Daniel Rosa-González

An Empirical Calibration of Star Formation Rate Estimators

ABSTRA C T The observational determination of the behaviour of the star formation rate (SFR) with lookback time or redshift has two main weaknesses: (i) the large uncertainty of the dust/extinction corrections, and (ii) that systematic errors may be introduced by the fact that the SFR is estimated using different methods at different redshifts. Most frequently, the luminosity of the Ha emission line, that of the forbidden line [O II] l3727 and that of the far-ultraviolet continuum are used with low-, intermediate- and high-redshift galaxies, respectively. To assess the possible systematic differences among the different SFR estimators and the role of dust, we have compared SFR estimates using Ha ,[ OII] l3727 A ˚ , ultraviolet (UV) and far-infrared (FIR) luminosities [SFR(Ha), SFR(O II), SFR(UV) and SFR(FIR), respectively] of a sample comprising the 31 nearby star-forming galaxies that have high-quality photometric data in the UV, optical and FIR. We review the different ‘standard’ methods for the estimation of the SFR and find that while the standard method provides good agreement between SFR(Ha) and SFR(FIR), both SFR(O II) and SFR(UV) are systematically higher than SFR(FIR), irrespective of the extinction law. We show that the excess in the SFR(O II) and SFR(UV) is mainly due to an overestimation of the extinction resulting from the effect of underlying stellar Balmer absorptions in the measured emission line fluxes. Taking this effect into consideration in the determination of the extinction brings the SFR(O II) and SFR(UV) in line with the SFR(FIR), and simultaneously reduces the internal scatter of the SFR estimations. Based on these results, we have derived ‘unbiased’ SFR expressions for the SFR(UV), SFR(O II) and SFR(Ha). We have used these estimators to recompute the SFR history of the Universe using the results of published surveys. The main results are that the use of the unbiased SFR estimators brings into agreement the results of all surveys. Particularly important is the agreement achieved for the SFR derived from the FIR/millimetre and optical/UV surveys. The ‘unbiased’ star formation history of the Universe shows a steep rise in the SFR from za 0t oza 1 with SFR/O1a zU 4:5 , followed by a decline for z . 2 where SFR/O1a zU 21:5 . Galaxy formation models tend to have a much flatter slope from za 0t o za 1.

Subarcsecond radio observations of the dwarf starburst galaxy NGC 3077

We present the first subarcsecond radio observations of the nearby dwarf starburst galaxy NGC 3077 obtained with the MERLIN interferometer. We have detected two resolved sources which are coincident with the positions of two discrete X-ray sources detected by Chandra. One of the radio sources is associated with a supernova remnant (SNR) and the observed radio flux is consistent with having a non-thermal origin. The age of the SNRs of about 760 yr is between the average age of the SNRs detected in M82 and those detected in the Milky Way and the Large Magellanic Cloud. We use this detection to calculate a star formation rate ‐ ‐

Optical/Near-IR spatially resolved study of the H ii galaxy Tol 02★

The main goal of this study is to characterise the stellar populations in very low metallicity galaxies. We have obtained broad U, B, R, I, J, H, K, intermediate Stromgren y and narrow H{\alpha} and [OIII] deep images of the Wolf-Rayet, Blue Compact Dwarf, H II galaxy Tol 02. We have analysed the low surface brightness component, the stellar cluster complexes and the H II regions. The stellar populations in the galaxy have been characterised by comparing the observed broad band colours with those of single stellar population models. The main results are consistent with Tol 02 being formed by a 1.5 Gyr old disk component at the centre of which a group of 8 massive

Weak X-ray Galaxies in the Chandra Deep Field South: Searching for an Early Stage of Star Formation

>10^4 \, M_{\odot}

On the Calibration of Star Formation Rates

stellar cluster clumps is located. Six of these clumps are 10 Myr old and their near infrared colours suggest that their light is dominated by Red Supergiant stars, the other two are young Wolf-Rayet cluster candidates of ages 3 and 5 Myr respectively. 12 H II regions in the star-forming region of the galaxy are also identified. These are immersed in a diffuse H{\alpha} and [OIII] emission that spreads towards the North and South covering the old stellar disk. Our spatial-temporal analysis shows that star formation is more likely stochastic and simultaneous within short time scales. The mismatch between observations and models cannot be attributed alone to a mistreat of the RSG phase and still needs to be further investigated.

How old are H ii galaxies

A set of bright UV galaxies showing weak X-ray emission was selected from the Chandra Deep Field South (CDFS). Using photometric redshifts from the COMBO-17 survey and a combination of optical and UV colors, together with Spitzer infrared fluxes, we estimate the current star formation rate (SFR) of the selected galaxies. The SFR was used to estimate the X-ray luminosities, finding that the selected galaxies are indeed weak X-ray emitters. We discuss the extreme nature of the selected sources concluding that we are probably observing an early transient phase when most of the high mass X-ray binaries have not yet been formed.

Integral field spectroscopy of H ii region complexes: the outer disc of NGC 6946

A central problem in the “Madau-Lilly” diagram, i.e. the evolution of the Star Formation Rates (SFR) with redshift, is that the SFR is estimated using several different estimators at different redshift ranges that potentially can have systematic differences among them. We will address the question: How much of the observed change of the SFR with redshift may be due to systematic differences in the SFR calibrators. We have used a well calibrated sample of star forming galaxies in the local Universe to establish a self-consistent system of SFR estimators. The application of the new system to a high redshift sample shows an increase of the SFR by a factor of 14 from the local Universe to redshift one and a slow decrease from redshift one to five with a median value of approximately 0.3 solar masses per year per cubic megaparsec (for h = 0.5, ΩM=1. and Λ=0).