Abstract by By Bethan James
The study of the chemical and physical evolution of Blue Compact Dwarf (BCDs) galaxies is fundamental in our understanding of galaxy formation in the early Universe. Being nearby, BCDs serve as excellent laboratories to study chemical enrichment processes in often unblemished environments, in comparison to primordial galaxies at high-redshift. It has been claimed in the past that in low-metallicity BCDs (12+log(O/H)<8.3), the N/O value is independent of metallicity (O/H ratio), implying the need to invoke a primary production of nitrogen in intermediate-mass stars, in addition to the secondary nitrogen produced from the CNO cycle in high-mass stars. In order to better understand this controversial issue, more extensive spatial information than that offered by traditional longslit spectroscopic methods, is required.
We undertook an integral field spectroscopic study of the nebular gas within a sample of BCDs previously thought to have anomalously high N/O values. Here we present the results of this study for 3 BCDs: two with anomalous N/O values (Mrk996 and UM420) and one with more normal N/O values (UM462). We describe in detail how we derived the physical conditions (Te, Ne) as a function of position within the galaxy, and as a consequence, how this revealed both revised metallicities and normal N/O ratios. Abundances for other elements will also be discussed. In the case of Mrk996, which displays both narrow and broad component emission lines, we associated an increased N/O ratio with the broad component in the nebular gas clearly correlated with WR emission features. This is one of the first clear evidence of nitrogen self-enrichment of an HII region from WR stars.
All the above mentioned results suggest that the nitrogen enrichment seen in the low-metallicity gas of these BCDs is not due to primary nitrogen production, as suggested by current theories. This investigation is also a direct proof that performing spatially resolved spectroscopy, and conducting a separate analysis of broad and narrow emission line components, are vital in deriving the 'true' chemical abundances of BCD galaxies.
Friday, March 18, 2011
Friday, March 4, 2011
Probing the stellar populations of z ~ 1.5-3.5 galaxies using rest-frame UV and optical morphology
Abstract by Nicholas Bond
I discuss some of my recent research studying the properties of the stellar populations of high-redshift galaxies by correlating their morphological properties with their spectral energy distributions. Using samples of star-forming galaxies at z ~ 1.5 - 3.5, including BX, BzK, Lyman break galaxies, and Lyman Alpha Emitters (LAEs), we implement comparative morphological diagnostics to spatially differentiate the young and old stars. Although the morphological differences between rest-frame UV and rest-frame optical light are generally small, they are statistically significant for the majority of star-forming galaxies at z >~ 2, and larger than for early-type galaxies at similar redshifts.
We then connect this work to LAEs, which are thought be among the most pristine of the high-redshift galaxies. Despite the common belief that a young, dust-free environment is needed to produce an LAE, there is increasing evidence for an underlying population of older stars in many LAEs, especially towards lower redshifts (z ~ 2).
Date: March 4th, 2010 (Friday)
Location: STScI, room N420
http://adsabs.harvard.edu/abs/2011ApJ...729...48B
http://adsabs.harvard.edu/abs/2011arXiv1101.3017G
http://adsabs.harvard.edu/abs/2010ApJ...716L.200B
http://adsabs.harvard.edu/abs/2010arXiv1005.3006G
http://adsabs.harvard.edu/abs/2010ApJ...714..255G
http://adsabs.harvard.edu/abs/2009ApJ...705..639B
http://adsabs.harvard.edu/abs/2003ApJ...598..827P
I discuss some of my recent research studying the properties of the stellar populations of high-redshift galaxies by correlating their morphological properties with their spectral energy distributions. Using samples of star-forming galaxies at z ~ 1.5 - 3.5, including BX, BzK, Lyman break galaxies, and Lyman Alpha Emitters (LAEs), we implement comparative morphological diagnostics to spatially differentiate the young and old stars. Although the morphological differences between rest-frame UV and rest-frame optical light are generally small, they are statistically significant for the majority of star-forming galaxies at z >~ 2, and larger than for early-type galaxies at similar redshifts.
We then connect this work to LAEs, which are thought be among the most pristine of the high-redshift galaxies. Despite the common belief that a young, dust-free environment is needed to produce an LAE, there is increasing evidence for an underlying population of older stars in many LAEs, especially towards lower redshifts (z ~ 2).
Date: March 4th, 2010 (Friday)
Location: STScI, room N420
http://adsabs.harvard.edu/abs/2011ApJ...729...48B
http://adsabs.harvard.edu/abs/2011arXiv1101.3017G
http://adsabs.harvard.edu/abs/2010ApJ...716L.200B
http://adsabs.harvard.edu/abs/2010arXiv1005.3006G
http://adsabs.harvard.edu/abs/2010ApJ...714..255G
http://adsabs.harvard.edu/abs/2009ApJ...705..639B
http://adsabs.harvard.edu/abs/2003ApJ...598..827P
Wednesday, February 2, 2011
Integrated spectroscopy of stellar clusters
Abstract by Andrea Ahumada
We present flux-calibrated integrated spectra in the optical spectral range (3600-6900 Å) of Galactic open clusters and Magellanic Clouds (MCs) stellar clusters mainly obtained with the 2.15 m telescope at "Complejo Astronómico El Leoncito" (CASLEO, San Juan, Argentina). Age and foreground reddening were simultaneously derived by comparing the continuum distribution and line strengths of the cluster spectra with those of template spectra. The present data will let us upgrade the spectral library of solar-metallicity and MCs metallicities which will be useful for several astrophysical applications.
Date: Friday, February 4th, 11:30 - 12.30 pm, room N420
Host: Nolan Walborn
We present flux-calibrated integrated spectra in the optical spectral range (3600-6900 Å) of Galactic open clusters and Magellanic Clouds (MCs) stellar clusters mainly obtained with the 2.15 m telescope at "Complejo Astronómico El Leoncito" (CASLEO, San Juan, Argentina). Age and foreground reddening were simultaneously derived by comparing the continuum distribution and line strengths of the cluster spectra with those of template spectra. The present data will let us upgrade the spectral library of solar-metallicity and MCs metallicities which will be useful for several astrophysical applications.
Date: Friday, February 4th, 11:30 - 12.30 pm, room N420
Host: Nolan Walborn
Monday, November 29, 2010
Modeling of X-ray emission line profiles in O stars
Abstract by M. Leutenegger
The radiatively driven winds of O stars are influential in the stars' environments, and are also of great importance in their evolution. The most important parameter of these winds is the stellar mass loss rate. Traditional diagnostics of the mass loss rate, such as H alpha recombination emission and UV P Cygni absorption lines, all have limitations and potential systematic errors. The broad X-ray emission line profiles of O stars offer an alternative mass loss rate diagnostic with different limitations and errors. I will argue that for some stars, X-ray mass loss rate measurements may be the most accurate available. Our group has pursued a modeling program aimed both at a thorough understanding of the underlying physics of X-ray profile formation, as well as robust measurements of mass loss rates. In this talk, I will review our recent work on mass loss rate measurements, and then go into greater detail on the physics of profile formation, including UV photoexcitation of metastable states, resonance scattering, and porosity.
Background Papers
Owocki & Cohen 2001
Feldmeier, Oskinova & Hamann 2003
Owocki & Cohen 2006
Leutenegger et al. 2006
Leutenegger et al. 2007
Cohen et al. 2010
Leutenegger et al. 2010
The radiatively driven winds of O stars are influential in the stars' environments, and are also of great importance in their evolution. The most important parameter of these winds is the stellar mass loss rate. Traditional diagnostics of the mass loss rate, such as H alpha recombination emission and UV P Cygni absorption lines, all have limitations and potential systematic errors. The broad X-ray emission line profiles of O stars offer an alternative mass loss rate diagnostic with different limitations and errors. I will argue that for some stars, X-ray mass loss rate measurements may be the most accurate available. Our group has pursued a modeling program aimed both at a thorough understanding of the underlying physics of X-ray profile formation, as well as robust measurements of mass loss rates. In this talk, I will review our recent work on mass loss rate measurements, and then go into greater detail on the physics of profile formation, including UV photoexcitation of metastable states, resonance scattering, and porosity.
Background Papers
Owocki & Cohen 2001
Feldmeier, Oskinova & Hamann 2003
Owocki & Cohen 2006
Leutenegger et al. 2006
Leutenegger et al. 2007
Cohen et al. 2010
Leutenegger et al. 2010
Friday, November 19, 2010
What Drives the Dynamics of Giant HII Regions? A Study of Stellar Feedback in 30 Doradus
Abstract by L. Lopez
Observations show that star formation is an inefficient and slow process. This result can be attributed to the injection of energy and momentum by stars that prevents free-fall collapse of molecular clouds. The dominant mechanism of this stellar feedback is debated theoretically: possible sources of pressure include the classical warm HII gas, the hot gas shock-heated by supernovae and stellar winds, the direct radiation from stars, and the dust-processed radiation field trapped inside the HII shell. In this talk, I will discuss how one can measure the pressures associated with these feedback processes using multiwavelength imaging (radio, infrared, optical, and X-ray), and I will present the results from applying these techniques to the giant HII region 30 Doradus, the largest star-forming region in the Local Group. For that source, I find that radiation pressure dominates close to the central star cluster, and I will consider the implications regarding the dynamics of 30 Dor and the regulation of star formation in the region.
Background papers
Lopez et al. 2010
Talk slides
pdf
Observations show that star formation is an inefficient and slow process. This result can be attributed to the injection of energy and momentum by stars that prevents free-fall collapse of molecular clouds. The dominant mechanism of this stellar feedback is debated theoretically: possible sources of pressure include the classical warm HII gas, the hot gas shock-heated by supernovae and stellar winds, the direct radiation from stars, and the dust-processed radiation field trapped inside the HII shell. In this talk, I will discuss how one can measure the pressures associated with these feedback processes using multiwavelength imaging (radio, infrared, optical, and X-ray), and I will present the results from applying these techniques to the giant HII region 30 Doradus, the largest star-forming region in the Local Group. For that source, I find that radiation pressure dominates close to the central star cluster, and I will consider the implications regarding the dynamics of 30 Dor and the regulation of star formation in the region.
Background papers
Lopez et al. 2010
Talk slides
Monday, November 15, 2010
Herschel PACS Spectroscopic Signatures of Evolution in Local Ultraluminous Galaxies
Abstract by J. Fischer
Coupling between radiation, gas, and dust is strong in the compact central regions of gas-rich mergers in their ultraluminous infrared galaxy (ULIRG) stage. I discuss the telltale signs of the effects of high ionization parameters, high far-infrared optical depths, and outflows that may be driven by this coupling, as well as the implications of new Herschel far-infrared observations for the evolutionary stage traced by these systems. The massive outflows detected via P-Cygni profile signatures of excited transitions of molecules such as OH and H2O are seen in early PACS observations of both warm and cold ULIRGs and provide evidence of feedback that may be capable of halting the star-formation process.
Background papers
Fischer et al. 2010
Abel et al. 2009
Gonzalez-Alfonso et al. 2008
Talk slides
pdf
Coupling between radiation, gas, and dust is strong in the compact central regions of gas-rich mergers in their ultraluminous infrared galaxy (ULIRG) stage. I discuss the telltale signs of the effects of high ionization parameters, high far-infrared optical depths, and outflows that may be driven by this coupling, as well as the implications of new Herschel far-infrared observations for the evolutionary stage traced by these systems. The massive outflows detected via P-Cygni profile signatures of excited transitions of molecules such as OH and H2O are seen in early PACS observations of both warm and cold ULIRGs and provide evidence of feedback that may be capable of halting the star-formation process.
Background papers
Fischer et al. 2010
Abel et al. 2009
Gonzalez-Alfonso et al. 2008
Talk slides
Monday, November 8, 2010
Weak Winds of Late-Type O Stars
Abstract by W. Marcolino
The radiatively driven wind theory provides a good agreement to measured
mass-loss rates for several O stars. However, low luminosity stars - of late spectral types - present rates much lower than expected, up to 2 orders of magnitude. Such huge discrepancy have serious consequences to stellar evolution theory and hydrodynamical models. In this work we present and discuss this so-called weak wind problem. We present results regarding five late-type O stars (O8-9V). The expanding atmosphere code CMFGEN was used to determine their stellar and wind properties. We discuss other mass-loss rate diagnostics besides the CIV 1549 line in the UV. We also show that although X-rays seem a nice way to recover the expected mass-loss rates and fit the observed spectra, the X-ray luminosity needed is in contrast with observations. Some perspectives and work in progress on the weak wind problem are discussed.
Background papers
Vink et al. 2000
Martins et al. 2005
Marcolino et al. 2009
Talk Slides
pdf
The radiatively driven wind theory provides a good agreement to measured
mass-loss rates for several O stars. However, low luminosity stars - of late spectral types - present rates much lower than expected, up to 2 orders of magnitude. Such huge discrepancy have serious consequences to stellar evolution theory and hydrodynamical models. In this work we present and discuss this so-called weak wind problem. We present results regarding five late-type O stars (O8-9V). The expanding atmosphere code CMFGEN was used to determine their stellar and wind properties. We discuss other mass-loss rate diagnostics besides the CIV 1549 line in the UV. We also show that although X-rays seem a nice way to recover the expected mass-loss rates and fit the observed spectra, the X-ray luminosity needed is in contrast with observations. Some perspectives and work in progress on the weak wind problem are discussed.
Background papers
Vink et al. 2000
Martins et al. 2005
Marcolino et al. 2009
Talk Slides
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