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We present second epoch optical spectra for 30 changing-look (CL) candidates found by searching for Type-1 optical variability in a sample of active galactic nuclei (AGNs) spectroscopically classified as Type 2. We use a random-forest-based light-curve classifier and spectroscopic follow-up, confirming 50 per cent of candidates as turning-on CLs. In order to improve this selection method and to better understand the nature of the not-confirmed CL candidates, we perform a multiwavelength variability analysis including optical, mid-infrared (MIR), and X-ray data, and compare the results from the confirmed and not-confirmed CLs identified in this work. We find that most of the not-confirmed CLs are consistent with weak Type 1s dominated by host-galaxy contributions, showing weaker optical and MIR variability. On the contrary, the confirmed CLs present stronger optical fluctuations and experience a long (from five to ten years) increase in their MIR fluxes and the colour W1–W2 over time. In the 0.2–2.3 keV band, at least four out of 11 CLs with available SRG / eROSITA detections have increased their flux in comparison with archival upper limits. These common features allow us to select the most promising CLs from our list of candidates, leading to nine sources with similar multiwavelength photometric properties to our CL sample. The use of machine learning algorithms with optical and MIR light curves will be very useful to identify CLs in future large-scale surveys. [ABSTRACT FROM AUTHOR]

We here study the multiband properties of a kiloparsec-size superbubble in the late-type spiral galaxy NGC 628. The superbubble is the largest of many holes seen in the early release images using James Webb Space Telescope (JWST)/MIRI filters that trace the polycyclic aromatic hydrocarbon (PAH) emissions. The superbubble is located in the interarm region ∼3 kpc from the Galactic Centre in the south-east direction. The shell surrounding the superbubble is detected in H  i , CO, and H α with an expansion velocity of 12 km s−1 and contains as much as 2 × 107 M⊙ of mass in gas that is mostly in molecular form. We find a clear excess of blue, bright stars inside the bubble as compared to the surrounding disc on the Hubble Space Telescope /Advanced Camera for Surveys images. These excess blue, bright stars are part of a stellar population of 105 M⊙ mass that is formed over the last 50 Myr in different star formation episodes, as determined from an analysis of colour–magnitude diagrams using a Bayesian technique. The mechanical power injected by the massive stars of these populations is sufficient to provide the energy necessary for the expansion of the shell gas. Slow and steady, rather than violent, injection of energy is probably the reason for the maintenance of the shell structure over the kiloparsec scale. The expanding shell is currently the site for triggered star formation as inferred from the JWST 21 µm (F2100W filter) and the H α images. [ABSTRACT FROM AUTHOR]

We present a spatially resolved spectroscopic study for the metal poor H  ii galaxy J084220+115000 using MEGARA Integral Field Unit observations at the Gran Telescopio Canarias. We estimated the gas metallicity using the direct method for oxygen, nitrogen, and helium and found a mean value of 12 + log (O/H) = 8.03 ± 0.06, and integrated electron density and temperature of ∼161 cm−3 and ∼15400 K, respectively. The metallicity distribution shows a large range of Δ(O/H) = 0.72 dex between the minimum and maximum (7.69 ± 0.06 and 8.42 ± 0.05) values, unusual in a dwarf star-forming galaxy. We derived an integrated log (N/O) ratio of −1.51 ± 0.05 and found that both N/O and O/H correspond to a primary production of metals. Spatially resolved maps indicate that the gas appears to be photoionized by massive stars according to the diagnostic line ratios. Between the possible mechanisms to explain the starburst activity and the large variation of oxygen abundance in this galaxy, our data support a possible scenario where we are witnessing an ongoing interaction triggering multiple star-forming regions localized in two dominant clumps. [ABSTRACT FROM AUTHOR]