Jorge Melnick

Over the past decade the relation between the Balmer-line luminosity of HII galaxies and the velocity width of the emission lines, the L - σ relation, has been painstakingly calibrated as a cosmological distance indicator with seemingly spectacular results: the Hubble constant and the energy-density of dark energy obtained using the L-σ indicator agree remarkably well with the values from canonical indicators. Since most of the luminosity of these young compact starburst galaxies is emitted by a few narrow emission-lines, they can be observed with good precision up redshifts z ~ 7 with JWST, making the L - σ indicator a potentially unique cosmological probe. However, the precision of the method remains too low to effectively constrain the relevant cosmological parameters, notably the equation of state of dark energy. The scatter of the L - σ relation is significantly larger than the random observational errors so we do not have a good handle on the systematics of the method. In a previous paper we posited that since the ionizing radiation of these young galaxies fades rapidly over time-scales of only a few million years, age differences could be the main underlying cause of the scatter. In this paper we explore several different ways to explain the scatter of the correlation, but without success. We show that the majority of HII galaxies are powered by multiple starbursts of slightly different ages, and therefore that the equivalent widths are not reliable chronometers to correct the luminosities for evolution. Thus, it is not likely that the accuracy of the L - σ distance indicator can be improved in the near future. Since we do not fully understand neither the systematics nor the underlying physics of the L - σ relation, using large samples of distant HII galaxies may or may not improve the accuracy of the method.

The tight correlation between turbulence and luminosity in Giant HII Regions is not well understood. While the luminosity is due to the UV radiation from the massive stars in the ionizing clusters, it is not clear what powers the turbulence. Observations of the two prototypical Giant HII Regions in the local Universe, 30 Doradus and NGC604, show that part of the kinetic energy of the nebular gas comes from the combined stellar winds of the most massive stars - the cluster winds, but not all. We present a study of the kinematics of 30 Doradus based on archival VLT FLAMES/GIRAFFE data and new high resolution observations with HARPS. We find that the nebular structure and kinematics are shaped by a hot cluster wind and not by the stellar winds of individual stars. The cluster wind powers most of the turbulence of the nebular gas, with a small but significant contribution from the combined gravitational potential of stars and gas. We estimate the total mass of 30 Doradus and we argue that the region does not contain significant amounts of neutral (HI) gas, and that the giant molecular cloud 30Dor-10 that is close to the center of the nebula in projection is in fact an inflating cloud tens of parsecs away from R136, the core of the ionizing cluster. We rule out a Kolmogorov-like turbulent kinetic energy cascade as the source of supersonic turbulence in Giant HII Regions.

The correlation between emission-line luminosity (L) and profile width (sigma) for HII Galaxies provides a powerful method to measure the distances to galaxies over a wide range of redshifts. In this paper we use SDSS spectrophotometry to explore the systematics of the correlation using the [OIII]5007 lines instead of Halpha or Hbeta to measure luminosities and line widths. We also examine possible systematic effects involved in measuring the profile-widths and the luminosities through different apertures. We find that the green L-sigma relation defined using [OIII]5007 luminosities is significantly more sensitive than Hbeta to the effects of age and the physical conditions of the nebulae, which more than offsets the advantage of the higher strength of the [OIII]5007 lines. We then explore the possibility of mixing [OIII]5007 profile-widths with SDSS Hbeta luminosities using the Hubble constant H0 to quantify the possible systematic effects. We find the mixed L(Hbeta) sigma[OIII] relation to be at least as powerful as the canonical L-sigma relation as a distance estimator, and we show that the evolutionary corrections do not change the slope and the scatter of the correlation, and therefore, do not bias the L-sigma distance indicator at high redshifts. Locally, however, the luminosities of the Giant HII Regions that provide the zero-point calibrators are sensitive to evolutionary corrections and may bias the Hubble constant if their mean ages, as measured by the equivalent widths of Hbeta, are significantly different from the mean age of the HII Galaxies. Using a small sample of 16 ad-hoc zero point calibrators we obtain a value of H0 = 66.4\pm4.5 km s^-1 Mpc^-1 for the Hubble constant, which is fully consistent with the best modern determinations, and that is not biased by evolutionary corrections.

The merger of two spiral galaxies is believed to be one of the main channels for the production of elliptical and early-type galaxies. In the process, the system becomes an (ultra) luminous infrared galaxy, or (U)LIRG, that morphs to a quasar, to a K+A galaxy, and finally to an early-type galaxy. The time scales for this metamorphosis are only loosely constrained by observations. In particular, the K+A phase should follow immediately after the quasi stellar object (QSO) phase during which the dust and gas remaining from the (U)LIRG phase are expelled by the active galactic nucleus (AGN). An intermediate class of QSOs with K+A spectral signatures, the post-starburst QSOs (PSQ), may represent the transitional phase between QSOs and K+As. We have compiled a sample of 72 bona fide z < 0.5 PSQ from the SDSS DR7 QSO catalogue. We find the intermediate age populations in this sample to be on average significantly weaker and metal poorer than their putative descendants, the K+A galaxies. The typical spectral energy distribution of PSQ is well fitted by three components: starlight; an obscured power-law; and a hot dust component required to reproduce the mid-IR fluxes. From the slope and bolometric luminosity of the power-law component we estimate typical masses and accretion rates of the AGN, but we find little evidence of powerful radio-loud or strong X-ray emitters in our sample. This may indicate that the power-law component originates in a nuclear starburst rather than in an AGN, as expected if the bulk of their young stars are still being formed, or that the AGN is still heavily enshrouded in dust and gas. We find that both alternatives are problematic and that more and better optical, X-ray, and mm-wave observations are needed to elucidate the evolutionary history of PSQ.

There are a number of stochastic effects that must be considered when comparing models to observations of starburst clusters: the IMF is never fully populated; the stars can never be strictly coeval; stars rotate and their photometric properties depend on orientation; a significant fraction of massive stars are in interacting binaries; and the extinction varies from star to star. The probability distributions of each of these effects are not a priori known, but must be extracted from the observations. Markov Chain Monte-Carlo methods appear to provide the best statistical approach. Here I present an example of stochastic age effects upon the upper mass limit of the IMF of the Arches cluster as derived from near-IR photometry.

We study spectral evolution of galaxies in a magnitude limited sample in a pencil beam of 10' x 7.5' from z = 1 down to 0.3. We bin our individual galaxy spectra to obtain representative high S/N spectra based upon large apparent cosmological structures. We divide the resulting average spectra in three groups: galaxies with pure absorption line spectra, galaxies with emission lines and blue continua, and galaxies with emission lines and red continua. We further divide emissionline galaxies in star-forming galaxies, Seyferts, and LINERs by using emission-line ratios and derive stellar fractions from population synthesis models. We estimate the downsizing in emission-line galaxies between z = 0.9 and z = 0.45 in our pencilbeam and the archeological dowsizing in a cluster at z = 0.29, and find the following results: (1) strong star formation in emission line galaxies, (2) aging in emission line galaxies, (3) aging in absorption systems, are shifting from bright to faint systems as cosmological time increases. Each redshift bin is repopulated in new starbursts. Therefore at redshifts z <1 galaxy formation is downsizing both in luminosity and number density. Cold Dark Matter (CDM) models are hierarchical in the sense that large halos are built from the merging of small halos. Our observations indicate that at z < 1 star formation and halo assemblage are no more in phase. Several analysis have shown that there is still a large gap between CDM simulations and observable parameters, and explored various solutions: the abundance of haloes of various masses forming at a given time is very broad, the gravitational sequence of halo formation may be modified by the galaxy formation physics which may change the efficiency of galaxy formation, simulations that incorporate a shutdown seem to be able to reproduce downsizing trends, gravitational shifts in halo formation time are enhanced by the inclusion of AGN feedback. All these attempts however point out a weakness in the CDM halo hypothesis: its low predictive power. It is as if dark matter is closer to baryons than CDM is.