Abstract
While the formation of stellar clumps in distant galaxies is usually attributed to gravitational violent disk instabilities, we show here that major mergers also represent a competitive mechanism to form bright clumps. Using ∼0.1″ resolution ACS F814W images in the entire COSMOS field, we measured the fraction of clumpy emission in 109 main sequence (MS) and 79 Herschel-detected starbursts (off-MS) galaxies at 0.5 < z < 0.9, representative of normal versus merger induced star-forming activity, respectively. We additionally identify merger samples from visual inspection and from Gini-M20 morphological parameters. Regardless of the merger criteria adopted, the clumpiness distribution of merging systems is different from that of normal isolated disks at a > 99.5% confidence level. The former reaches higher clumpiness values up to 20% of the total galaxy emission. We confirm the merger induced clumpiness enhancement with novel hydrodynamical simulations of colliding galaxies with gas fractions typical of z ∼ 0.7. Multi-wavelength images of three starbursts in the CANDELS field support the young nature of clumps, which are likely merger products rather than older preexisting structures. Finally, for a subset of 19 starbursts with existing near-infrared rest frame spectroscopy, we find that the clumpiness is mildly anti-correlated with the merger phase, which decreases toward final coalescence. Our result can explain recent ALMA detections of clumps in hyperluminous high-z starbursts, while normal objects are smooth. This work raises a question as to the role of mergers on the origin of clumps in high redshift galaxies in general.
Highlights
While local galaxies have well defined morphological types described by the so called Hubble sequence, higher redshift systems are more irregular and clumpy, which makes it increasingly more difficult to associate one of the Hubble classes to them
We present the results of our clumpiness measurements, and compare the properties of the starburst and main sequence populations that we have taken as representatives of two star-formation modes: a higher efficiency stellar production induced by merger events in the first case, and a normal starformation activity associated with secularly evolving disks in the latter
This translates into a median clumpiness for SBs that is more than a factor of two higher compared to the main sequence (MS) population (0.05 and 0.022, respectively)
Summary
While local galaxies have well defined morphological types described by the so called Hubble sequence, higher redshift systems are more irregular and clumpy, which makes it increasingly more difficult to associate one of the Hubble classes to them. Typical clumps at z > 1 might live between 100 and 650 Myr, depending on their stellar mass (Zanella et al 2015, 2019) As a consequence, they could migrate toward the center and eventually contribute to the formation of a central bar (Immeli et al 2004a; Sheth et al 2012; Kraljic et al 2012) or to the stellar bulge growth (Noguchi 1999; Elmegreen et al 2008; Bournaud et al 2014; Bournaud 2016). Clumps in distant galaxies may lead to the formation of super star clusters and globular clusters (Shapiro et al 2010)
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