Publications

The so-called "Galactic center lobe" (GCL) is an extended (~1°) radio continuum feature situated above the Galactic plane, the nature and location of which have been subject to varying claims in the literature. Using new optical integral field spectroscopic observations from the SDSS-V Local Volume Mapper, we confirm the characterization of the GCL as a foreground photoionized H II region, not associated with the Galactic center. We present a new analysis of the ionized gas morphology, line ratio diagnostics, and kinematics. From our [S III]λ9532 emission line map, which suffers the least extinction, we identify ionized gas emission throughout a closed outer loop, which does not fill the GCL interior. All optical line ratio diagnostics are consistent with photoionization. By comparing the ionized gas reddening from the Balmer decrement with 3D dust maps, we directly constrain the distance to the GCL to ~2 kpc. [N II]λ6583 line kinematics show a uniform velocity structure across the GCL, further confirming that the entire bubble is one structure. The size and emission line morphology is strongly reminiscent of that seen in the nearby Barnard's Loop, providing a possible analog to explain how this outer shell may be photoionized by a more distant and off-center embedded young cluster. We suggest that the acronym GCL be repurposed to instead abbreviate the name "greatly confused loop".

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM) of near solar metallicity galaxies, where they play a critical role in ISM heating, cooling, and reprocessing stellar radiation. The PAH fraction, the abundance of PAHs relative to total dust mass, is a key parameter in ISM physics. Using JWST and MUSE observations of 42 galaxies from the PHANGS survey, we analyzed the PAH fraction in over 17 000 H II regions spanning a gas-phase oxygen abundance of 12 + log(O/H) = 8.0–8.8 (Z ∼ 0.2–1.3 Z_⊙), and ∼400 isolated supernova remnants (SNRs). We find a significantly lower PAH fraction toward H II regions compared to a reference sample of diffuse ISM areas at matched metallicity. At 12 + log(O/H) > 8.2, the PAH fraction toward H II regions is strongly anti-correlated with the local ionization parameter, suggesting that PAH destruction is correlated with ionized gas and/or hydrogen-ionizing UV radiation. At lower metallicities, the PAH fraction declines steeply in H II regions and in the diffuse ISM, likely reflecting less efficient PAH formation in metal-poor environments. Carefully isolating dust emission from the vicinity of optically identified supernova remnants, we see evidence of selective PAH destruction from measurements of lower PAH fractions, which is, however, indistinguishable at ∼50 pc scales. Overall, our results point to ionizing radiation as the dominant agent of PAH destruction within H II regions; metallicity plays a key role in their global abundance in galaxies.

Context. 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. Aims. 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). Methods. 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. Results. 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.

Context. Stellar feedback is a key contributor to the morphology and dynamics of the interstellar medium in star-forming galaxies. In particular, energy and momentum input from massive stars can drive the turbulent motions in the gas, but the dominance and efficiency of this process are unclear. The study of ionised superbubbles enables quantitative constraints to be placed on the energetics of stellar feedback. Aims: 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 such a comparison, we aim to determine whether the stellar feedback is responsible for the observed turbulent motions and to quantify the fraction of mechanical energy retained in the superbubbles. Methods: We investigated the ionised gas morphology, excitation properties, and kinematics in 19 nearby star-forming galaxies from the PHANGS-MUSE survey. Based on the distribution of the flux and velocity dispersion in the Hα line, we selected 1484 regions of locally elevated velocity dispersion (σ(Hα) > 45 km s^‒1), including at least 171 expanding superbubbles. We analysed these regions and related their properties to those of the young stellar associations and star clusters identified in PHANGS-HST data. Results: 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. At the same time, the contribution of mechanical energy injected 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 and thermal pressure as well as winds is necessary. We find that the gas kinetic energy decreases with metallicity for our sample covering Z = 0.5 ‒ 1.0 Z_☉, reflecting the lower impact of stellar feedback. For the sample of well-resolved 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 (< 3 Myr) stellar associations in the rims of the superbubbles than in the centres and the opposite trend for older associations, which implies possible propagation or triggering of star formation. Conclusions: Stellar feedback is the dominant source for powering the ionised gas in regions of locally (on a 50-500 pc scale) elevated velocity dispersion, with a typical coupling efficiency of 10 ‒ 20%. Accounting for pre-supernovae feedback is required to set up the energy balance between gas and stars.

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.

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∼ 0.07 Z_⊙) 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.

We present the results of a multiwavelength study of the nearby dwarf galaxy DDO 53 - a relatively isolated member of the M 81 group. We analyse the atomic and ionized gas kinematics (based on the observations with Fabry-Perot interferometer in H α line and archival data in H I 21 cm line), distribution, excitation, and oxygen abundance of the ionized gas (based on the long-slit and integral-field spectroscopy and on imaging with narrow-band filters), and their relation with the young massive stars (based on archival HST data). We detect a faint 2-kpc sized supershell of ionized gas surrounding the galaxy. Most probably, this structure represents a large-scale gas outflow, however, it could be also created by the ionizing quanta leaking from star-forming regions to the marginally detected atomic hydrogen surrounding the galactic disc. We analyse the properties of the anomalous H I in the north part of the galaxy and find that its peculiar kinematics is also traced by ionized gas. We argue that this H I feature is related to the accreting gas cloud captured from the intergalactic medium or remaining after the merger event occurred >1 Gyr ago. The infalling gas produces shocks in the interstellar medium and could support the star formation activity in the brightest region in DDO 53.

The ionization state and oxygen abundance distribution in a sample of polar-ring galaxies (PRGs) were studied from the long-slit spectroscopic observations carried out with the SCORPIO-2 focal reducer at the Russian 6-m telescope. The sample consists of 15 PRGs classified as `the best candidates' in the SDSS-based Polar Ring Catalogue. The distributions of line-of-sight velocities of stellar and gaseous components have given kinematic confirmation of polar structures in 13 galaxies in the sample. We show that ionization by young stars dominates in the external parts of polar discs, while shocks have a significant contribution to gas excitation in the inner parts of polar structures. This picture was predicted earlier in a toy model implying the collision between gaseous clouds on polar orbits with the stellar disc gravitational potential well. The exception is a moderately inclined ring to the host galaxy NGC 5014: the accreted gas in the centre has already settled on the main plane and ionized by young stars, while the gas in the internal part of the ring is excited by shocks. The present study three times increases the number of polar structures with available oxygen abundance estimation. The measured values of the gas metallicity almost do not depend on the galaxy luminosity. The radial [O/H] gradient in the considered polar rings is shallow or absent. No metal-poor gas was detected. We ruled out the scenario of the formation of polar rings due to cold accretion from cosmic filaments for the considered sample of PRGs.

We present the results of observations of the galaxy Holmberg I carried out at the Russian 6-m telescope in the narrow-band imaging, long-slit spectroscopy, and scanning Fabry-Perot interferometer modes. A detailed analysis of gas kinematics, ionization conditions, and metallicity of star-forming regions in the galaxy is presented. The aim of the paper is to analyse the propagation of star formation in the galaxy and to understand the role of the ongoing star formation in the evolution of the central `galaxy-sized' supergiant H I shell (SGS), where all regions of star formation are observed. We show that star formation in the galaxy occurs in large unified complexes rather than in individual giant H II regions. Evidence of the triggered star formation is observed both on scales of individual complexes and of the whole galaxy. We identified two supernova-remnant candidates and one late-type WN star and analysed their spectrum and surrounding-gas kinematics. We provide arguments indicating that the SGS in Holmberg I is destructing by the influence of star formation occurring on its rims.

We report a detailed analysis of all regions of current star formation in the walls of the supergiant H I shell (SGS) in the galaxy Holmberg II based on observations with a scanning Fabry-Perot interferometer at the Russian 6-m telescope. We compare the structure and kinematics of ionized gas with that of atomic hydrogen and with the stellar population of the SGS. Our deep Hα images and archival images taken by the Hubble Space Telescope demonstrate that current star formation episodes are larger and more complicated than previously thought: they represent unified star-forming complexes with sizes of several hundred pc rather than `chains' of separate bright nebulae in the walls of the SGS. The fact that we are dealing with unified complexes is evidenced by identified faint shell-like structures of ionized and neutral gas which connect several distinct bright H II regions. Formation of such complexes is due to the feedback of stars with very inhomogeneous ambient gas in the walls of the SGS. The arguments supporting an idea about the triggering of star formation in SGS by the H I supershells collision are presented. We also found a faint ionized supershell inside the H I SGS expanding with a velocity of no greater than 10-15 km s^-1. Five OB stars located inside the inner supershell are sufficient to account for its radiation, although a possibility of leakage of ionizing photons from bright H II regions is not ruled out as well.