The nature of giant clumps in distant galaxies probed by the anatomy of the cosmic snake

Abstract

Giant stellar clumps are ubiquitous in high-redshift galaxies. 1,2 They are thought to play an important role in the build-up of galactic bulges 3 and as diagnostics of star formation feedback in galactic discs. M and linear sizes > ∼ 1 kpc. 5,6 Recently, gravitational lensing has also been used to get higher spatial resolution. [7][8][9] However, both recent lensed observations 10,11 and models 12,13 suggest that the clumps properties may be overestimated by the limited resolution of standard imaging techniques. A definitive proof of this observational bias is nevertheless still missing. Here we investigate 1 arXiv:1711.03977v1 [astro-ph.GA] 10 Nov 2017 directly the effect of resolution on clump properties by analysing multiple gravitationallylensed images of the same galaxy at different spatial resolutions, down to 30 pc. We show that the typical mass and size of giant clumps, generally observed at ∼1 kpc resolution in high-redshift galaxies, are systematically overestimated. The high spatial resolution data, only enabled by strong gravitational lensing using currently available facilities, support smaller scales of clump formation by fragmentation of the galactic gas disk via gravitational instabilities.The multiply imaged galaxy situated behind the central region of the cluster MACSJ1206.2-084747 14 (Figure 1a) is the perfect target for our experiment. Figure 1b shows two images of this galaxy: a strongly lensed one whose peculiar shape led us to name it the "Cosmic Snake", and a more regular and less amplified one (the "Counterimage"). For the source galaxy we have estimated, from spectral energy distribution (SED) fitting (see Methods), a total stellar mass M * ∼ 4×10 10 M , and total SFR∼ 30M yrThis places the galaxy on the main sequence of star-forming galaxies at z ∼ 1-2, providing a target with physical properties comparable to typical high redshift clumpy galaxies. 15,16 According to our tailored lensing model (see Methods) the Cosmic Snake is composed by four elongated and stretched images (Figure 1c) of the southern half of the source galaxy, imaged with magnifications covering a wide range from a few to hundreds times ( Figure 1d). In contrast, the Counterimage is located in a region of nearly constant amplification, with an average magnification of µ 4.5, and shows the entire source galaxy. These extremely different magnifications allow us to inspect features of the galaxy on very different intrinsic physical scales.Using high-quality data from the Cluster Lensing And Supernova survey with Hubble (CLA-SH; 17), we have identified clumps in the Cosmic Snake and the Counterimage starting from the rest-frame UV band (HST-WFC3 filter F390W). This filter shows clumps with a high 2 Figure 1 | Overview of the Cosmic Snake and the Counterimage. a, Portion of the HST field of view showing an RGB color composite image (R = F160W, G = F110W, B =F606W) of the cluster MACSJ1206.2-0847 including the giant arc (Cosmic Snake) and its Counterimage. b, Zoomed view of the Cosmic Snake (bottom) and the Counterimage(to...

Origin of giant stellar clumps in high-redshift galaxies

“…Comparison of clump masses in high-redshift field galaxies with those magnified by gravitational lensing points to a similar overestimation due to insufficient resolution. The observed clump stellar mass increases from around 10 7 M for lensed galaxies to ∼ 10 9 M for field galaxies (Dessauges-Zavadsky et al 2017b;Cava et al 2018). Thus angular resolution is critical for correct characterization of giant clumps in high-redshift galaxies.…”

“…The interpretation of clump origins is complicated by possible overestimation of the clump masses and sizes due to limited angular resolution and sensitivity (e.g., Tamburello et al 2015Tamburello et al , 2017Cava et al 2018). The observed kpc-scale clumps may also be clusters of clumps or blending of smaller structures (Behrendt et al 2016).…”

The continuous rise of bulges out of galactic disks

“…e.g., Tamburello et al 2017;Cava et al 2017) and a detailed treatment of stellar-dynamical processes, such as stellar migration and its observational imprints (e.g., Sánchez-Blázquez et al 2009;Roskar et al 2012;Ruiz-Lara et al 2016), the dissolution and regeneration of stellar bars (e.g., Berentzen et al 2004), and, more generally, of all intrinsic and environmental parameters that shape the dynamical coupling of stars, gas and dark matter (DM) in an evolving galaxy ecosystem. For example, a process that might act toward steepening the density profile of a bulge, thereby possibly promoting nuclear starburst activity (Loose et al 1982), is adiabatic contraction of the stellar component in response to gas inflow.…”

Giant star-forming clumps?

“…Even more robust, though difficult to visualise because of the extreme gravitational amplification that gives rise to its name, are the dozen or more molecular clouds uncovered by recent 0.2-arcsec FWHM imaging in CO(4-3) of the Cosmic Snake, at z = 1.036, by Dessauges-Zavadsky et al (2019). Each of these clouds is at least an order of magnitude more massive and turbulent than those in the Milky Way today, and there is a substantial spatial disconnect between the gas and the twenty clumps seen in Hubble Space Telescope imaging (Cava et al 2018) which Dessauges-Zavadsky et al had expected to detect in CO.…”