arXiv subjects

A. Nersesian

Publications and source records attributed to A. Nersesian.

32 records · Page 2Linked to original sources

arXiv2022

Star-forming early- and quiescent late-type galaxies in the local Universe

The general consensus is that LTGs undergo intense star-formation activity, while ETGs are mostly inactive. We question this general rule and investigate the existence of star-forming ETGs and quiescent LTGs in the local Universe. By computing the physical properties of 2,209 such galaxies in the GAMA survey being morphologically classified and using information on their structural properties as well as the density of their local environment, we seek for understanding the differences from their 'typical' counterparts. We separate galaxies into subsets based on their dominant ionising process, making use of criteria based on the WH$_α$ width and the [NII/H$_α$] ratio. Taking advantage of the SED fitting code CIGALE we derive galaxy properties, such as the $M_\text{star}$, $M_\text{dust}$, and SFR and also estimate the unattenuated and the dust-absorbed stellar emission, for both the young and old stellar populations. Ongoing star-formation activity is found in 47% of ETGs and 8% of LTGs are quiescent. The star-forming E galaxies, together with the LBSs, constitute a population that follows very well the SFMS of spiral galaxies. The fraction of the luminosity originating from young stars in the star-forming ETGs is quite substantial ($\sim$ 25%) and similar to that of the star-forming LTGs. Investigating possible differences between star-forming and quiescent galaxies we find that the intrinsic shape of the SED of the star-forming galaxies is, on average, very similar for all morphological types. Concerning their structural parameters, quiescent galaxies tend to show larger values of the Sérsic index and larger $R_\text{eff}$ (compared to star-forming galaxies). Finally, we find that star-forming galaxies preferably reside in lower-density environments compared to the quiescent ones, which exhibit a higher percentage of sources being members of groups.

astro-ph.GA↗

arXiv2022

Dust emissivity in resolved spiral galaxies

Context: The far-infrared (FIR) and sub-millimeter (submm) emissivity of the Milky Way (MW) cirrus is an important benchmark for dust grain models. Dust masses in other galaxies are generally derived from the FIR/submm using the emission properties of these MW-calibrated models. Aims: We seek to derive the FIR/submm emissivity in nine nearby spiral galaxies to check its compatibility with MW cirrus measurements. Methods: We obtained values of the emissivity at 70 to 500 um, using maps of dust emission from the Herschel satellite and of gas surface density from the THINGS and HERACLES surveys on a scale generally corresponding to 440 pc. We studied the variation of the emissivity with the surface brightness ratio I(250um)/I(500um), a proxy for the intensity of the interstellar radiation field heating the dust. Results: We find that the average value of the emissivity agrees with MW estimates for pixels sharing the same color as the cirrus, namely, for I(250um)/I(500um) = 4.5. For I(250um)/I(500um) > 5, the measured emissivity is instead up to a factor ~2 lower than predicted from MW dust models heated by stronger radiation fields. Regions with higher I(250um)/I(500um) are preferentially closer to the galactic center and have a higher overall (stellar+gas) surface density and molecular fraction. The results do not depend strongly on the adopted CO-to-molecular conversion factor and do not appear to be affected by the mixing of heating conditions. Conclusions: Our results confirm the validity of MW dust models at low density, but are at odds with predictions for grain evolution in higher density environments. If the lower-than-expected emissivity at high I(250um)/I(500um) is the result of intrinsic variations in the dust properties, it would imply an underestimation of the dust mass surface density of up to a factor ~2 when using current dust models.

astro-ph.GA↗

arXiv2021

Dust Emission in Galaxies at Millimeter Wavelengths: Cooling of star forming regions in NGC6946

Interstellar dust plays an important role in the formation of molecular gas and the heating and cooling of the interstellar medium. The spatial distribution of the mm-wavelength dust emission from galaxies is largely unexplored. The NIKA2 Guaranteed Time Project IMEGIN (Interpreting the Millimeter Emission of Galaxies with IRAM and NIKA2) has recently mapped the mm emission in the grand design spiral galaxy NGC6946. By subtracting the contributions from the free-free, synchrotron, and CO line emission, we map the distribution of the pure dust emission at 1:15mm and 2mm. Separating the arm/interarm regions, we find a dominant 2mm emission from interarms indicating the significant role of the general interstellar radiation field in heating the cold dust. Finally, we present maps of the dust mass, temperature, and emissivity index using the Bayesian MCMC modeling of the spectral energy distribution in NGC6946.

astro-ph.GA↗

arXiv2021

Exploring the millimetre emission in nearby galaxies: analysis of the edge-on galaxy NGC 891

New observations of the edge-on galaxy NGC 891, at 1.15 and 2 mm obtained with the IRAM 30-m telescope and the NIKA2 camera, within the framework of the IMEGIN (Interpreting the Millimetre Emission of Galaxies with IRAM and NIKA2) Large Program, are presented in this work. By using multiwavelength maps (from the mid-IR to the cm wavelengths) we perform SED fitting in order to extract the physical properties of the galaxy on both global and local ($\sim$kpc) scales. For the interpretation of the observations we make use of a state-of-the-art SED fitting code, HerBIE (HiERarchical Bayesian Inference for dust Emission). The observations indicate a galaxy morphology, at mm wavelengths, similar to that of the cold dust emission traced by sub-mm observations and to that of the molecular gas. The contribution of the radio emission at the NIKA2 bands is very small (negligible at 1.15 mm and $\sim10\%$ at 2 mm) while it dominates the total energy budget at longer wavelengths (beyond 5 mm). On local scales, the distribution of the free-free emission resembles that of the dust thermal emission while the distribution of the synchrotron emission shows a deficiency along the major axis of the disc of the galaxy.

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arXiv2021

The physical properties of local (U)LIRGs: a comparison with nearby early- and late-type galaxies

In order to pinpoint the place of the (U)LIRGs in the local Universe we examine the properties of a sample of 67 such systems and compare them with those of 268 ETGs and 542 LTGs from the DustPedia database. We make use of multi-wavelength photometric data and the CIGALE SED fitting code to extract their physical parameters. The median SEDs as well as the values of the derived parameters were compared to those of the local ETGs and LTGs. In addition to that, (U)LIRGs were divided into seven classes, according to the merging stage of each system, and variations in the derived parameters were investigated. (U)LIRGs occupy the `high-end' on the dust and stellar mass, and SFR in the local Universe with median values of 5.2$\times10^7~M_{\odot}$, 6.3$\times10^{10}~M_{\odot}$ and 52$~M_{\odot}$yr$^{-1}$, respectively. The PDR-dust emission in (U)LIRGs is 11.7% of the total dust luminosity, significantly higher than ETGs (1.6%) and the LTGs (5.2%). The median value of the dust temperature in (U)LIRGs is 32 K, which is higher compared to both the ETGs (28 K) and the LTGs (22 K). Small differences, in the derived parameters, are seen for the seven merging classes of our sample of (U)LIRGs with the most evident one being on the star-formation rate, where in systems in late merging stages the median SFR reaches up to 99 M$_{\odot}$ yr$^{-1}$ compared to 26 M$_{\odot}$ yr$^{-1}$ for the isolated ones. In contrast to the local normal galaxies where old stars dominate the stellar emission, the young stars in (U)LIRGs contribute with 64% of their luminosity to the total stellar luminosity. The fraction of the dust-absorbed stellar luminosity is extremely high in (U)LIRGs (78%) compared to 7% and 25% in ETGs and ETGs, respectively. The fraction of the stellar luminosity used to heat up the dust grains is very high in (U)LIRGs, while 74% of the dust emission comes from the young stars.

astro-ph.GA↗

arXiv2020

High-resolution, 3D radiative transfer modelling IV. AGN-powered dust heating in NGC 1068

Dust emission, an important diagnostic of star formation and ISM mass throughout the Universe, can be powered by sources unrelated to ongoing star formation. In the framework of the DustPedia project we have set out to disentangle the radiation of the ongoing star formation from that of the older stellar populations. This is done through detailed, 3D radiative transfer simulations of face-on spiral galaxies. In this particular study, we focus on NGC 1068, which contains an active galactic nucleus (AGN). The effect of diffuse dust heating by AGN (beyond the torus) was so far only investigated for quasars. This additional dust heating source further contaminates the broadband fluxes on which classic galaxy modelling tools rely to derive physical properties. We aim to fit a realistic model to the observations of NGC 1068 and quantify the contribution of the several dust heating sources. Our model is able to reproduce the global spectral energy distribution of the galaxy. It matches the resolved optical and infrared images fairly well, but deviates in the UV and the submm. We find a strong wavelength dependency of AGN contamination to the broadband fluxes. It peaks in the MIR, drops in the FIR, but rises again at submm wavelengths. We quantify the contribution of the dust heating sources in each 3D dust cell and find a median value of 83% for the star formation component. The AGN contribution is measurable at the percentage level in the disc, but quickly increases in the inner few 100 pc, peaking above 90%. This is the first time the phenomenon of an AGN heating the diffuse dust beyond its torus is quantified in a nearby star-forming galaxy. NGC 1068 only contains a weak AGN, meaning this effect can be stronger in galaxies with a more luminous AGN. This could significantly impact the derived star formation rates and ISM masses for such systems.

astro-ph.GA↗

arXiv2019

A systematic metallicity study of DustPedia galaxies reveals evolution in the dust-to-metal ratios

Observations of evolution in the dust-to-metal ratio allow us to constrain the dominant dust processing mechanisms. In this work, we present a study of the dust-to-metal and dust-to-gas ratios in a sub-sample of ~500 DustPedia galaxies. Using literature and MUSE emission line fluxes, we derived gas-phase metallicities (oxygen abundances) for over 10000 individual regions and determine characteristic metallicities for each galaxy. We study how the relative dust, gas, and metal contents of galaxies evolve by using metallicity and gas fraction as proxies for evolutionary state. The global oxygen abundance and nitrogen-to-oxygen ratio are found to increase monotonically as galaxies evolve. Additionally, unevolved galaxies (gas fraction > 60%, metallicity 12 + log(O/H) < 8.2) have dust-to-metal ratios that are about a factor of 2.1 lower (a factor of six lower for galaxies with gas fraction > 80%) than the typical dust-to-metal ratio (Md/MZ ~ 0.214) for more evolved sources. However, for high gas fractions, the scatter is larger due to larger observational uncertainties as well as a potential dependence of the dust grain growth timescale and supernova dust yield on local conditions and star formation histories. We find chemical evolution models with a strong contribution from dust grain growth describe these observations reasonably well. The dust-to-metal ratio is also found to be lower for low stellar masses and high specific star formation rates (with the exception of some sources undergoing a starburst). Finally, the metallicity gradient correlates weakly with the HI-to-stellar mass ratio, the effective radius and the dust-to-stellar mass ratio, but not with stellar mass.

astro-ph.GA↗

arXiv2019

Predicting the global far-infrared SED of galaxies via machine learning techniques

Dust plays an important role in shaping a galaxy's spectral energy distribution (SED). It absorbs ultraviolet (UV) to near-infrared (NIR) radiation and re-emits this energy in the far-infrared (FIR). The FIR is essential to understand dust in galaxies. However, deep FIR observations require a space mission, none of which are still active today. We aim to infer the FIR emission across six Herschel bands, along with dust luminosity, mass, and effective temperature, based on the available UV to mid-infrared (MIR) observations. We also want to estimate the uncertainties of these predictions, compare our method to energy balance SED fitting, and determine possible limitations of the model. We propose a machine learning framework to predict the FIR fluxes from 14 UV-MIR broadband fluxes. We used a low redshift sample by combining DustPedia and H-ATLAS, and extracted Bayesian flux posteriors through SED fitting. We trained shallow neural networks to predict the far-infrared fluxes, uncertainties, and dust properties. We evaluated them on a test set using a root mean square error (RMSE) in log-space. Our results (RMSE = 0.19 dex) significantly outperform UV-MIR energy balance SED fitting (RMSE = 0.38 dex), and are inherently unbiased. We can identify when the predictions are off, for example when the input has large uncertainties on WISE 22, or when the input does not resemble the training set. The galaxies for which we have UV-FIR observations can be used as a blueprint for galaxies that lack FIR data. This results in a 'virtual FIR telescope', which can be applied to large optical-MIR galaxy samples. This helps bridge the gap until the next FIR mission.

astro-ph.GA↗

arXiv2019

Dust emissivity and absorption cross section in DustPedia late-type galaxies

Aims: We compare the far-infrared to sub-millimetre dust emission properties measured in high Galactic latitude cirrus with those determined in a sample of 204 late-type DustPedia galaxies. The aim is to verify if it is appropriate to use Milky Way dust properties to derive dust masses in external galaxies. Methods: We used Herschel observations and atomic and molecular gas masses to estimate the disc-averaged dust emissivity at 250 micrometres, and from this, the absorption cross section per H atom and per dust mass. The emissivity requires one assumption, which is the CO-to-H_2 conversion factor, and the dust temperature is additionally required for the absorption cross section per H atom; yet another constraint on the dust-to-hydrogen ratio D/H, depending on metallicity, is required for the absorption cross section dust mass. Results: We find epsilon(250) = 0.82 +/- 0.07 MJy sr^-1 (1E20 H cm^-2)^-1 for galaxies with 4 < F(250)/F(500) < 5. This depends only weakly on the adopted CO-to-H_2 conversion factor. The value is almost the same as that for the Milky Way at the same colour ratio. Instead, for F(250)/F(500) > 6, epsilon(250) is lower than predicted by its dependence on the heating conditions. The reduction suggests a variation in dust emission properties for spirals of earlier type, higher metallicity, and with a higher fraction of molecular gas. When the standard emission properties of Galactic cirrus are used for these galaxies, their dust masses might be underestimated by up to a factor of two. Values for the absorption cross sections at the Milky Way metallicity are also close to those of the cirrus. Mild trends of the absorption cross sections with metallicity are found, although the results depend on the assumptions made.

astro-ph.GA↗

arXiv2019

DustPedia - the relationships between stars, gas and dust for galaxies residing in different environments

We use a sub-set of the DustPedia galaxy sample (461 galaxies) to investigate the effect the environment has had on galaxies. We consider Virgo cluster and field samples and also assign a density contrast parameter to each galaxy, as defined by the local density of SDSS galaxies. We consider their chemical evolution (using M_{Dust}/M_{Baryon} and M_{Gas}/M_{Baryon}), their specific star formation rate (SFR/M_{Stars}), star formation efficiency (SFR/M_{Gas}), stars-to-dust mass ratio (M_{Stars}/M_{Dust}), gas-to-dust mass ratio (M_{Gas}/M_{Dust}) and the relationship between star formation rate per unit mass of dust and dust temperature (SFR/M_{Dust} and T_{Dust}). Late type galaxies (later than Sc) in all of the environments can be modelled using simple closed box chemical evolution and a simple star formation history (SFR(t) \propto t\exp{-t/τ}). For earlier type galaxies the physical mechanisms that give rise to their properties are clearly much more varied and require a more complicated model (mergers, gas in or outflow). However, we find little or no difference in the properties of galaxies of the same morphological type within the cluster, field or with different density contrasts. It appears that it is morphology, how and whenever this is laid down, and consistent internal physical processes that primarily determine the derived properties of galaxies in the DustPedia sample and not processes related to differences in the local environment.

astro-ph.GA↗

arXiv2019

Old and young stellar populations in DustPedia galaxies and their role in dust heating

Within the framework of the DustPedia project we investigate the properties of cosmic dust and its interaction with the stellar radiation (originating from different stellar populations) for 814 galaxies in the nearby Universe, all observed by the Herschel Space Observatory. We take advantage of the widely used galaxy SED fitting code CIGALE, properly adapted to include the state-of-the-art dust model THEMIS. Using the DustPedia photometry we determine the physical properties of the galaxies, such as, the dust and stellar mass, the star-formation rate, the bolometric luminosity as well as the unattenuated and the absorbed by dust stellar light, for both the old (> 200 Myr) and young (<= 200 Myr) stellar populations. We show how the mass of stars, dust, and atomic gas, as well as the star-formation rate and the dust temperature vary between galaxies of different morphologies and provide recipes to estimate these parameters given their Hubble stage (T). We find a mild correlation between the mass fraction of the small a-C(:H) grains with the specific star-formation rate. On average, young stars are very efficient in heating the dust, with absorption fractions reaching as high as ~77% of the total, unattenuated luminosity of this population. On the other hand, the maximum absorption fraction of old stars is ~24%. Dust heating in early-type galaxies is mainly due to old stars, up to a level of ~90%. Young stars progressively contribute more for `typical' spiral galaxies and they become the dominant source of dust heating for Sm type and irregular galaxies, donating up to ~60% of their luminosity to this purpose. Finally, we find a strong correlation of the dust heating fraction by young stars with morphology and the specific star-formation rate.

astro-ph.GA↗

arXiv2018

Dust emission profiles of DustPedia galaxies

Most radiative transfer models assume that dust in spiral galaxies is distributed exponentially. In this paper our goal is to verify this assumption by analysing the two-dimensional large-scale distribution of dust in galaxies from the DustPedia sample. For this purpose, we make use of Herschel imaging in five bands, from 100 to 500μm, in which the cold dust constituent is primarily traced and makes up the bulk of the dust mass in spiral galaxies. For a subsample of 320 disc galaxies, we successfully perform a simultaneous fitting with a single Sérsic model of the Herschel images in all five bands using the multiband modelling code GALFITM. We report that the Sérsic index $n$, which characterises the shape of the Sérsic profile, lies systematically below 1 in all Herschel bands and is almost constant with wavelength. The average value at 250μm is $0.67\pm0.37$ (187 galaxies are fitted with $n_{250}\leq0.75$, 87 galaxies have $0.75 1.25$). Most observed profiles exhibit a depletion in the inner region (at $r<0.3-0.4$ of the optical radius $r_{25}$ ) and are more or less exponential in the outer part. We also find breaks in the dust emission profiles at longer distances $(0.5-0.6)r_{25}$ which are associated with the breaks in the optical and near-infrared. We assume that the observed deficit of dust emission in the inner galaxy region is related to the depression in the radial profile of the HI surface density in the same region because the atomic gas reaches high enough surface densities there to be transformed into molecular gas. If a galaxy has a triggered star formation in the inner region (for example, because of a strong bar instability, which transfers the gas inwards to the centre, or a pseudobulge formation), no depletion or even an excess of dust emission in the centre is observed.

astro-ph.GA↗

arXiv2018

The fraction of bolometric luminosity absorbed by dust in DustPedia galaxies

We study the fraction of stellar radiation absorbed by dust, f_abs, in 814 galaxies of different morphological types. The targets constitute the vast majority (93%) of the DustPedia sample, including almost all large (optical diameter larger than 1'), nearby (v <= 3000 km/s) galaxies observed with the Herschel Space Observatory. For each object, we model the spectral energy distribution from the ultraviolet to the sub-millimetre using the dedicated, aperture-matched DustPedia photometry and the fitting code CIGALE. The value of f_abs is obtained from the total luminosity emitted by dust and from the bolometric luminosity, which are estimated by the fit. On average, 19% of the stellar radiation is absorbed by dust in DustPedia galaxies. The fraction rises to 25% if only late-type galaxies are considered. The dependence of f_abs on morphology, showing a peak for Sb-Sc galaxies, is weak; it reflects a stronger, yet broad, positive correlation with the bolometric luminosity, which is identified for late-type, disk-dominated, high-specific-star-formation rate, gas-rich objects. We find no variation of f_abs with inclination, at odds with radiative transfer models of edge-on galaxies. These results call for a self-consistent modelling of the evolution of the dust mass and geometry along the build-up of the stellar content. We also provide template spectral energy distributions in bins of morphology and luminosity and study the variation of f_abs with stellar mass and specific star formation rate. We confirm that the local Universe is missing the high f_abs}, luminous and actively star-forming objects necessary to explain the energy budget in observations of the extragalactic background light.

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arXiv2016

Long-term optical variability of high-mass X-ray binaries. II. Spectroscopy

We present the results of our monitoring program to study the long-term variability of the Halpha line in high-mass X-ray binaries. We have carried out the most complete optical spectroscopic study of the global properties of high-mass X-ray binaries so far with the analysis of more than 1100 spectra of 20 sources. Our aim is to characterise the optical variability timescales and study the interaction between the neutron star and the accreting material. Our results can be summarised as follows: i) we find that Be/X-ray binaries with narrow orbits are more variable than systems with long orbital periods, ii) we show that a Keplerian distribution of the gas particles provides a good description of the disks in Be/X-ray binaries, as it does in classical Be stars, iii) a decrease in the Halpha equivalent width is generally observed after major X-ray outbursts, iv) we confirm that the Halpha equivalent width correlates with disk radius, v) while systems with supergiant companions display, multi-structured profiles, most of the Be/X-ray binaries show at some epoch double-peak asymmetric profiles, indicating that density inhomogeneities is a common property in the disk of Be/X-ray binaries, vi) the profile variability (V/R ratio) timescales are shorter and the Halpha equivalent width are smaller in Be/X-ray binaries than in isolated Be stars, and vii) we provide new evidence that the disk in Be/X-ray binaries is on average denser than in classical Be stars.

astro-ph.HE↗