arXiv subjects

E. Congiu

Publications and source records attributed to E. Congiu.

At least 37 records · Page 2Linked to original sources

arXiv2021

Star formation scaling relations at ~100 pc from PHANGS: Impact of completeness and spatial scale

Aims: The complexity of star formation at the physical scale of molecular clouds is not yet fully understood. We investigate the mechanisms regulating the formation of stars in different environments within nearby star-forming galaxies from the PHANGS sample. Methods: Integral field spectroscopic data and radio-interferometric observations of 18 galaxies were combined to explore the existence of the resolved star formation main sequence (rSFMS), resolved Kennicutt-Schmidt relation (rKS), and resolved molecular gas main sequence (rMGMS), and we derived their slope and scatter at spatial resolutions from 100 pc to 1 kpc (under various assumptions). Results: All three relations were recovered at the highest spatial resolution (100 pc). Furthermore, significant variations in these scaling relations were observed across different galactic environments. The exclusion of non-detections has a systematic impact on the inferred slope as a function of the spatial scale. Finally, the scatter of the $Σ_\mathrm{mol. gas + stellar}$ versus $Σ_\mathrm{SFR}$ correlation is smaller than that of the rSFMS, but higher than that found for the rKS. Conclusions: The rMGMS has the tightest relation at a spatial scale of 100 pc (scatter of 0.34 dex), followed by the rKS (0.41 dex) and then the rSFMS (0.51 dex). This is consistent with expectations from the timescales involved in the evolutionary cycle of molecular clouds. Surprisingly, the rKS shows the least variation across galaxies and environments, suggesting a tight link between molecular gas and subsequent star formation. The scatter of the three relations decreases at lower spatial resolutions, with the rKS being the tightest (0.27 dex) at a spatial scale of 1 kpc. Variation in the slope of the rSFMS among galaxies is partially due to different detection fractions of $Σ_\mathrm{SFR}$ with respect to $Σ_\mathrm{stellar}$.

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arXiv2021

Ammonia snow-lines and ammonium salts desorption

Context. The nitrogen reservoir in planetary systems is a long standing problem. Part of the N-bearing molecules is probably incorporated into the ice bulk during the cold phases of the stellar evolution, and may be gradually released into the gas phase when the ice is heated, such as in active comets. The chemical nature of the N-reservoir should greatly influence how, when and in what form N returns to the gas phase, or is incorporated into the refractory material forming planetary bodies. Aims. We present the study the thermal desorption of two ammonium salts: ammonium formate and ammonium acetate from a gold surface and from a water ice substrate. Methods. Temperature-programmed desorption experiments and Fourier transform infrared reflection spectroscopy were conducted to investigate the desorption behavior of ammonium salts. Results. Ammonium salts are semi-volatile species releasing neutral species as major components upon desorption, that is ammonia and the corresponding organic acid (HCOOH and CH3COOH), at temperatures higher than the temperature of thermal desorption of water ice. Their desorption follows a first-order Wigner-Polanyi law. We find the first order kinetic parameters A = 7.7 $\pm$ 0.6 $\times$ 10$^{15}$ s$^{-1}$ and E$_{bind}$ = 68.9 $\pm$ 0.1 kJ~mol$^{-1}$ for ammonium formate and A = 3.0 $\pm$ 0.4 $\times$ 10$^{20}$ s$^{-1}$ and E$_{bind}$ = 83.0 $\pm$ 0.2 kJ~mol$^{-1}$ for ammonium acetate. The presence of a water ice substrate does not influence the desorption kinetics. Ammonia molecules locked in salts desorb as neutral molecules at temperatures much higher than previously expected that are usually attributed to refractory materials. Conclusions. Ammonia snow-line has a smaller radius than the water snow-line. As a result, the NH3/H2O ratio content in solar system bodies can be a hint as to where they formed and subsequently migrated.

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arXiv2020

Dorado and its member galaxies. Ha imaging of the group backbone

ABRIDGED: Dorado is a nearby, rich and clumpy galaxy group that extends for several degrees in the Southern Hemisphere. For the first time, we map the Ha distribution as a possible indicator of star formation (SF) activity of Dorado members a large fraction of which show interaction and merging signatures. With the 2.5m du Pont and the 1m Swope telescopes we obtained narrow-band, calibrated images of 14 galaxies, forming the backbone of the group, mapping Ha+[N II] down to few 10$^{-17}$ erg~cm$^{-2}$~s$^{-1}$~arcsec$^{-2}$. We estimated the galaxy star formation rate (SFR) from the Ha fluxes, corrected for Galaxy foreground extinction and [N II] contamination. Ha+[N II] emission has been detected in all galaxies. HII regions clearly emerge in late-type galaxies (LTGs) , while in early-type galaxies (ETGs) the Ha+[N II] emission is dominated by [N II], especially in the central regions. However, HII complexes are also revealed in four ETGs. Considering the Dorado group as a whole, we notice that the Ha+[N II] equivalent width, a measure of the specific SF, increases with the morphological type, from early to late-type members, although it remains lower that what observed in similar surveys of spiral galaxies. The SFR of the spiral members is in the range of what observed in similar galaxies surveys James+ (2004), although, in three spirals the SFR is well below the median for their morphological classes. The SFR of some early-type members tends, at odds, to be higher than the average derived from Ha+[N II] surveys of this morphological family. We detected in Ha+[N II] all the ETGs observed and half of them show HII regions. These findings suggest that ETGs in this group are not dead galaxies: their SF has not shut down yet. Mechanisms such as gas stripping and gas accretion, through galaxy-galaxy interaction, seem relevant in modifying the SF in this evolutionary phase of Dorado.

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arXiv2020

The Radio Structure of the Narrow-Line Seyfert 1 Mrk 783 with VLBA and e-MERLIN

In this paper, we present the analysis of new radio and optical observations of the narrow-line Seyfert 1 galaxy Mrk 783. 1.6 GHz observations performed with the e-MERLIN interferometer confirm the presence of the diffuse emission previously observed. The Very Long Baseline Array (VLBA) also detects the nuclear source both at 1.6 GHz (L-Band) and 5 GHz (C-band). While the L-band image shows only an unresolved core, the C-band image shows the presence of a partially resolved structure, at a position angle of 60°. The brightness temperature of the emission in both bands ($>10^6$ K) suggests that it is a pc-scale jet produced by the AGN. The relatively steep VLBA spectral index ($α_{VLBA} = 0.63\pm0.03$) is consistent with the presence of optically thin emission on milliarcsecond scales. Finally, we investigated two possible scenarios that can result in the misalignment between the kpc and pc-scale radio structure detected in the galaxy. We also analysed the optical morphology of the galaxy, which suggests that Mrk 783 underwent a merging in relatively recent times.

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arXiv2020

Absorbed relativistic jets in radio-quiet narrow-line Seyfert 1 galaxies

Narrow-line Seyfert 1 (NLS1) galaxies are peculiar active galactic nuclei (AGN). Most of them do not show strong radio emission, but recently seven radio-quiet (or -silent) NLS1s have been detected flaring multiple times at 37 GHz by the Metsähovi Radio Telescope, indicating the presence of relativistic jets in these peculiar sources. We observed them with the Karl G. Jansky Very Large Array (JVLA) in A configuration at 1.6, 5.2, and 9.0 GHz. Our results show that these sources are either extremely faint or not detected in the JVLA bands. At those frequencies, the radio emission from their relativistic jet must be absorbed, either via synchrotron self-absorption as it occurs in gigahertz-peaked sources or, more likely, via free-free absorption by a screen of ionized gas associated with starburst activity or shocks. Our findings cast new shadows on the radio-loudness criterion, which seems to be more and more frequently a misleading parameter. New high-frequency and high-resolution radio observations are essential to test our hypotheses.

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arXiv2019

Mapping metallicity variations across nearby galaxy disks

The distribution of metals within a galaxy traces the baryon cycle and the buildup of galactic disks, but the detailed gas phase metallicity distribution remains poorly sampled. We have determined the gas phase oxygen abundances for 7,138 HII regions across the disks of eight nearby galaxies using VLT/MUSE optical integral field spectroscopy as part of the PHANGS-MUSE survey. After removing the first order radial gradients present in each galaxy, we look at the statistics of the metallicity offset (Delta O/H) and explore azimuthal variations. Across each galaxy, we find low (sigma=0.03-0.05 dex) scatter at any given radius, indicative of efficient mixing. We compare physical parameters for those HII regions that are 1 sigma outliers towards both enhanced and reduced abundances. Regions with enhanced abundances have high ionization parameter, higher Halpha luminosity, lower Halpha velocity dispersion, younger star clusters and associated molecular gas clouds show higher molecular gas densities. This indicates recent star formation has locally enriched the material. Regions with reduced abundances show increased Halpha velocity dispersions, suggestive of mixing introducing more pristine material. We observe subtle azimuthal variations in half of the sample, but can not always cleanly associate this with the spiral pattern. Regions with enhanced and reduced abundances are found distributed throughout the disk, and in half of our galaxies we can identify subsections of spiral arms with clearly associated metallicity gradients. This suggests spiral arms play a role in organizing and mixing the ISM.

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arXiv2019

A correlation between [O III] line property and X-ray spectral complexity in narrow-line Seyfert 1 galaxies?

We present a detailed study of 11 narrow-line Seyfert 1 galaxies (NLS1s) from the Six-degree Field Galaxy Survey (6dFGS) that both have optical and X-ray spectroscopic observations. There are five complex NLS1s (C-NLS1s) and six simple NLS1s (S-NLS1s). We propose a possible correlation between [O III] line asymmetry and X-ray complexity. The outflow or wind from the inner accretion disk is commonly present in NLS1s and mostly directed along the system axis. In C-NLS1s only weak wind effects are measured, the X-ray spectral complexity might be caused by the presence of ionized material in the wind. On the contrary, the wind in S-NLS1s is fast, the ionized material could be swept by such a strong wind, thus the complex feature is missing which results in a simple X-ray spectrum. Furthermore, this outflow scenario seems to be an inclination effect. Since the speed of the wind is higher in a small inclination while lower in a large inclination, S-NLS1s might be sources viewed at small angles while C-NLS1s might be sources viewed at large angles.

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arXiv2019

Line shapes in narrow-line Seyfert 1 galaxies: a tracer of physical properties?

Line profiles can provide fundamental information on the physics of active galactic nuclei (AGN). In the case of narrow-line Seyfert 1 galaxies (NLS1s) this is of particular importance since past studies revealed how their permitted line profiles are well reproduced by a Lorentzian function instead of a Gaussian. This has been explained with different properties of the broad-line region (BLR), which may present a more pronounced turbulent motions in NLS1s with respect to other AGN. We investigated the line profiles in a recent large NLS1 sample classified using SDSS, and we divided the sources into two subsamples according to their line shapes, Gaussian or Lorentzian. The line profiles clearly separate all the properties of NLS1s. Black hole mass, Eddington ratio, [O III], and Fe II strength are all very different in the Lorentzian and Gaussian samples. We interpret this in terms of evolution within the class of NLS1s. The Lorentzian sources may be the youngest objects, while Gaussian profiles may be typically associated to more evolved objects. Further detailed spectroscopic studies are needed to fully confirm our hypothesis.

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arXiv2019

A luminous stellar outburst during a long-lasting eruptive phase first, and then SN IIn 2018cnf

We present the results of the monitoring campaign of the Type IIn supernova (SN) 2018cnf (aka ASASSN-18mr). It was discovered about 10 days before the maximum light (on MJD = 58293.4+-5.7 in the V band, with MV = -18.13+-0.15 mag). The multiband light curves show an immediate post-peak decline with some minor luminosity fluctuations, followed by a flattening starting about 40 days after maximum. The early spectra are relatively blue and show narrow Balmer lines with P Cygni profiles. Additionally, Fe II, O I, He I and Ca II are detected. The spectra show little evolution with time, with intermediate-width features becoming progressively more prominent, indicating stronger interaction of the SN ejecta with the circumstellar medium. The inspection of archival images from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey has revealed a variable source at the SN position, with a brightest detection in December 2015 at Mr = -14.66+-0.17 mag. This was likely an eruptive phase from the massive progenitor star started from at least mid-2011, and that produced the circumstellar environment within which the star exploded as a Type IIn SN. The overall properties of SN 2018cnf closely resemble those of transients such as SN 2009ip. This similarity favours a massive hypergiant, perhaps a luminous blue variable, as progenitor for SN 2018cnf.

astro-ph.SR↗

arXiv2019

Luminous Red Novae: Stellar Mergers or Giant Eruptions?

We present extensive datasets for a class of intermediate-luminosity optical transients known as "luminous red novae" (LRNe). They show double-peaked light curves, with an initial rapid luminosity rise to a blue peak (at -13 to -15 mag), which is followed by a longer-duration red peak that sometimes is attenuated, resembling a plateau. The progenitors of three of them (NGC4490-2011OT1, M101-2015OT1, and SNhunt248), likely relatively massive blue to yellow stars, were also observed in a pre-eruptive stage when their luminosity was slowly increasing. Early spectra obtained during the first peak show a blue continuum with superposed prominent narrow Balmer lines, with P Cygni profiles. Lines of Fe II are also clearly observed, mostly in emission. During the second peak, the spectral continuum becomes much redder, Halpha is barely detected, and a forest of narrow metal lines is observed in absorption. Very late-time spectra (~6 months after blue peak) show an extremely red spectral continuum, peaking in the infrared (IR) domain. Halpha is detected in pure emission at such late phases, along with broad absorption bands due to molecular overtones (such as TiO, VO). We discuss a few alternative scenarios for LRNe. Although major instabilities of single massive stars cannot be definitely ruled out, we favour a common envelope ejection in a close binary system, with possibly a final coalescence of the two stars. The similarity between LRNe and the outburst observed a few months before the explosion of the Type IIn SN 2011ht is also discussed.

astro-ph.SR↗

arXiv2019

SN 2017gmr: An energetic Type II-P supernova with asymmetries

We present high-cadence ultraviolet (UV), optical, and near-infrared (NIR) data on the luminous Type II-P supernova SN 2017gmr from hours after discovery through the first 180 days. SN 2017gmr does not show signs of narrow, high-ionization emission lines in the early optical spectra, yet the optical lightcurve evolution suggests that an extra energy source from circumstellar medium (CSM) interaction must be present for at least 2 days after explosion. Modeling of the early lightcurve indicates a ~500R$_{\odot}$ progenitor radius, consistent with a rather compact red supergiant, and late-time luminosities indicate up to 0.130 $\pm$ 0.026 M$_{\odot}$ of $^{56}$Ni are present, if the lightcurve is solely powered by radioactive decay, although the $^{56}$Ni mass may be lower if CSM interaction contributes to the post-plateau luminosity. Prominent multi-peaked emission lines of H$α$ and [O I] emerge after day 154, as a result of either an asymmetric explosion or asymmetries in the CSM. The lack of narrow lines within the first two days of explosion in the likely presence of CSM interaction may be an example of close, dense, asymmetric CSM that is quickly enveloped by the spherical supernova ejecta.

astro-ph.HE↗

arXiv2019

Monte Carlo studies for the optimisation of the Cherenkov Telescope Array layout

The Cherenkov Telescope Array (CTA) is the major next-generation observatory for ground-based very-high-energy gamma-ray astronomy. It will improve the sensitivity of current ground-based instruments by a factor of five to twenty, depending on the energy, greatly improving both their angular and energy resolutions over four decades in energy (from 20 GeV to 300 TeV). This achievement will be possible by using tens of imaging Cherenkov telescopes of three successive sizes. They will be arranged into two arrays, one per hemisphere, located on the La Palma island (Spain) and in Paranal (Chile). We present here the optimised and final telescope arrays for both CTA sites, as well as their foreseen performance, resulting from the analysis of three different large-scale Monte Carlo productions.

astro-ph.IM↗

arXiv2018

The interacting late-type host galaxy of the radio-loud narrow-line Seyfert 1 IRAS 20181-2244

Narrow-line Seyfert 1 galaxies (NLS1s) are a class of active galactic nuclei (AGN) which are known to be one of the few sources of $γ$ rays, which originate in a relativistic beamed jet. Becuase of their relatively large distance, a poorly investigated aspect of these jetted NLS1s is their environment, and in particular their host galaxy. In this work we present the results of a morphological analysis of the host galaxy of the jetted NLS1 IRAS 20181-2244 observed with the 6.5m Baade Telescope of the Las Campanas Observatory. The GALFIT analysis ran on the Ks image, along with additional spectroscopic observations performed with the Nordic Optical Telescope, clearly revealed the presence of an interacting system of two galaxies. The data suggest that this NLS1 is hosted by a late-type galaxy, although the result is not conclusive. This analysis, along with other results in the literature, might suggest that two populations of jetted NLS1 exist. Further morphological studies are needed to confirm or disprove this hypothesis.

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arXiv2018

The link between X-ray complexity and optical lines in NLS1s

Narrow-line Seyfert 1 galaxies (NLS1s) are a subclass of active galactic nuclei (AGN). It is often believed that these AGN have small black hole mass, which is responsible for the narrowness of the permitted lines. They are also characterised by a high accretion rate, typically closer to the Eddington limit. Nevertheless, narrow permitted lines might also be caused by a disk-like broad-line region (BLR) viewed pole-on. This class of objects presents strong X-ray emission, which is characterised by a very steep spectral index described by a single power law. In particular, some of them exhibit particular features around the iron K-shell energy at 6-8 keV. Recently, this different spectral behaviour was attributed to inclination. In this work we are going to analyse optical spectra to measure in different ways the width of H$β$, which is another potential inclination indicator. Our aim is to search for a correlation between the high-energy spectral complexity and FWHM of H$β$, in order to verify whether or not the broad-line region could be flattened.

astro-ph.GA↗

arXiv2018

A catalog of narrow-line Seyfert 1 galaxies in the southern hemisphere

We present a new accurate sample of narrow-line Seyfert 1 galaxies (NLS1s) in the southern hemisphere from the Six-degree Field Galaxy Survey (6dFGS). Based on the optical spectral features, 167 sources were classified as NLS1s. We derived flux-calibrated spectra in the sample for the first time. Strong luminosity correlations between the continuum and the emission lines were found. We estimated their central black hole masses and Eddington ratios, which are lying in a typical range of NLS1s. In the sample, 23 NLS1s were detected at radio frequencies and 12 of them are radio-loud. We publish the X-ray data analysis of a campaign of observations carried out by the Swift X-ray Telescope.

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arXiv2018

Probing narrow-line Seyfert 1 galaxies in the southern hemisphere

We present a new accurate catalog of narrow-line Seyfert 1 galaxies (NLS1s) in the southern hemisphere from the Six-degree Field Galaxy Survey (6dFGS) final data release, which is currently the most extensive spectroscopic survey available in the southern sky whose database has not yet been systematically explored. We classified 167 sources as NLS1s based on their optical spectral properties. We derived flux-calibrated spectra for the first time that the 6dFGS indeed does not provide. By analyzing these spectra, we obtained strong correlations between the monochromatic luminosity at 5100 A and the luminosity of Hbeta and [O III]5007 lines. The estimated central black hole mass and Eddington ratio have an average value of 8.6 x 10^6 M_Sun and 0.96 L_Edd respectively, which is a typical value for NLS1s. In the sample, 23 (13.8%) NLS1s were detected at radio frequencies, and 12 (7.0%) of them are radio-loud. Our results confirmed that radio-loud sources tend to have a higher redshift, more massive black hole, and higher radio and optical luminosity than radio-quiet sources.

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arXiv2018

Models of Optical Emission Lines To Investigate Narrow-Line Seyfert 1 Galaxies in Spectroscopic Databases

Thanks to the execution of extensive spectroscopic surveys that have covered large fractions of the sky down to magnitudes as faint as $i \approx 19$, it has been possible to identify several narrow-line Seyfert 1 galaxies (NLS1s) and to investigate their properties over a large range of the electro-magnetic spectrum. The interpretation of their nature, however, is still hampered by the statistical uncertainties related to the way in which NLS1 candidates are selected. In this contribution, we present a study to detect and to model emission lines in optical spectra extracted from the Sloan Digital Sky Survey (SDSS), adopting the most proper strategy to identify the source of line excitation and to produce a detailed model with measurements of several emission-line parameters. We demonstrate the application of this technique to explore fundamental questions, such as the presence of gas and dust around the core of AGNs and the spectral energy distribution of their ionizing radiation. We compare the spectral properties of NLS1s with those of other type 1 active galaxies and we summarize the potential of this approach to identify NLS1s in present day and future spectroscopic surveys. We finally consider the implications of multi-frequency data analysis in the debate concerning the intrinsic nature of NLS1s.

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arXiv2018

The flat-spectrum radio quasar 3C 345 from the high to the low emission state

We report simultaneous observations at different energy bands in radio, optical, UV, X-rays and $γ$ rays of the flat-spectrum radio-quasar 3C 345. We built the light curve of the source at different frequencies from 2008, the beginning of the \textit{Fermi} all-sky survey, to 2016, using new data and public archives. In particular we obtained several optical spectra, to study the behavior of emission lines and the continuum in different activity states and to derive the black hole mass. 3C 345 showed two flaring episodes in 2009, which occurred simultaneously in $γ$ ray, optical/UV and X-rays, and were later followed in radio. The source shows an inverse Compton dominated spectral energy distribution, which moved from higher to lower frequencies from the high to the low state. The reverberation of emission lines during one outburst event allowed us to constrain the location of production of $γ$ rays very close to the broad-line region, and possibly in the jet-base. We report the observation of an increased accretion after the outburst, possibly induced by the decrease of magnetic field intensity with respect to the low state.

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