Measuring the Gas in Galaxies in the Distant Past

Philip Lah

Too late. Here comes

the SKA.

Who am I?

• I am astronomer with a joint appointment at the:

National Centre for Radio Astrophysics,

Tata Institute of Fundamental Research, India

and the

Research School of Astronomy & Astrophysics,

Mount Stromlo Observatory,

Australian National University

• I use both radio and optical telescopes for my research

Atmospheric TransmissionA

tmos

pher

ic O

paci

ty

0 %

50 %

100 %

1 nm 1 μm 1 mm 1 m 1 km

Wavelength →← Frequency

radio

optical

infraredgamma rays, X-rays

& ultraviolet

Hydrogen Gasin the

Universe

Hydrogen Gas in the Universe

Hydrogen 76% by mass of the baryons – normal matter

(that is not the weird stuff – Dark Matter)

Neutral Atomic

Hydrogen Gas

produces emission at a wavelength of 21 cm

Hydrogen Gas in the Universe

Hydrogen 76% by mass of the baryons – normal matter

(that is not the weird stuff – Dark Matter)

Neutral Atomic

Hydrogen Gas (HI)

produces emission at a wavelength of 21 cm

21 cm

Atmospheric TransmissionA

tmos

pher

ic O

paci

ty

0 %

50 %

100 %

1 nm 1 μm 1 mm 1 m 1 km

Wavelength →← Frequency

radio

optical

infraredgamma rays, X-rays

& ultraviolet

HI 21-cm Emission From Galaxies

Galaxy M33: optical

Galaxy M33: HI 21-cm emission

Galaxy M33: optical and HI

Galaxy M33: optical

Why Study Neutral Atomic Hydrogen Gas?

HI Gas and Star Formation

neutral atomic hydrogen gas

cloud (HI)

molecular gas cloud (H

2)

star formation

Redshift, Observing the Past

The Expanding Universe

Hubble`s Lawredshift velocity related to distance

distance related to age(due to light travel time)

Hubble`s Lawredshift velocity related to distance

distance related to age(due to light travel time)

21 cm

Atmospheric TransmissionA

tmos

pher

ic O

paci

ty

0 %

50 %

100 %

1 nm 1 μm 1 mm 1 m 1 km

Wavelength →← Frequency

radio

optical

infraredgamma rays, X-rays

& ultraviolet

redshifted

The History of

Star Formation in the Universe

Star Formation Rate Density

Star Formation Rate Density

~10 times current rate

The Amount of

HI Gas in the Universe

HI Gas Density

HI Gas Density

~2 times current density

HI Gas Density

HI Gas Density

region where I have been

working

Current Observations

Giant Metrewave Radio Telescope

Giant Metrewave Radio Telescope

Coadding HI signals

Coadding HI signals

RA

DEC

Radio Data Cube

Frequen

cy

HI red

shift

Coadding HI signals

RA

DEC

Radio Data Cube

Frequen

cy

HI red

shift

positions of optical galaxies

Coadding HI signals

frequency

flux

Coadding HI signals

frequency

flux

z2

z1

z3

z1, z2 & z3 optical redshifts

of galaxies

Coadding HI signals

velocity

z1

z2

z3

flux

z1, z2 & z3 optical redshifts

of galaxies

Coadding HI signals

velocity

z1

z2

z3

flux

velocity

Coadded HI signal

z1, z2 & z3 optical redshifts

of galaxies

My Current Best Observation

HI blue outside x-ray gasblue galaxies surrounding the core of a large galaxy cluster

galaxies observed as they were 4 billion years

ago

coadding signal from 94 galaxies

HI blue outside x-ray gasblue galaxies surrounding the core of a large galaxy cluster

galaxies observed as they were 4 billion years

ago

coadding signal from 94 galaxies

Future Observations

The SKA and Me

• SKA`s collecting area 20GMRT`s collecting area

• SKA better receivers

• SKA larger field of view

• SKA better RFI environment

The SKA and Me• my current research looking at galaxies 3 to 4 billion years ago –

SKA directly detect HI signal

• originally SKA proposed to detect HI gas from 8 billion years ago

• stacking with SKA ~12.7 billion years ago, the era of first galaxies

SKA

fully operational

2024

The SKA pathfinders

MeerKAT

South African SKA pathfinder

• 13.5 m diameter dishes

• 64 dishes

• finished 2014 (2018)

ASKAP• 12 m diameter dishes

• 30 dishes

• focal plane array

• finished 2013