ν = 6.7 and 12.2 GHz 🡪 λ ~ 4.5 cm and 2.5 cm

In the Milky Way and in external galaxies

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Rare species

H₂CO (Formaldehyde)

ν = 4.8 and 14.5 GHz 🡪 λ ~ 6.2 cm and 2.0 cm

NH₃ ,HCN, SiS, CO,CO₂, HC₃N, CH, CS

Masers as astronomical tools

• Properties of astronomical masers:
• High brigthness temperatures (T_B~10¹² K)
• Small size of individual spots (≤10¹⁴ cm)
• Narrow linewidths (few km/s)
• They can be mapped at mas resolution with VLBI
• From Doppler shift we can infer the velocity of the gas
• The allow us to study the structure and dynamics of the emitting gas in a variety of astrophysical environments:
• Star forming regions, SNRs, and evolved stars in the Milky Way
• Nuclear and off-nuclear extragalactic star forming regions
• AGN
• The brightest and most common extragalactic masers

🡪 H₂O and OH

Water masers

Gray 2012

Water masers

• Star forming regions:
• Shock-excited filaments in the protostellar outflows

(Elitzur et al. 1989) L_H2O ~ 10^-4 L_sun but can reach also 1 L_sun (W49N)

They are collisionally pumped and form in “dense” (10⁷- 10¹¹ cm^-3) and “warm” (300 – 1000 K) gas

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In the Milky Way they can be found in:

• Evolved stars:
• Shock-excited gas in the expanding evelope of red giants or supergiants (L_H2O ~ 10^-6 L_sun, can reach also 10^-4 L_sun)

Extragalactic H₂O masers

• ~ 200 masers in more than 5000 galaxies observed
• So far they have been found mostly in radio quiet AGN (Seyfert 2 or LINER) in the local Universe (z < 0.06) with few exceptions (3C403 at z=0.06, SDSS J0804+3607 at z=0.66, MGJ0414+0534 at z=2.64)
• Can reach 30,000 L_sun (!!!) and can be associated with:

Active Galactic Nuclei (AGNs)

(accretion disks, radio jets, nuclear outflows)

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Megamasers

L_H2O >10 L_sun

Excitation mechanism:

X-ray heating in the accretion disk

(Neufeld et al. 1994)

Star forming regions

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Kilomasers

L_H2O < 10 L_sun

Excitation mechanism:

Shocks in protostellar outflows

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(e.g. M 33, IC 10, IC 342, NGC 2146)

Star formation: M 33

VLBA measurements of proper motions (Brunthaler et al. 2005)

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Comparing the relative angular motion with the expected linear motion one derives:

And from maser proper motions one can derive the proper motion of M33:

Disk-masers

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3 groups of water maser lines

• systemic lines
• high-velocity lines

Disk-masers

Maser clouds are rotating around a mass of:

~ 3.6 x 10⁷ M_sun

Enclosed in a region of:

~ 0.13 pc

SMBH!

(Miyoshi et al. 1995, Nature)

NGC 4258

D_N4258 = 7.2±0.5 Mpc

M_BH = (3.9±0.3) × 10⁷M_SUN

(Herrnstein et al. 1999, Nature)

Disk-masers

High-velocity lines (tangential points)

V_HV = V_R

Systemic lines (l.o.s to the core)

dV_S/ dt =V_R²/ R_S

If R_S=R_HV single-dish observations can give an estimate of the disk radius!

Disk-masers

• PV diagram 🡪 θ_S,V_R
• Velocity drift 🡪 R_S

D = R_S / θ_S

D_U3789 = 49.9±7.0 Mpc

M_BH = 1.09 × 10⁷M_SUN

(Braatz et al. 2010)

Disk-masers

3D structure of accretion disks

• Warps, height of the disk

SMBH masses

• L_edd, ε, M-σ relation

🡪 AGN models, coevolution of SMBHs and host galaxies

Geometric distances of galaxies

• Calibration of the standard candles, H₀

H₀ =73.9±3.0 km s^-1 Mpc^-1 (Pesce et al. 2020, MCP)

• Test of cosmological models

(for recent reviews: Greenhill 2007, Tarchi 2012)

Jet-masers

Jet-cloud interaction

Mrk 348

(Peck et al. 2003)

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• Continuum and line flux densities are correlated
• Reverberation maps give:

V_shock, ρ_j, ρ₀

Wind-masers

• Circinus: thin warped disk + wide-angle outflow

(Greenhill et al. 2003)

• NGC 3079:

clumpy thick disk + outflow (Kondratko et al. 2005)

Millimeter/submm masers

NGC 3079: first extragalactic detection!

183 GHz (SMA)

7σ detection

Same velocity range of the 22 GHz lines

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439 GHz

(tentative, JCMT)

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Humphreys et al. (2005)

Millimeter/submm masers

Star formation activity: ~10 ULIRGs

(e.g. Arp220, 183 and 325 GHz; Cernicharo et al. 2006, Koenig et al. 2017)

AGN activity: ~20 Seyferts/LINERs

(183, 321 and 380 GHz; Pesce et al. 2023, Tarchi et al. 2024)

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Circinus: 321 GHz and 183 GHz (ALMA)

(Hagiwara et al . 2013, Pesce et al. 2023)

It is the brightest mm maser so far!

Coarse mapping with ALMA 🡪 Likely a disk-maser!

Larger radius w.r.t. 22 GHz

Millimeter/submm masers

TXS2226-184:

• The first unambiguous detection of the 380 GHz line (S/N > 100)!
• First case where 22, 183 and 380 GHz transitions are detected in the same object! (ALMA; Tarchi et al. 2024)

Millimeter/submm masers

TXS2226-184:

• Location of the (sub)mm masers is coincident with the 22 GHz maser spots (the emission is, however, unresolved at the angular resolution of the data!)
• Co-spatiality and overall spectral structure suggest a common origin for all transitions 🡪 possibly amplification of the bright nuclear continumm through dense and hot gas in a disk or torus

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Millimiter/submm masers

Why they are important:

• Study of gas structure and dynamics as the 22 GHz ones but with finer resolution (when mm VLBI observations will be possible!)
• Line ratios can constrain radiative transfer models (Humphreys et al. 2005)

Extragalactic H₂O maser with existing and upcoming facilities�

Current instruments used:

• Large single-dish: Effelsberg, GBT, SRT
• VLBI array: VLBA, EVN

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Fig. 5: Areal view of the 64-m Sardinia Radio Telescope (SRT), San Basilio (CA), Italy. Credits: P. Soletta, INAF

The 64-m Sardinia Radio Telescope (SRT)

Credits: P. Soletta, INAF

Extragalactic H₂O maser with existing and upcoming facilities�

More (mega)maser sources are needed (detection rates are low):

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@ 22 GHz: ngVLA, SKA, ...

(see, e.g., SKA Memo #20-01;

ngVLA Science Use Case NGA18;

Tarchi+ 2020,2024)

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@ mm/sub-mm: ALMA, NOEMA, LMT, ...

Maser studies with existing and upcoming facilities

High resolution is necessary for maser followup studies:

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• cm-VLBI (also from Space)

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• ALMA long baselines

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• mm-VLBI

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• EHT

OH (mega)masers

Luminous OH masers found in ~120 galaxies:

• Commonly found in LIRGs (L_IR > 10¹¹ L_sun) and ULIRGs (L_IR > 10¹² L_sun)
• Radiatively pumped by IR photons
• Trace a relatively warm and dense gas

100 K < T_kin < 300 K)

10⁴ cm^-3 < N(H₂) < 10⁶ cm^-3

(Tarchi 2012 and references therein)

• Mostly associated with compact and extreme starburst likely triggered by merger events (Darling 2007)

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OH (mega)masers

OH megamasers in AGN:

• IIIZw35 (LIRG, Sy2)

Diffuse emission

🡪 ring-like structure with r ~22 pc

Compact emission

🡪 clouds at the tangent points

(Pihlstroem et al. 2021)

• Mrk231 (ULIRG, LINER)

Trace a rotating dusty molecular torus

(Kloeckner, Baan & Garret 2003)

IIIZw35

OH (mega)masers

Zw049.057

(LIRG, hosting a CON)

• OH and H₂CO megamaser emission
• H₂CO trace the nuclear disk
• OH a slow and wide-angle outflow likely evolving in a fountain flow

🡪 common in CONs?

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OH (mega)masers

Future of OH megamaser discoveries

Identifynig OHM sources in upcoming HI surveys

may unveil thousands of new sources!

Roberts & Darling 2024

Take-home messages

• Extragalactic masers

single-dish + VLBI 🡪 structure and kinematics of the gas

• H₂O masers
• 22 GHz masers

accretion-disk geometry, SMBH masses, geometrical distances

jet-ISM interaction, shock velocity, wind geometry

• mm/sum-mm masers

same potential as 22 GHz + radiative transfer models

• OH masers

starburst regions, torus geometry, outflows in Compact Obsured Nuclei

Future perspectives

Discovery of new extragalactic masers

• H₂O masers

22 GHz masers: NgVLA, SKA

mm/sum-mm masers: ALMA, Noema, LMT

• OH masers

HI surveys with MeerKAT and SKA

High resolution follow-ups

cm-VLBI (space VLBI), mm-VLBI, EHT