Galaxies and Extragalactic Astronomy
D. F. de Mello along with J. P. Gardner (GSFC) and the GOODS team have analyzed HST/WFPC2 observations covering 88 % of the GOODS fields. Most of their 268 identified objects have redshifts 0.2<z<0.8. They used the spectral types obtained by photometric redshift fitting to identify starburst galaxies and found that 45 % of the UV-selected galaxies are starbursts, nearly 75 % of the starbursts have tidal tails or show some peculiarity typical of interactions or mergers, and 50 % have companions. D. F. de Mello, B. Siana, H. Teplitz (Caltech) and collaborators estimated the escape fraction of the Lyman continuum radiation from starburst galaxies at z~1.3 and demonstrated that previous high-redshift studies likely have underestimated the relative escape fraction by a factor 2.
In a project led by R. Kennicutt (IoA/Cambridge), D. F. de Mello together with the SINGS team have studied the relationship between the star formation rate, surface density and gas surface density in the spiral galaxy M51a using multi-wavelength data.
D. F. de Mello, J. Gallagher (U. Wisconsin-Madison), M. Mountain, L. Smith and E. Sabbi (STScI) studied the environment of the interacting triplet M81, M82 and NGC 3077. They identified eight young star-forming regions with GALEX observations and determined the star-formation history of these regions using HST ACS images. Their results imply that the old stellar population was formed in the stellar disk of M82 and/or M81 and ejected into the intergalactic medium during a tidal passage about 200-300 Myr ago, whereas the young UV-bright stars have formed in the tidal debris. The tidal bridge between M81 and M82 appears to have star formation greater than the low levels observed in the Magellanic bridge, but lower than the active star-forming tidal tails associated with major galaxy mergers. They have also studied the M81 companion dwarf irregular galaxy Holmberg IX and classified it as the nearest young galaxy. At least 20 % of Holmberg IX's mass was produced during the last 200 Myr, giving it the youngest stellar population of any nearby galaxy.
D. F. de Mello worked in collaboration with a group led by C. Mendes de Oliveira (USP) in obtaining and analyzing images of compact groups of galaxies. They found that young star-forming regions are located in the intergalactic H I clouds of the compact group which extend over 130 kpc from the main galaxies. A tidal dwarf galaxy candidate is located in the densest region of the H I tail. Fifteen other intra-group H II regions and tidal dwarf galaxy candidates are detected.
D. F. de Mello, with Catholic University graduate student S. Petty, J. Gallagher (U.Wisconsin-Madison), M. Mountain and L. Smith (STScI) have been artificially redshifting images of local interacting and starburst galaxies, including M82, to determine which types of starburst galaxies might represent similar morphologies as the higher redshift starburst galaxies at z~1.5 and the Lyman Break Galaxies at z~4. They also investigated possible biases in morphological analysis due to dimming of the surface brightness with redshift.
D. F. de Mello, with Catholic University graduate student E. Voyer, J. P. Gardner B., Siana H., Teplitz (Caltech) are analyzing the U-band image of the HST Ultra Deep Field. The U-band serves as a key component in understanding galactic star formation environments at intermediate redshifts. Recently, Catholic University graduate student C. Quirk joined the team to analyze GALEX detections in the HST Ultra Deep Field images.
D. F. de Mello in collaboration with A. Rawat (IUCA) and Y. Wadadekar (NCRA) are analyzing the deepest U-band image ever taken with HST and obtaining morphological information in the UV rest frame. They give a possible explanation to that high-z Lyman Break Galaxies show significant higher.
D. F. de Mello in a program led by S. Oey (U. Michigan) is clarifying the conditions that promote the escape of Lyman continuum radiation from star-forming galaxies. The results will constrain the energy budget in starbursts and clarify the radiation transfer in these galaxies and their environments.
G. Williger has in collaboration with Haberzettl and collaborators studied UV-selected galaxies in a large quasar group at z~1.2. This is the first result of a number of observing proposals for a multi-wavelength survey of a high quasar density region. They found that the UV-selected galaxies, on average, have flat spectra with up to 2.5 Gyr ages.
G. Williger and Karen Collins co-authored a technical report (internally refereed by STScI) on a ghost in the HST ACS solar blind channel. The work was performed during the analysis of HD 169142 and HD 100453. The position and the strength of the ghost has been entered into the instrumental calibration record for permanent reference.
S. Kraemer assisted J. P. Dunn, D. M. Crenshaw (GSU) and J. R. Gable (Creighton U.) in a FUSE survey of intrinsic absorption in active galaxies. They found that within their sample of 72 Seyfert galaxies and quasars, 50 % show evidence of intrinsics absorption in the O VI λλ1031.9, 1037.6 and H I Ly&beta. This is a slightly smaller fraction than what was found in earlier UV surveys. They determined a global covering factor of the absorbing gas with respect to the central nucleus. This is the, to date, largest search for intrinsic UV absorption in low-redshift active galactic nucleus (AGN) with high spectral resolution.
S. Kraemer led a team including D. M. Crenshaw, J. R. Gabel, G. A. Kriss (STScI), who obtained simultaneous UV and X-ray observations of the Seyfert 1 galaxy NGC 4151, using HST/STIS, FUSE and Chandra. In the analysis of the Chandraspectra they determined that the X-ray absorption is dominated by two components: a high-ionization absorber observed in H and He-like lines of Mg, Si, and S, and a lower ionization absorber with lines of the same elements in lower ionization states. With the UV spectra, they derived tighter constrains on the line-of-sight covering factors, densities, and radial distances of the absorbers. They determined line-of-sight covering factors for three sub-components, and evidence for a fourth component characterized by low ionization. The complexity of the UV absorption in NGC 4151 may be a consequence of the fact that the system is vied a high inclination and, therefore, may be detecting the most dense part of the outflow. They found that the equivalent widths of the low-ionization lines varied over the period from 1999 July to 2002 May, accompanied by a drop in covering factor. As suggested in their previous study, it is likely that the absorption arises in a disk-driven wind.
D. M. Crenshaw and S. Kraemer analyzed UV and optical STIS spectra of NGC 4151. They identified a kinematic component of the emission lines with a FWHM of 1170 km/s, intermediate between those from the broad and narrow emission line regions. They presented evidence that the emission lines arise from the same gas responsible for most of the high-column UV and X-ray absorption. The gas in this intermediate-line region shields the narrow line region and has a global covering factor of 0.4, indicating mass outflow over a large solid angle centered on the accretion disk's axis. Based on physical conditions in the gas and dynamical considerations, models that invoke magneto-centrifugal acceleration (e.g. in an accretion disk wind) are favored over those that rely on radiation driving or thermal expansion as the principal driving mechanism.
S. Kraemer with H. R. Schmitt (NRL) and D. M. Crenshaw used narrow-band O III λ5007 and O II λ3727 images obtained with HST/WFPC2, to probe the ionization structure of the narrow line region in NGC 4151. They found that, while the narrow line region exhibits an overall structure consistent with the unified model of Seyfert galaxies, the WFPC2 images show significant emission outside the emission-line bi-cone. The lower [O III]/[O II] ratios in these regions are consistent with a weaker ionizing flux. The photoionization models suggests that the gas is ionized by radiation that has been attenuated by a highly ionized absorber, which completely covers the source, and a lower ionization absorber with variable covering factor.
S. Kraemer with V. Das, and D. M. Crenshaw used archival STIS spectra to study the dynamics of the narrow line region of the Seyfert 2 galaxy NGC 1068. The dynamical models consider the radiative force due to the AGN, gravitational forces from the supermassive black hole, nuclear stellar cluster, and galactic bulge, and a drag force due to the narrow line regions clouds interaction with a hot ambient medium. The derived velocity profile of the narrow line region gas and compared the results to kinematic models of the narrow line region, using a bi-conical geometry for the outflow. The results show that the acceleration profile due to radiative line driving is too steep to fit the data and that gravitational forces alone cannot slow the clouds down. With drag forces included, the clouds can slow down to the systemic velocity over the range 100 to 400 pc, as observed.
B. Armentrout, S. Kraemer, and T. J. Turner (UMBC) performed a photoionization modeling analysis of archival XMM-Newton observations of NGC 4151. Spectral data from the RGS instruments reveal several strong soft X-ray emission lines, chiefly from H and He-like O, N, Ne and C. Radiative recombination continua from O and C are also detected. Their analysis suggests that the emission data are consistent with photoionization. Using the CLOUDY photoionization code, they found that, while a two-component, high column density model, with low covering factor, proved adequate in reproducing all detected Lyman series lines, it was insufficient in modeling the He-like triplets observed. If resonance line data were ignored, the two-component model was sufficient to match the intercombination and forbidden lines. However, with the inclusion of resonance line data, He-like triplets could no longer be modeled with only two components. They succeeded in modeling the observed spectrum with the addition of a third, lower column density component with non-zero microturbulence and high covering factor. Two of the three emission components have similar physical conditions to the absorption components detected in the Chandra spectra of NGC 4151.
S. Kraemer, M. C. Bottorff (Southwestern U.) and D. M. Crenshaw examined, with CLOUDY, the effects of dissipative turbulence on the narrow emission-line ratios in Seyfert galaxies. While earlier studies of narrow line region spectra generally found good agreement between the observations and the model predictions for most strong emission lines the strengths of lines from species with ionization potentials greater than that of He+ (54.4 eV ) were often underestimated. Among the explanations suggested for these discrepancies were (selectively) enhanced elemental abundances and contributions from shock-heated gas.
S. Kraemer, as part of a team with R. P. Deo, D. M. Crenshaw, M. Dietrich (OSU), M. Elitzur (UKy), H. Telplitz (Caltech), and T. J. Turner analyzed Spitzer/IRS observations of Seyfert 1.8 and 1.9s. They compared the spectral characteristics of a sample of 12 1.8s and 1.9s to those of 58 Seyfert 1 and 2 galaxies from archived Spitzer spectra. An analysis of the spectral shapes, the silicate 10 μm feature and the emission-line fluxes have enabled them to characterize the mid-IR properties of Seyfert 1.8/1.9s.
M. Melendez, S. Kraemer, B. Armentrout, R. P. Deo, D. M. Crenshaw, H. R. Schmitt, R. F Mushotzky, J. Tueller, C. B. Markwardt (GSFC), and L. Winter (UMCP), explored the correlations between [O IV] 25.89 μm, from Spitzer/IRS spectra, and [O III] λ5007, from ground-based observations, with the hard X-ray luminosities in the 2-10 keV (ASCA) and 14-195 keV (Swift/BAT) bands. They found that both the [O IV] and [O III] luminosities are well correlated with the BAT luminosities. When comparing [O IV] and [O II] luminosities for the different types of galaxies, they found that the Seyfert 2s have significantly lower [O III] to [O IV] ratios than the Seyfert 1s. They suggest that this is due to more reddening of the narrow-line region of the Seyfert 2s, since the [O IV] line is much less affected by extinction. The combined effects of reddening and the X-ray absorption is the probable reason why the [O III] versus 2-10 keV correlation is better than the [O IV] versus 2-10 kev correlation. Based on photoionization models, they found that the [O IV] comes from higher ionization states and lower density regions than previous studies had determined for [O III]. Overall, the [O IV] appears to be an accurate indicator of the power of the AGN.
S. Kraemer, in team with L. Miller (Oxford), T. J. Turner, J. N. Reeves (Keele), I. M. George (IMBC) and B. Wingert (UMBC) performed a principal component analysis using a long XMM observation of the narrow-line Seyfert 1 galaxy Mrk 766. The source spectral variability is well-explained by additive variations, with smaller extra contributions, most likely from variable absorption. The principal varying component, eigenvector 1, is found to have a steep photon index (Γ~2.4) power-law shape, affected by a low column of ionized absorption. Eigenvector 1 varies by a factor of 10 on time-scales of days and appears to have broad ionized Fe Kα associated with it. There is also a component of near-constant emission that dominates in the low state, whose spectrum is extremely hard above 1 keV, has a soft excesses at lower energies, and has an edge at Fe K, but little Fe Kα emission. This could arise in either an ionized reflector from an extended region or as part of a disc wind with a variable covering factor.
S. Kraemer assisted T. J. Turner, L. Miller and J. N. Reeves in the analysis of time-resolved XMM and Suzaku spectroscopy of Mrk 766. They found that the spectral variability of Mrk 766 can be explained by either of the two interpretations from the principle components analysis. Detailed investigation confirmed rapid changes in the relative strengths of scattered and direct emission or rapid changes in the covering fraction of the absorption. However, a strong correlation between the 6.97 KeV absorption line and the primary continuum, together with the rapid opacity changes, show that variations in the absorber, most likely a disk-wind, are the dominant source of spectral variability in Mrk 766.
Figure: Spitzer telescope 3.6 mu image of the Vela X-1 bow shock. Bow shocks provide a way to probe their energeic environments and reveal properties of both the stellar wind and the ambient stellar medium. The presence of a bow shock indicates that the system has a high (supersonic) velocity with respect to the interstellar medium, in the process of which gas and dust is accumulated (Spitzer Program ID 30174 PI Iping).
S. Kraemer, T. J. Turner, Reeves, and L. Miller analyzed combined XMM and Chandra/HETG spectra of the Seyfert galaxy NGC 3516. New H and He-like emission and absorption features in the Fe K regime reveal a previously unknown zone of highly ionized circumnuclear gas in NGC 3516. These observations confirmed the presence of a lower ionization gas, with a covering factor of ~0.5. Changes in the covering factor of this component provide a explanation for a deep dip observed in the light-curve of the source. Constraints on the radial locations of the absorbers suggest an origin as part of a disk wind.R. C. Iping worked on FUSE observations of three Early-type, double-lined spectroscopic binaries in the Magellanic Clouds. With collaborators, she investigated mass loss from the central star of NGC 7009. With FUSE obervations of the high mass X-ray Binaries, SMC X-1 and 4U1700-37. R. C. Iping with collaborators analyzed FUSE observations of supernovae 1987A and determined the O IV emission from the shocked circumstellar ring of SN 1987A.
Cosmology
S. Odenwald and S. Kashlinsky (SSAI) have continued a 5-year, NSF-funded, research program to investigae the cosmic infrared background at near-IR wavelengths using the 2MASS survey. Their detection of this background in 2001 using 2MASS and COBE/DIRBE data revealed a level of IR background structure using power spectrum techniques, that strongly constrain models of galaxy clustering at redshifts of z>1. Their current investigation makes use of 2MASS Standard Star Calibration Fields that have effective integration times of over 15,000 s, and reach magnitudes of mK fainter than +20.0. It is expected that the new data will, not only corroborate earlier measurements, but substantially improve galaxy clustering model constraints at larger redshifts.
Figure: Spitzer telescope 3.6 mu image of the Vela X-1 bow shock. Bow shocks provide a way to probe their energeic environments and reveal properties of both the stellar wind and the ambient stellar medium. The presence of a bow shock indicates that the system has a high (supersonic) velocity with respect to the interstellar medium, in the process of which gas and dust is accumulated (Spitzer Program ID 30174 PI Iping).
Stellar Astrophysics
A search for lobes associated with other symbiotic stars is being pursued using Spitzer imaging data. F. Bruhweiler, G. Wahlgren and E. Verner in collaboration with B. McCollum (IPAC) found a large lobe using Spitzer, that may be associated with the symbiotic star BI Cru. This star has previously been reported to possess a much smaller lobe based on optical imaging. The origin of the multiple lobe structure can be a result of repeated episodic eruptive events.
G. Wahlgren is investigating the chemical composition of the cool, supergiant α Ori through analysis of near-infrared high-resolution spectra. In support of this program, laboratory spectra are being recorded at NIST in collaboration with G. Nave. G. Wahlgren have been analyzing the ultraviolet spectrum of hot Am star HR 3383 for abundance analysis of post-iron group elements. Spectral lines from a number of elements have been detected for which optical transitions are not observed.
G. Wahlgren continues to investigate weak emission lines in the red spectral region of main-sequence B stars. A model atom is being created for the ion P+ for inclusion in non-LTE investigations into the origin of emission lines of this ion in the stars 3 Cen A and HR 7264.
M. Smith has used archival IUE data to search for variations in He I λ1640 in 74 early type Be stars. He found at least two instances of variability in 10 stars of this sample. Most of these variabilities occur on time-scales longer than the rotational periods of the stars but in several cases rapid variations occur over a few hours. These variation fall into several distinct classes and manifest themselves as emission or absorptions. In some cases, the wind-sensitive C IV and/or Si IV doublets are affected, suggesting a heating of parts of the wind to about 35,000 K. In other cases the doublets are unaffected, suggesting a short term and localized heating near the photosphere.
M. Smith, G. Wade (Royal Military College), D. Bohlender (Herzberg Institute) and C. T. Bolton (U. Toronto) used the model atmosphere and disk radiative transfer codes SYNSPEC and CIRCUS, to determine physical and geometric properties of the magnetic B8p star 36 Lyn. They modeled IUE spectra and their analysis showed that the two segments of 36 Lyn's disk are situated at different distances from the star and have different geometrical thicknesses in the direction perpendicular to the magnetic plane. M. Smith, D. Bohlender derived similar geometric and thermodynamic properties of the magnetic disk rotating around the B2p star ζ Cas. This disk is heated by magnetically focused wind streams, impacting pre-existing disk material in the magnetic plane. The results indicate that heating does not decrease radially outward from the star but rather is concentrated at some distance from the star, probably just within the star's magnetic Alfven radius. M. Smith and D. Bohlender continued this work with the prototypical Bp star with a magnetic disk, σ Ori E, by mapping the density structure from the strengthenings of high level Balmer lines as the disk occults the star. As with the case of ζ Cas, they found the disk segments reside at different distances from the star and have different thicknesses. This suggests that the star's magnetic dipole is decentered with respect to its geometric center.
M. Smith and G. Henry have presented optical light-curves of the X-ray peculiar star γ Cas over a span of 10 years. Their data show that the light-curve undergoes cylical variations of a few percent and with periods that vary between 2 and 3 months. In addition, the light-curve shows a robust strictly period of 1.21 d over the entire timespan of observations. The amplitudes of the star are the same in both V and B filters, and the waveform is highly non-sinusoidal. This result confirms that the X-ray peculiarities in this star arise from magnetic interactions between the Be star and its surrounding decretion disk.
L. Balona (SAAO) and M. Smith analyzed time-serial observations of the B1e star BZ Cru and showed that migrating subfeatures moved across the lines of neutral He in this star's spectrum, but in a non-periodic and unpredictable fashion. The observation implies the presence of small occulting features forced into co-rotaton over the stellar disk, which is also the hallmark of optical lines in the X-ray peculiar Be star γ Cas. In a subsequent optical study led by R. Lopes de Oliveira and C. Motch (Strasbourg), M Smith and coworkers compared a high resolution X-ray spectrum obtained by \xmm\ and showed that indeed BZ Cru shows the same X-ray abnormalities as γ Cas. The combined X-ray and optical similarities argue that there exists a small class of X-ray peculiar "γ Cas stars", of which BZ Cru is the second member.
K. Nielsen, G. Vieira Kober, E. Verner, T. R. Gull (GSFC) and former Catholic University research associate S. Ivarsson have investigated Luminous Blue Variables and in particular the massive η Car. The overall goal is to improve the current knowledge of the evolution of massive stars, especially during their Luminous Blue Variable phase, and how they recycle stellar material into the interstellar medium. Eta Car's geometry is characterized by dramatic outbursts in the 1840s and 1890s. The system is exhibiting 5.54 year spectroscopic period with a ionization low-state that last for 3 months and a higher ionization state during the remaining of the period. The periodicity is believed to be caused by the presence of a companion star in a highly eccentric orbit, where the systems low state is related to the companions periastron passage. Observations over the 5.54 year period with HST/STIS and VLT/UVES have been used to shed some light on this peculiar system that is enshrouded in ejected material. The data has resulted in a better understanding of the system, both regarding the physical character of the ejecta and the systems interior, including the two massive objects. The complete spectrum between 1215 and 10,430 Å has been analyzed. In particular the absorption shells were used for determining the physical parameters of the ejecta such as the temperature, density and its location in reference to the radiative source. A comprehensive abundance analysis of the ejecta is currently on its way using standard curve of growths and CLOUDY modeling. Potential, circumstellar ejecta around other Luminous Blue Variables, such as P Cyg and AG Car, are investigated with similar methods.
K. Nielsen has in collaboration with T. R. Gull, M. Corcoran, K. Hamaguchi (GSFC) and D. J. Hillier (U. Pitts) searched for η Car's companion to derive its orbital parameters. They traced He I P Cygni shaped lines and constructed a radial velocity curve based on the absorption portion of the line. The He I emission was, based on their velocity variability and excitation determined, to be formed in the wind-wind interface region between the two massive objects. However, the absorption component was not easily interpreted and more work is needed to tie the velocity variations to the motion of the companion.
The η Car FUSE spectrum is currently being analyzed by G. Vieira Kober in collaboration with K. Nielsen, R. Iping and G. Sonneborn (GSFC) to further understand the ejecta but in particular in a search for direct evidence of the hot companion.
Circumstellar Discs
G. Williger and his graduate student K. Collins (U. Louisville) have, in collaboration with C. Grady (GSFC), investigated the disk and environment of HD 169142. They found from the surface brightness profile that the system is consistent with a centrally cleared disk with extended polycyclic aromatic hydrocarbon emission, which means a gas disk with organic molecules. They date the system to ~6 Myr by using a spatially associated M-star. They also found evidence that the system is a rapid rotator which is contradictory to the previous understanding of the system. With its central clearing and rapid rotation, it resembles a young Vega system. Characterizing the chemistry in the planet-forming regions of young circumstellar disks is important to understanding planetary system formation. B. Bonev and G. Villanueva in team with A. M Mandell, M. J. Mumma (GSFC), G. A. Blake, and Salyk (Caltech) detected emission from multiple low-excitation ro-vibrational transitions of OH from the two Herbig Ae stars AB Aur and MWC 758 in the 3.0-3.7 micron wavelength range, using Keck 2/NIRSPEC. The inner radius for the emitting region in both stars is close to 1 AU. The authors compared an optically thin LTE model and a thin-wedge fluorescence model, finding rotational temperatures of 650-800 K and OH abundances of 1042-1045 molecules for the two stars. Comparisons with current chemical models supports a fluorescence excitation model for AB Aur and possibly MWC 758, but additional observations and detailed modeling are necessary to improve the constraints on OH emission in different disk environments.