DYNAMO-HST survey: clumps in nearby massive turbulent discs and the effects of clump clustering on kiloparsec scale measurements of clumps

Abstract

We present $\\sim$100 pc resolution Hubble Space Telescope H$\\alpha$ images of\n10 galaxies from the DYnamics of Newly-Assembled Massive Objects (DYNAMO)\nsurvey of low-$z$ turbulent disk galaxies, and use these to undertake the first\ndetailed systematic study of the effects of resolution and clump clustering on\nobservations of clumps in turbulent disks. In the DYNAMO-{\\em HST} sample we\nmeasure clump diameters spanning the range $d_{clump} \\sim 100-800$~pc, and\nindividual clump star formation rates as high as $\\sim5$~M$_{\\odot}$~yr$^{-1}$.\nDYNAMO clumps have very high SFR surface densities, $\\Sigma_{SFR}\\sim\n15$~M$_{\\odot}$~yr$^{-1}$~kpc$^{-2}$, $\\sim100\\times$ higher than in H{\\sc ii}\nregions of nearby spirals. Indeed, SFR surface density provides a simple\ndividing line between massive star forming clumps and local star forming\nregions, where massive star forming clumps have $\\Sigma_{SFR}>\n0.5$~M$_{\\odot}$~yr$^{-1}$~kpc$^{-2}$. When degraded to match the observations\nof galaxies in $z\\sim 1-3$ surveys, DYNAMO galaxies are similar in morphology\nand measured clump properties to clumpy galaxies observed in the high-$z$\nUniverse. Emission peaks in the simulated high-redshift maps typically\ncorrespond to multiple clumps in full resolution images. This clustering of\nclumps systematically increases the apparent size and SFR of clumps in 1~kpc\nresolution maps, and decreases the measured SFR surface density of clumps by as\nmuch as a factor of 20$\\times$. From these results we can infer that clump\nclustering is likely to strongly effect the measured properties of clumps in\nhigh-$z$ galaxies, which commonly have kiloparsec scale resolution.\n