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Clumpy galaxies seen in H α: inflated observed clump properties due to limited spatial resolution and sensitivity


Tamburello, Valentina; Rahmati, Alireza; Mayer, Lucio; Cava, Antonio; Dessauges-Zavadsky, Miroslava; Schaerer, Daniel (2017). Clumpy galaxies seen in H α: inflated observed clump properties due to limited spatial resolution and sensitivity. Monthly Notices of the Royal Astronomical Society, 468(4):4792-4800.

Abstract

High-resolution simulations of star-forming massive galactic discs have shown that clumps form with a characteristic baryonic mass in the range 107–108 M⊙, with a small tail exceeding 109 M⊙ produced by clump–clump mergers. This is in contrast with the observed kpc-size clumps with masses up to 1010 M⊙ in high-redshift star-forming galaxies. In this paper, we show that the comparison between simulated and observed star-forming clumps is hindered by limited observational spatial resolution and sensitivity. We post-process high-resolution hydrodynamical simulations of clumpy discs using accurate radiative transfer to model the effect of ionizing radiation from young stars and to compute H α emission maps. By comparing the intrinsic clump size and mass distributions with those inferred from convolving the H α maps with different Gaussian apertures, we mimic the typical resolution used in observations. We found that with 100 pc resolution, mock observations can recover the intrinsic clump radii and stellar masses, in agreement with those found by lensing observations. Instead, using a 1 kpc resolution smears out individual clumps, resulting in their apparent merging. This causes significant overestimations of the clump radii and therefore masses derived using methods that use their observed sizes. We show that limited sensitivity can also force observations to significantly overestimate the clump masses. We conclude that a significant fraction of giant clumps detected in the observations may result from artificially inflated radii and masses, and that ≈100 pc spatial resolution is required to capture correctly the physical characteristics of star-forming clumps if they are coherent structures produced by disc fragmentation.

Abstract

High-resolution simulations of star-forming massive galactic discs have shown that clumps form with a characteristic baryonic mass in the range 107–108 M⊙, with a small tail exceeding 109 M⊙ produced by clump–clump mergers. This is in contrast with the observed kpc-size clumps with masses up to 1010 M⊙ in high-redshift star-forming galaxies. In this paper, we show that the comparison between simulated and observed star-forming clumps is hindered by limited observational spatial resolution and sensitivity. We post-process high-resolution hydrodynamical simulations of clumpy discs using accurate radiative transfer to model the effect of ionizing radiation from young stars and to compute H α emission maps. By comparing the intrinsic clump size and mass distributions with those inferred from convolving the H α maps with different Gaussian apertures, we mimic the typical resolution used in observations. We found that with 100 pc resolution, mock observations can recover the intrinsic clump radii and stellar masses, in agreement with those found by lensing observations. Instead, using a 1 kpc resolution smears out individual clumps, resulting in their apparent merging. This causes significant overestimations of the clump radii and therefore masses derived using methods that use their observed sizes. We show that limited sensitivity can also force observations to significantly overestimate the clump masses. We conclude that a significant fraction of giant clumps detected in the observations may result from artificially inflated radii and masses, and that ≈100 pc spatial resolution is required to capture correctly the physical characteristics of star-forming clumps if they are coherent structures produced by disc fragmentation.

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Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Institute for Computational Science
Dewey Decimal Classification:530 Physics
Language:English
Date:March 2017
Deposited On:09 Jan 2018 20:47
Last Modified:19 Feb 2018 09:49
Publisher:Oxford University Press
ISSN:0035-8711
Additional Information:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2017 The Authors Published by Oxford University Press on behalf of Royal Astronomical Society. All rights reserved.
OA Status:Green
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1093/mnras/stx784

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