The ASKAP-FLASH survey�Progress update and recent results

Elizabeth Mahony + ASKAP-FLASH team

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The ASKAP-FLASH survey

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

The First Large Absorption Survey in HI: a wide-area 21cm survey with ASKAP

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Searching for HI in absorption between 0.42<z<1.0

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Probing the cool, neutral gas at intermediate redshifts in an untargeted survey

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International team consisting of >50 members

- PIs: Elaine Sadler + Elizabeth Mahony

- Project Scientist: Hyein Yoon

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The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

Advantages of ASKAP:

• Large f.o.v – can survey the whole sky quickly
• Radio quiet site – opens up frequency range 712-1000 MHz
• Wide bandwidth – probe redshift range 0.4<z<1.0 in single observation

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We acknowledge the Wajarri Yamaji as the traditional owners and native title holders of Inyarrimanha Ilgari Bundara, CSIRO’s Murchison Radio-astronomy Observatory

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FLASH survey design

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

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FLASH key science aims

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

Intervening absorbers

Study the cosmic evolution of HI, testing current galaxy evolution and mass assembly models

Associated absorbers

Study AGN fuelling and feedback processes in powerful radio galaxies

schematics: Hyein Yoon

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FLASH key science aims

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

Intervening absorbers

Study the cosmic evolution of HI, testing current galaxy evolution and mass assembly models

Associated absorbers

Study AGN fuelling and feedback processes in powerful radio galaxies

Morganti+ 2013

Allison+ 2022

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Progress update

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

• 200hrs observed as part of FLASH pilot survey
• Full survey started May 2023
• 174 fields completed (29% complete) +70 more fields recently observed
• 120+ new detections from full survey so far
• Data processed by ASKAPsoft pipeline + uploaded to CASDA
• Becomes public after data validation by the FLASH team
• Uploaded to FLASHdb hosted @ AusSRC
• Post-processing + running linefinder (FLASH team access only)

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The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

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Image: Hyein Yoon

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The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

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Postprocessing + analysis�

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

FLASHfinder: Bayesian approach to the one-dimensional line-finding problem (Allison+ 2012)

+ ML analysis to separate real lines from artefacts

Ln(B)

Ln(B)

Line width (km/s)

-> Ivy’s talk in Source Finding Breakout

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First results: the FLASH pilot survey

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

30 new detections of HI in absorption at 0.42<z<1.0 (Yoon+ 2025)

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First results 1: the FLASH pilot survey

• Detection rate:
• Pilot 1: 17 new lines out of 37 fields (0.46 lines per field)
• Pilot 2: 12 new lines out of 38 fields (0.32 lines per fields, lower due to artefacts)
• Lower than the ~1 line/field predicted, but only counting high-confidence lines from pilot survey data

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• Probing high HI column-densities
• Majority of detections >10²¹ cm^-2

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• Most (28/30) have optical counterparts detected in DESI legacy survey (Sadler+ in prep)
• These are mainly bright and massive galaxies (based on WISE mid-IR data), often in groups or with close companions
• Working on a photo-z catalogue for all FLASH continuum targets (Roster+ in prep)

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The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

Yoon+ 2025

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First results 2: FLASH detections dominated by young, compact radio galaxies

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

Image: Emily Kerrison

• Fit using RadioSED (Kerrison et al. 2024) 
• At least 65% of detections well characterised by a peaked spectrum model → young radio AGN (O’Dea & Saikia 2021)
• FLASH is detecting radio galaxies at a specific evolutionary phase

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First results 3: VLBI follow-up of FLASH detections

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

• LBA+VLBA follow-up of FLASH detections shows most sources resolved on VLBI scales
• can also provide a more accurate estimate of f and N_HI

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JNHS Aditya+, subm. 2025

400pc

450pc

550pc

400pc

350pc

1kpc

2.5kpc

1.5kpc

300pc

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The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

• Observed a pilot sample of 16 FLASH detections with MeerKAT
• Covered range of redshifts + optical depths
• Aim to detect additional features below ASKAP sensitivity.
• Each target observed for between 1-3 hrs

First results 4: MeerKAT Follow-up

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The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

z = 0.68

S₈₈₈ = 1.6 Jy

v = -200 km/s

N_HI = 5x10²⁰ cm^-2

Ṁ ~ 0.8 M_◉/yr

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z = 0.87

S₈₈₈ = 780 mJy

v = -500 km/s

N_HI = 2x10²⁰ cm^-2

Ṁ ~ 4-7 M_◉/yr

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z = 0.81

S₈₈₈ = 143 mJy

v = -1000 km/s

N_HI = 1.5x10²¹ cm^-2

Ṁ ~ 30 M_◉/yr

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• Are fast outflows of neutral gas more prevalent at z~1?
• What role does the radio jet play in driving these outflows?

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Challenges + Lessons Learned

Challenges:

• Troposheric Ducting of RFI
• Solar interference
• Large datasets, reliance on observatory pipeline + external supercomputing facilities
• Leads to delay between observations and checking data
• Distinguishing real lines from artefacts
• ML algorithms (see Ivy’s talk Thursday)
• Matching to optical data with photo-z’s (Roster+ in prep)

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What’s worked well:

• AusSRC connection and FLASHdb

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The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

-> FLASH meeting on Thursday

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Summary

The ASKAP-FLASH Survey @ PHISCC 2025 | Elizabeth Mahony

• The ASKAP-FLASH survey provides a HI-selected sample at 0.4<z<1.0
• 29% of the full survey already completed

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• 100+ detections of HI absorption from the FLASH pilot surveys (Yoon+ 2025)
• 120+ detections from full survey so far
• Detections predominately found in young, compact radio galaxies.
• Rare population of high column density objects

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• Follow-up observations vital to understand these complex systems

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