Physical Conditions in the Narrow-Line Region of Markarian 3. II. Photoionization Modeling Results

We have examined the physical conditions in the narrow-line region (NLR) ofthe Seyfert 2 galaxy Markarian 3, using long-slit spectra obtained with theHubble Space Telescope/Space Telescope Imaging Spectrograph and photoionizationmodels. We find three components of photoionized gas in the NLR. Two of thesecomponents, characterized by emission lines such as [NeV] 3426 and [OIII] 5007,lie within the envelope of the bi-conical region described in our previouskinematic study. A component of lower ionization gas, in which lines such as[OII] 3727 arise, is found to lie outside the bi-cone. Each of these componentsis irradiated by a power-law continuum which is attenuated by intervening gas,presumably closer to the central source. The radiation incident upon the lowionization gas, external to the bi-cone, is much more heavily absorbed. Theseabsorbers are similar to the intrinsic UV and X-ray absorbers detected in manySeyfert 1 galaxies, which suggests that the collimation of the ionizingradiation occurs in a circumnuclear wind, rather than a thick, molecular torus.We estimate the mass for the observed NLR emitting gas to be 2 millionsolar-masses. It is likely that Markarian 3 acquired this gas through anon-going interaction with the spiral galaxy UGC 3422.

INVERTING COLOR-MAGNITUDE DIAGRAMS TO ACCESS PRECISE STAR CLUSTER PARAMETERS: A NEW WHITE DWARF AGE FOR THE HYADES

We have extended our Bayesian modeling of stellar clusters—which uses main-sequence stellar evolution models, a mapping between initial masses and white dwarf (WD) masses, WD cooling models, and WD atmospheres—to include binary stars, field stars, and two additional main-sequence stellar evolution models. As a critical test of our Bayesian modeling technique, we apply it to Hyades UBV photometry, with membership priors based on proper motions and radial velocities, where available. Under the assumption of a particular set of WD cooling models and atmosphere models, we estimate the age of the Hyades based on cooling WDs to be 648 ± 45 Myr, consistent with the best prior analysis of the cluster main-sequence turnoff (MSTO) age by Perryman et al. Since the faintest WDs have most likely evaporated from the Hyades, prior work provided only a lower limit to the cluster’s WD age. Our result demonstrates the power of the bright WD technique for deriving ages and further demonstrates complete age consistency between WD cooling and MSTO ages for seven out of seven clusters analyzed to date, ranging from 150 Myr to 4 Gyr.

A HIRES/KECK SPECTROSCOPIC INVESTIGATION OF THE MEASUREMENT OF SODIUM IN THE ATMOSPHERE OF HD 209458b

A Neptune-mass Planet Orbiting the Nearby G Dwarf HD16417

Precision Doppler measurements from an intensive 48 night "Rocky PlanetSearch" observing campaign on the Anglo-Australian Telescope (AAT) haverevealed the presence of a low-mass exoplanet orbiting the G1 dwarf HD16417.Subsequent Doppler observations with the AAT, as well as independentobservations obtained by the Keck Planet Search, have confirmed this initialdetection and refine the orbital parameters to period 17.24+/-0.01 d,eccentricity 0.20+/-0.09, orbital semi-major axis 0.14+/-0.01 AU and minimumplanet mass 22.1+/-2.0 Mearth. HD 16417 raises the number of publishedexoplanets with minimum masses of less than 25 Mearth to eighteen.Interestingly, the distribution of detected sub-25 Mearth planets over thespectral types G, K and M is almost uniform. The detection of HD 16417b by anintensive observing campaign clearly demonstrates the need for extended andcontiguous observing campaigns when aiming to detect low-amplitude Dopplerplanets in short period orbits. Perhaps most critically it demonstrates thatthe search for low-mass Doppler planets will eventually require thesetraditional "bright-time" projects to extend throughout dark lunations.

Time-Distance Seismology of the Solar Corona with CoMP

We employ a sequence of Doppler images obtained with the CoronalMulti-channel Polarimeter (CoMP) instrument to perform time-distance seismologyof the solar corona. We construct the first k-omega diagrams of the region.These allow us to separate outward and inward propagating waves and estimatethe spatial variation of the plane-of-sky projected phase speed, and therelative amount of outward and inward directed wave power. The disparitybetween outward and inward wave power and the slope of the observed power lawspectrum indicate that low-frequency Alfvenic motions suffer significantattenuation as they propagate, consistent with isotropic MHD turbulence.

History of Galaxy Interactions and their Impact on Star Formation over the Last 7 Gyr from GEMS

We perform a comprehensive estimate of the frequency of galaxy mergers andtheir impact on star formation over z~0.24--0.80 (lookback time T_b~3--7 Gyr)using 3698 (M*>=1e9 Msun) galaxies with GEMS HST, COMBO-17, and Spitzer data.Our results are: (1) Among 790 high mass (M*>=2.5e10 Msun) galaxies, thevisually-based merger fraction over z~0.24--0.80, ranges from 9%+-5% to 8%+-2%.Lower limits on the major and minor merger fractions over this interval rangefrom 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the firstapproximate empirical estimate of the frequency of minor mergers at z1/10, with ~16%, 45%,and 7% of these corresponding respectively to major, minor, and ambiguous`major or minor' mergers. The mean merger rate is a few x 1e-4 Gyr-1 Mpc-3. (2)We compare the empirical merger fraction and rate for high mass galaxies to asuite of Lambda CDM-based models: halo occupation distribution models,semi-analytic models, and hydrodynamic SPH simulations. We find qualitativeagreement between observations and models such that the (major+minor) mergerfraction or rate from different models bracket the observations, and show afactor of five dispersion. Near-future improvements can now start to rule outcertain merger scenarios. (3) Among ~3698 M*>=1e9 Msun galaxies, we find thatthe mean SFR of visibly merging systems is only modestly enhanced compared tonon-interacting galaxies over z~0.24--0.80. Visibly merging systems onlyaccount for less than 30% of the cosmic SFR density over T_b~3--7 Gyr. Thissuggests that the behavior of the cosmic SFR density over the last 7 Gyr ispredominantly shaped by non-interacting galaxies.

The Connection Between Diffuse Light and Intracluster Planetary Nebulae in the Virgo Cluster

We compare the distribution of diffuse intracluster light detected in theVirgo Cluster via broadband imaging with that inferred from searches forintracluster planetary nebulae (IPNe). We find a rough correspondence on largescales (~ 100 kpc) between the two, but with very large scatter (~ 1.3mag/arcsec^2). On smaller scales (1 -- 10 kpc), the presence or absence ofcorrelation is clearly dependent on the underlying surface brightness. On thesescales, we find a correlation in regions of higher surface brightness (mu_V <~27) which are dominated by the halos of large galaxies such as M87, M86, andM84. In those cases, we are likely tracing PNe associated with galaxies ratherthan true IPNe. In true intracluster fields, at lower surface brightness, thecorrelation between luminosity and IPN candidates is much weaker. While acorrelation between broadband light and IPNe is expected based on stellarpopulations, a variety of statistical, physical, and methodological effects canact to wash out this correlation and explain the lack of a strong correlationat lower surface brightness found here. [abridged]

Evolution of Lyman Alpha Galaxies: Stellar Populations at z ~ 0.3

We present the results of a stellar population analysis of 30 Lyman alphaemitting galaxies (LAEs) at z ~ 0.3, previously discovered with the GalaxyEvolution Explorer (GALEX). With a few exceptions, we can accurately fit modelspectral energy distributions to these objects, representing the first timethis has been done for a large sample of LAEs at z < 3, a gap of ~ 8 Gyr in thehistory of the Universe. From the 26/30 LAEs which we can fit, we find an ageand stellar mass range of 200 Myr - 10 Gyr and 10^9 - 10^11 Msol, respectively.These objects thus appear to be significantly older and more massive than LAEsat high-redshift. We also find that these LAEs show a mild trend towards highermetallicity than those at high redshift, as well as a tighter range of dustattenuation and interstellar medium geometry. These results suggest thatlow-redshift LAEs have evolved significantly from those at high redshift.

Turning Back the Clock: Inferring the History of the Eight O'clock Arc

We present the results from an optical and near-infrared spectroscopic studyof the ultraviolet-luminous z = 2.73 galaxy, the 8 o'clock arc. Due togravitational lensing, this galaxy is magnified by a factor of > 10, allowingin-depth measurements which are usually unfeasible at such redshifts. In theoptical spectra, we measured the systemic redshift of the galaxy, z = 2.7322+/- 0.0012, using stellar photospheric lines. This differs from the redshift ofabsorption lines in the interstellar medium, z = 2.7302 +/- 0.0006, implyinggas outflows on the order of 160 km/s. With H and K-band near-infrared spectra,we have measured nebular emission lines of Halpha, Hbeta, Hgamma, [N II] and [OIII], which have a redshift z = 2.7333 +/- 0.0001, consistent with the derivedsystemic redshift. From the Balmer decrement, we measured the dust extinctionto be A_5500 = 1.17 +/- 0.36 mag. Correcting Halpha for dust extinction and theassumed lensing factor, we measure a star-formation rate of ~ 270 Msol/yr,which is higher than ~ 85% of star-forming galaxies at z ~ 2-3. Usingcombinations of all detected emission lines, we find that the 8 o'clock arc hasa gas-phase metallicity of ~ 0.8 Zsol, showing that enrichment at high-redshiftis not rare, even in blue, star-forming galaxies. Studying spectra from two ofthe arc components separately, we find that one component dominates the dustextinction and star-formation rate, although the metallicities between the twocomponents are similar. We derive the mass via stellar population modeling, andfind that the arc has a total stellar mass of ~ 4 x 10^11 Msol, which falls onthe mass-metallicity relation at z ~ 2. Finally, we estimate the total gasmass, and find it to be only ~ 12% of the stellar mass, implying that the 8o'clock arc is likely nearing the end of a starburst.

Spitzer Observations of the HII Region NGC 2467: An Analysis of Triggered Star Formation

We present new Spitzer Space Telescope observations of the region NGC 2467,and use these observations to determine how the environment of an HII regionaffects the process of star formation. Our observations comprise IRAC (3.6,4.5, 5.8, and 8.0 um) and MIPS (24 um) maps of the region, coveringapproximately 400 square arcminutes. The images show a region of ionized gaspushing out into the surrounding molecular cloud, powered by an O6V star andtwo clusters of massive stars in the region. We have identified as candidateYoung Stellar Objects (YSOs) 45 sources in NGC 2467 with infrared excesses inat least two mid-infrared colors. We have constructed color-color diagrams ofthese sources and have quantified their spatial distribution within the region.We find that the YSOs are not randomly distributed in NGC 2467; rather, over75% of the sources are distributed at the edge of the HII region, alongionization fronts driven by the nearby massive stars. The high fraction of YSOsin NGC 2467 that are found in proximity to gas that has been compressed byionization fronts supports the hypothesis that a significant fraction of thestar formation in NGC 2467 is triggered by the massive stars and the expansionof the HII region. At the current rate of star formation, we estimate at least25-50% of the total population of YSOs formed by this process.