Research Projects

Development of Astrochemical and Radiative Transfer Codes

     I am the main developer of the astrochemical code 3D-PDR. This is an open-source OpenMP-parallelized 3D code. It is specifically designed to treat Photodissociation Regions (PDRs) of arbitrary density distributions. It uses the Large Velocity Gradient approximation to compute the cooling functions, using a HEALPIX-based ray-tracing scheme. The code solves for the thermal balance self-consistently with the chemistry, to determine the gas temperature. It includes heating due to photoionisation and photodissociation reactions, in addition to the standard gas-phase chemistry and it accounts for self-shielding of H2 and CO against photodissociation. Suprathermal formation of CO via CH+ is included. Along with Dr. T. Haworth, we embedded 3D-PDR in the photoionisation-hydrodynamics code TORUS, creating the MPI-parallelized TORUS-3DPDR hydro-photochemical code. The tool is able to perform photoionisation calculations using an iterative Monte Carlo photon packet propagating routine. It can calculate complicated 3D UV radiation fields from single or multiple sources, including their diffuse component of the radiation field. TORUS-3DPDR is therefore able to simulate -for the first time- HII/PDR complexes of arbitrary density distributions. This makes it superior to synthetic observations obtained by post-processing hydrodynamical simulations and allows for directly making the important connection between the numerical and observational data cubes.

     Further reading:



Cosmic-ray induced destruction of CO

     Ordinary boosts of cosmic-rays (CRs) are able to destroy the molecule of CO very efficiently but not H2, creating vast amounts of the so-called ‘CO-dark’ molecular gas. We have found that this CR-induced destruction of CO is effective to interstellar medium environments expected in numerous star-forming systems throughout the Universe, particularly in the high-redshift galaxies where star formation activity is very high. It is a density-dependent effect and operates volumetrically rather than only on molecular cloud surfaces. This effect is strong enough to render Milky-Way-type giant molecular clouds CO-untraceable. Using advanced three-dimensional astrochemical models, we have found that the CO molecule may be destroyed so thoroughly that only the densest regions of the giant molecular cloud remain CO-rich. The CO/H2 abundance ratio is sensitive to the temperature of the gas (once exceeding 50K), since a significant production of the OH molecule, acting as an intermediary, determines that ratio. CO rotational line imaging of extragalactic disks, may then reveal much clumpier structures than the actual ones. As a consequence, the wide-spread CR destruction of CO expected in such systems, will make the calibration of their CO-to-H2 conversion factor challenging. It is therefore expected significant effects of CR-induced destruction of CO to occur in the so-called ‘main-sequence’ (MS) galaxies, the systems where most of the cosmic history of star formation unfolds. This is a result of their high star forming rates (implying high CR rates) and seemingly Galactic-type molecular clouds.

     Further reading:



New X-ray extragalactic observations

     I have an approved 80ks time (66k USD awarded) to observe the IRAS 18293-3413 galaxy in X-rays using the Chandra space telescope. This galaxy is a local Luminous Infrared Galaxy with high star-formation rate. It is one of the best local galaxies that represent the conditions of those in the distant Universe, just 3 billion years after Big Bang. The observations are scheduled for mid-2019. The immediate objective is to spatially map out the X-ray flux and hot gas for comparison with other wavelengths. The observations will enable to determine the fraction of CO that could be destroyed by X-rays vs cosmic-rays and to identify the dominant agent of CO-destruction. The new data will provide insights for the nature of the highly ionized gas, appearing to exist mainly along the minor axis of the system as defined by the emission map of molecular gas; this indicates the existence of an unprecedented outflow driven by stron and pure star formation activity. The new observations will generate a number of different projects, appropriate for postgraduate students: 1) testing CO-destruction theories (X-rays versus cosmic-rays), 2) multiple-gas phases and thermal balance studies, 3) studying of star formaction activity, 4) studying outflows and galaxy quenching. This project is in close collaboration with the Harvard-Smithsonian Centre for Astrophysics.



New high-redshift ALMA data

     I have [CI] (1-0), (2-1) and CO(4-3) data of three ‘main-sequence’ galaxies (D3a-15504, zC-406690, zC-400569), at a redshift of z~2.3 (age of the Universe ~3 billion years old).  I also have a second observational program to observe zC-400569 and a nearby galaxy in CO(4-3) and CO(3-2), corresponding to CO(3-2) and CO(2-1) for the nearby one. These galaxies have been observed in Hα lines by other groups who claim to have identified falling rotation curves. This suggests that they are baryon-dominated disks, which is in contradiction with the ΛCDM Cosmology model. The targeted objects have very high star-forming rates and are thus expected to have high cosmic-ray ionization rates. Thus according to the cosmic-ray induced destruction of CO (see above), the [CI] fine-structure lines may identify molecular gas at large galactocentric radii and further than  ~10 kpc as currently probed by Hα. This will reveal the properties of molecular gas in these galaxies, which is the next best diagnostic to study the kinematical structure after atomic hydrogen (HI). It is therefore expected that these observations will provide the most unbiased picture of the morphology and the resulting rotation curves of these galaxies, thus significantly contributing to the current research of Dark Matter distribution in the Early Universe. The findings from these observations will inevitably have a huge potential to the community studying galaxy evolution models.

     Further reading:



Fine-structure lines as diagnostic for cloud-cloud collision activity

     The dominant process for the galactic star formation is believed to be the collision between giant molecular clouds. The collision triggers the gravitational collapse of the merged clouds, creating star clusters. Identifying these collisions is a complicated task, involving a particular pattern in the position-velocity diagrams, the so-called "bridge-effect". So far the community has focused on searching for this signature in molecular lines i.e. in low-J CO lines. State-of-the-art astrochemical simulations have shown that apart from molecular lines, atomic and ionic lines (known as "fine-structure lines") can also be used as diagnostics, particularly in the case when the collision is already evolved and the signature in molecular lines has been diminished. This is because fine-structure lines are predominantly emitted from the outer envelope of the clouds, thus carrying the bridge-effect signature longer than molecular lines do (the latter emitted from the innermost parts of the cloud). Recent observations in the [CII] 158μm line using the Stratospheric Observatory for Infrared Astronomy (SOFIA) strongly support the case that the Infrared Dark Cloud G035.39-00.33, may currently undergo collision. These latter observations are the first ones suggesting that fine-structure lines can be used as diagnostic for cloud-cloud collision activity.

     Further reading:

Dr. Thomas G. Bisbas