Based on optical/near-IR Magellan FIRE spectra of 25 starburst galaxies at 0.5 < z < 0.9 , Calabrò et al .
( 16 ) showed that their attenuation properties can be explained by a single-parameter sequence of total obscurations ranging from A _ { V } = 2 to A _ { V } = 30 towards the starburst core centers in a mixed stars and dust configuration .
We investigate here the origin of this sequence for the same sample .
We show that total attenuations anti-correlate with the starburst sizes in radio ( 3 GHz ) with a significance larger than 5 \sigma and a scatter of 0.26 dex .
More obscured and compact starbursts also show enhanced N2 ( = [ NII ] /H \alpha ) ratios and larger line velocity widths that we attribute to an increasing shock contribution toward later merger phases , driven by deeper gravitational potential wells at the coalescence .
Additionally , the attenuation is also linked to the equivalent width ( EW ) of hydrogen recombination lines , which is sensitive to the luminosity weighted age of the relatively unobscured stellar populations .
Overall , the correlations among A _ { \text { V } } , radio size , line width , N2 and EW of Balmer/Paschen lines converge towards suggesting an evolutionary sequence of merger stages : all of these quantities are likely to be good time-tracers of the merger phenomenon , and their large spanned range appears to be characteristic of the different merger phases .
Half of our sample at higher obscurations have radio sizes approximately 3 times smaller than early type galaxies at the same redshift , suggesting that , in analogy with local Ultraluminous Infrared galaxies ( ULIRGs ) , these cores can not be directly forming elliptical galaxies .
Finally , we detect mid-IR AGN torus for half of our sample and additional X-ray emission for 6 starbursts ; intriguingly , the latter have systematically more compact sizes , suggestive of emerging AGNs towards later merger stages , possibly precursors of a later QSO phase .