Browsing by Author "Concas, A."

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    Gas, dust, and the CO-to-molecular gas conversion factor in low-metallicity starbursts⋆
    (2023) Hunt, L. K.; Belfiore, F.; Lelli, F.; Draine, B. T.; Marasco, A.; Garcia-Burillo, S.; Venturi, G.; Combes, F.; Weiss, A.; Henkel, C.; Menten, K. M.; Annibali, F.; Casasola, V.; Cignoni, M.; McLeod, A.; Tosi, M.; Beltran, M.; Concas, A.; Cresci, G.; Ginolfi, M.; Kumari, N.; Mannucci, F.
    The factor relating CO emission to molecular hydrogen column density, X-CO, is still subject to uncertainty, in particular at low metallicity. In this paper, to quantify X-CO at two different spatial resolutions, we exploited a dust-based method together with ALMA 12-m and ACA data and H I maps of three nearby metal-poor starbursts, NGC 625, NGC 1705, and NGC 5253. Dust opacity at 250 pc resolution was derived based on dust temperatures estimated by fitting two-temperature modified blackbodies to Herschel PACS data. By using the HI maps, we were then able to estimate dust-to-gas ratios in the regions dominated by atomic gas, and, throughout the galaxy, to infer total gas column densities and H-2 column densities as the difference with HI. Finally, from the ACA CO(1-0) maps, we derived X-CO. We used a similar technique with 40 pc ALMA 12-m data for the three galaxies, but instead derived dust attenuation at 40 pc resolution from reddening maps based on VLT/MUSE data. At 250 pc resolution, we find X-CO  & SIM; 10(22) - 10(23) cm(-2)/K km s(-1), 5-1000 times the Milky Way value, with much larger values than would be expected from a simple metallicity dependence. Instead, at 40 pc resolution, X-CO again shows large variation, but is roughly consistent with a power-law metallicity dependence, given the Z  & SIM; 1/3 Z(& ODOT;) metal abundances of our targets. The large scatter in both estimations could imply additional parameter dependence, which we have investigated by comparing X-CO with the observed velocity-integrated brightness temperatures, I-CO, as predicted by recent simulations. Indeed, larger X-CO is significantly correlated with smaller I-CO, but with slightly different slopes and normalizations than predicted by theory. Such behavior can be attributed to the increasing fraction of CO-faint (or dark) H-2 gas with lower spatial resolution (larger beams). This confirms the idea the X-CO is multivariate, depending not only on metallicity but also on the CO brightness temperature and beam size. Future work is needed to consolidate these empirical results by sampling galaxies with different metal abundances observed at varying spatial resolutions.
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    SDSS1335+0728: The awakening of a ∼106 M⊙ black hole
    (2024) Sanchez-Saez, P.; Hernandez-Garcia, L.; Bernal, S.; Bayo, A.; Calistro Rivera, G.; Bauer, F. E.; Ricci, C.; Merloni, A.; Graham, M. J.; Cartier, R.; Arevalo, P.; Assef, R. J.; Concas, A.; Homan, D.; Krumpe, M.; Lira, P.; Malyali, A.; Martinez-Aldama, M. L.; Arancibia, A. M. Munoz; Rau, A.; Bruni, G.; Foerster, F.; Pavez-Herrera, M.; Tubin-Arenas, D.; Brightman, M.
    Context. The early-type galaxy SDSS J133519.91+072807.4 (hereafter SDSS1335+0728), which had exhibited no prior optical variations during the preceding two decades, began showing significant nuclear variability in the Zwicky Transient Facility (ZTF) alert stream from December 2019 (as ZTF19acnskyy). This variability behaviour, coupled with the host-galaxy properties, suggests that SDSS1335+0728 hosts a similar to 106 M-circle dot black hole (BH) that is currently in the process of "turning on". Aims. We present a multi-wavelength photometric analysis and spectroscopic follow-up performed with the aim of better understanding the origin of the nuclear variations detected in SDSS1335+0728. Methods. We used archival photometry (from WISE, 2MASS, SDSS, GALEX, eROSITA) and spectroscopic data (from SDSS and LAMOST) to study the state of SDSS1335+0728 prior to December 2019, and new observations from Swift, SOAR/Goodman, VLT/X-shooter, and Keck/LRIS taken after its turn-on to characterise its current state. We analysed the variability of SDSS1335+0728 in the X-ray/UV/optical/mid-infrared range, modelled its spectral energy distribution prior to and after December 2019, and studied the evolution of its UV/optical spectra. Results. From our multi-wavelength photometric analysis, we find that: (a) since 2021, the UV flux (from Swift/UVOT observations) is four times brighter than the flux reported by GALEX in 2004; (b) since June 2022, the mid-infrared flux has risen more than two times, and the W1 - W2 WISE colour has become redder; and (c) since February 2024, the source has begun showing X-ray emission. From our spectroscopic follow-up, we see that (i) the narrow emission line ratios are now consistent with a more energetic ionising continuum; (ii) broad emission lines are not detected; and (iii) the [OIII] line increased its flux similar to 3.6 years after the first ZTF alert, which implies a relatively compact narrow-line-emitting region. Conclusions. We conclude that the variations observed in SDSS1335+0728 could be either explained by a similar to 106 M-circle dot AGN that is just turning on or by an exotic tidal disruption event (TDE). If the former is true, SDSS1335+0728 is one of the strongest cases of an AGN observed in the process of activating. If the latter were found to be the case, it would correspond to the longest and faintest TDE ever observed (or another class of still unknown nuclear transient). Future observations of SDSS1335+0728 are crucial to further understand its behaviour.
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    Shaken, but not expelled: Gentle baryonic feedback from nearby starburst dwarf galaxies
    (2023) Marasco, A.; Belfiore, F.; Cresci, G.; Lelli, F.; Venturi, G.; Hunt, L. K.; Concas, A.; Marconi, A.; Mannucci, F.; Mingozzi, M.; McLeod, A. F.; Kumari, N.; Carniani, S.; Vanzi, L.; Ginolfi, M.
    Baryonic feedback is expected to play a key role in regulating the star formation of low-mass galaxies by producing galaxy-scale winds associated with mass-loading factors of beta similar to 1-50. We test this prediction using a sample of 19 nearby systems with stellar masses of 10(7) M-star/M-circle dot < 10(10), mostly lying above the main sequence of star-forming galaxies. We used MUSE at VLT optical integral field spectroscopy to study the warm ionised gas kinematics of these galaxies via a detailed modelling of their H alpha emission line. The ionised gas is characterised by irregular velocity fields, indicating the presence of non-circular motions of a few tens of km s(-1) within galaxy discs, but with intrinsic velocity dispersion of 40-60 km s(-1) that are only marginally larger than those measured in main-sequence galaxies. Galactic winds, defined as gas at velocities larger than the galaxy escape speed, encompass only a few percent of the observed fluxes. Mass outflow rates and loading factors are strongly dependent on M-star, the star formation rate (SFR), SFR surface density, and specific SFR (sSFR). For M-star of 10(8) M-circle dot we find beta similar or equal to 0.02, which is more than two orders of magnitude smaller than the values predicted by theoretical models of galaxy evolution. In our galaxy sample, baryonic feedback stimulates a gentle gas cycle rather than causing a large-scale blow-out.