Neutral gas in galaxies: Synergies between ALMA and ASKAP

Elaine M. Sadler

Outline of this talk

• Big picture questions: Gas and galaxy evolution

• ASKAP: Why an all-sky HI absorption survey?

• ALMA synergies: CO observations of an HI-selected galaxy sample at 0.5 < z < 1

• Pathfinder work: An ATCA/CABB search for associatedHI absorption in young radio galaxies

Neutral hydrogen in galaxies

HI observations provide a powerful probe of the mass distribution in galaxies (dark matter) and the fuel supply available for new star formation

Stellar light HI gas distribution

[NGC 4631, Rand 1994]

Cosmic star-formation history

The rate at which new stars form in galaxies has decreased by about a factor of 20 over the past 7-8 billion years (from redshift z~1 to 0).

What caused this? A decline in the supply of cold neutral gas in galaxies?

We don’t know!

(Hopkins & Beacom 2006)

Neutral (atomic and molecular) hydrogen is the missing link in our current models of galaxy evolution

• We know almost nothing about the HI content of galaxies at 0.2<z<2.

• A wide range of models and simulations exist, none of which fit all the data.

• Unknown HI/H2 fraction at z > 0.

Cosmic HI mass density

Nearby Sbc spiral galaxy NGC 5055 (Wong & Blitz 2002)

HI, CO and star formation in nearby galaxies

LJ Tacconi et al. Nature 463, 781-784 (2010) doi:10.1038/nature08773

CO in star-forming galaxies at z~1

(Ellison et al. 2001)

Optical: Damped Ly Absorbers

DLAs: Intervening absorbers with high HI column density (NHI > 2 x 1020 cm-2) can be

used to detect and study neutral hydrogen in the very distant universe. Ground-based observations of the Lyman- line are only possible at redshift z > 1.7

DLA

Lyman forest

Cosmic HI mass density

HI column density distribution

Zwaan et al. (2005)

Optical DLA surveys at z > 2 do not detect the highest column-density absorbers expected on ~0.1% of sightlines, and “do not trace the majority of star-forming gas in the universe” (Ledoux et al. 2003). Dust obscuration?

WSRT: Lane et al. 2001, targeted, z = 0.436

Radio 21cm measurements are particularly sensitive to cold HI (TS < 200K)

NHI/Ts.V for observed optical depth

Probability of intercepting a DLA system (NHI > 2 x 1020 cm-2) on a random line of sight?

dN/dZ=0.055 (1+z)1.11

(Storrie-Lombardi & Wolfe 2000)

i.e. ~6% for z=0.7, 300 MHz band

Radio: Intervening HI absorption lines

Unlike optical, no redshift limit for detecting radio 21cm absorption lines.

But do need many targets, wide bandwidth

Team members: Currently 37 members from 18 institutions in 7 countries Elaine Sadler (Sydney, PI), James Allison (Oxford), Chris Blake (Swinburne), Joss Bland-Hawthorn (Sydney), Robert Braun (ATNF), Matthew Colless (AAO), Rob Crain (Swinburne), Scott Croom (Sydney), Darren Croton (Swinburne), Stephen Curran (UNSW), Jeremy Darling (USA), John Dickey (Tasmania), Michael Drinkwater (Qld), Ron Ekers (ATNF), Sara Ellison (Canada), Bjorn Emonts (ATNF), Ilana Feain (ATNF), Ken Freeman (ANU), Bryan Gaensler (Sydney), Dick Hunstead (Sydney), Helen Johnston (Sydney), Baerbel Koribalski (ATNF), Philip Lah (ANU), Tom Mauch (Oxford), Martin Meyer (UWA), Raffaella Morganti (Netherlands), Tom Oosterloo (Netherlands), Max Pettini (UK), Kevin Pimbblet (Monash), Michael Pracy (Swinburne), Steve Rawlings (Oxford), Tim Robishaw (Sydney), D.J. Saikia (Pune, India), Lister Staveley-Smith (UWA), Matthew Whiting (ATNF), Richard Wilman (Melbourne), Martin Zwaan (ESO)

FLASH: the First Large Absorption Survey in HI

Key science goals:

• To provide the first systematic probe of the neutral hydrogen (HI) content of individual galaxies in the redshift range 0.5 < z < 1.0

• To make tests of current galaxy evolution and mass assembly models in this redshift range, using the observed and predicted distributions of quantities like HI optical depth and line width.

The advantages of ASKAP

ASKAP’s

• Wide field of view

• Wide spectral bandwidth

• Radio-quiet site

make it possible to carry out the first blind radio survey for HI absorption

Only about 400 sightlines have so far been searched for HI absorption in the radio (roughly 200 each with single-dish (blind) and interferometer (targeted) programs).

In the ASKAP-FLASH survey, we will target more than 150,000 sightlines to bright background continuum sources, an increase of more than two orders of magnitude over previous work.

Survey parameters

FLASH WALLABY piggyback

Survey area (deg2) 25,000 30,000

Observing freq. (MHz) 700-1000 1130-1430

HI redshift range 0.5 <z< 1.0 0 <z< 0.26

Angular resol. (arcsec) 30 30

Spectral resol. (kHz) 18 18

Bandwidth (MHz) 300 300

Integration time per field (hr) 2 8

Total obs. time (hr) 1600 (9600)

Expected detections (intervening) ~450 ~400

Cross-comparison of emission, absorption and stacking measurements at z<0.26 is a new and important aspect of the ASKAP HI surveys

LJ Tacconi et al. Nature 463, 781-784 (2010) doi:10.1038/nature08773

Integrated CO spectra of SF galaxies at z~1 to 2

CO spectral-line detections should require only short integration times with ALMA for HI-selected galaxies out to z=1

ATCA: Oosterloo et al., targeted, z = 0.01

Associated HI absorption in radio galaxies

Nearby galaxy NGC 6868, continuum flux density ~120 mJy at 1.4 GHz.

Associated HI absorption at or near the galaxy redshift is particularly common in compact, flat-spectrum radio galaxies (Vermeulen et al. 2003)

ATCA/CABB HI absorption (C2434)

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(Orienti et al. 2006)

• Target sample of 60 nearby (z < 0.08) ‘young’ compact (<0.5 kpc) radio galaxies selected from the AT20G survey.

• 30 galaxies so far observed in HI line with ATCA/CABB (Feb and Apr 2011), analysis in progress

• Probe distribution and kinematics of gas in innermost regions of AGN

• Wideband test data for ASKAP pipeline

HI in PKS 1814-637

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ATCA C2434: James AllisonATCA: Morganti et al. 2001

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