Welcome to the Mandelker Group 

Hi, I'm Nir Mandelker, senior lecturer at the Racah Institute of Physics at the Hebrew University of Jerusalem. My research group focuses on galaxy formation during its peak phase, at "Cosmic Noon" during the first few billion years after the Big Bang. We are interested in how these early galaxies acquire fresh gas from the intergalactic medium, and how hydrodynamic and gravitational instabilities help turn this gas into stars.


Galaxies are not closed boxes

Rather, they are intimately linked to large reservoirs of gas surrounding them within their host dark matter halos (the circumgalactic medium, CGM), and outside the halos (the intergalactic medium, IGM). These shape galaxy evolution through cycles of gas accretion, star-formation, galactic outflows and re-accretion, known as the cosmic baryon cycle. A detailed understanding of the physical properties of the C/IGM is thus crucial for understanding galaxy evolution. During cosmic noon, the peak phase of galaxy formation at redshift z~(1-6), the baryon cycle is far more intense than in the local Universe. The growth of large-scale structure triggers thermal instabilities and creates a multiphase IGM. Cold streams of dense gas flowing along cosmic web filaments penetrate the hot CGM, filling the halo with Lyman alpha emission and potentially leading to globular cluster formation. Intense accretion triggers violent disk instability (VDI) in galactic disks, resulting in the formation of giant clumps and intense starbursts.

Cosmic Fog: Formation, evolution, and implications of Multiphase gas in the high-z cosmic web

The IGM contains most of the baryons in the Universe and sets the boundary conditions for gas flows into galactic halos. However, resolving it in cosmological simulations is notoriously difficult, since resolution elements typically have fixed mass yielding poor spatial resolution in low density regions.

Our group has led several collaborations pioneering novel methods to greatly enhance the resolution in the CGM and the IGM, without affecting the resolution in the galaxy and without prohibitive computational cost.  In one pilot study, we used this method to simulate a Milky-Way mass halo to z = 0, revealing much more small scale structure and increased HI in the CGM, in better agreement with observations. In another, we zoomed-in on a large region of the cosmic-web, intergalactic sheets and filaments surrounding galaxies, revealing for the first time the complex, multiphase, internal structure of these cosmic environments and their impact on galaxy evolution.

The multiphase IGM. Shown is the evolution of gas temperature in the IGM surrounding two massive halos in a high-resolution cosmological simulation. The halos lie in a cosmic sheet shown edge-on and face-on in the left and right panels respectively. The gravitational collapse of the sheet and the growth of large-scale structure triggers an accretion shock, visible around the sheet in the edge-on view and traveling from left-to-right in the face-on view. Most halos, marked with circles, lie along cold filaments making up the cosmic web, while the sheet in between is multiphase with hot and cold regions. Adapted from Mandelker et al. 2019b

Stability of cold flows

Idealized studies of the evolution of cold streams in the CGM of massive high-z galaxies. Shown are density maps at late times from Ramses simulations of cold, dense cylindrical streams flowing supersonically through a hot, diffuse medium, representing astrophysical streams in halos. The top row, adapted from Aung et al. 2019, studies the effect of self-gravity. The middle row, adapted from Mandelker et al. 2020a, studies the effect of cooling. The bottom row shows the impact of magneitc fields together with cooling. Each of these three effects can prevent stream disruption by KHI, with many implications for the thermal, morphological, and dynamic state of the gas that joins the galaxy. 

In the developing picture of galaxy formation, the main mode of accretion in massive star-forming galaxies (SFGs) at z>~2 is through cold streams of dense gas flowing along cosmic web filaments. The high density in these streams allows them to penetrate the hot CGM and reach the central galaxy. Recent observations in both gas absorption and emission reveal large quantities of cool, dense gas in the CGM with spatial and kinematic properties resembling cold streams. However, the evolution of these streams in the CGM is still widely disputed, and it is unclear whether or not they penetrate all the way to the galaxy, what their thermal and morphological properties are, and how these affect the emission and absorption signatures of the CGM or galaxy growth. As current cosmological simulations do not properly resolve the streams, different computational methods disagree on their fate. 

As an alternative to cosmological simulations, I have been spearheading a collaboration studying the evolution of cold streams using detailed analytic models and high resolution,
idealized simulations, run with the AMR code RAMSES, where new physical effects are added and studied one at a time. 

Formation of Metal-Poor Globular Clusters in Cold Streams

Globular clusters are some of the most enigatic objects in the Universe. These are extremely dense stellar clusters, packing roughly one-million suns into a ball just a few lightyears across. For comparison, the distance between the sun and our next closest star is roughly 6 light years... Globular clusters can be found in and around basically every galaxy in the local Universe, yet their origin remains an ipen question, as does the reason for seemingly different types (populations) of globular clusters. 

We are studying the formation of globular clusters in the high redshift Universe, both in gravitationally unstable intergalactic streams, and in gian star-forming clumps within violently unstable galactic discs.

In Mandelker et al 2018, we proposed that cold filamentary accretion in high-z galaxies can lead to the formation of star-forming clumps in the CGM without associated dark-matter substructure, and that these clumps could be the birth places of metal-poor globular clusters (GCs), whose formation had long been a mystery. I derived an analytic model for the properties of streams as a function of halo mass and redshift, assessed when streams would be gravitationally unstable, when this could lead to star-formation in the halo, and when it may result in GC formation. This was found to be possible at z>~4.5, particularly in the turbulent “eyewall” at ~0.3Rv where stream collisions can drive very large densities. My model can account for numerous observed properties of GC systems, and the prediction that GCs form along circumgalactic filaments at high-redshift will be testable with JWST.