Calibration of star formation rate tracers for short- and long-lived star formation episodes

To derive the history of star formation in the Universe a set of calibrated star formation rate tracers at different wavelengths is required. The calibration has to consistently take into account the effects of extinction, star formation regime (short or long-lived) and evolutionary state to avoid biases at different redshift ranges. We use evolutionary synthesis models optimized for intense episodes of star formation in order to compute a consistent calibration of the most usual star formation rate tracers at different energy ranges, from X-ray to radio luminosities. Nearly-instantaneous and continuous star formation regimes, and the effect of interstellar extinction are considered, as well as the effect of metallicity on the calibration of the different estimators. A consistent calibration of a complete set of star formation rate tracers is presented, computed for the most usual star-forming regions conditions: evolutionary state, star formation regime, interstellar extinction and initial mass function. We discuss the validity of the different tracers in different star formation scenarios and compare our predictions with previous calibrations of general use. Nearly-instantaneous and continuous star formation regimes must be distinguished. While the Star Formation Strength (\msun) should be used for the former, the more common Star Formation Rate (\msun yr$^{-1}$) is only valid for episodes forming stars at a constant rate during extended periods for time. Moreover, even for the latter, the evolutionary state should be taken into account, since most SFR tracers stabilize only after 100 Myr of evolution.Comment: Accepted for publication in A&A, webtool in http://www.laeff.cab.inta-csic.es/research/sfr/, 19 pages, 10 figures, 14 tables. New version including language style revisio

Improving the Estimation of Star formation Rates and Stellar Population Ages of High-redshift Galaxies from Broadband Photometry

We explore methods to improve the estimates of star formation rates and mean stellar population ages from broadband photometry of high redshift star-forming galaxies. We use synthetic spectral templates with a variety of simple parametric star formation histories to fit broadband spectral energy distributions. These parametric models are used to infer ages, star formation rates and stellar masses for a mock data set drawn from a hierarchical semi-analytic model of galaxy evolution. Traditional parametric models generally assume an exponentially declining rate of star-formation after an initial instantaneous rise. Our results show that star formation histories with a much more gradual rise in the star formation rate are likely to be better templates, and are likely to give better overall estimates of the age distribution and star formation rate distribution of Lyman break galaxies. For B- and V-dropouts, we find the best simple parametric model to be one where the star formation rate increases linearly with time. The exponentially-declining model overpredicts the age by 100 % and 120 % for B- and V-dropouts, on average, while for a linearly-increasing model, the age is overpredicted by 9 % and 16 %, respectively. Similarly, the exponential model underpredicts star-formation rates by 56 % and 60 %, while the linearly-increasing model underpredicts by 15 % 22 %, respectively. For U-dropouts, the models where the star-formation rate has a peak (near z ~ 3) provide the best match for age -- overprediction is reduced from 110 % to 26 % -- and star-formation rate -- underprediction is reduced from 58 % to 22 %. We classify different types of star-formation histories in the semi-analytic models and show how the biases behave for the different classes. We also provide two-band calibration formulae for stellar mass and star formation rate estimations.Comment: 28 pages, 7 figures, minor changes; published in Ap

The Spatial Extent and Distribution of Star Formation in 3D-HST Mergers at z~1.5

We present an analysis of the spatial distribution of star formation in a sample of 60 visually identified galaxy merger candidates at z>1. Our sample, drawn from the 3D-HST survey, is flux-limited and was selected to have high star formation rates based on fits of their broad-band, low spatial resolution spectral energy distributions. It includes plausible pre-merger (close pairs) and post-merger (single objects with tidal features) systems, with total stellar masses and star formation rates derived from multi-wavelength photometry. Here we use near-infrared slitless spectra from 3D-HST which produce Halpha or [OIII] emission line maps as proxies for star-formation maps. This provides a first comprehensive high-resolution, empirical picture of where star formation occurred in galaxy mergers at the epoch of peak cosmic star formation rate. We find that detectable star formation can occur in one or both galaxy centres, or in tidal tails. The most common case (58%) is that star formation is largely concentrated in a single, compact region, coincident with the centre of (one of) the merger components. No correlations between star formation morphology and redshift, total stellar mass, or star formation rate are found. A restricted set of hydrodynamical merger simulations between similarly massive and gas-rich objects implies that star formation should be detectable in both merger components, when the gas fractions of the individual components are the same. This suggests that z~1.5 mergers typically occur between galaxies whose gas fractions, masses, and/or star formation rates are distinctly different from one another.Comment: Accepted for publication in MNRAS, 16 pages, 10 figure

Star Formation in the LMC: Gravitational Instability and Dynamical Triggering

Evidence for triggered star formation is difficult to establish because energy feedback from massive stars tend to erase the interstellar conditions that led to the star formation. Young stellar objects (YSOs) mark sites of {\it current} star formation whose ambient conditions have not been significantly altered. Spitzer observations of the Large Magellanic Cloud (LMC) effectively reveal massive YSOs. The inventory of massive YSOs, in conjunction with surveys of interstellar medium, allows us to examine the conditions for star formation: spontaneous or triggered. We examine the relationship between star formation and gravitational instability on a global scale, and we present evidence of triggered star formation on local scales in the LMC.Comment: 6 pages, 6 figures, IAU Symposium 237, Triggered Star Formation in a Turbulent Medium, eds. Elmegreen and Palou

Massive Star Formation

The enormous radiative and mechanical luminosities of massive stars impact a vast range of scales and processes, from the reionization of the universe, to the evolution of galaxies, to the regulation of the interstellar medium, to the formation of star clusters, and even to the formation of planets around stars in such clusters. Two main classes of massive star formation theory are under active study, Core Accretion and Competitive Accretion. In Core Accretion, the initial conditions are self-gravitating, centrally concentrated cores that condense with a range of masses from the surrounding, fragmenting clump environment. They then undergo relatively ordered collapse via a central disk to form a single star or a small-N multiple. In this case, the pre-stellar core mass function has a similar form to the stellar initial mass function. In Competitive Accretion, the material that forms a massive star is drawn more chaotically from a wider region of the clump without passing through a phase of being in a massive, coherent core. In this case, massive star formation must proceed hand in hand with star cluster formation. If stellar densities become very high near the cluster center, then collisions between stars may also help to form the most massive stars. We review recent theoretical and observational progress towards understanding massive star formation, considering physical and chemical processes, comparisons with low and intermediate-mass stars, and connections to star cluster formation.Comment: Accepted for publication as a chapter in Protostars and Planets VI, University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C. Dullemond, Th. Hennin

Intergalactic Star Formation

Star formation in interacting systems may take place in various locations, from the dust--enshrouded core of Ultraluminous Infrared Galaxies to more unusual places such as the debris of colliding galaxies expelled into the intergalactic medium. Determining whether star-formation proceeds in the latter environment, far from the parent galaxies, in a similar way as in spiral disks has motivated the multi--wavelength study presented here. We collected VLA/HI, UV/GALEX, optical Halpha and MIR/Spitzer images of a few nearby interacting systems chosen for their prominent "intergalactic" star formation activity. Preliminary results on the spectacular collisional HI ring around NGC 5291 are presented.Comment: 4 pages, 1 fig., tp appear in conference proceedings "Studying Galaxy Evolution with Spitzer and Herschel", eds. V. Charmandaris, D. Rigopoulou & N. Kylafi

Radio Triggered Star Formation in Cooling Flows

The giant galaxies located at the centers of cluster cooling flows are frequently sites of vigorous star formation. In some instances, star formation appears to have been triggered by the galaxy's radio source. The colors and spectral indices of the young populations are generally consistent with short duration bursts or continuous star formation for durations much less than 1 Gyr, which is less than the presumed ages of cooling flows. The star formation properties are inconsistent with fueling by a continuously accreting cooling flow, although the prevalence of star formation is consistent with repeated bursts and periodic refueling. Star formation may be fueled, in some cases, by cold material stripped from neighboring cluster galaxies