arXiv subjects

M. H. Gevorgyan

Publications and source records attributed to M. H. Gevorgyan.

6 recordsLinked to original sources

arXiv2026

Supernovae Ia ejecta velocities and host galaxy environments: the role of survey-selection effects

The origin of the near-maximum-light Si II $λ$6355 velocity diversity among supernovae Ia (SNe Ia) remains uncertain. Previous studies have suggested that high-velocity (HV) and normal-velocity (NV) SNe Ia occupy systematically different host galaxy environments, implying differences in their progenitor populations. We re-examine this hypothesis using a sample of 354 nearby ($z\leq0.04$) spectroscopically normal SNe Ia, comprising 239 NV and 115 HV events, identified in targeted and untargeted surveys. For each SN, we determine the host galaxy morphology, galactocentric distance, and physical size, and obtain homogeneous stellar mass estimates while assessing the influence of survey-selection effects. The Si II velocity distribution is well described by two Gaussian components, confirming the NV and HV populations. We find no statistically significant differences between the galactocentric distance, host galaxy size, or stellar mass distributions of the two velocity subgroups. The environmental trends reported in previous studies are recovered only with marginal statistical significance in the targeted subsample, whereas they disappear almost entirely in the untargeted subsample, consistent with survey-selection effects playing a major role in the previously inferred environmental differences between the NV and HV events. Our results weaken the interpretation that the observed Si II velocity diversity is primarily driven by systematic differences in the global host properties examined here. Instead, they favour a scenario in which intrinsic explosion asymmetries and viewing-angle effects account for a substantial fraction of the observed velocity diversity, without excluding a possible contribution from local environmental conditions or progenitor properties.

astro-ph.GA↗

arXiv2026

Active galactic nucleus activity in nearby disk galaxies: The roles of bar strength and spiral arm morphology

(abridged) We investigate the dependence of AGN activity on bar strength and spiral arm morphology in nearby spiral galaxies, with a particular emphasis on disentangling their individual and joint effects while controlling for galaxy stellar mass and color. We constructed a sample of 843 morphologically undisturbed Sa-Sd galaxies from the MaNGA survey in the redshift range $0.026 \leq z \leq 0.1$. Bar strengths, stellar masses, colors, and AGN classes were adopted from the literature, while spiral arm classes (ACs), i.e., flocculent (FL), multi-armed (MA), and grand design (GD) systems, were visually determined for the entire sample using optical imaging together with bulge-disk decomposition residual maps. The AGN fraction increases systematically with bar strength, from $0.10^{+0.02}_{-0.02}$ in unbarred galaxies to $0.34^{+0.03}_{-0.03}$ in strongly barred systems. After controlling for stellar mass and color, a statistically significant enhancement remains detectable primarily at intermediate stellar masses ($10.5 \lesssim \log(M_\star/M_\odot) \lesssim 11.0$), whereas no significant dependence is found at higher masses. In contrast, spiral arm morphology alone does not show a robust independent connection with AGN activity once stellar mass and color are controlled, although a weak enhancement is present in intermediate-mass GD and MA systems. When bar strength and spiral AC are considered jointly, the highest AGN fraction is observed in strongly barred GD$+$MA galaxies $(0.39^{+0.04}_{-0.04})$, while the lowest AGN fractions are found in unbarred FL systems $(0.09^{+0.03}_{-0.02})$. Our results suggest that stellar mass and color drive the primary trend in AGN activity in nearby disk galaxies, while stellar bars and spiral arms act as secondary structural drivers.

astro-ph.GA↗

arXiv2025

Calcium versus silicon ejecta velocities and decline rates in supernovae Ia: The role of high-velocity features

Photospheric and high-velocity features (PVFs and HVFs) of Si II $λ$6355 and Ca II IR3 lines in supernova Ia (SN Ia) spectra provide insights into ejecta structure, energetics, and circumstellar interaction, yet their interplay remains poorly understood. We analyse a representative sample of 145 nearby SNe Ia observed within $\pm$5 days of B-band maximum light, including normal, 91T-, and 91bg-like events with measured light-curve decline rates ($Δm_{15}$) and Si II and Ca II line properties from the literature. We model PVF and HVF velocity distributions using Gaussian Mixture Models, compare Si II and Ca II PVF velocity distributions, assess Ca II HVF properties, and test correlations between Si II PVF velocities and $Δm_{15}$, with emphasis on HVF effects. For the first time, we show that the Ca II PVF velocity distribution, measured for the same events at the same phases as Si II, is predominantly unimodal, in contrast to the well-known bimodal Si II PVF distribution that supports the high-velocity/normal-velocity division. This contrast likely reflects a subclass-dependent formation depth of the Ca II line, as supported by a positive correlation ($>3.3σ$) between $Δm_{15}$ and the velocity offset between Ca II and Si II PVFs, particularly in faster-declining SNe Ia. Importantly, HVFs do not significantly bias PVF velocity distributions. A significant negative correlation ($>3.3σ$) between Si II PVF velocity and $Δm_{15}$ is found only for HVF-weak SNe Ia, consistent with more energetic explosions yielding faster ejecta, while this trend vanishes in HVF-strong events, likely due to circumstellar interaction. These results underscore the critical role of HVFs and SN Ia subclass in interpreting ejecta kinematics in both models and observations.

astro-ph.GA↗

arXiv2021

Type Ia supernovae in the star formation deserts of spiral host galaxies

Using a sample of nearby spiral galaxies hosting 185 supernovae (SNe) Ia, we perform a comparative analysis of the locations and light curve decline rates $(Δm_{15})$ of normal and peculiar SNe Ia in the star formation deserts (SFDs) and beyond. To accomplish this, we present a simple visual classification approach based on the UV/H$α$ images of the discs of host galaxies. We demonstrate that, from the perspective of the dynamical timescale of the SFD, where the star formation (SF) is suppressed by the bar evolution, the $Δm_{15}$ of SN Ia and progenitor age can be related. The SFD phenomenon gives an excellent possibility to separate a subpopulation of SN Ia progenitors with the ages older than a few Gyr. We show, for the first time, that the SFDs contain mostly faster declining SNe Ia $(Δm_{15} > 1.25)$. For the galaxies without SFDs, the region within the bar radius, and outer disc contain mostly slower declining SNe Ia. To better constrain the delay times of SNe Ia, we encourage new studies (e.g. integral field observations) using the SFD phenomenon on larger and more robust datasets of SNe Ia and their host galaxies.

astro-ph.GA↗

arXiv2020

Supernovae and their host galaxies -- VII. The diversity of Type Ia supernova progenitors

We present an analysis of the light curve (LC) decline rates $(Δm_{15})$ of 407 normal and peculiar supernovae (SNe) Ia and global parameters of their host galaxies. As previously known, there is a significant correlation between the $Δm_{15}$ of normal SNe Ia and global ages (morphologies, colours, masses) of their hosts. On average, those normal SNe Ia that are in galaxies from the Red Sequence (early-type, massive, old hosts) have faster declining LCs in comparison with those from the Blue Cloud (late-type, less massive, younger hosts) of the colour-mass diagram. The observed correlations between the $Δm_{15}$ of normal SNe Ia and hosts' parameters appear to be due to the superposition of at least two distinct populations of faster and slower declining normal SNe Ia from older and younger stellar components. We show, for the first time, that the $Δm_{15}$ of 91bg- and 91T-like SNe is independent of host morphology and colour. The distribution of hosts on the colour-mass diagram confirms the known tendency for 91bg-like SNe to occur in globally red/old galaxies while 91T-like events prefer blue/younger hosts. On average, the youngest global ages of 02cx-like SNe hosts and their positions in the colour-mass diagram hint that these events likely originate from young population, but they differ from 91T-like events in the LC decline rate. Finally, we discuss the possible explosion channels and present our favoured SN Ia models that have the potential to explain the observed SN-host relations.

astro-ph.GA↗

arXiv2019

New powerful outburst of the unusual young star V1318 Cyg S (LkHα 225)

Young double star V1318 Cyg, which is associated with a small isolated star-forming region around HAeBe star BD+40o4124, has very unusual photometric and spectral behavior. We present results of photometric and spectroscopic observations in the optical range. We carried out BVRI CCD photometric observations of V1318 Cyg from 2015 Sept. to 2017 July. For the same period we acquired medium- and low-resolution spectra. Observations were performed with the 2.6 m telescope. We analyze the historical light curve for V1318 Cyg and demonstrate that the southern component, V1318 Cyg S, after being rather bright in the 1970s (V$\approx$14 mag) started to lower its brightness and in 1990 became practically invisible in the optical. After its reappearance in the second half of the 1990s the star started to become very slowly brighter. Between 2006 and 2010 V1318 Cyg S started brightening more quickly, and in 2015 had become brighter by more than five magnitudes in visible light. Since this time V1318 Cyg S has remained at this maximum. Its spectrum shows little variability and consists of a mixture of emission and absorption lines, which has allowed for estimates of its spectral type as early Ae, with obvious evidence of matter outflow. We derive its current A(V)$\approx$ 7.2 and L = 750 L(sun) thus confirming that V1318 Cyg S should belong to the Herbig Ae stars, making it, along with BD+40o4124 and V1686 Cyg, the third luminous young star in the group. It is very probable that we observe V1318 Cyg S near the pole and that the inclination of its dense and slow ($\approx$ 100 km/s) outflow is low. The unusual variability and other features of V1318 Cyg S make it difficult to classify this star among known types of eruptive young stars. It could be an extreme, higher-mass example of an EXor, or an object of intermediate class between EXors and FUors, like V1647 Ori.

astro-ph.SR↗