What do I do?
I am an astrophysicist interested in applying state-of-the-art computational tools to
(i) the problems in galaxy formation and evolution and large-scale structure cosmology
(ii) Developing hydrodynamical simulations, N-body simulations, and semi-analytical models of galaxy formation to make predictions and create mock catalogues to compare with observations
(iii) analyzing observational data and making robust observational strategies
(iv) constraining theory using observations.
My Research Interests
I'm primarily interested in uncovering "The Synergistic Effects of Physical Processes on Galaxy Evolution", as described below.
Recent Works
An Atlas of Gas Motions in the TNG-Cluster Simulation: from Cluster Cores to the Outskirts (Ayromlou et al. 2023c)
Galaxy clusters are unique laboratories for studying astrophysical processes and their impact on halo gas kinematics. Despite their importance, the full complexity of gas motion within and around these clusters remains poorly known. This paper is part of a series presenting first results from the new TNG-Cluster simulation, a suite comprising 352 high-mass galaxy clusters including the full cosmological context, mergers and accretion, baryonic processes and feedback, and magnetic fields. Studying the dynamics and coherence of gas flows, we find that gas motions in galaxy cluster cores and intermediate regions are largely balanced between inflows and outflows, exhibiting a Gaussian distribution centered at zero velocity. In the outskirts, even the net velocity distribution becomes asymmetric, featuring a double peak where the second peak reflects cosmic accretion. Across all cluster regions, the resulting net flow distribution reveals complex gas dynamics. These are strongly correlated with halo properties: at a given total cluster mass, unrelaxed, late-forming halos with less massive black holes and lower accretion rates exhibit a more dynamic behavior. Our analysis shows no clear relationship between line-of-sight and radial gas velocities, suggesting that line-of-sight velocity alone is insufficient to distinguish between inflowing and outflowing gas. Additional properties, such as temperature, can help break this degeneracy. A velocity structure function (VSF) analysis indicates more coherent gas motion in the outskirts and more disturbed kinematics towards halo centers. In all cluster regions, the VSF shows a slope close to the theoretical models of Kolmogorov (1/3), except within 50 kpc of the cluster centers, where the slope is significantly steeper. The outcome of TNG-Cluster broadly aligns with observations of the VSF of multiphase gas across different scales in galaxy clusters, ranging from 1 kpc to Megaparsec scales.
Figure: Gas kinematics (radial velocity) in and around a massive halo of the TNG-Cluster simulation. The halo is closed, meaning all of its baryons are confined within its boundary (the closure radius is smaller than the virial radius). In contrast, its kinematics is in a state of significant evolution.
The Closure Radius: Feedback reshapes the distribution of baryons (Ayromlou et al. 2023b)
We explore three sets of cosmological hydrodynamical simulations, IllustrisTNG, EAGLE, and SIMBA, to investigate the physical processes impacting the distribution of baryons in and around haloes across an unprecedented mass range of 108<M200c/M⊙<1015, from the halo centre out to scales as large as 30R200c. We demonstrate that baryonic feedback mechanisms significantly redistribute gas, lowering the baryon fractions inside haloes while simultaneously accumulating this material outside the virial radius. To understand this large-scale baryonic redistribution and identify the dominant physical processes responsible, we examine several variants of TNG that selectively exclude stellar and AGN feedback, cooling, and radiation. We find that heating from the UV background in low-mass haloes, stellar feedback in intermediate-mass haloes, and AGN feedback in groups (1012≤M200c/M⊙<1014) are the dominant processes. Galaxy clusters are the least influenced by these processes on large scales. We introduce a new halo mass-dependent characteristic scale, the closure radius Rc, within which all baryons associated with haloes are found. For groups and clusters, we introduce a universal relation between this scale and the halo baryon fraction: Rc/R200c,500c−1=β(z)(1−fb(<R200c,500c)/fb,cosmic), where β(z)=α(1+z)γ, and α and γ are free parameters fit using the simulations. Accordingly, we predict that all baryons associated with observed X-ray haloes can be found within Rc∼1.5−2.5R200c. Our results can be used to constrain theoretical models, particularly the physics of supernova and AGN feedback, as well as their interplay with environmental processes, through comparison with current and future X-ray and SZ observations..
Figure: The first illustration of the baryon fraction in a cosmological simulation. The blue/red regions have baryon fraction below/above the cosmic value. The circle shows the halo virial radius within which the halo is dark matter dominated. The missing baryons can be found beyond the halo boundary within a characteristic scale, called the closure radius.
The physical origin of galactic conformity (Ayromlou et al. 2023a)
We employ several galaxy formation models, particularly, L-GALAXIES, IllustrisTNG, and EAGLE, as well as observational samples from SDSS and dark energy spectroscopic intstrument (DESI), to investigate galactic conformity, the observed correlation between the star-formation properties of central (primary) galaxies and those of their neighbours. To analyse the models and observations uniformly, we introduce CENSAT, a new algorithm to define whether a galaxy is a central or a satellite system. We find that the conformity signal is present, up to at least 5 Mpc from the centres of low- and intermediate-mass centrals in the latest version of L-GALAXIES (Ayromlou et al. 2021b), IllustrisTNG, and EAGLE, as well as in SDSS and DESI observational samples. In comparison, the conformity signal is substantially weaker in an older version of L-GALAXIES (Henriques et al. 2020). One of the main differences between this older model and the other models is that except for satellites within the boundaries of massive cluster haloes, it neglects ram-pressure stripping of the gas reservoirs of galaxies (e.g. in groups and cluster outskirts). Our observational comparisons demonstrate that this difference significantly affects the observed large-scale conformity signal. Furthermore, by examining the contribution of backsplash, fly-by, central, and satellite galaxies to the conformity signal, we show that much, but not all, of it arises from primary galaxies near massive systems. Remaining tensions between the models and observations may be solved by modifying the physical prescriptions for how feedback processes affect the distribution and kinematics of gas and the environment around galaxies out to scales of several Megaparsecs.
Figure: The galactic conformity signal. The L-Galaxies model (Ayromlou et al. 2021b) is in very good agreement with observations.
Galaxy Formation with L-GALAXIES (Ayromlou et al. 2021b)
In this work, we present a variation of the recently updated Munich semi-analytical galaxy formation model, L-Galaxies, with a new gas stripping method. Extending earlier work, we directly measure the local environmental properties of galaxies to formulate a more accurate treatment of ram-pressure stripping for all galaxies. We fully re-calibrate the modified L-Galaxies model using a Markov Chain Monte Carlo (MCMC) method with the stellar mass function and quenched fraction of galaxies at 0<z<2 as constraints. Due to this re-calibration, global galaxy population relations, including the stellar mass function, quenched fractions versus galaxy mass, and HI mass function are all largely unchanged and remain consistent with observations. By comparing to data on galaxy properties in different environments from the SDSS and HSC surveys, we demonstrate that our modified model improves the agreement with the quenched fractions and star formation rates of galaxies as a function of environment, stellar mass, and redshift. Overall, in the vicinity of clusters and groups at z=0, our new model produces higher quenched fractions and stronger environmental dependencies, better recovering observed trends with halocentric distance up to several virial radii. By analysing the actual amount of gas stripped from galaxies in our model, we show that those in the vicinity of massive haloes lose a large fraction of their hot halo gas before they become satellites. We demonstrate that this affects galaxy quenching both within and beyond the halo boundary. This is likely to influence the correlations between galaxies up to tens of megaparsecs.
Figure: Evolution of a cluster and five sample galaxies in its vicinity through cosmic time. The sample galaxies are experiencing strong ram-pressure stripping.
Main Paper Model Description Download Catalogs
Comparing Galaxy Formation Models (Ayromlou et al. 2021a)
In this work, we perform a comparison, object-by-object and statistically, between the Munich semi-analytical model, L-Galaxies, and the IllustrisTNG hydrodynamical simulations. By running L-Galaxies on the IllustrisTNG dark matter-only merger trees, we identify the same galaxies in the two models. This allows us to compare the stellar mass, star formation rate and gas content of galaxies, as well as the baryonic content of subhaloes and haloes in the two models. We find that both the stellar mass functions and the stellar masses of individual galaxies agree to better than ∼0.2dex. On the other hand, specific star formation rates and gas contents can differ more substantially. At z=0 the transition between low-mass star-forming galaxies and high-mass, quenched galaxies occurs at a stellar mass scale ∼0.5dex lower in IllustrisTNG than in L-Galaxies. IllustrisTNG also produces substantially more quenched galaxies at higher redshifts. Both models predict a halo baryon fraction close to the cosmic value for clusters, but IllustrisTNG predicts lower baryon fractions in group environments. These differences are due primarily to differences in modeling feedback from stars and supermassive black holes. The gas content and star formation rates of galaxies in and around clusters and groups differ substantially, with IllustrisTNG satellites less star-forming and less gas-rich. We show that environmental processes such as ram-pressure stripping are stronger and operate to larger distances and for a broader host mass range in IllustrisTNG. We suggest that the treatment of galaxy evolution in the semi-analytic model needs to be improved by prescriptions that capture local environmental effects more accurately.
Figure: Visual overview of galaxy stellar mass (colors), comparing the results of L-Galaxies (left column) vs. IllustrisTNG (right column).
A New Method to Quantify Environment and Model Ram-Pressure Stripping (Ayromlou et al. 2019)
In this work, we introduce a local background environment (LBE) estimator that can be measured in and around every galaxy or its dark matter subhalo in high-resolution cosmological simulations. The LBE is designed to capture the influence of environmental effects such as ram-pressure stripping (RPS) on the formation and evolution of galaxies in semi-analytical models. We define the LBE directly from the particle data within an adaptive spherical shell, and devise a Gaussian mixture estimator to separate background particles from previously unidentified subhalo particles. Analyzing the LBE properties, we find that the LBE of satellite galaxies is not at rest with respect to their host halo, in contrast to typical assumptions. The orientations of the velocities of a subhalo and its LBE are well aligned in the outer infall regions of haloes, but decorrelated near halo center. Significantly, there is no abrupt change in LBE velocity or density at the halo virial radius. This suggests that stripping should also happen beyond this radius. Therefore, we use the time-evolving LBE of galaxies to develop a method to better account for ram-pressure stripping of hot gas within the Munich semi-analytical model, L-GALAXIES. Overall, our new approach results in a significant increase in gas stripping across cosmic time. Central galaxies, as well as satellites beyond the virial radius, can lose a significant fraction of their hot halo gas. As a result, the gas fractions and star formation rates of satellite galaxies are suppressed relative to the fiducial model, although the stellar masses and global stellar mass functions are largely unchanged.
Figure: Schematic visualization of the local background environment surrounding a galaxy and its subhalo.