Exploring Galaxy Evolution
from Atomic-Molecular Gas
Tsutomu T. TAKEUCHI
with Kai KONO, Moe YODA, Shuntaro YOSHIDA &Chihiro KONDO
Division of Particle and Astrophysical Science, Nagoya
University, Japan
Galaxy Evolution and Distant Universe 2019,
Yokohama, 11-13 March, 2019

1 Galaxy Evolution in the Evolving Universe

Cosmic star formation density

1.1 The cosmic star formation history

Hopkins & Beacom (2006)

Cosmic star formation density

1.1 The cosmic star formation history

The Cosmic
Dawn

Hopkins & Beacom (2006)

Cosmic star formation density

1.1 The cosmic star formation history

The Cosmic
Noon

Hopkins & Beacom (2006)

Cosmic star formation density

1.1 The cosmic star formation history?

Hopkins & Beacom (2006)

Cosmic star formation density

1.1 The cosmic star formation history

The Cosmic
Dusk

I propose

Hopkins & Beacom (2006)

1.2 Cosmic star formation from the gaseous side
Important quantity ever focused: star formation rate (SFR)
Observational SFR indicators• Ionizing photons from OB stars• Recombination lines from HII regions• Forbidden lines from HII regions• Non-ionizing UV photons• IR reemission from dust
• PAH band emission from photodissociation regions• Synchrotron radiation• X-ray from binaries, etc.

1.2 Cosmic star formation from the gaseous side
Important quantity ever focused: star formation rate (SFR)
Observational SFR indicators
They give information on the SFR, but do NOT tellanything about the transition from gas to stars, which isfundamental in galaxy formation and evolution.
⇒ Importance of gas phase (ISM) observations
Gas will complement the picture of galaxy evolution.
We will have a unified picture of the physics ofgalaxy evolution.

1.3 Formal goal of the galaxy evolution studies
Galaxy evolution equation in a formal expression:

1.3 Formal goal of the galaxy evolution studies
Galaxy evolution equation in a formal expression:x = x(T|T < t)

1.3 Formal goal of the galaxy evolution studies
Galaxy evolution equation in a formal expression:

This is the formal and ultimate goal of the studies ongalaxy evolution, but clearly it is a substantiallycomplicated problem. New ideas on the methodology areneeded to tackle, as well as the physical ideas.

1.3 Formal goal of the galaxy evolution studies
Galaxy evolution equation in a formal expression:

Especially, the gaseous side should be investigatedthoroughly.

1.3 Formal goal of the galaxy evolution studies
Sometimes, some people tend to say1.3 Formal goal of the galaxy evolution studies
Sometimes, some people tend to say
Galaxy evolution = high-z studies
as if low-z studies on galaxy evolution had been completed.

1.3 Formal goal of the galaxy evolution studies
Sometimes, some people tend to say
Galaxy evolution = high-z studies

DEFINITELYNOT!
We have many fundamental unsolved problems at eachredshift (cosmic age).

2 Cosmic Dusk

2 Cosmic Dusk
2.1 Gas mass function
Today, the HI surveys of galaxies are shallow (> mJy), withpoor angular resolution.
The HI Parkes All Sky Survey (HIPASS)
The HI Jodrell All Sky Survey (HIJASS)The Arecibo Legacy Fast ALFA Survey (ALFALFA)etc.
< z > ~ 0.01-0.06
For some dedicated observations, z < 0.5 (Kanekar etal. 2016; Fernández et al. 2016)

2.1 Gas mass function
The situation for the molecular gas mass function is more
serious because of the lack of large-area surveys.
Molecular gas (CO)Verter et al. (1987): compilation of surveys
Kereš et al. (2003): FCRAO (but optical or FIR selected)
No systematic dedicated survey exists.

⇒ We can discuss statistically only nearby galaxies, and it isdifficult to explore their evolutionary aspects.

HI mass function (HIPASS)

Zwaan et al. (2003)

Molecular (CO) luminosity function(Kereš et al. 2003)

Need for systematic deep surveys
Only a few systematic studies on the gas mass function
have been made mainly because of the limitation from theobservational side.

Need for much larger and deeper surveys, both for atomic
and molecular gas!

Need for systematic deep surveys
Only a few systematic studies on the gas mass function
have been made mainly because of the limitation from theobservational side.

Need for much larger and deeper surveys, both for atomic
and molecular gas!

Theoretical (semi-analytic) models are studied, but still quitepremature to reproduce the gas mass functions.

2.2 Transition from atomic to molecular hydrogenH2 formation in galaxies is determined by the balance between
production and dissociation.
Production

Dissociation• 2-atom conjugate reaction• Photodissociation by UV• 3-atom collision reaction• Dust surface reaction⇒most efficient in galaxies

Not efficient in a dense
dusty molecular clouds
because of self-shielding.

• Dissociation by cosmic rays

Dissociate H2 in molecularclouds.

• Dissociation by collision

H atomContribution is small.

(1) sticking (2) diffusion (3) reaction (4) ejection

Takahashi (2000)(e.g. Gould & Salpeter 1963;
Draine & Bertoldi 1996)

H2 and HI in galaxies

ΣHI [M☉ pc-2]• For late types, fmol ~ 25-30 %• Radial decrease.

ΣH2 [M☉ pc-2]

ΣH2+HI [M☉ pc-2]

At galactic center, fmol ~1

Radius [kpc]Radius [kpc]Bigiel & Blitz (2012); Boselli et al. (2014)

Transition from HI to H2Column density of gas required to have enough shielding(assuming Z = Z☉):
ΣH2 ~ 10 M☉pc-2
NHI ~ 1021 cm2
These values agree with observation of nearby late-typegalaxies.

(Bigiel et al. 2008)

Transition from HI to H2
Transition column density is determined by metallicity(Gnedin et al. 2009): Metal-poor molecular clouds do notcontain much dust
⇒ Critical NHI becomes higher.
Various topics are yet to be solved.
Z☉0.3 Z☉

0.1 Z☉

Gnedin et al. (2009)

3 Cosmic Noon

3 Cosmic Noon
3.1 Scaling laws including gas properties
Total baryon mass [M☉]Stellar mass [M☉]Baryonic Tully-Fisher (BTF) relation

McGaugh et al. (2000)
Circular velocity [kms-1]
If we consider the HI mass and evaluate the TF relation withbaryonic mass, the linearity is regained (McGaugh et al. 2000).

The “extended” BTF
The extended BTF has different
slopes for the dSphs, normalgalaxies, and galaxy clusters. The
slope becomes shallower forlarger objects.
⇒ Feedback?
However in current studies, gasrich dwarfs are missing in thesample.
Toward lower HI masses!
McGaugh et al. (2010)

HI or baryonic mass

BTF as an HI-dynamical bivariate mass function

Dynamical massKennicutt-Schmidt law
By considering the size of a galaxy, we can discuss therelation between surface densities of gas and SFR. This isknow as the Kennicutt-Schmidt law.

The classical Kennicutt-Schmidtlaw is the relation between thesurface densities of gas and SFR.
A single power law is found in awide range of gas surface density,
but the slope is still a matter ofdebate.
Kennicutt & Evans (2012)Kennicutt-Schmidt law
What the K-S law shows is the relation between total gas mass
(HI + H2) and SFR.
We need observations of HI
as deep as CO (1 < z < 2), toexplore the evolution of theK-S law.
Synergy with observationsof molecules is important!

Bigiel et al. (2008)

SFR

K-Slaw as an Mgas-SFR bivariate function

Gas mass

Star forming galaxy main sequence

SFRSince the SFR is the most interesting quantity, we want toexamine the scaling relations including the SFR.

Stellar mass

Stellar mass

Peng et al. (2010)

A prominent sequence of SF galaxies is found on the stellar
mass-SFR plane: star formation main sequence (SFMS).

Star forming galaxy main sequence
The star formation main sequence consists of secularly
evolving star-forming galaxies (i.e., not merging galaxies).
Relation to various physical properties are examined.
Especially the dependence on the gas amount is of current
interest (Genzel et al. 2012; Fernández et al. 2016).
Observations of CO reach 1 < z < 2, but are not yet a survey.
HI is far from reachable at these redshifts.

Star forming galaxy main sequence

Kanekar et al. (2016)

SFR

SFMS as an M*-SFR bivariate function

Stellar mass

A bridge from resolved to global properties
Scaling relations themselves do not tell anything about theirgoverning physics. We need various additional observations
and sophisticated analysis to reveal the relevant physicalprocesses (e.g., Kennicutt & Evans 2012).
Resolved observation⇒HI-H2 transition
Relation to metallicity (or rather dust amount)
Emerging scale of the global scaling laws
Global scaling laws⇒
Unification of all the scaling laws as a multivariate
distribution function⇒
Fundamental (multidimensional) relation

4 Cosmic Dawn

4 Cosmic Dawn
4.1 Gas observation of ultra-high redshift galaxies?
Perhaps first galaxies might have formed from halos
containing a large amount of HI.

4 Cosmic Dawn
4.1 Gas observation of ultra-high redshift galaxies?
Perhaps first galaxies might have formed from halos
containing a large amount of HI.
However…
Even with SKA2, it would not be easy to detect emissions
from individual galaxies at z > 4.
Should we give up a direct observation of such objects?

4.2 Observation of high-z galaxies through absorption
Observation of gas-dominated galaxies
In optical, gas that is not yet turned into galaxies, or gasdominant young galaxies can be efficiently detected throughQSO absorption lines.

QuasarObserver4.2 Observation of high-z galaxies through absorption
Observation of gas-dominated galaxies
In optical, gas that is not yet turned into galaxies, or gasdominant young galaxies can be efficiently detected throughQSO absorption lines.

QuasarObserver

QSO absorption line systems with particularly high HIcolumn density are observed as damped Lyman  systems(DLAs: NHI > 2 × 1020 cm−2). Such systems are thought tobe a progenitor of present-day giant galaxies.

Observation of gas-dominated galaxies

QuasarObserverObservations showed that these systems are gas-rich andmetal-poor (e.g., Ledoux et al. 2003).
Also, DLAs can be a probe to explore the power spectrum of
the large-scale structure at smaller scales.
However, a possible fundamental problem in optical/UVbased observation was pointed out.

Observation of gas-dominated galaxies

QuasarObserver

We want to detect absorption line systems. However, since
the continuum emission from background quasars would bevery strongly extinguished through the systems withextremely high column density, such systems would be
dropped from the initial selection (Vladilo & Péroux 2005).

Observation of gas-dominated galaxies

Observer

Quasar

We want to detect absorption line systems. However, since
the continuum emission from background quasars would bevery strongly extinguished through the systems withextremely high column density, such systems would be
dropped from the initial selection (Vladilo & Péroux 2005).
But such a high column density systems are very possibly
just before the initial starburst. Namely they are the systems
fundamental to understand the cosmic SF history and what
we indeed want to observe.
This selection bias is fatal!

Observation of gas-dominated galaxies

QuasarHow do we solve this fundamental problem?

ObserverObservation of gas-dominated galaxies

QuasarObserverHow do we solve this fundamental problem?
Select quasar continuum at radio, and explore 21-cm
absorption line systems: best topic for SKA2.

Observation of gas-dominated galaxies

QuasarObserverHow do we solve this fundamental problem?
Select quasar continuum at radio, and explore 21-cm
absorption line systems: best topic for SKA2.
Advantage to optical/UV absorption line observation:
1. At radio, dust extinction is negligible.
2. Because of small cross section, very high column density
systems can be observed.

Observation of gas-dominated galaxies

QuasarObserverNot only the continuum observation but also ancillaryobservations like radio emission, optical etc. will provide us
with more information on the physics of the systems (e.g.,Kanekar et al. 2014; Neeleman et al. 2018).
Theoretical models are also important to compare withthe observed results.

Observation of gas-dominated galaxies

QuasarObserver

York et al. (2007)

4.3 Background Continuum Sources
We also need to develop a model of the background light
continuum sources.
Particularly important is to predict the radio spectraof the background objects, their luminosity function,
frequency of occurrence, and observational feasibility.
1. Radio loud quasars are the most natural candidate
continuum sources.
2. Since at z > 4, quasar density is expected to decreasesignificantly, GRBs are good candidate radio sources.
3. Star-forming galaxies are also a good candidate radiocontinuum sources because of their overwhelmingly
large number at high-z.

GRBs

Inoue, Omukai, & Ciardi (2007)

Radio continuum of SF galaxies
M82
SynchrotronRadio

DustFree-free(Condon 1992)

The 1.4 GHz LF of SF galaxies and AGNs
The 1.4 GHz LF is divided
into two populations: star
forming galaxies and AGNs
(radio galaxies). The LF ofeach population can be wellapproximated by a double
power-law like function
similar to FIR galaxies.
(Machalski & Godlowski
2000; Mauch & Sadler
2007)
Mauch & Sadler (2007)⇒ The ratio of the two
populations evolve?

Number of sample

Luminosity function

The 3 GHz LF at = 1.3−1.7

Luminosity
Error: bootstrap resampling with 95% confidence interval
Kono et al. (2019).

/The 3 GHz number counts of SF galaxies and AGNs

Kono et al. (2019)
Flux(See also Novak et al. 2017)

5 Summary
1. Galaxy evolution is one of the most important topics inthe studies of the cosmic evolution. However, galaxy
evolution in terms of gas has just been started. But exact
themes on galaxy evolution are not deeply discussed.
2. The detection limit of wide surveys of the HI emission bycurrent facilities is of order of mJy, and angularresolution is not sufficient.
3. Statistical analysis of galaxies with HI is limited to z ~ 0,and the evolution of the HI mass function is not known.
The situation for the molecular mass function is evenmore serious. They will provide the information on the“gaseous side” of galaxy evolution, including thetransition from atomic to molecular gas.

5 Summary
4. Transition from HI to H2 in nearby galaxies can be
discussed with spatially resolved gas clouds. We expect
SKA1 contributes significantly.
5. Studies on the scaling relation including gas are also
limited to nearby, massive galaxies. Baryon Tully-Fisherrelation (BTF) is suggested to deviate from single powerlaw at smallest mass galaxies. It is interesting to clarify
what caused this feature.
6. The tight correlation between the stellar mass and SFR isreferred to as the SF main sequence. The gaseous aspectof the emergence of the SF main sequence is yet to bestudied. HI and molecular observation at 1 < z < 3 isdesired.

5 Summary
7. Kennicutt-Schmidt law is a relation between surface
densities of HI+H2 and SFR in galaxies. Some details aregradually understood through the spatially resolved K-Slaw. This study is also limited to nearby galaxies up tonow. The evolution will be traced by SKA and ALMA.
8. Scaling relations themselves do not tell anything about
their governing physics. We need various additional
observations and sophisticated analysis to reveal therelevant physical processes.
9. Direct detection of the HI emission line from galaxies atz > 4 might be very difficult. Extremely gaseous galaxies
and their progenitors can be explored through HI
absorption line systems. This is a plausible topic for theSKA2.

5 Summary
10. Radio-loud QSOs are the first candidate for the radiocontinuum sources. However since they are expected to
be rare at z > 4, we should also consider GRBs and starforming galaxies as a candidate radio sources.

Are you ready to SKA?

From Tokyo Ska Paradise Orchestra feat. Hiroji Miyamoto