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Astrophysics behind the gravitational wave sources

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GW190814 is a merger of a mass gap compact object with a much more massive black hole. The mass gap refers to compact objects with a mass between 2-5 solar masses. This is the first confirmed detection of a mass gap objects in the universe! The formation path of GW190814 remains a mystery. With Avi Loeb, we suggested a simple mechanism for the formation of such objects next to very massive black holes resembling GW190814. 

Please read our take on this event here to appear in ApJ Letters:

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With Adrian Hamers, Avi Loeb, and Edo Berger, we showed mass-gap black hole mergers, defined as a binary black hole merger in which one of the black holes have a mass between 2-5 solar masses, could be formed in a wide hierarchical quadruple system through secular processes involving Lidov-Kozai oscillations.

In this system, the mass-gap BH is formed in the merging of two neutron stars, which subsequently merges with a more massive BH, both due to the induced Lidov-Kozai effect.


GW190412 is a peculiar GW event with three unique properties: 1) low mass ratio between the two BHs. 2) high effective spin, and 3) a hint of orbital precession. 

With Adrian Hamers, we showed such properties could all come naturally in a hierarchical quadruple structure, as shown on the right. Such configuration, albeit not common, seems to have great potential to explain the most peculiar events. 

Please read our take on this here

In a separate work with Kenta Hotokezaka, we argue that such systems are not likely to be born in the field formation scenarios through a tidal spin-up process as the rate of such events is too high compared to the theoretical expectations.

Please read our take on this here

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As the most massive binary neutron star

detected through gravitational waves,

GW190425 challenges our assumptions

regarding the formation of binary neutron stars.

Please read our take on this event here, where we suggest an alternative formation pathway for this enigmatic event:

Our paper was featured in Quanta Magazine


Stochastic Gravitational Wave Background and Merger History of Binary Black Holes

Detection reach of Binary Black Hole (BBH) mergers is limited to the local universe. Most of the BBH mergers go undetected and only contribute to the background noise of the detectors. With Sylvia Biscoveanu and Avi Loeb we show the lack of detection of this background can inform us about the merger history of the BBHs in the universe.

Please read our take on this here

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The branching ratio of LIGO Binary Black Holes

Binary Black Holes (BBHs) could be assembled dynamically in dense stellar clusters or through field formation scenarios. The branching ratio refers to the fraction of each of these two channels to the total BBH population that LIGO observes. One can use the spin of the BBHs to differentiate between the two formation scenarios. Given this simple clue, I conclude that more than half of the LIGO BBHs have assembled dynamically in dense stellar systems. 

Please read my take on this here

Our paper was featured in AAS NOVA

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A trend in the effective spin distribution of LIGO binary black holes with mass

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With Will Farr and Enrico Ramirez-Ruiz, we showed that the ten binary black holes detected by LSC in O1/O2 runs exhibit a trend that is their mean effective spin decrease with their chirp mass while their dispersion increase with mass. We suggest that this indicates a strong contribution from both the field and dynamically assembled binary black holes to the ensemble population! Please read our work here:

On the upper mass limit of LIGO Binary Black Holes

With Will Farr, we showed that bounds on the upper mass limit of LIGO binary black holes depend on the assumptions regarding the metallicity evolution of the universe at high redshifts

We show that the bounds are not as strict as has previously been claimed in the literature if this effect is properly taken into account.

dark matter

Nature of Dark Matter

With David Spergel, we showed that the ultra-faint dwarf galaxies' density profiles are incompatible with the predicted density profiles in ultra-light dark matter models. We showed that the dynamical friction timescales for the satellites of the MW are in disagreement with such models. Read about our work here:

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Safarzadeh & Spergel (2019)

With Arif Babul and Evan Scannapieco, we showed that if the EDGES redshifted 21 cm signal is confirmed, one can set the most stringent limit on the mass of a warm dark matter particle to be above 3 Kev.

Read about our work here:

Safarzadeh et al. (2018)


Delay Time Distribution of Binary Neutron Stars

While it is assumed that Binary Neutron Stars (BNS) population follows a power-law delay time distribution (DTD), the exact shape of this DTD is not constrained. In a series of papers, we investigated how the upcoming gravitational wave (GW) events can shed light on the shape of the DTD.

In paper I, we show that using scaling relations of the host galaxies of the BNS merger events, at least 1,000 host galaxies need to be identified to constrain the DTD shape to within a 10% accuracy while large degeneracy between the slope and the minimum delay time remains unconstrained.

Read more about this in our paper:

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Safarzadeh & Berger (2019)

In paper II, we show that using the GW events' redshift distribution with the upcoming third-generation GW detector network of Cosmic Explorer and Einstein Telescope would overcome the degeneracy within one year of observations. Read more about this in our paper:

Safarzadeh et al. (2019)

In paper III, we show that the order of a few hundred

host galaxies of the GW events would suffice to constrain the DTD models if we have detailed knowledge about their star formation histories. However, this would depend on the assumed priors for the shape of the galaxies' star formation histories.

Read more about our work here:

Our paper was highlighted in AAS Nova.

Please read a popular article about this work here.

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Safarzadeh et al. (2019)


r-process enrichment of MW and its satellites with neutron star mergers

The movie below shows a zoom cosmological simulation of r-process enrichment of an MW type halo with binary neutron stars. From the top-left panel, clocks-wise are dark matter, Hydrogen, metals, and r-process, respectively. The binary neutron stars are born stochastically and receive natal kicks and, therefore, can fly out of their host halos before they merge and generate r-process elements. These simulations show the need for a prompt channel for binary neutron stars if they are the main source of r-process enrichment in the universe at early times. Read more about this in our paper here:

Safarzadeh et al. (2019)


Supersonic Turbulence

A Markov Model

For the first time, with Evan Scannapieco, we modeled the motion of gas parcels in a supersonic turbulent medium as a Markov process. The formulation of this approach is the newest breakthrough in our understanding of turbulence.

we show how addition of a gravity term the Langevin equation also reproduces the rate of material collapsing to high densities seen in turbulent simulations including

self-gravity. Read more about our work here:

The figure to the right shows the evolution of a volume density weighted PDF of a supersonic gas with time. A power-law tail develops with the same characteristics as observed in 3D hydro-dynamical simulations of supersonic turbulence.


Scannapieco & Safarzadeh (2018)

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Escape of Ly continuum photons

What happens when a photon tries to escape a turbulent medium?

Would it be easier for a photon to escape a highly turbulent medium or harder?

read more about the story of how this question is linked to the re-ionization of the universe at high redshifts in our paper published in ApJL.

Safarzadeh & Scannapieco (2016)


Primordial Magnetic Fields

Upper limit on comoving Magnetic Field

The presence of primordial magnetic fields increases the minimum halo mass in which star formation is possible at high redshifts.

With Avi Loeb, we showed that the collapse redshift of the ultra-faint dwarf galaxies can set an upper limit on the strength of a comoving magnetic field.

This is the most stringent limit achieved so far comparable to the limits from CMB bi-spectrum. Read about our work here:

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Safarzadeh & Loeb (2019)

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Biermann battery and accretion disks

I showed that accretion disks around primordial black holes

can be the origin of large scale structure magnetic field in the universe. The proposed mechanism is through Biermann battery and I showed the magnitude of this process is large such that the launched fields from the black hole can act as the origin of the magnetic field in the universe. Read about my work here:

Safarzadeh (2018)


Satellites in Galaxy Clusters

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JellyFish Galaxies

We explain the observed enhanced star formation in certain class of galaxy cluster satellites called jellyfish galaxies.
They're called Jellyfish because of their look on the sky as their gas content is being ram pressure stripped away.


We suggest that a pressurized ISM can explain the observed sample of these satellites if their molecular clouds are shielded from the hot ICM pressure by magnetic fields in their ISM. Read about our work here:

Safarzadeh & Loeb (2019)

What happens to a small galaxy that is full of gas when it enters the very hot medium of a galaxy cluster? What happens to its stellar system when its cold gas is stripped away? Would that possibly lead to the formation of ultra diffuse galaxies? Read more about this in our paper published in ApJ.

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Scannapieco & Safarzadeh (2017)

Ultra Diffuse Galaxies


New model for Quasars' host dark matter halos

Renyue Cen and I introduced a new theory on dark matter halos hosting quasars. In our model, quasars DM halo are about an order of magnitude less massive than previously suggested and quasars live in congested environments. Our model can explain the observed 2PCF between qusars and galaxies up to z=3 (where we have data to constrain the models). Read more about our work here:

This Figures shows the 2PCF at z=3 for different scenarios of our model compared with the observed quasars auto-correlation function at z=3 from Shen et al 2007. 

Improved observations at scales smaller than 1 Mpc/h can help

constrain the models further.


Thermal SZ effect

Statistical analysis of the stacked Compton-y maps

of quasars based on both HOD and CS models for the host dark matter halos, strongly constrains the 

feedback energy in quasars. We show the data can be explained without invoking feed-back energy in quasars. We show sub-arc min beam surveys can

distinguish between CS and HOD models.

Read about our work here:

This figure shows the average Compton-y map of 10,000 individual maps centered on the quasar hosts sampled from CS model at z = 1.4. 

Cen & Safarzadeh (2015a)

Cen & Safarzadeh (2015b)

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Strong lensing  effect

We further investigated how future strong lensing studies could differentiate between our model and the HOD model for quasar host halos.  HOD model predict wider image separation angles compared to ours. Read more about our work here:

Cen & Safarzadeh (2016)


Far Infrared emission from simulated galaxies​

With Yu Lu and Chris Hayward we showed that there is no need to invoke a top-heavy IMF to explain the observed sub mm number count of the galaxies in 850 micron. We showed how the observed number counts could be explained by implementing the 2D FIR SEDs of Safarzadeh et al. (2017).

Read mode about our work here:

Top heavy IMF and sub-mm emission

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Safarzadeh, Lu, Hayward (2017)

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IRX-ß relation

With Chris Hayward and Henry Ferguson, we analyzed the behavior of simulated isolated disks and mergers in IRX-ß plane that related the UV part of their SED to their infrared part. We show that the dusty star forming galaxies could be either isolated or merger systems, as both occupy the same space in IRX-ß plane. Read more about our work here:

Safarzadeh, Hayward, Ferguson (2017)

Far Infrared SEDs

With Chris Hayward, Henry Ferguson and Rachel Somerville, we analyzed the FIR SEDs of a set of hydro-dynamically simulated galaxies representative of both local and high redshift universe. We find that infrared luminosity and dust mass together are the main driver for the shape of the FIR SEDs. This can potentially solve the long standing problem of sub-mm number counts where current SAMs largely underpredict.

Read more about our work here:

This Figure shows the compilation of the SEDs

from SUNRISE in a plane of IR luminosity and Dust mass. At fixed dust mass, SEDs become hotter with

increasing LIR. At fixed LIR, SEDs become cooler

with increasing the dust mass. This is the consequnece of thermal equilibrium of dust with the

radiation field.


Safarzadeh et al. (2016)

Safarzadeh et al. (2015)

Confusion noise

High number density of the sources and large beams of FIR telescopes, leads to confusion noise. We introduce a novel method to overcome confusion noise by using Bayesian priors on the flux of the emitting sources and strong priors on their positions from HST observations.

Read more about our work here:

This Figure shows how many sources in a tiny patch of a simulated PACS160 Herschel image are connected

together, meaning they influence each other's photometry. Our method finds the blended groups and fits the pixels around them separately from the rest of the imege (the blue contour) 

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