The classification of the optical spectra of active galactic nuclei (AGN) into different types is well founded on AGN physics, but it involves some degree of human oversight and cannot be reliably scaled to large data sets. Machine learning (ML) tack
les such a classification problem in a fast and reproducible way, but is often perceived as a black box. However, ML interpretability and explainability are active research areas in computer science, increasingly providing us with tools to alleviate this issue. We applied ML interpretability tools to a classifier trained to predict AGN type from spectra, to demonstrate the use of such tools in this context. We trained a support-vector machine on 3346 high-quality, low redshift AGN spectra from SDSS DR15 with an existing reliable classification as type 1, type 2, or intermediate type. On a selection of test-set spectra, we computed the gradient of the predicted class probability and we built saliency maps. We also visualized the high-dimensional space of AGN spectra using t-distributed stochastic neighbor embedding (t-SNE), showing where the spectra for which we computed a saliency map are located. Regions that affect the predicted AGN type often coincide with physically relevant features, such as spectral lines. t-SNE visualization shows good separability of type 1 and type 2 spectra, while intermediate-type spectra either lie in-between as expected or appear mixed with type 2 spectra. Saliency maps show why a given AGN type was predicted by our classifier, resulting in a physical interpretation in terms of regions of the spectrum that affected its decision, making it no longer a black box. These regions coincide with those used by human experts such as relevant spectral lines, and are even used in a similar way, with the classifier e.g. effectively measuring the width of a line by weighing its center and its tails oppositely.
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 f
raction 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.
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 hi
gh 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$beta$, which is another potential inclination indicator. Our aim is to search for a correlation between the high-energy spectral complexity and FWHM of H$beta$, in order to verify whether or not the broad-line region could be flattened.
We have monitored the return to quiescence of novae previously observed in outburst as supersoft X-ray sources, with optical photometry of the intermediate polar (IP) V4743 Sgr and candidate IP V2491 Cyg, and optical spectroscopy of these two and sev
en other systems. Our sample includes classical and recurrent novae, short period (few hours), intermediate period (1-2 days) and long period (symbiotic) binaries. The light curves of V4743 Sgr and V2491 Cyg present clear periodic modulations. For V4743 Sgr, the modulation occurs with the beat of the rotational and orbital periods. If the period measured for V2491 Cyg is also the beat of these two periods, the orbital one should be almost 17 hours. The recurrent nova T Pyx already shows fragmentation of the nebular shell less than 3 years after the outburst. While this nova still had strong [OIII] at this post-outburst epoch, these lines had already faded after 3 to 7 years in all the others. We did not find any difference in the ratio of equivalent widths of high ionization/excitation lines to that of the Hbeta line in novae with short and long orbital period, indicating that irradiation does not trigger high mass transfer rate from secondaries with small orbital separation. An important difference between the spectra of RS Oph and V3890 Sgr and those of many symbiotic persistent supersoft sources is the absence of forbidden coronal lines. With the X-rays turn-off, we interpret this as an indication that mass transfer in symbiotics recurrent novae is intermittent.
We investigated four luminous supersoft X-ray sources (SSS) in the Magellanic Clouds suspected to have optical counterparts of Be spectral type. If the origin of the X-rays is in a very hot atmosphere heated by hydrogen burning in accreted envelopes
of white dwarfs (WDs), like in the majority of SSS, these objects are close binaries, with very massive WD primaries. Using the South African Large Telescope (SALT), we obtained the first optical spectra of the proposed optical counterparts of two candidate Be stars associated with SUZAKU J0105-72 and XMMU J010147.5-715550, respectively a transient and a recurrent SSS, and confirmed the proposed Be classification and Small Magellanic Clouds membership. We also obtained new optical spectra of two other Be stars proposed as optical counterparts of the transient SSS XMMU J052016.0-692505 and MAXI-J0158-744. The optical spectra with double peaked emission line profiles, are typical of Be stars and present characteristics similar to many high mass X-ray binaries with excretion disks, truncated by the tidal interaction with a compact object. The presence of a massive WD that sporadically ignites nuclear burning, accreting only at certain orbital or evolutionary phases, explains the supersoft X-ray flares. We measured equivalent widths and distances between lines peaks, and investigated the variability of the prominent emission lines profiles. The excretion disks seem to be small in size, and are likely to be differentially rotating. We discuss possible future observations and the relevance of these objects as a new class of type Ia supernovae progenitors.
We revisited the spectroscopic characteristics of narrow-line Seyfert 1 galaxies (NLS1s) by analysing a homogeneous sample of 296 NLS1s at redshift between 0.028 and 0.345, extracted from the Sloan Digital Sky Survey (SDSS-DR7) public archive. We con
firm that NLS1s are mostly characterized by Balmer lines with Lorentzian profiles, lower black hole masses and higher Eddington ratios than classic broad-line Seyfert 1 (BLS1s), but they also appear to be active galactic nuclei (AGNs) contiguous with BLS1s and sharing with them common properties. Strong Fe II emission does not seem to be a distinctive property of NLS1s, as low values of Fe II/H$beta$ are equally observed in these AGNs. Our data indicate that Fe II and Ca II kinematics are consistent with the one of H$beta$. On the contrary, O I $lambda$8446 seems to be systematically narrower and it is likely emitted by gas of the broad-line region more distant from the ionizing source and showing different physical properties. Finally, almost all NLS1s of our sample show radial motions of the narrow-line region highly-ionised gas. The mechanism responsible for this effect is not yet clear, but there are hints that very fast outflows require high continuum luminosities (> $10^{44}$ erg/s) or high Eddington ratios (log(L$_{rm bol}$/L$_{rm Edd}$) > -0.1).
Narrow-line Seyfert 1 galaxies (NLS1s) are active galactic nuclei (AGN) recently identified as a new class of $gamma$-ray sources. The high energy emission is explained by the presence of a relativistic jet observed at small angles, just like in the
case of blazars. When the latter are observed at larger angles they appear as radio-galaxies, but an analogue parent population for beamed NLS1s has not yet been determined. In this work we analyze this problem by studying the physical properties of three different samples of parent sources candidates: steep-spectrum radio-loud NLS1s, radio-quiet NLS1s, and disk-hosted radio-galaxies, along with compact steep-spectrum sources. In our approach, we first derived black hole mass and Eddington ratio from the optical spectra, then we investigated the interaction between the jet and the narrow-line region from the [O III] $lambdalambda$4959,5007 lines. Finally, the radio luminosity function allowed us to compare their jet luminosity and hence determine the relations between the samples.
The aim of this work is to understand the nature of the parent population of beamed narrow-line Seyfert 1 galaxies (NLS1s), by studying the physical properties of three parent candidates samples: steep-spectrum radio-loud NLS1s, radio-quiet NLS1s and
disk-hosted radio-galaxies. In particular, we focused on the black hole mass and Eddington ratio distribution and on the interactions between the jet and the narrow-line region.
We have collected continuum data of a sample of D-type symbiotic stars. By modelling their spectral energy distribution in a colliding-wind theoretical scenario we have found the common characteristics to all the systems: 1) at least two dust shells
are clearly present, one at sim 1000 K and the other at sim 400 K; they dominate the emission in the IR; 2) the radio data are explained by thermal self-absorbed emission from the reverse shock between the stars; while 3) the data in the long wavelength tail come from the expanding shock outwards the system; 4) in some symbiotic stars, the contribution from the WD in the UV is directly seen. Finally, 5) for some objects soft X-ray emitted by bremsstrahlung downstream of the reverse-shock between the stars are predicted. The results thus confirm the validity of the colliding wind model and the important role of the shocks. The comparison of the fluxes calculated at the nebula with those observed at Earth reveals the distribution throughout the system of the different components, in particular the nebulae and the dust shells. The correlation of shell radii with the orbital period shows that larger radii are found at larger periods. Moreover, the temperatures of the dust shells regarding the sample are found at 1000 K and <=400 K, while, in the case of late giants, they spread more uniformly throughout the same range.
MASYS is the AKARI spectroscopic survey of Symbiotic Stars in the Magellanic Clouds, and one of the European Open Time Observing Programmes approved for the AKARI (Post-Helium) Phase-3. It is providing the first ever near-IR spectra of extragalactic
symbiotic stars. The observations are scheduled to be completed in July 2009.
