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Zoom-in on the dust-oscured phase of galaxy formation with gravitational lenses

Periodic Reporting for period 1 - ZoomInTheDust (Zoom-in on the dust-oscured phase of galaxy formation with gravitational lenses)

Reporting period: 2016-05-01 to 2018-04-30

This project exploits gravitational lensing to investigate the early, dust-obscured, phase in the formation of elliptical galaxies in order to better understand the mechanisms driving their intense star formation and their subsequent evolution. Elliptical galaxies are in fact the oldest and most massive galaxies observed in the Universe today and, as such, they represent a challenge for the standard bottom-up scenario for dark matter structure formation, according to which massive galaxies formed late. The early stages of the formation of elliptical galaxies are best probed at far-IR/sub-mm/mm wavelengths, where the UV/optical radiation of the newly formed stars is reprocessed by dust. The project exploits observations at those wavelengths performed with the Herschel Space Observatory as part of the Cardiff-led Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) to identify proto-ellipticals that have been gravitationally lensed. The identification of lensing events is done using a new methodology I proposed and validated on the first data collected with H-ATLAS. Thanks to the increase in spatial resolution and in apparent luminosity provided by gravitational lensing, the derived sample of lensed proto-ellipticals is used to test different scenarios of galaxy formation.

The project has three main objectives:
1. Generating a large sample of gravitationally lensed, dusty star forming, proto-ellipticals from the Herschel-ATLAS data
2. Writing a state-of-the-art lens modelling code that can deal with both imaging and interferometric data.
3. Applying the code to available near-infrared/sub-millimetre/millimetre data of lensed proto-ellipticals.
4. Testing models for the formation and evolution of elliptical galaxies against the new observational results.
THE LARGEST SAMPLE OF SUB-MM SELECTED, GRAVITATIONALLY LENSED, STAR FORMING GALAXIES PRODUCED TO DATE
Work done. I have produced a complete sample of 80 candidate, gravitationally lensed, proto-ellipticals, from the H-ATLAS (Negrello et al. 2017, MNRAS, 465, 3558) and investigated their properties. I have also obtained new follow-up observations with the Sub-Millimeter Array (SMA) (Enia, Negrello et al. 2018, MNRAS, 475, 3467).
Main results. IThe statistical properties of the H-ATLAS lensed galaxies can be reproduced by a scenario in which the formation of elliptical galaxies is primarily controlled by self-regulated baryonic processes, with mergers playing only a minor role (Fig.1). The model also predicts values of the magnification factors and of the source sizes consistent with the ones we have derived from the lens modelling of individual lensed galaxies (Enia, Negrello et al. 2018, MNRAS, 475, 3467).

A STATE-OF-THE-ART LENS MODELLING CODE
Work done. I have implemented in Python a state-of-the-art lens modelling formalism, where the background source is pixellated. I have then extended the formalism to work with interferometric data, as those obtained with instruments like the SMA and the Atacama Large Millimeter Array (ALMA). I have tested the code against simulated images (Examples in Fig. 2 and Fig. 3).
Together with my PhD student Andrea Enia I have applied the new code to the SMA data of the H-ATLAS lensed galaxies (Enia, Negrello et al. 2018, MNRAS, 475, 3467; see also Fig. 4).
Main results. The derived magnification factors and source sizes are in very good agreement with what predicted by the self-regulated scenario for the formation of elliptical galaxies.

A MODEL FOR THE STATISTICS OF DUSTY PROTO-CLUSTERS
Work done. Since the most massive ellipticals are found today in the cores of rich galaxy clusters, it is expected that dusty star forming proto-ellipticals at z>1 will be associated with forming clusters cores, which are usually referred to as dusty proto-clusters. The Planck collaboration has recently published a catalogue of >2000 candidate dusty proto-clusters, with photometric redshifts z>2. I have used such a sample to further test the self-regulated formation and evolution model of elliptical galaxies by improving a formalism I developed in 2005 (Negrello et al. 2017, MNRAS, 470, 2253).
Main results. The statistics of Planck sources can be explained by the model once Poisson fluctuations in the number of high-redshift proto-clusters within the same Planck beam are accounted for. Interestingly, the only Planck overdensity for which spectroscopic redshifts of member galaxies have been obtained so far, was found to consist of two physically unrelated structures at z ∼ 1.7 and z ∼ 2, thus confirming my predictions.

PROBING THE EARLY CO-EVOLUTION BETWEEN GALAXIES AND SUPERMASSIVE BLACK HOLES
Work done. Elliptical galaxies host a supermassive black hole (SMBH) at their centre. It is believed that a past mutual interaction between the black hole growth and the build up of the mass in stars took place during the early stages of the formation of ellipticals. Gravitational lensing can potentially reveal the faint nuclear activity in these early phase. Together with my collaborators we carried out a pilot study to observe, in the X-ray, two H-ATLAS lensed galaxies at z>1.5 with specific star formation rates indicative of young ages. We successfully detected the X-ray emission associated with the SMBH (Massardi, Enia, Negrello et al. 2018, A&A, 610, 53; Fig.6). For the two targets I have performed lens modelling on HST data, while my student Adrea Enia has modelled ALMA observations (Fig. 7).
Main results. We have directly probed the SMBH-galaxy co-evolution in the early stages of the formation of elliptical galaxies.

DISSEMINATION
I have given about 15 seminars, including 6 public talks . I have also been invited to South Africa in 2017 and in 2018 to give talks at UCT, SAAO and UWC and to dis
The systematic search of gravitationally lensed galaxies in the sub-mm and their exploitation represents a new frontier in the fields of galaxy formation and evolution and, in particular, of gravitational lensing, which, so far, has been the exclusive domain of optical and radio astronomy. I have created the largest sample of sub-mm selected lensed galaxies to date by exploiting the data from the widest-area extragalactic survey ever conducted with the Herschel space observatory. This sample has already helped to advance our understanding of how massive elliptical form by supporting a self-regulated rather than a major merger driven formation scenario. It also has a huge legacy value by offering a large number of suitable targets for future, highly detailed, astrophysical and cosmological studies.
Integral number counts of lensed galaxies: measurements versus model predictions
Application of the source reconstruction code to simulated interferometric data
Application of the source reconstruction code to ALMA data
Application of the source reconstruction code to simulated imaging data
Reconstructed source using HST + ALMA + Chandra data
HST (grey) vs SMA (red) vs Chandra (blue) data in 2 H-ATLAS lensed galaxies
Application of the source reconstruction code to SMA data