Renato Falomo

BL Lac objects (BLLs) are defined by the presence of very weak (typically < 5 Å) or even absent spectral lines. This makes determining their distance particularly challenging, especially at high redshift, where the sources are fainter and the host galaxy contribution in the optical band becomes negligible. Yet measuring their distance is crucial for deriving and modelling their luminosity, notably in the gamma-ray band, where BLLs dominate the extragalactic sky. In this work, we re-examine the reported high-redshift ( z > 0.6) BLLs, many of which are commonly cited in the literature despite appearing questionable. We present new spectra for 52 objects obtained with the 10.4 m GTC. For 16 of them we propose a new redshift or provide a spectroscopic lower limit, while for 14 sources we confirm previously published values. In 22 cases the spectra remain featureless, even with high S/N observations. These objects are likely to lie at 0.3 < z < 1.4; the lack of host-galaxy features sets a lower limit to their distance, while the absence of intervening absorption systems argues against substantially higher redshifts. We compare our findings with the previous robustly established cases of BLLs at z > 0.6 that meet our selection criteria.

Modifications to Newtonian dynamics at low accelerations have long been proposed as an alternative to dark matter to explain galaxy rotation curves. More recently, similar corrections have been invoked to interpret anomalies in Cavendish-type laboratory experiments and in the dynamics of wide binary stars, although the latter remain affected by ongoing observational debate. We show that, if deviations from Newtonian gravity occur in the low-acceleration regime, the available data are broadly consistent with a modification of Newtonian dynamics-like interpretation. In this framework, wide binary systems appear to extend the Tully–Fisher relation – well established for galaxies – to much smaller mass scales. Taken together, departures from Newtonian predictions at low accelerations in galaxies, wide binaries, and laboratory experiments may point to a common physical scenario.

We investigate the relationship between quasars (QSOs) and close companion galaxies using photometry and spectroscopy from the large dataset of the SDSS DR16 survey. Using the SDSS-QSO catalogue of quasars, we selected objects with 0.1 < z ≤ 0.35 and absolute magnitude M ( r ) < − 21.3. For all these targets, we searched for candidate companion galaxies located at a projected distance from the target of < 700 kpc and a radial velocity difference from the QSO of Δ V < 1000 km/s. We find that in 447 QSOs at least one companion galaxy was found to be associated. A total of 691 associated galaxies were found in these QSO fields. In the majority there was just one associated galaxy however in several cases a number of companion were also discovered and in two cases the QSO was found in a rich galaxy environments. We note that the possible contamination attributed to a chance projection of the companion galaxies is less than 5%. Based on the available data we expected to find on average ∼2 associated companion galaxies for each QSO. A small fraction (13%) of the companion galaxies exhibit [OII] emission lines as signature of recent star formation. However a similar fraction (16%) of unassociated galaxies in the same QSO fields exhibit [OII] emission. Overall this study suggests that there is no significant link between the presence of these close companion galaxies or the signature of recent star formation and QSO nuclear activity.

We investigate the relationship between quasars and close companion galaxies using photometry and spectroscopy from the large data set of the SDSS DR16 survey. From the SDSS-QSO catalog of quasars, we selected objects with 0.1 < z < 0.35 and absolute magnitude M(r) <-21.3. For all these targets, we searched for candidate companion galaxies that have a projected distance from the target < 700 kpc and a radial velocity difference from the QSO DeltaV < 1000 km/sec. We find that in 447 QSO at least one companion galaxy is associated. A total of 691 associated galaxies are found in these QSO fields. In the majority of them there is just one associated galaxy but in few cases, many companions are also discovered and in two cases the QSO is found in rich galaxy environments. The possible contamination due to a chance projection of the companion galaxies is found less than 5%. Based on the available data we expect on average to find \sim2 associated companion galaxies for each QSO. A small fraction (13%) of the companion galaxies exhibits [OII] emission lines as signature of recent star formation. However a similar fraction (16%) of unassociated galaxies in the same QSO fields show [OII] emission. This study suggests that there is no significant link between the presence of these close companion galaxies or the signature of recent star formation and the QSO nuclear activity.

We present UV and X-ray observations, obtained with the Swift Observatory, of Gaia BH3, a binary system containing a 33 M _⊙ black hole discovered through Gaia astrometry. The system is well detected in all UV and optical filters (1700–6500 Å). We compare our results with the modeling of the noncollapsed component using synthetic stellar libraries, and a good agreement is found with our UV observations.

We present UV and X-ray observations, obtained with the Swift Observatory, of Gaia BH3 a binary system containing a 33 solar masses black hole discovered through Gaia astrometry. The system is well detected in all UV and optical filters (1700-6500 Å). We compare our results with the modeling of the non collapsed component using synthetic stellar libraries, a good agreement is found with our UV observations

Following the high activity of the γ-ray Fermi source 4FGL J0449.1+1121 (PKS 0446+112), possibly associated with a IceCube neutrino event IC-240105A, we obtained optical spectroscopy with the Gran Telescopio Canarias of the counterpart. We detect a clear emission line at 3830 Å identified as Lyα that confirms the redshift of source at z = 2.153. Comparing with previous spectroscopy, we find an increase of the continuum by a factor ∼10, and a significant decrease of the C iv 1550 emission line flux by a factor ∼5. This produces a dramatic drop of the equivalent width from ∼20 to 0.8 Å, which is suggestive of a very high jet activity. The full width half maximum of the emission lines are midway (1000–2000 km s ⁻¹) between those typical of the broad and narrow regions of quasars. Based on this, the source classification is intermediate between flat spectrum radio quasar and BL Lac object.

A correlation between astrophysical high-energy neutrinos and blazars has been suggested by various authors. In particular, a likely association between IceCube events and intermediate and high-energy peaked BL Lac objects has led to a sample of 47 objects having a high probability of being neutrino sources. In the first paper of this series we reported optical spectroscopy of 17 objects, which together with data taken from the literature covered 80 per cent of the sample. Here we present spectroscopy obtained at large aperture telescopes of a further 17 objects (plus four additional targets), which completes the sample coverage. For twelve objects we are able to determine the redshift (0.07 < z < 1.6), while for the others we set a lower limit on it, based on either the robust detection of intervening absorption systems or on an estimation derived from the absence of spectral signatures of the host galaxy. With these new data we expand and reinforce the main results of our previous papers, namely the fact that in terms of their broad-band properties our sources appear to be indistinguishable from the rest of the blazar population and the relatively large (> 34 per cent and possibly as high as 80 per cent) fraction of masquerading BL Lac objects, for which the low equivalent width of the emission lines is due to the brightness of the boosted continuum, rather than being an intrinsic property, in our sample.

We present multi-object optical spectroscopy of the galaxies in the environment of 12 low-redshift (z < 0.5) quasars and of 11 inactive massive galaxies chosen to match the properties of the quasar host galaxies to probe physical association and possible events of recent star formation (SF). The quasars are selected from a sample of QSOs in the Sloan Digital Sky Survey (SDSS) Stripe 82 region for which both the host galaxy and the large-scale environments were previously investigated. The new observations complement those reported in our previous works on close companion galaxies of nearby quasars. For the whole data set, we find that for about half (19 out of 44) of the observed QSOs, there is at least one associated companion galaxy. In addition to the new spectroscopic observations, we add data from the SDSS data base for the full sample of objects. We find that the incidence of companion galaxies in the fields of QSO (17 per cent) is not significantly different from that of inactive galaxies (19 per cent) similar to quasar hosts in redshift and mass. Nevertheless, the companions of quasars exhibit more frequently emission lines than those of inactive galaxies, suggesting a moderate link between the nuclear activity and recent SF in their environments.

Examining a catalogue of isolated galaxy pairs, a preferred orbital intervelocity of ∼ 150 km s⁻¹ was recently reported. This discovery is difficult to reconcile with the expectations from Newtonian numerical simulations of cosmological structure formations. In a previous paper we have shown that a preferred intervelocity for galaxy pairs is expected in Modified Newtonian Dynamics (MOND). Here a detailed analysis of the MOND predictions is presented, showing that a remarkable agreement with the observations can be achieved for a mass to light ratio M/L∼1 in solar units. This agrees with the expectations for a typical stellar population, without requiring non-baryonic dark matter for these systems.