NGC 253 starburst at low radio frequencies

Anna D. Kapińska ICRAR/University of Western Australia CAASTRO Introduction: NGC 253 – the galaxy

Archetypal nuclear .

Located in the nearby at a distance ~3.9 Mpc, cz = 240 km/s

ESO (R,V,Hα,OIII) Introduction: NGC 253 – the Sculptor galaxy

Archetypal nuclear starburst galaxy.

➢ 10 Located in the nearby Sculptor Group stellar mass 4 x 10 M⊙

➢ 9 at a distance ~3.9 Mpc, cz = 240 km/s stellar halo of 2.5 x 10 M⊙

➢ formation rate ~5 M⊙/yr (Bailin+2011; Lucero+2015)

ESO (R,V,Hα,OIII) G.Meurer (Hα) GaLactic and Extragalactic All-sky MWA Survey – GLEAM

Murchison Radio Observatory Western Australia

➢ 72 – 230 MHz (20 bands) ➢ sky south of declination +30° Kapinska

➢ depth: ~10 mJy/beam @200MHz ➢ resolution: ~2 arcmin @200MHz

GLEAM: Wayth+ 2015

Data Release 1 (DR1): Hurley-Walker+ 2017 Hurley-Walker Talk outline

➢ Synchroton radio halo

➢ Broadband radio spectral energy distribution (SED)

➢ Spectral index map (α) & multi-frequency context NGC 253 – low frequency radio

GLEAM MWA/EoR0

Offringa+2016, MWA/EoR collab.

GLEAM Hα + GLEAM 200 MHz contours Synchrotron radio halo

78.5°

Projected scale-height (maximum vertical extent) Spectral energy distribution – radio

Kapinska et al. ApJ under review

Total flux density

Central starburst (300-500 pc) Spectral energy distribution – radio

Kapinska et al. ApJ under review GLEAM

EoR Total flux density

TGSS

Central starburst (300-500 pc) Spectral energy distribution – radio

Kapinska et al. ApJ under review

Disk & halo

Central starburst Radio emission – models synchrotron emission free-free emission

absorption at low frequencies optically thick regime

self-absorption Radio emission – models synchrotron emission free-free emission

absorption at low frequencies optically thick regime

Internal or external free-free absorption of synchrotron emission self-absorption galaxy

Line of sight

H II galaxy

external internal Spectral energy distribution – radio

Kapinska et al. ApJ under review

Disk & halo curved synchrotron emission

cf. Carilli 1996 Central starburst – internal free-free absorbed synchrotron emission, turnover: ~230 MHz Radio spectral index map

Kapinska et al. ApJ under review

α

200 MHz – 1.46 GHz Multiwavelength view

Kapinska et al. ApJ under review

Soft X-ray (<1 keV; Bauer+2008)

Extraplanar HI (KAT-7; Lucero+2015)

Spectral index (0.2–1.46 GHz) Multiwavelength view

Kapinska et al. ApJ under review

Soft X-ray (<1 keV; Bauer+2008)

Extraplanar HI (KAT-7; Lucero+2015)

Spectral index (0.2–1.46 GHz)

Spectral flattening * e– low-frequency cut off? * strong superwind collimating cold gas? → extraplanar HI

Summary

● New, deep low-frequency radio images of NGC 253 → amazing capabilities of MWA – SKA precursor!

● Radio emission at MHz frequencies a combination of central starburst (internally free-free absorbed synchrotron) and extended emission of disk and halo (synchrotron flattening at low radio frequencies)

● Spectral turnover of central starburst for the first time statistically constrained, at ~230 MHz

● Extensive synchrotron radio halo of NGC 253 imaged by MWA, extending up to 8 kpc at 200 MHz