Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/~wofford/research.html
Äàòà èçìåíåíèÿ: Wed Nov 28 21:04:12 2012
Äàòà èíäåêñèðîâàíèÿ: Sun Feb 3 03:03:55 2013
Êîäèðîâêà: ISO8859-5

Postdoc - Completed Projects

Postdoc - Ongoing Projects


Ultraviolet spectroscopy of circumnuclear star clusters in M83

Wofford et al. 2011

Abstract. We analyze archival HST/STIS/FUV-MAMA imaging and spectroscopy of 13 compact star clusters within the circumnuclear starburst region of M83, the closest such example. We compare the observed spectra with semi-empirical models, which are based on an empirical library of Galactic O and B stars observed with IUE, and with theoretical models, which are based on a new theoretical UV library of hot massive stars computed with WM-Basic. The models were generated with Starburst99 for metallicities of Z = 0.020 and Z = 0.040, and for stellar initial mass functions (IMFs) with upper mass limits of 10, 30, 50, and 100 M sun. We estimate the ages and masses of the clusters from the best-fit model spectra and find that the ages derived from the semi-empirical and theoretical models agree within a factor of 1.2 on average. A comparison of the spectroscopic age estimates with values derived from HST/WFC3/UVIS multi-band photometry shows a similar level of agreement for all but one cluster. The clusters have a range of ages from about 3 to 20 Myr and do not appear to have an age gradient along M83's starburst. Clusters with strong P-Cygni profiles have masses of a few x 10^4 M_sun, seem to have formed stars more massive than 30 M sun, and are consistent with a Kroupa IMF from 0.1 to 100 M_sun. Field regions in the starburst lack P-Cygni profiles and are dominated by B stars.

Lyα escape from z~0.03 star-forming galaxies: the dominant role of outflows

Wofford et al. 2012, subm. to ApJ, talk_slides, poster

Abstract. The usefulness of H I Lyα for characterizing star formation in the high-redshift uni- verse is limited by our understanding of the astrophysical processes that regulate the escape of LyÞÁ photons from galaxies. These processes can only be observed in detail out to a few Ó?100 Mpc. Past nearby (z < 0.3) spectroscopic studies are based on small samples and/or kinematically unresolved data. We observed a representative sample of 20 galaxies located at ò??zò?? = 0.03 with the HST COS. We studied correlations be- tween the Lyα line properties and fundamental properties of the galaxies. The strongest correlations are with distance, gas kinematics, and star formation rate. For the seven galaxies with net LyÞÁ emission, the ranges in the rest-frame LyÞÁ equivalent width and the LyÞÁ escape fraction are 1 ò?? 12 Å and 1 ò?? 12 %. KISSR 298 shows double-peaked LyÞÁ such that the blue peak is twice as strong as the red peak and the peaks are 370 km sò??1 apart. For the emitters excluding KISSR 298, Lyα and O i λ1302 are Doppler- shifted to mean velocites with respect to HÞÁ of 172 ÒÁ 49 km sò??1 and ò??117 ÒÁ 47 km sò??1, respectively. For the non-emitters, the corresponding offsets are 28 ÒÁ 101 and ò??23 ÒÁ 28 km sò??1.

Star formation properties of local star-forming galaxies

Wofford et al. 2013, in prep., poster

Abstract. Star-forming galaxies host populations of newly formed massive stars that emit the peak of their radiation in the ultraviolet (UV). Such radiation is heavily absorbed and scattered by dust. Studying the opacity of dust in local star-forming galaxies is necessary for understanding the nature and stellar content of these systems, as well as for determining the fraction of UV-starlight that leaks out of the galaxies. It is also important for correctly interpreting observations of more distant galaxies (z~3-4) that have rest-frame UV properties similar to those of local objects. We propose the analysis of archival UV data taken with HUT, FUSE, and IUE, for a sample of ~70 low-redshift (z<0.1) star-forming galaxies, including isolated and interacting spiral, irregular, blue-compact, blue-dwarf, and HII galaxies, with metallicities ranging from subsolar to supersolar. Due to their non-stellar UV component, Seyfert galaxies are excluded. The data will be complemented with supporting HST data (calibrated at no cost to the project). We wish to produce a homogeneous low-redshift study of the attenuation of UV-radiation by dust, down to the Lyman-limit (912 Å), based on a large sample of objects that combines most NASA UV sets. Leakage of Lyman-continuum (Lyc) photons (λ≤912 Å) and Lyman-alpha (Lyα) photons (λ=1216 Å), which can be observed simultaneously for a significant number of objects in our sample, will also be studied. The three specific questions regarding local star-forming galaxies to be addressed follow. (i) What are their dust obscuration properties? (ii) What controls UV-photon escape from these systems? (iii) What information can be gained by combining UV data from instruments with different aperture sizes, spectral resolutions, and wavelength ranges? Work on (i)-(iii) will involve comparing observations with stellar population models developed by our group over the past decade, as well as computing a grid of photoionization models specially tailored for our study.

Is the extraordinary super star cluster NGC 3125-1 an imposter?

Wofford et al. 2013, in prep., talk slides

Abstract. We propose a short, 4 orbit COS+STIS spectroscopic program to observe the extraordinary super star cluster in the local starburst galaxy NGC 3125. The cluster has the strongest He II 1640 emission ever observed in a starburst region in the local universe. This line is the tell-tale sign of Wolf-Rayet stars, the evolved descendants of very massive O stars. Taken at face value, the anomalous He II 1640 line indciates a Wolf-Rayet population that is very different from other starburst regions. However, previous attempts to interpret the observational data of the super star cluster were hampered by the low resolution of the ultraviolet spectra and the lack of co-spatial panchromatic data. As a result, the suggestion of the extraordinary nature of this super star cluster is still not unambiguous. The proposed program will settle the matter. We will test the upper initial mass function from several angles: the N V and Si IV stellar wind-lines, the elusive O V line associated with the hottest, most massive stars, and the ionizing radiation as probed by recombination lines. We will determine the dust redening with three independent methods: the SED, the Balmer decrement, and the He II 4686/1640 ratio. The STIS long-slit capabilities will allow us to perform a comparative study with a nearby super star cluster in the host galaxy. The ultraviolet spectrum of the super star cluster may be the missing link between local starbursts and star-forming galaxies at cosmological redshift. The UV spectra of the two classes of objects are rather similar, except for the He II 1640 line, which is much stronger at high redshift. Detailed observations of NGC 3125 may help shed light on understanding the details of star-formation at high redshift.

Feedback between Stars, ISM and IGM in IR-luminous galaxies

Leitherer et al. 2013, in prep.

Abstract. We propose COS G130M mode spectroscopy between 1150 and 1450 Angstroms of four ultraviolet-bright, infrared-luminous starburst galaxies. Our selected sightlines towards the starburst nuclei probe the physical conditions in the starburst-driven galactic superwinds. The spectral resolution of about 20 km/s is an order of magnitude improvement over existing HST data and allows a study of the complex gas conditions. Previous observations at lower resolution found strong, blueshifted interstellar absorption lines whose strength and velocity indicate signficant mass outflow. High-resolution data are required to verify or reject suggestions of velocity dispersion and/or covering factor variations being the prime mechanism responsible for the line properties. The proposed observations will constrain the energetics of the cool and warm gas and help determine if the outflow material can escape from the galaxies. Several weak lines will be measured at sufficiently high S/N to derive the column densities and abundances in the dominant ions entrained within the outflow. In combination with the kinematic properties we will estimate the mechanical energy and mass outflow rate and perform a comparison with the star-formation rate. We will investigate whether the outflows quench star formation and ultimately regulate the starburst as has been suggested for high-redshift galaxies. The chosen sightlines will allow us to generalize our results to the overall properties of galaxy outflows, to search for implications for similar galaxies at cosmological distances, and to add to our understanding of QSO absorption lines. Outflows may be the long-sought mechanism for distributing the products of stellar nucleosynthesis over large volumes in the universe.

The cosmic carbon conundrum

Peña-Guerrero et al. 2013, in prep.

Abstract. We propose STIS G230L ultraviolet spectroscopy of a carefully chosen sample of starburst galaxies to measure the strength of the C III] 1909 and other diagnostic lines. The C III] line is a key indicator of the carbon abundance. Atomic physics and ionization conditions in galaxies preclude any determination of nebular carbon abundances in the optical. Only HST's ultraviolet capabilities allow us to document, explore, model, and interpret the behavior of the 4th most abundant element in the universe in these galaxies. Our sample of 18 galaxies exhibits the signatures of Wolf-Rayet stars, whose presence will greatly facilitate population synthesis modeling and at the same time will provide clues on possible short-comings in stellar evolution modeling. The sample is large enough for meaningful statistics and parameter space coverage but still allows us to reach our science goals in an economic way. Together with short but deep co-spatial optical spectroscopy we will use the data for comparison with Mappings III photoionization models to determine abundances of the major elements. The full star-formation history will be established using the Starburst99 code. We will investigate the behavior of, e.g., C/O vs. O/H in order to constrain the production mechanism of carbon. We will test the predictions of chemical evolution models of galaxies and address the importance of starburst durations, infall, and outflows. The proposed study will help us understand the puzzling carbon abundances in objects as diverse as low-mass Galactic halo stars or distant damped Lyman-alpha absorbers.

Do Lyman-alpha photons escape through dust holes?

PI Wofford

Abstract. The hydrogen Lyman-alpha line is arguably the most important signature of galaxies undergoing their first violent burst of star formation. Although Lya photons are easily destroyed by dust, candidate Lya emitters have been detected at z>5. Thus the line can potentially be used to probe galaxy formation and evolution, as long as the astrophysical processes that regulate the escape of Lya photons from star-forming galaxies are well understood. We request 15 orbits for imaging in Lya and the FUV continuum with ACS/SBC, and in the H-beta/H-alpha ratio (proxy for dust extinction) with WFC3/UVIS, a sample of isolated non-AGN face-on spirals for which our team previously obtained and analyzed COS FUV spectroscopy of the central regions. Each target shows a different Lya profile, i.e., pure absorption, P-Cygni like, and multiple-emission. From the COS data, we already know the starburst phase and H I gas velocity. The images would greatly increase the impact of our spectroscopic study by enabling us to 1) conclusively determine if Lya photons escape through dust-holes, 2) assess the relative importance of dust extinction, ISM kinematics, and starburst phase in regulating the Lya escape, 3) clarify what we can really learn from the Lya equivalent width, and 4) provide constraints on the dust extinction to Lya 3D radiative transfer models. Ultimately this program will inform our understanding of the Lya escape at high redshift by providing spatially resolved views of the local conditions within star-forming galaxies that favor escape.

PhD


For my PhD I addressed two problems in galactic chemical evolution (GCE), 1) which stellar mass range dominates the cosmic production of nitrogen at low metallicity (12+log(O/H)ò??8.1), and 2) what is the time-scale for the homogeneous mixing of massive star products with the ISM. For these purposes, I studied the global N/H, O/H, and N/O number abundance ratios of H II regions in ~70 metal-poor galaxies selected from the literature and I computed single-star and giant-H II region models. This work resulted in three main findings.