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HIDDeN - Exploring the Hidden Dusty Nuclei of Galaxies

Periodic Reporting for period 2 - HIDDeN (HIDDeN - Exploring the Hidden Dusty Nuclei of Galaxies)

Reporting period: 2020-04-01 to 2021-09-30

Luminous infrared galaxies (LIRGs) emit most of their luminosity in the far-infrared part of the spectrum. They are mainly powered by extreme bursts of star formation and/or Active Galactic Nuclei (AGNs; accreting/growing supermassive black holes (SMBHs)) in their centers. LIRGs are the closest examples of rapid evolution in galaxies and a detailed study of LIRGs is critical for our understanding of the cosmic evolution of galaxies and SMBHs. Centers of some LIRGs are deeply obscured and unreachable at optical, infrared and even X-ray wavelengths. These hidden nuclei therefore represent a largely unexplored phase of the growth of central regions with their SMBHs. Large growth spurts are suspected to occur when the SMBHs are deeply embedded. Deeply obscured nuclei thus can provide new constraints on the AGN duty cycle, give the full range of environments and astrophysical processes that drive the growth of SMBHs and the central stellar distributions, and help to complete the picture of connections between the host galaxy and the SMBH.

In the HIDDeN project we use mm and submm observational methods to reach behind the curtain of dust in the most obscured centres of LIRGs, allowing us to undertake ground-breaking studies of heretofore hidden rapid evolutionary phases of nearby galaxy nuclei. HIDDeN takes advantage of new and emerging opportunities to address the nature of near-field, and redshift z=1-2, obscured AGNs/starbursts and their associated molecular inflows and outflows in the context of their evolution and the starburst-AGN connection.

To understand how the Universe evolves and what the main driving mechanisms is of fundamental importance also for the evolution of our societies. Humans have always looked up to the stars to wonder what is out there – it has shaped our view of ourselves, what we can become, and our place in the Universe. Consider the importance of Astronomy for us finding our way: People have been astronomers for a long time and used the stars to navigate. Today, navigation is dependent on our observations of growing supermassive black holes at the beginning of the Universe. To always reach further lies in our nature.
The HIDDeN project helps us see behind the veil of dust that enshrouds the dark hearts of galaxies. It reveals what lies hidden, and which role that buried activity plays in the overall evolution of galaxies.
We obtained observing time at ALMA for the “CONquest” project where we used emission from vibrationally excited HCN (HCN-vib) to detect Compact Obscured Nuclei (CONs) in a sample of LIRGs in the local Universe. The main result from CONquest, so far, is that extreme levels of nuclear dust and gas (hiding an invisible luminosity source) are far more common among LIRGs than previously thought. The incidence is strongly luminosity dependent with as many as 40% of the most highly luminous galaxies harbouring CONs. We have also learned that they are in a state of rapid evolution with massive flows of gas fuelling their centres. These inflows coexist with unusual, dense and sometimes collimated outflows.
Two more studies are being completed from the CONquest survey. One of them investigates the dense gas properties of the most luminous LIRGs and their outflows. The other focuses on the lower luminosity, and more nearby, objects in CONquest to study the link between dense gas concentrations, dynamics and nuclear activity.

With ALMA we have also carried out high-resolution observations of HCN-vib emission in the LIRG IC860. This is a pre-cursor to the upcoming “CONfirm” study, which will map the inner regions on a sample of nearby CON-LIRGs. In IC860 we find that the inflowing gas actually obscures the outflow – which shows that both in- and outflows are very dense and gas-rich. The unprecedented high resolution allows us to determine dynamical masses and put limits on the nature of the embedded activity. Detailed, high-resolution (2-3 pc, 20-30 milli arcseconds), Detailed, high-resolution (2-3 pc, 20-30 milli arcseconds), ALMA studies of one of the most nearby galaxies with a buried nucleus, NGC1377, reveal an extremely collimated molecular jet-like molecular outflow (seen in CO emission) – while no radio jet has yet been detected. Our results suggest that the outflow of NGC1377 may be a new form of feedback process, different from that normally seen in AGNs and starbursts. This process may be part of a feeding cycle that helps the nuclear region grow.

We have also carried out APEX, NOEMA and ALMA studies of H2S and HCN in the outflows of nearby LIRGs. Discovered (using VLA observations at radio wavelengths) that a new maser molecule, CH2NH, is a promising new probe of obscured galaxy nuclei. We are also completing detailed VLBI, VLA and eMerlin studies of the radio continuum and cm-wave l-doubling HCN-vib absorption in a sample of northern LIRG-CONs.
Our observational programmes exploit new techniques to probe behind the veil of dust in extremely obscured galaxies. The ALMA CONquest survey uses a complete, sample that gives the first systematic view of how common levels of the extreme obscuration are. We use the ability of the HCN molecule to couple to a buried, high surface-brightness radiation field and subsequently reemit this emission at longer wavelengths. Thus the HCN-vib line serves as as a probe of hidden luminosity sources. We are using the highest spatial resolution available in today, where ALMA and NOEMA are the most recent state-of-the art additions to the facilities, and crucial to the success of HIDDeN.

We have accepted A-rated high-resolution projects (e.g. CONfirm) that are key in revealing the nature of the hidden luminosity sources in the most obscured objects. The combination of high sensitivity, spatial and spectral resolution offered by ALMA allow us to measure dynamical masses, separate in- and outflows, study the structure of the gas and dust and map out the intensity distribution. The CONs are disk-outflow systems, and detailed knowledge of the source structure let us exploit new radiative transfer approaches.

We also have accepted programmes on the upcoming JWST to explore the star formation properties that surround the embedded activity of nearby CON-LIRGs. In addition, we aim to investigate if the opaque nuclei really are completely inaccessible in the infrared, or if emission can escape at infrared wavelengths. JWST will offer the best sensitivity and resolution at near and mid-infrared wavelengths to date. We also have accepted programmes on the highly competitive ESO MUSE instrument to probe the larger scale optical gas and dust structures of the CON-LIRGs. With these observations we will test the notion that the growth in CON-LIRGs is episodic and that we can find the remnants of past activity, as well as potential evidence of feedback process that emerge from larger scale starbursts. Our recent HST optical data on LIRG-CONs provides further, crucial information on the host galaxy properties.


The fact that we already have A-rated programmes on some of the worlds most competitive and state-of-the art telescopes ensures the continued success of HIDDeN.

Glossary:
Atacama Large mm/submm Array=ALMA
Atacama Pathfinder EXperiment=APEX
NOrthern Extended Millimeter ARRAY=NOEMA
VLA=Very Large Array
VLBI=Very Long Baseline Interferometry
EVN=European VLBI Network
JWST= James Webb Space Telescope
MUSE=The Multi Unit Spectroscopic Explorer
HST= Hubble Space Telescope
ALMA CO 3-2 image of NGC1377 showing molecular jet, narrow wind and disk