Giant star-forming clumps?

Abstract

With the spatial resolution of the Atacama Large Millimetre Array (ALMA), dusty galaxies in the distant Universe typically appear as single, compact blobs of dust emission, with a median half-light radius, ≈ 1 kpc. Occasionally, strong gravitational lensing by foreground galaxies or galaxy clusters has probed spatial scales 1-2 orders of magnitude smaller, often revealing late-stage mergers, sometimes with tantalising hints of sub-structure. One lensed galaxy in particular, the Cosmic Eyelash at z = 2.3, has been cited extensively as an example of where the interstellar medium exhibits obvious, pronounced clumps, on a spatial scale of ≈ 100 pc. Seven orders of magnitude more luminous than giant molecular clouds in the local Universe, these features are presented as circumstantial evidence that the blue clumps observed in many z ∼ 2-3 galaxies are important sites of ongoing star formation, with significant masses of gas and stars. Here, we present data from ALMA which reveal that the dust continuum of the Cosmic Eyelash is in fact smooth and can be reproduced using two Sérsic profiles with effective radii, 1.2 and 4.4 kpc, with no evidence of significant star-forming clumps down to a spatial scale of ≈ 80 pc and a star-formation rate of < 3 M ⊙ yr −1 .

Global torques and stochasticity as the drivers of massive black hole pairing in the young Universe

“…In addition, it is unclear whether the blobs found in observations of galaxies at z ∼ 1-2, that have been long interpreted as giant clumps, are instead underresolved smaller object (e.g. Ivison et al 2020). The existence of bars and spiral structures is obvious even in the low-redshift Universe; thus, we believe the source of stochasticity in the current study to be more general, and potentially acting at all redshifts.…”

[N ii] Fine-structure Emission at 122 and 205 μm in a Galaxy at z = 2.6: A Globally Dense Star-forming Interstellar Medium

“…More recently, resolved imaging of strongly lensed dusty star-forming systems suggests the presence of distinct regions of embedded high-density star formation on scales of ∼100 pc, with luminosity densities comparable to the cores of local giant molecular clouds (e.g. Swinbank et al 2010Swinbank et al , 2015Hatsukade et al 2015, though see Ivison et al 2020). This led to a 'giant clump' model of star formation in the most vigorously star-forming galaxies at high redshift, where the 100 pc-scale clumps thought to be present in objects like the Cosmic Eyelash resemble scaledup versions of the dense, 1 pc-scale cores within local giant molecular clouds.…”

“…Recent work on an-other well studied lensed system, SDP 81 (Rybak et al 2020), also suggests that a large fraction of the molecular ISM has been driven to high density, with star-formation occurring throughout the galaxy, but localised in dense star-forming complexes of size ∼200 pc with conditions comparable to the Orion Trapezium cluster in the Milky Way (see also Swinbank et al 2015). We note, however, that the interpretation of 'clumps' detected in high resolution interferometric imaging needs to be treated with caution, with Ivison et al (2020) demonstrating that previously reported star-forming clumps in the Cosmic Eyelash are in fact spurious artifacts, a result of over-cleaning of relatively low signal-to-noise data.…”

Low-frequency radio spectra of submillimetre galaxies in the Lockman Hole

“…However, we caution the reader that there has been debate as to whether observed 'clumpy' substructure is biased by effects relating to interferometric observations. The case for ∼100 pc scale substructure in the 'Cosmic Eyelash' (Swinbank et al 2010) has recently been challenged, with work showing that the inferred structure may be due to filtering and resolution effects amplifying spurious features in low S/N interferometric images (Cava et al 2018;Gullberg et al 2018;Ivison et al 2020).…”

“…However, we caution the reader that there has been debate as to whether observed 'clumpy' substructure is biased by effects relating to interferometric observations. The case for ∼100 pc scale substructure in the 'Cosmic Eyelash'(Swinbank et al 2010) has recently been challenged, with work showing that the inferred structure may be due to filtering and resolution effects amplifying spurious features in low S/N interferometric images(Cava et al 2018;Gullberg et al 2018;Ivison et al 2020).Our extreme low-frequency spectral flattened sample with α > −0.25 consists of only nine of the 42 sources in the whole sample, implying that if our assumption that this spectral flattening is caused by free-free absorption is correct, the majority of the population of submillimetre-bright sources might not be expected to show strong evidence of dense, clumpy star-forming regions on these scales. The effect of absorption on our unresolved galaxy-averaged observations must depend on the fraction of low-frequency radio emission that is embedded within and absorbed by high density gas; if a sufficient fraction is able to escape, we will see this flattening to a lesser degree.…”