Selected Research Projects
Highlights from my Postdoc research at Harvard-CfA
Highlights from my PhD years at Tel Aviv University
The far-UV Interstellar Radiation Field in Galactic Disks
Bialy (2020), ApJ submitted ArXiv paper link
Analytic (top) and numerical (bottom) models for the intensity of the far-UV interstellar radiation field relative to the star-formation rate, as a function of properties of the galactic disk: dust-to-gas ratio, galactic radius, gas density and star-formation rate.
The far-ultraviolet (FUV) interstellar radiation field is formed by the summed flux arriving from the most massive (O,B stellar types) stars in galaxies. The intensity of the FUV radiation is very important both for determining the chemical state of the gas (i.e., atomic versus molecular), and the thermal state (i.e., cold T~100K or warm T~10,000K). Theoretical models suggest that heating by the FUV interstellar radiation field may be a mechanism for regulation of star formation in galaxies.
Using a numerical model which follows the distributions of OB associations in galactic disks, in combination with an analytic model,
I have characterized the FUV interstellar radiation field in galactic disks of various properties: disk scale radius, gas density, dust abundance, and star-formation rate. The dust plays a key role as galaxies that are more dusty, result in more absorption of the FUV radiation and thus a smaller FUV intensity. Following these models, we see that galaxies in early cosmic epochs in which heavy elements and dust were less common, have systematically higher FUV interstellar radiation fields, which may potentially result in more gas heating and less efficient star-formation.
Cold Clouds as Cosmic-Ray Detectors
Bialy (2020): Nature Communication Physics, 3, 32 (2020)
Cosmic Rays are energetic particles: protons, electrons, and heavy nuclei, traveling throughout the Galaxy, and also partly escaping to intergalactic space. The low energy cosmic rays (E<1 GeV) (which are still very energetic compared to particles with thermal energies) play a key role in the physics of the interstellar medium.
The dark cloud Barnard 68 in visible and near-infrared wavelength (composite).
These cosmic rays penetrate into the cold dark interiors of molecular clouds (for instance, see the image above), where they interact with the gas, resulting in ionizations, dissociation, and excitations of molecular hydrogen, H2 (H2 dominates the bulk of the interstellar cloud mass). These cosmic-ray ionizations, are the dominant gas heating mechanism in the cloud, controlling its temperature, and thus also affecting its stability against gravitational collapse and star formation. The penetrating cosmic-rays also drive a rich chemical network leading to the formation of various molecules (e.g., CO, OH, and water), and the ionization also introduce a coupling mechanism of the clouds to the magnetic field of the Galaxy.
Unfortunately, the cosmic-ray ionization rate is highly uncertain. This is because these cosmic rays are efficiently deflected by the solar wind, and thus direct measurements of their flux are unreliable. The cosmic-ray ionization rate is deduced instead by indirect methods, relying on observations of rare molecular species (such as H3+, OH+, H2O+) and using chemical models that include various unknown parameters and require various assumptions. Over the last decades, estimates of the cosmic ray-ionization rate resulted in different contradicting values, ranging over ~3 orders of magnitude.
However, there may be a better way to constrain this uncertain parameter: H2, the bulk of the cloud mass, may reveal the true rate of cosmic ray ionization, and also the properties of the low energy cosmic rays.
I have shown (Bialy 2020, Nature Communication Physics, 3, 32) that the cosmic rays that penetrate into the molecular cloud interior, not only ionize the H2 molecules but also they lead to excitations of the various rotational and vibrational energy levels of H2. These excited states decay to the ground state via the emission of infrared photons. These infrared transitions may be observed with ground-based and space-based telescopes.
As the flux in these infrared transitions is directly proportional to the cosmic ray ionization rate in the clouds, these clouds may be used as giant "cosmic-ray detectors"
(this is in analogy to the giant terrestrial [neutrino] detectors, e.g.: Super-Kamiokande)
An observational program to detect this cosmic-ray signal in four neighboring interstellar clouds is underway (3 nights were assigned on the MMT observatory in Arizona for Dec. 2020).
If successful, this would be the first-ever measurement of these lines in cold dark clouds, and will open a new way for constraining the properties of the mysterious low-energy cosmic rays in the Galaxy!
Chemistry in the Turbulent Interstellar Medium
Density slices through subsonic (left) & supersonic (right) magnetohydrodynamic simulations. As the gas becomes supersonic, strong density fluctuations develop. These fluctuations modify the formation-destruction rates of molecules resulting in fluctuations in their abundances. This effect may be observed through spectroscopic observations of clouds in the Galaxy, in the infrared, sub-mm and radio wavelengths.
From Harvard's press release article: (Friday, November 22, 2019)
Over two hundred molecules have been discovered in space, some (like Buckminsterfullerene) very complex with carbon atoms. Besides being intrinsically interesting, these molecules radiate away heat, helping giant clouds of interstellar material cool and contract to form new stars. Moreover, astronomers use the radiation from these molecules to study the local conditions, for example, as planets form in disks around young stars.
The relative abundance of these molecular species is an important but longstanding puzzle, dependent on many factors from the abundances of the basic elements and the strength of the ultraviolet radiation field to a cloud’s density, temperature, and age. The abundances of the small molecules (those with two or three atoms) are particularly important since they form stepping stones to larger species, and among these the ones that carry a net charge are even more important since they undergo chemical reactions more readily. Current models of the diffuse interstellar medium assume uniform layers of ultraviolet illuminated gas with either a constant density or a density that varies smoothly with depth into the cloud. The problem is that the models' predictions often disagree with observations.
Decades of observations have also shown, however, that the interstellar medium is not uniform but rather turbulent, with large variations in density and temperature over small distances. CfA astronomer Shmuel Bialy led a team of scientists investigating the abundances of four key molecules -- H2, OH+, H2O+, and ArH+ -- in a supersonic (with motions exceeding the speed of sound) and turbulent medium. These particular molecules are both useful astronomical probes and highly sensitive to the density fluctuations that naturally arise in turbulent media. Building on their previous studies of the behavior of molecular hydrogen (H2) in turbulent media, the scientists performed detailed computer simulations that incorporate a wide range of chemical pathways together with models of supersonic turbulent motions under a variety of excitation scenarios driven by ultraviolet radiation and cosmic rays. Their results, when compared to extensive observations of molecules, show good agreement. The range of turbulent conditions is wide and the predictions correspondingly wide, however, so that while the new models do a better job of explaining the observed ranges, they can be ambiguous and explain a particular situation with several different combinations of parameters. The authors make a case for additional observations and a next-generation of models to constrain the conclusions more tightly.
Catalogue for Astrophysical Turbulence Simulations (CATS)
I am excited to be part of the CATS collaboration, with the aim of making science more accessible and open-source!
CATS (Catalog for Astrophysical Turbulence Simulations)
is an open-source database of magneto-hydrodynamical simulations, open, and free for download, ready for science!
Check out our website https://www.mhdturbulence.com/
The paper is coming soon (ApJ. accepted!)
The HI-to-H2 Transition in a Turbulent Medium
The calculated HI map for a supersonically turbulent slab irradiated by isotropic (left) or beamed (right) UV radiation field. The HI is formed by H2 photodissociation. The structure seen in the two panels is a result of the density fluctuations produced in supersonic turbulence.
Observations in the Galaxy, as well as observations in external galaxies, show that the formation of new stars occurs within the molecular phase of the turbulent interstellar medium (ISM) of galaxies. The atomic-to-molecular (the HI-to-H2) transition is thus of fundamental importance for star-formation and the evolution of galaxies.
In the ISM, H2 is formed primarily on the surfaces of dust grains and is destroyed by UV photons emitted from stars. At the surface of molecular clouds, the gas is primarily atomic owning to the effective UV destruction. However, as one enters deeper and deeper into a cloud, the UV radiation is getting absorbed by gas and dust particles until at some point the UV is attenuated enough so that the gas becomes predominantly molecular - this is the point of "the HI-to-H2 transition".
If the medium is also supersonically turbulent, the gas is constantly being compressed by the turbulent motions, and strong density perturbations arise. The effect of the density perturbations is to alter the rates of H2 formation and destruction (as both depend on density), which change the H/H2 structure.
In this study, we modeled the HI-to-H2 transition in a turbulent medium. For that, we used a set of numerical simulations that solve the ideal magnetohydrodynamic (MHD) equations, together with the H2 formation-destruction rate equations.
Using our models of the HI and H2 structures, we have derived a novel method how radio (21 cm) observations of the HI gas may be used to constrain important properties of the interstellar turbulence, these are: (1) the sonic Mach number, and (2) the length-scale of the turbulence driving mechanism. This may shed light on the source of turbulence in the ISM.
Reference: ApJ 843 92
arXiv 1703.08549 (open access)
HI-to-H2 Transition Layers in the Star-Forming Region W43
Observed atomic and molecular mass surface densities. The HI density is ~10 larger than expected from HI-to-H2 theories.
W43 is an extremely active star-forming region towards the center of our Galaxy. Recent observations reveal extremely large atomic mass surface densities, much larger than what is expected from theory. In this work, we propose an explanation for this theory-observation discrepancy.
Reference: ApJ 835 126
arXiv 1612.02428 (open access)
Water Formation During the Epoch of First Metal Enrichment
The water fractional abundance as a function of temperature and UV-to-density ratio, as calculated by the model for 0.1% solar metallicity. The black line is the Milky Way abundance, showing that water may be abundant even at extremely low metallicity, when temperature is above ~200 K.
When did water first formed in the Universe? In this theoretical study, we show that water could have formed at early cosmic times, during first-galaxy-assembly, some 13 billion years ago.
arXiv (open access)
Reference: ApJ Letters 804 29