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Register a new userStar formation rates and efficiencies in the Galactic Centre
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The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionisation rate orders of magnitude higher than the disc; akin to the environment found in star-forming galaxies at high-redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics - both direct counting of young stellar objects and integrated light measurements - are in agreement within a factor two, hence the low star formation rate is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different timescales, from $0.1 - 5$ Myr, we conclude that the star formation rate has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on $sim$ parsec scales and find $1 - 4$ per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered volumetric star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories.
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We present radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds, modeling the formation of individual massive stars, including their UV radiation feedback. The set of simulations have cloud masses between $m_{rm gas}=10^3$~M$_odot$ to $3 times 10^5$~M$_odot$ and gas densities typical of clouds in the local universe ($overline n_{rm gas} sim 1.8times 10^2$~cm$^{-3}$) and 10$times$ and 100$times$ denser, expected to exist in high-redshift galaxies. The main results are: {it i}) The observed Salpeter power-law slope and normalisation of the stellar initial mass function at the high-mass end can be reproduced if we assume that each star-forming gas clump (sink particle) fragments into stars producing on average a maximum stellar mass about $40%$ of the mass of the sink particle, while the remaining $60%$ is distributed into smaller mass stars. Assuming that the sinks fragment according to a power-law mass function flatter than Salpeter, with log-slope $0.8$, satisfy this empirical prescription. {it ii}) The star formation law that best describes our set of simulation is $drho_*/dt propto rho_{gas}^{1.5}$ if $overline n_{gas}<n_{cri}approx 10^3$~cm$^{-3}$, and $drho_*/dt propto rho_{rm gas}^{2.5}$ otherwise. The duration of the star formation episode is roughly $6$ clouds sound crossing times (with $c_s=10$~km/s). {it iii}) The total star formation efficiency in the cloud is $f_*=2% (m_{rm gas}/10^4~M_odot)^{0.4}(1+overline n_{rm gas}/n_{rm cri})^{0.91}$, for gas at solar metallicity, while for metallicity $Z<0.1$~Z$_odot$, based on our limited sample, $f_*$ is reduced by a factor of $sim 5$. {it iv)} The most compact and massive clouds appear to form globular cluster progenitors, in the sense that star clusters remain gravitationally bound after the gas has been expelled.
We examine the HI-based star formation efficiency (SFE_HI), the ratio of star formation rate to the atomic Hydrogen (HI) mass, in the context of a constant stability star-forming disk model. Our observations of HI-selected galaxies show SFE to be fairly constant (log SFE_HI = -9.65 yr-1 with a dispersion of 0.3 dex) across ~5 orders of magnitude in stellar masses. We present a model to account for this result, whose main principle is that the gas within galaxies forms a uniform stability disk and that stars form within the molecular gas in this disk. We test t
The slope of the star formation rate/stellar mass relation (the SFR Main Sequence; ${rm SFR}-M_*$) is not quite unity: specific star formation rates $({rm SFR}/M_*)$ are weakly-but-significantly anti-correlated with $M_*$. Here we demonstrate that this trend may simply reflect the well-known increase in bulge mass-fractions -- portions of a galaxy not forming stars -- with $M_*$. Using a large set of bulge/disk decompositions and SFR estimates derived from the Sloan Digital Sky Survey, we show that re-normalizing SFR by disk stellar mass $({rm sSFR_{rm disk}equiv SFR}/M_{*,{rm disk}})$ reduces the $M_*$-dependence of SF efficiency by $sim0.25$ dex per dex, erasing it entirely in some subsamples. Quantitatively, we find $log {rm sSFR_{disk}}-log M_*$ to have a slope $beta_{rm disk}in[-0.20,0.00]pm0.02$ (depending on SFR estimator and Main Sequence definition) for star-forming galaxies with $M_*geq10^{10}M_{odot}$ and bulge mass-fractions $B/Tlesssim0.6$, generally consistent with a pure-disk control sample ($beta_{rm control}=-0.05pm0.04$). That $langle{rm SFR}/M_{*,{rm disk}}rangle$ is (largely) independent of host mass for star-forming disks has strong implications for aspects of galaxy evolution inferred from any ${rm SFR}-M_*$ relation, including: manifestations of mass quenching (bulge growth), factors shaping the star-forming stellar mass function (uniform $dlog M_*/dt$ for low-mass, disk-dominated galaxies), and diversity in star formation histories (dispersion in ${rm SFR}(M_*,t)$). Our results emphasize the need to treat galaxies as composite systems -- not integrated masses -- in observational and theoretical work.
Studies of the Galactic Centre suggest that in-situ star formation may have given rise to the observed stellar population near the central supermassive black hole (SMBH). Direct evidence for a recent starburst is provided by the currently observed young stellar disc (2-7 Myr) in the central 0.5 pc of the Galaxy. This result suggests that star formation in galactic nuclei may occur close to the SMBH and produce initially flattened stellar discs. Here we explore the possible build-up and evolution of nuclear stellar clusters near SMBHs through in-situ star formation producing stellar discs similar to those observed in the Galactic Centre and other nuclei. We make use of N-body simulations to model the evolution of multiple young stellar discs and explore the potential observable signatures imprinted by such processes. Each of the five simulated discs is evolved for 100 Myr before the next one is introduced in the system. We find that populations born at different epochs show different morphologies and kinematics. Older and presumably more metal poor populations are more relaxed and extended, while younger populations show a larger amount of rotation and flattening. We conclude that star formation in central discs can reproduce the observed properties of multiple stellar populations in galactic nuclei differing in age, metallicity and kinematic properties.
NGC 3311 is the central galaxy of the Hydra I galaxy cluster. It has a hot interstellar medium and hosts a central dust lane with emission lines. These dust lanes are frequent in elliptical galaxies, but the case of NGC 3311 might be particularly interesting for problems of dust lifetime and the role of cool gas in the central parts. We aim to use archival HST images and MUSE data to investigate the central dust structure of NGC 3311. We used the tool PyParadise to model the stellar population and extract the emission lines. The HST/ACS colour map reveals the known dust structures, but also blue spots, which are places of strong line emission. A dusty mini-jet emanates from the centre. The distribution of the emission line gas matches the dust silhouette almost exactly. Close to the brightest Halpha emission, the ratio [NII]/Halpha resembles that of HII-regions; in the outer parts, [NII] gets stronger and is similar to LINER-like spectra. The gas kinematics is consistent with that of a rotating disc. The Doppler shifts of the strongest line emissions, which indicate the areas of highest star formation activity, smoothly fit into the disc symmetry. The metallicity is supersolar. The presence of neutral gas is indicated by the fit residuals of the stellar NaI D absorption line, which we interpret as interstellar absorption. We estimate the mass of the neutral gas to be of the order of the X-ray mass. The dynamical mass infers a stellar population of intermediate age, whose globular clusters have already been identified. Our findings can be harmonised in a scenario in which the star formation is triggered by the accretion of cold gas onto a pre-existing gas/dust disc or ring. Newly produced dust then contributes to the longevity of the dust.
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