Star formation

About: Star formation is a research topic. Over the lifetime, 37405 publications have been published within this topic receiving 1808161 citations. The topic is also known as: astrogenesis.

Gusty, gaseous flows of FIRE: Galactic winds in cosmological simulations with explicit stellar feedback

Abstract: We present an analysis of the galaxy-scale gaseous outflows from the Feedback in Realistic Environments (FIRE) simulations. This suite of hydrodynamic cosmological zoom simulations resolves formation of star-forming giant molecular clouds to z = 0, and features an explicit stellar feedback model on small scales. Our simulations reveal that high-redshift galaxies undergo bursts of star formation followed by powerful gusts of galactic outflows that eject much of the interstellar medium and temporarily suppress star formation. At low redshift, however, sufficiently massive galaxies corresponding to L* progenitors develop stable discs and switch into a continuous and quiescent mode of star formation that does not drive outflows far into the halo. Mass-loading factors for winds in L* progenitors are η ≈ 10 at high redshift, but decrease to η ≪ 1 at low redshift. Although lower values of η are expected as haloes grow in mass over time, we show that the strong suppression of outflows with decreasing redshift cannot be explained by mass evolution alone. Circumgalactic outflow velocities are variable and broadly distributed, but typically range between one and three times the circular velocity of the halo. Much of the ejected material builds a reservoir of enriched gas within the circumgalactic medium, some of which could be later recycled to fuel further star formation. However, a fraction of the gas that leaves the virial radius through galactic winds is never regained, causing most haloes with mass M_h ≤ 10^(12) M_⊙ to be deficient in baryons compared to the cosmic mean by z = 0.

Hubble space telescope observations of young star clusters in NGC-4038/4039, 'the antennae' galaxies

Abstract: New, high-resolution images of the disks of NGC 4038/4039 obtained with the Wide Field Camera of the Hubble Space Telescope (HST) are presented NGC 4038/4039, nicknamed The Antennae, is a prototypical example of a pair of colliding galaxies believed to be at an early stage of a merger Down to the limiting magnitude of V approximately 23 mag, the HST images reveal a population of over 700 blue pointlike objects within the disks The mean absolute magnitude of these objects is M(sub V) = -11 mag, with the brightest objects reaching M(sub V) approximately -15 Their mean apparent color indices ar U - V = -07 mag and V - 1 = 08 mag on the Johnson UVI passband system, while their mean indices corrected for internal reddening are (u - v)(sub 0) = -10 mag and (V - I(sub 0) = 05 Their mean effective radius, determined from slightly resolved images, is 18 pc (for H(sub 0) = 50 km/s /Mpc) Based on their luminosities and resolution, most of these objects cannot be individual stars, but are likely young compact star clusters The brighter ones are similar to the objects found in NGC 1275 and NGC 7252, which appear to be young globular clusters formed during recent galazy mergers Based on their U - V and V - I colors, the brightest, bluest clusters of NGC 4038/4039 appear to be less than 10 Myr old Most of these bright clusters are relatively tightly clustered themselves, with typically a dozen individual clusters belonging to a complex identified as a giant H II region from ground-based observations The cluster luminosity function (LF) is approximately a power law, phi(L)dL proportional to L(exp -178+/-005)dL, with no hint of a turnover at fainter magnitudes This power-law shape agrees with the LF of Magellanic Cloud clusters and Galactic open clusters, but differs from the LF of old globular cluster systems that is typically Gaussian with a Full Width at Half Maximum (FWHM) of approximately 3 mag Besides the blue clusters, we also find about a dozen extremely red objects with V - I greater than 30 The highest number density of these red objects is found in the SE quadrant, where star formation appears to be most recent We propose that these objects may be very young star clusters still embedded in their placental dust cocoons

Magnetic fields in interstellar clouds from zeeman observations: inference of total field strengths by bayesian analysis

Abstract: The only direct measurements of interstellar magnetic field strengths depend on the Zeeman effect, which samples the line-of-sight component B{sub z} of the magnetic vector. In this paper, we use a Bayesian approach to analyze the observed probability density function (PDF) of B{sub z} from Zeeman surveys of H I, OH, and CN spectral lines in order to infer a density-dependent stochastic model of the total field strength B in diffuse and molecular clouds. We find that at n 300 cm{sup -3}, with an uncertainty at the 50% level in the power-law exponent of about {+-}0.05. This break-point density could be interpreted as the average density at which parsec-scale clouds become self-gravitating. Both the uniform PDF of total field strengths and the scalingmore » with density suggest that magnetic fields in molecular clouds are often too weak to dominate the star formation process. The stochasticity of the total field strength B implies that many fields are so weak that the mass/flux ratio in many clouds must be significantly supercritical. A two-thirds power law comes from isotropic contraction of gas too weakly magnetized for the magnetic field to affect the morphology of the collapse. On the other hand, our study does not rule out some clouds having strong magnetic fields with critical mass/flux ratios.« less

CIGALE: a python Code Investigating GALaxy Emission

Abstract: Context . Measuring how the physical properties of galaxies change across cosmic times is essential to understand galaxy formation and evolution. With the advent of numerous ground-based and space-borne instruments launched over the past few decades we now have exquisite multi-wavelength observations of galaxies from the far-ultraviolet (FUV) to the radio domain. To tap into this mine of data and obtain new insight into the formation and evolution of galaxies, it is essential that we are able to extract information from their spectral energy distribution (SED).Aims . We present a completely new implementation of Code Investigating GALaxy Emission (CIGALE). Written in python, its main aims are to easily and efficiently model the FUV to radio spectrum of galaxies and estimate their physical properties such as star formation rate, attenuation, dust luminosity, stellar mass, and many other physical quantities.Methods . To compute the spectral models, CIGALE builds composite stellar populations from simple stellar populations combined with highly flexible star formation histories, calculates the emission from gas ionised by massive stars, and attenuates both the stars and the ionised gas with a highly flexible attenuation curve. Based on an energy balance principle, the absorbed energy is then re-emitted by the dust in the mid- and far-infrared domains while thermal and non-thermal components are also included, extending the spectrum far into the radio range. A large grid of models is then fitted to the data and the physical properties are estimated through the analysis of the likelihood distribution.Results . CIGALE is a versatile and easy-to-use tool that makes full use of the architecture of multi-core computers, building grids of millions of models and analysing samples of thousands of galaxies, both at high speed. Beyond fitting the SEDs of galaxies and parameter estimations, it can also be used as a model-generation tool or serve as a library to build new applications.

Heavy-Element Abundances in Blue Compact Galaxies

Abstract: We present high-quality ground-based spectroscopic observations of 54 supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen abundances 12+log O/H between 7.1 and 8.3. We use the data to determine abundances for the elements N, O, Ne, S, Ar, and Fe. We also analyze Hubble Space Telescope (HST) Faint Object Spectrograph archival spectra of 10 supergiant H II regions to derive C and Si abundances in a subsample of seven BCGs. The main result of the present study is that none of the heavy element-to-oxygen abundance ratios studied here (C/O, N/O, Ne/O, Si/O, S/O, Ar/O, Fe/O) depend on oxygen abundance for BCGs with 12+log O/H≤7.6 (Z≤Z☉/20). This constancy implies that all of these heavy elements have a primary origin and are produced by the same massive (M≥10 M☉) stars responsible for O production. The dispersion of the ratios C/O and N/O in these galaxies is found to be remarkably small, being only ±0.03 and ±0.02 dex, respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest metallicity BCGs (secondary N production is negligible at these low metallicities). The absence of a time-delayed production of C and N is consistent with the scenario that galaxies with 12+logO/H≤7.6 are now undergoing their first burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicity BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3 M☉≤M≤9 M☉) stars at very low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies. In higher metallicity BCGs (7.6 7.6; (3) by the time intermediate-mass stars have evolved and released their nucleosynthetic products (100-500 Myr), all galaxies have become enriched to 7.6 8.2, secondary N production becomes important. BCGs show the same O/Fe overabundance with respect to the Sun (~0.4 dex) as Galactic halo stars, suggesting the same chemical enrichment history. We compare heavy elements yields derived from the observed abundance ratios with theoretical yields for massive stars and find general good agreement. However, the theoretical models are unable to reproduce the observed N/O and Fe/O. Further theoretical developments are necessary, in particular to solve the problem of primary nitrogen production in low-metallicity massive stars. We discuss the apparent discrepancy between abundance ratios N/O measured in BCGs and those in high-redshift damped Lyα galaxies, which are up to 1 order of magnitude smaller. We argue that this large discrepancy may arise from the unknown physical conditions of the gas responsible for the metallic absorption lines in high-redshift damped Lyα systems. While it is widely assumed that the absorbing gas is neutral, we propose that it could be ionized. In this case, ionization correction factors can boost N/O in damped Lyα galaxies into the range of those measured in BCGs.