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...

We study the effect of sub-grid physics, galaxy mass, structural parameters and resolution on the fragmentation of gas-rich galaxy discs into massive star forming clumps. The initial conditions are set up with the aid of the ARGO cosmological hydrodynamical simulation. Blast-wave feedback does not suppress fragmentation, but reduces both the number of clumps and the duration of the unstable phase. Once formed, bound clumps cannot be destroyed by our feedback model. Widespread fragmentation is promoted by high gas fractions and low halo concentrations. Yet giant clumps M > 10 8 M lasting several hundred Myr are rare and mainly produced by clump-clump mergers. They occur in massive discs with maximum rotational velocities V max > 250 km/s at z ∼ 2, at the high mass end of the observed galaxy population at those redshifts. The typical gaseous and stellar masses of clumps in all runs are in the range ∼ 10 7 − 10 8 M for galaxies with disc mass in the range 10 10 − 8 × 10 10 M . Clumps sizes are usually in the range 100 − 400 pc, in agreement with recent clump observations in lensed high-z galaxies. We argue that many of the giant clumps identified in observations are not due to in-situ fragmetation, or are the result of blending of smaller structures owing to insufficient resolution. Using an analytical model describing local collapse inside spiral arms, we can predict the characteristic gaseous masses of clumps at the onset of fragmentation (∼ 3 − 5 × 10 7 M ) quite accurately, while the conventional Toomre mass overestimates them. Due to their moderate masses, clumps which migrate to the centre have marginal effect on bulge growth.

We analyse stellar masses of clumps drawn from a compilation of star-forming galaxies at 1.1 < z < 3.6. Comparing clumps selected in different ways, and in lensed or blank field galaxies, we examine the effects of spatial resolution and sensitivity on the inferred stellar masses. Large differences are found, with median stellar masses ranging from ∼ 10 9 M for clumps in the often-referenced field galaxies to ∼ 10 7 M for fainter clumps selected in deep-field or lensed galaxies. We argue that the clump masses, observed in non-lensed galaxies with a limited spatial resolution of ∼ 1 kpc, are artificially increased due to the clustering of clumps of smaller mass. Furthermore, we show that the sensitivity threshold used for the clump selection affects the inferred masses even more strongly than resolution, biasing clumps at the low mass end. Both improved spatial resolution and sensitivity appear to shift the clump stellar mass distribution to lower masses, qualitatively in agreement with clump masses found in recent high-resolution simulations of disk fragmentation. We discuss the nature of the most massive clumps, and we conclude that it is currently not possible to properly establish a meaningful clump stellar mass distribution from observations and to infer the existence and value of a characteristic clump mass scale.

Massive gas-rich galaxy discs at z ∼ 1−3 host massive star-forming clumps with typical baryonic masses in the range 10 7 − 10 8 M ⊙ which can affect the orbital decay and concurrent growth of supermassive black hole (BH) pairs. Using a set of high-resolution simulations of isolated clumpy galaxies hosting a pair of unequal-mass BHs, we study the interaction between massive clumps and a BH pair at kpc scales, during the early phase of the orbital decay. We find that both the interaction with massive clumps and the heating of the cold gas layer of the disc by BH feedback tend to delay significantly the orbital decay of the secondary, which in many cases is ejected and then hovers for a whole Gyr around a separation of 1-2 kpc. In the envelope, dynamical friction is weak and there is no contribution of disc torques: these lead to the fastest decay once the orbit of the secondary BH has circularised in the disc midplane. In runs with larger eccentricities the delay is stronger, although there are some exceptions. We also show that, even in discs with very sporadic transient clump formation, a strong spiral pattern affects the decay time-scale for BHs on eccentric orbits. We conclude that, contrary to previous belief, a gas-rich background is not necessarily conducive to a fast BH decay and binary formation, which prompts more extensive investigations aimed at calibrating event-rate forecasts for ongoing and future gravitational-wave searches, such as with Pulsar Timing Arrays and the future evolved Laser Interferometer Space Antenna.

There are growing amount of very high-resolution polarized scattered light images of circumstellar disks. Naturally, the question arises whether the circumplanetary disk forming around nascent planets can be detected with the same technique. Here we created scattered light mock observations at 1.2 and 1.6 microns for instruments like SPHERE and GPI, for various planetary masses and disk inclinations. We found that the detection of a circumplanetary disk is significantly favored if the planet is massive (≥ 5M Jup ) and the system is nearly face-on (≤ 30 • ). Its detection is hindered by the neighboring circumstellar disk that also provides a strong polarized flux. However, the comparison between the P I and the Q φ maps, as well as the contrasts between the J and H bands are viable tools to pinpoint the presence of the circumplanetary disk within the circumstellar disk, as the two disks are behaving differently on those images.