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- Della Bruna, Lorenza, 1992-
(författare)
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Star Formation and feedback at key physical scales for galaxy evolution
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Feedback from young, massive stars plays an essential role in the self-regulation of star formation in galaxies, and in shaping the galaxies' global properties. This phenomenon originates at small scales, surrounding the stars, but has been observed to be effective up to galactic-wide scales. The exact mechanism which allows the ionising radiation to escape the star-forming regions (HII regions), initially still embedded in their natal molecular hydrogen gas, is still unknown. Constraining the escape of ionising photons from HII regions is also relevant in order to explain the origin of the diffuse ionised gas (DIG) that is observed to contribute up to 50% to the Ha luminosity of nearby galaxies. I present the results of the study of stellar feedback in two nearby galaxies (NGC 7793 and M83), at spatial scales that critically connect the sources of ionisation with their immediate surroundings. We determine the fraction of DIG and study its properties and origin. We find that in NGC 7793 ionising sources located in the DIG are producing a sufficient amount of hydrogen-ionising (LyC) photons to explain the diffuse gas emission. In M83, on the other hand, the DIG is ionised by a mixed contribution of photoionisation and shocks. We investigate the link between LyC leakage from HII regions and their stellar and gas properties. We find that the age spread of the stellar population in the region does not seem to imply a higher leakage. Also the ionisation structure of the regions (e.g. the presence of "channels" that are transparent to the LyC photons) appears to be uncorrelated with escape in our sample. In M83, we also study the relative importance of different types of stellar feedback. We find that the pressure exerted by the ionised gas is always dominant over the direct radiation pressure. When the total HII region pressure is compared to the environmental pressure, we observe that regions near the galactic centre are in equilibrium with the surroundings, whereas regions in the disk are overpressured and are therefore expanding. We also find that changes in the local environmental conditions are the dominant factor in setting the ionised gas pressure, and that the pressure terms are linked to the physical properties (age and mass) of the young star clusters powering the regions. In the near future, observations from the James Webb Space Telescope will allow us to study the most embedded star-forming regions with a resolution comparable to the present one.
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| 2. |
- Della Bruna, Lorenza, et al.
(författare)
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Stellar feedback in M 83 as observed with MUSE II. Analysis of the H II region population : Ionisation budget and pre-SN feedback
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 666
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Tidskriftsartikel (refereegranskat)abstract
- Context. Energy and momentum injected by young, massive stars into the surrounding gas play an important role in regulating further star formation and in determining the galaxy's global properties. Before supernovae begin to explode, stellar feedback consists of two main processes: radiation pressure and photoionisation.Aims. We study pre-supernova feedback and constrain the leakage of Lyman continuum (LyC) radiation in a sample of similar to 4700 H II regions in the nearby spiral galaxy M 83. We explore the impact that the galactic environment and intrinsic physical properties (metallicity, extinction, and stellar content) have on the early phases of H II region evolution.Methods. We combined VLT/MUSE observations of the ionised gas with young star cluster physical properties derived from HST multiwavelength data. We identified H II regions based on their Hα emission, and cross-matched the sample with planetary nebulae and supernova remnants to assess contaminant sources and identify evolved H II regions. We also spectroscopically identified Wolf-Rayet (WR) stars populating the star-forming regions. We estimated the physical properties of the H II regions (luminosity, size, oxygen abundance, and electron density). For each H II region, we computed the pressure of ionised gas (Pion) and the direct radiation pressure (Pdir) acting in the region, and investigated how they vary with galactocentric distance, with the physical properties of the region, and with the pressure of the galactic environment (PDE). For a subset of similar to 500 regions, we also investigated the link between the pressure terms and the properties of the cluster population (age, mass, and LyC flux). By comparing the LyC flux derived from Hα emission with the one modelled from their clusters and WRs, we furthermore constrained any escape of LyC radiation (fesc).Results. We find that Pion dominates over Pdir by at least a factor of 10 on average over the disk. Both pressure terms are strongly enhanced and become almost comparable in the central starburst region. In the disk (R ≥ 0.15Re), we observe that Pdir stays approximately constant with galactocentric distance. We note that Pdir is positively correlated with an increase in radiation field strength (linked to the negative metallicity gradient in the galaxy), while it decreases in low extinction regions, as is expected if the amount of dust to which the momentum can be imparted decreases. In addition, Pion decreases constantly for increasing galactocentric distances; this trend correlates with the decrease in extinction - indicative of more evolved and thus less compact regions - and with changes in the galactic environment (traced by a decrease in PDE). In general, we observe that H II regions near the centre are underpressured with respect to their surroundings, whereas regions in the rest of the disk are overpressured and hence expanding. We find that regions hosting younger clusters or those that have more mass in young star clusters have a higher internal pressure, indicating that clustered star formation likely plays a dominant role in setting the pressure. Finally, we estimate that only 13% of H II regions hosting young clusters and WR stars have fesc ≥ 0, which suggests that star formation taking place outside young clusters makes a non-negligible contribution to ionising H II regions.
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| 5. |
- Della Bruna, Lorenza, et al.
(författare)
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Stellar feedback in M83 as observed with MUSE I. Overview, an unprecedented view of the stellar and gas kinematics and evidence of outflowing gas
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 660
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Tidskriftsartikel (refereegranskat)abstract
- Context. Young massive stars inject energy and momentum into the surrounding gas, creating a multi-phase interstellar medium (ISM) and regulating further star formation. The main challenge of studying stellar feedback proves to be the variety of scales spanned by this phenomenon, ranging from the immediate surrounding of the stars (H II regions, 10s pc scales) to galactic-wide kiloparsec scales.Aims. We present a large mosaic (3.8 × 3.8 kpc) of the nearby spiral galaxy M83, obtained with the MUSE instrument at ESO Very Large Telescope. The integral field spectroscopy data cover a large portion of the optical disk at a resolution of ∼20 pc, allowing the characterisation of single H II regions while sampling diverse dynamical regions in the galaxy.Methods. We obtained the kinematics of the stars and ionised gas, and compared them with molecular gas kinematics observed in CO(2-1) with the ALMA telescope array. We separated the ionised gas into H II regions and diffuse ionised gas (DIG) and investigated how the fraction of Hα luminosity originating from the DIG (fDIG) varies with galactic radius.Results. We observe that both stars and gas trace the galactic disk rotation, as well as a fast-rotating nuclear component (30″ ≃ 700 pc in diameter), likely connected to secular processes driven by the galactic bar. In the gas kinematics, we observe a stream east of the nucleus (50″ ≃ 1250 pc in size), redshifted with respect to the disk. The stream is surrounded by an extended ionised gas region (1000 × 1600 pc) with enhanced velocity dispersion and a high ionisation state, which is largely consistent with being ionised by slow shocks. We interpret this feature as either the superposition of the disk and an extraplanar layer of DIG, or as a bar-driven inflow of shocked gas. A double Gaussian component fit to the Hα line also reveals the presence of a nuclear biconic structure whose axis of symmetry is perpendicular to the bar. The two cones (20″ ≃ 500 pc in size) appear blue- and redshifted along the line of sight. The cones stand out for having an Hα emission separated by up to 200 km s−1 from that of the disk, and a high velocity dispersion ∼80–200 km s−1. At the far end of the cones, we observe that the gas is consistent with being ionised by shocks. These features had never been observed before in M83; we postulate that they are tracing a starburst-driven outflow shocking into the surrounding ISM. Finally, we obtain fDIG ∼ 13% in our field of view, and observe that the DIG contribution varies radially between 0.8 and 46%, peaking in the interarm region. We inspect the emission of the H II regions and DIG in ‘BPT’ diagrams, finding that in H II regions photoionisation accounts for 99.8% of the Hα flux, whereas the DIG has a mixed contribution from photoionisation (94.9%) and shocks (5.1%).
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| 6. |
- Della Bruna, Lorenza, et al.
(författare)
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Studying the ISM at ∼10 pc scale in NGC 7793 with MUSE : I. Data description and properties of the ionised gas
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Tidskriftsartikel (refereegranskat)abstract
- Context. Studies of nearby galaxies reveal that around 50% of the total H alpha luminosity in late-type spirals originates from diffuse ionised gas (DIG), which is a warm, diffuse component of the interstellar medium that can be associated with various mechanisms, the most important ones being leaking HII regions, evolved field stars, and shocks.Aims. Using MUSE Wide Field Mode adaptive optics-assisted data, we study the condition of the ionised medium in the nearby (D=3.4 Mpc) flocculent spiral galaxy NGC 7793 at a spatial resolution of similar to 10 pc. We construct a sample of HII regions and investigate the properties and origin of the DIG component.Methods. We obtained stellar and gas kinematics by modelling the stellar continuum and fitting the H alpha emission line. We identified the boundaries of resolved HII regions based on their H alpha surface brightness. As a way of comparison, we also selected regions according to the H alpha/[SII] line ratio; this results in more conservative boundaries. Using characteristic line ratios and the gas velocity dispersion, we excluded potential contaminants, such as supernova remnants (SNRs) and planetary nebulae (PNe). The continuum subtracted HeII map was used to spectroscopically identify Wolf Rayet stars (WR) in our field of view. Finally, we computed electron densities and temperatures using the line ratio [SII]6716/6731 and [SIII]6312/9069, respectively. We studied the properties of the ionised gas through BPT emission line diagrams combined with velocity dispersion of the gas.Results. We spectroscopically confirm two previously detected WR and SNR candidates and report the discovery of the other seven WR candidates, one SNR, and two PNe within our field of view. The resulting DIG fraction is between similar to 27 and 42% depending on the method used to define the boundaries of the HII regions (flux brightness cut in H alpha = 6.7x10(-18) erg s(-1) cm(-2) or H alpha/[SII] = 2.1, respectively). In agreement with previous studies, we find that the DIG exhibits enhanced [SII]/H alpha and [NII]/H alpha ratios and a median temperature that is similar to 3000 K higher than in HII regions. We also observe an apparent inverse correlation between temperature and H alpha surface brightness. In the majority of our field of view, the observed [SII]6716/6731 ratio is consistent within 1 sigma with n(e)< 30 cm(-3), with an almost identical distribution for the DIG and HII regions. The velocity dispersion of the ionised gas indicates that the DIG has a higher degree of turbulence than the HII regions. Comparison with photoionisation and shock models reveals that, overall, the diffuse component can only partially be explained via shocks and that it is most likely consistent with photons leaking from density bounded HII regions or with radiation from evolved field stars. Further investigation will be conducted in a follow-up paper.
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| 7. |
- Della Bruna, Lorenza, et al.
(författare)
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Studying the ISM at ∼10 pc scale in NGC 7793 with MUSEII : Constraints on the oxygen abundance and ionising radiation escape (Corrigendum)
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 663
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Tidskriftsartikel (refereegranskat)abstract
- Table 3 in the original paper reports inaccurate values of Q0exp;YSC and Q0obs(and hence of the related quantities Qexp;tot, Qexptot=Qobs, and fesc). The error in Q0exp;YSC results from a mistake in the sampling of the cluster probability distribution functions (PDFs), whereas Q0obswas mistakenly computed from non-dereddened values of L(Hα). In Table 1 we provide a revised version of the original Table 3, and in Figs. 1 and 2 we show revised versions of the original Figs. 10 and 11. The revised values do not change any of the conclusions stated in the original paper. In the following we provide revised text for the affected sections. (Figure Presented).
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| 8. |
- Della Bruna, Lorenza, et al.
(författare)
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Studying the ISM at similar to 10 pc scale in NGC 7793 with MUSE : II. Constraints on the oxygen abundance and ionising radiation escape
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 650
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Tidskriftsartikel (refereegranskat)abstract
- Context. Feedback from massive stars a ffects the interstellar medium (ISM) from the immediate surroundings of the stars (parsec scales) to galactic (kiloparsec) scales. High-spatial resolution studies of H ii regions are critical to investigate how this mechanism operates.Aims. We study the ionised ISM in NGC7793 with the MUSE instrument at ESO Very Large Telescope (VLT), over a field of view (FoV) of similar to 2 kpc2 and at a spatial resolution of similar to 10 pc. The aim is to link the physical conditions of the ionised gas (reddening, ionisation status, abundance measurements) within the spatially resolved H ii regions to the properties of the stellar populations producing Lyman continuum photons.Methods. The analysis of the MUSE dataset, which provides a map of the ionised gas and a census of Wolf Rayet stars, is complemented with a sample of young star clusters (YSCs) and O star candidates observed with the Hubble Space Telescope (HST) and of giant molecular clouds traced in CO(2-1) emission with the Atacama Large Millimeter /submillimeter Array (ALMA). We estimated the oxygen abundance using a temperature-independent strong-line method. We determined the observed total amount of ionising photons ( Q(H0)) from the extinction corrected H ff luminosity. This estimate was then compared to the expected Q(H0) obtained by summing the contributions of YSCs and massive stars. The ratio of the two values gives an estimate for the escape fraction ( fesc) of photons in the region of interest. We used the [S ii] /[O iii] ratio as a proxy for the optical depth of the gas and classified H ii regions into ionisation bounded, or as featuring channels of optically thin gas. We compared the resulting ionisation structure with the computed fesc. We also investigated the dependence of fesc on the age spanned by the stellar population in each region.Results. We find a median oxygen abundance of 12 + log (O =H) similar to 8 :37, with a scatter of 0.25 dex, which is in agreement with previous estimates for our target. We furthermore observe that the abundance map of H ii regions is rich in substructures, surrounding clusters and massive stars, although clear degeneracies with photoionisation are also observed. From the population synthesis analysis, we find that YSCs located in H ii regions have a higher probability of being younger and less massive as well as of emitting a higher number of ionising photons than clusters in the rest of the field. Overall, we find fesc;H ii = 0:67+0:08 0:12 for the population of H ii regions. We also conclude that the sources of ionisation observed within the FoV are more than su fficient to explain the amount of di ffuse ionised gas (DIG) observed in this region of the galaxy. We do not observe a systematic trend between the visual appearance of H ii regions and fesc, pointing to the e ffect of 3D geometry in the small sample probed.
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| 9. |
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| 10. |
- Hannon, Stephen, et al.
(författare)
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H α morphologies of star clusters in 16 LEGUS galaxies : Constraints on H II region evolution time-scales
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 512:1, s. 1294-1316
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Tidskriftsartikel (refereegranskat)abstract
- The analysis of star cluster ages in tandem with the morphology of their H II regions can provide insight into the processes that clear a cluster’s natal gas, as well as the accuracy of cluster ages and dust reddening derived from Spectral Energy Distribution (SED) fitting. We classify 3757 star clusters in 16 nearby galaxies according to their H α morphology (concentrated, partially exposed, no emission), using Hubble Space Telescope (HST) imaging from the Legacy ExtraGalactic Ultraviolet Survey (LEGUS). We find: (1) The mean SED ages of clusters with concentrated (1–2 Myr) and partially exposed H II region morphologies (2–3 Myr) indicate a relatively early onset of gas clearing and a short (1–2 Myr) clearing time-scale. (2) The reddening of clusters can be overestimated due to the presence of red supergiants, which is a result of stochastic sampling of the IMF in low mass clusters. (3) The age-reddening degeneracy impacts the results of the SED fitting – out of 1408 clusters with M* ≥ 5000 M⊙, we find that at least 46 (3 per cent) have SED ages which appear significantly underestimated or overestimated based on H α and their environment, while the total percentage of poor age estimates is expected to be several times larger. (4) Lastly, we examine the dependence of the morphological classifications on spatial resolution. At HST resolution, our conclusions are robust to the distance range spanned by the sample (3–10 Mpc). However, analysis of ground-based H α images shows that compact and partially exposed morphologies frequently cannot be distinguished from each other.
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