Dr. Oleg Egorov
Research Fellow at Heidelberg University
Research interests
The main focus of my research is how feedback from massive stars regulates the appearance of the interstellar medium (ISM), ongoing star formation and the chemical evolution of nearby galaxies (with a particular interest in the Local Volume dwarf galaxies).
Besides, I am interested in how external gas inflows (gas accretion, tidal interactions, mergers) regulate the star formation activity and evolution of galaxies.
I use the multiwavelength observations (in particular, optical integral-field spectroscopy) from different telescopes and instruments to analyse the morphology, kinematics, physical conditions and chemical abundances of gas and dust at the scales from individual HII regions to giant kpc-sized star-forming complexes, superbubbles and outflows.
I am a member of the PHANGS collaboration (co-chair of the Ionized ISM working group) and the Local Volume Mapper (SDSS-V/LVM) science team.
The Orion Molecular Cloud complex, one of the nearest (D = 406 pc) and most extensively studied massive star-forming regions, is ideal for constraining the physics of stellar feedback, but its ~12 deg diameter on the sky requires a dedicated approach to mapping ionized gas structures within and around the nebula.... The Sloan Digital Sky Survey (SDSS-V) Local Volume Mapper (LVM) is a new optical integral field unit (IFU) that will map the ionized gas within the Milky Way and Local Group galaxies, covering 4300 deg2 of the sky with the new LVM Instrument (LMV-I). We showcase optical emission line maps from LVM covering 12 deg2 inside of the Orion belt region, with 195 000 individual spectra combined to produce images at 0.07 pc (35.3″) resolution. This is the largest IFU map made (to date) of the Milky Way, and contains well-known nebulae (the Horsehead Nebula, Flame Nebula, IC 434, and IC 432), as well as ionized interfaces with the neighboring dense Orion B molecular cloud. We resolve the ionization structure of each nebula, and map the increase in both the [S II]/Hα and [N II]/Hα line ratios at the outskirts of nebulae and along the ionization front with Orion B. [O III] line emission is only spatially resolved within the center of the Flame Nebula and IC 434, and our ~0.1 pc scale line ratio diagrams show how variations in these diagnostics are lost as we move from the resolved to the integrated view of each nebula. We detect ionized gas emission associated with the dusty bow wave driven ahead of the star σ Orionis, where the stellar wind interacts with the ambient interstellar medium. The Horsehead Nebula is seen as a dark occlusion of the bright surrounding photo-disassociation region. This small glimpse into Orion only hints at the rich science that will be enabled by the LVM.
Stellar feedback impact on the ionized gas kinematics in the dwarf galaxy Sextans B
We investigated the ionised and atomic gas kinematics and excitation state in the central region of ongoing star formation of the nearby low-metallicity dwarf galaxy Sextans B. The analysis is based on the new observations performed in the Hα emission line with high resolution (R ~ 16000) scanning Fabry-Perot interferometer at the 6-m BTA SAO RAS telescope, and on the long-slit spectral observations at the 9.2-m SALT... and 2.5-m CMO SAI MSU telescopes. Strong non-circular gas motions detected in the studied regions probably resulted from the off-plane gas motions and impact of stellar feedback. We identified six regions of elevated Hα velocity dispersion, five of which exhibit asymmetric or two-component Hα line profiles. Three of these regions are young (<1.1 Myr) expanding (Vexp ~ 25 - 50 km s-1) superbubbles. We argue that at least three regions in the galaxy could be supernova remnants. We conclude that supernovae feedback is the dominant source of energy for superbubbles in Sextans B, which is expected for such a low metallicity, although we cannot rule out a strong impact of pre-supernova feedback for one superbubble.
Quantifying the energy balance between the turbulent ionised gas and young stars
We investigate the ionised gas morphology, excitation properties, and kinematics in 19 nearby star-forming galaxies from the PHANGS-MUSE survey. We directly compare the kinetic energy of expanding superbubbles and the turbulent motions in the interstellar medium with the mechanical energy deposited by massive stars in the form of winds and supernovae, ... with the aim to answer whether the stellar feedback is responsible for the observed turbulent motions and to quantify the fraction of mechanical energy retained in the superbubbles.
Based on the distribution of the flux and velocity dispersion in the Hα
line, we select 1484 regions of locally elevated velocity dispersion (σ(Hα)>45 km/s), including at least 171 expanding superbubbles. We analyse these regions and relate their properties to those of the young stellar associations and star clusters identified in PHANGS-HST data. We find a good correlation between the kinetic energy of the ionised gas and the total mechanical energy input from supernovae and stellar winds from the stellar associations, with a typical coupling efficiency of 10-20%. The contribution of mechanical energy by the supernovae alone is not sufficient to explain the measured kinetic energy of the ionised gas, which implies that pre-supernova feedback in the form of radiation/thermal pressure and winds is necessary. We find that the gas kinetic energy decreases with metallicity for our sample covering Z=0.5-1.0 Zsun, reflecting the lower impact of stellar feedback. For the sample of superbubbles, we find that about 40% of the young stellar associations are preferentially located in their rims. We also find a slightly higher (by ~15%) fraction of the youngest (1-3 Myr) stellar associations in the rims of the superbubbles than in the centres, and the opposite for older associations, which implies possible propagation or triggering of star formation.
PHANGS-JWST First Results: Destruction of the PAH Molecules in H II Regions Probed by JWST and MUSE
Polycyclic aromatic hydrocarbons (PAHs) play a critical role in the reprocessing of stellar radiation and balancing the heating and cooling processes in the interstellar medium but appear to be destroyed in H II regions. However, the mechanisms driving their destruction are still not completely understood. Using PHANGS-JWST and PHANGS-MUSE observations, we investigate how the PAH fraction changes in about 1500 H II regions across four nearby star-forming galaxies ... (NGC 628, NGC 1365, NGC 7496, and IC 5332). We find a strong anticorrelation between the PAH fraction and the ionization parameter (the ratio between the ionizing photon flux and the hydrogen density) of H II regions. This relation becomes steeper for more luminous H II regions. The metallicity of H II regions has only a minor impact on these results in our galaxy sample. We find that the PAH fraction decreases with the Hα equivalent width-a proxy for the age of the H II regions-although this trend is much weaker than the one identified using the ionization parameter. Our results are consistent with a scenario where hydrogen-ionizing UV radiation is the dominant source of PAH destruction in star-forming regions.
Unveiling the nitrogen-rich massive star in the metal-poor galaxy NGC 4068
We report the identification of the unusual emission-line stellar-like object in the nearby low-metallicity (Z∼0.1Z⊙) dwarf galaxy NGC 4068. Our observations performed with long-slit spectrograph and Fabry-Perot interferometer demonstrate high velocity dispersion in H α line, presence of He II λ4686Å line and peculiarly low [S II]/[N II] fluxes ratio for this object. From observational data, we derived that the object represents a single star of high bolometric luminosity (L* ~1.5e6 L⊙) surrounded by an expanding nebula with kinematical age of t~0.5 Myr. The nebula exhibits significant nitrogen overabundance [log (N/O)~-0.05, that is by ~1.4 dex higher than expected for low-metallicity galaxies]. ... We suggested that this is a massive blue supergiant (BSG) or Wolf-Rayet (WR) star surrounded by its ejecta interacting with the interstellar medium. We calculated the models of the nebula using CLOUDY photoionization code, applying CMFGEN-modelled BSG and WR stars as ionization sources. We found a best agreement between the modelled and observed spectra for the model assuming ionization by low-metallicity WR star of mass M∗≈80M⊙, ionizing the nebula through the strong wind and enriching the interstellar medium with nitrogen.
Stellar feedback impact on the ionized gas kinematics in the dwarf galaxy Sextans A
Feedback from massive stars shapes the interstellar medium (ISM) and affects the evolution of galaxies, but its mechanisms acting at the small scales (~10 pc) are still not well constrained observationally, especially in the low-metallicity environments. We present the analysis of the ionized gas (focusing on its kinematics, which were never studied before) and its connection to the massive stars in the nearby (D ~ 1.4 Mpc) star-forming very metal poor (Z∼0.07Z⊙) galaxy Sextans A.... The analysis is based on the observations with a scanning Fabry-Perot interferometer, long-slit spectroscopy, and imaging in emission lines with narrow-band tunable filters. We found 10 expanding superbubbles of ionized gas with ages of 1-3 Myr. We argue that three of them are probable supernova remnants, while the pre-supernova feedback is an important source of energy for blowing out the remaining superbubbles. The two brightest sites of star formation exhibit signs of outflowing ionized gas, which is traced by its ionized and atomic gas kinematics and (in one case) by its emission line flux ratios. Overall, the ionized gas kinematics in Sextans A is highly affected by the feedback from several generations of massive stars and inconsistent with the mere solid-body rotation observed in atomic hydrogen.
