arXiv subjects

William C. Keel

Publications and source records attributed to William C. Keel.

At least 73 records · Page 4Linked to original sources

arXiv2010

Galaxy Zoo: The fundamentally different co-evolution of supermassive black holes and their early- and late-type host galaxies

We use data from the Sloan Digital Sky Survey and visual classifications of morphology from the Galaxy Zoo project to study black hole growth in the nearby Universe (z < 0.05) and to break down the AGN host galaxy population by color, stellar mass and morphology. We find that black hole growth at luminosities L_OIII >1E40 erg/s in early- and late-type galaxies is fundamentally different. AGN host galaxies as a population have a broad range of stellar masses (1E10-1E11 Msun), reside in the green valley of the color-mass diagram and their central black holes have median masses around 1E6.5 Msun. However, by comparing early- and late-type AGN host galaxies to their non-active counterparts, we find several key differences: in early-type galaxies, it is preferentially the galaxies with the least massive black holes that are growing, while late-type galaxies, it is preferentially the most massive}black holes that are growing. The duty cycle of AGN in early-type galaxies is strongly peaked in the green valley below the low-mass end (1E10 Msun) of the red sequence at stellar masses where there is a steady supply of blue cloud progenitors. The duty cycle of AGN in late-type galaxies on the other hand peaks in massive (1E11 Msun) green and red late-types which generally do not have a corresponding blue cloud population of similar mass. At high Eddington ratios (L/L_Edd > 0.1), the only population with a substantial fraction of AGN are the low-mass green valley early-type galaxies. Finally, the Milky Way likely resides in the "sweet spot" on the color-mass diagram where the AGN duty cycle of late-type galaxies is highest. We discuss the implications of these results for our understanding of the role of AGN in the evolution of galaxies

astro-ph.CO↗

arXiv2009

Unveiling Galaxy Interactions: Watching the Tides Roll

I set the stage for discussion of the stellar populations in interacting galaxies by looking back over the slow development of our understanding of these systems. From early anecdotal collections, to systematic cataloging, and finally to increasingly sophisticated n-body calculations, we have seen how gravity in distributed systems can produce the stunning variety of structures we see. At the same time, measures across the spectrum have made it clear that galaxy interactions are linked to star formation, albeit with the physical mechanisms much less clear. Improved data sets, including HST imaging, deep IR data, and large samples with well-defined selection criteria, have started to reveal correlations with dynamical parameters pointing to detailed histories of starbirth during collisions. The merger hypothesis for elliptical galaxies has broadened into seeing interactions and mergers as important parts of the overall evolution of galaxies. The connection becomes more important as we look to higher redshift, where more frequent interactions can drive the evolution of galaxies in multiple ways. Links between the properties of central black holes and surrounding galaxies makes it important likewise to understand the connections between AGN and interactions, which has remained more ambiguous due to the strong role of sample selection. Finally, contemporary data reach deep enough to show that most galaxies have interacted in the observable past; we must consider these events to be a normal part of galaxy history.

astro-ph.CO↗

arXiv2009

Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies

We investigate a class of rapidly growing emission line galaxies, known as "Green Peas", first noted by volunteers in the Galaxy Zoo project because of their peculiar bright green colour and small size, unresolved in SDSS imaging. Their appearance is due to very strong optical emission lines, namely [O III] 5007 A, with an unusually large equivalent width of up to ~1000 A. We discuss a well-defined sample of 251 colour-selected objects, most of which are strongly star forming, although there are some AGN interlopers including 8 newly discovered narrow Line Seyfert 1 galaxies. The star-forming Peas are low mass galaxies (M~10^8.5 - 10^10 M_sun) with high star formation rates (~10 M_sun/yr), low metallicities (log[O/H] + 12 ~ 8.7) and low reddening (E(B-V) < 0.25) and they reside in low density environments. They have some of the highest specific star formation rates (up to ~10^{-8} yr^{-1}) seen in the local Universe, yielding doubling times for their stellar mass of hundreds of Myrs. The few star-forming Peas with HST imaging appear to have several clumps of bright star-forming regions and low surface density features that may indicate recent or ongoing mergers. The Peas are similar in size, mass, luminosity and metallicity to Luminous Blue Compact Galaxies. They are also similar to high redshift UV-luminous galaxies, e.g., Lyman-break galaxies and Lyman-alpha emitters, and therefore provide a local laboratory with which to study the extreme star formation processes that occur in high-redshift galaxies. Studying starbursting galaxies as a function of redshift is essential to understanding the build up of stellar mass in the Universe.

astro-ph.CO↗

arXiv2009

The disappearance of Lyman alpha blobs: a GALEX search at z=0.8

Lyman alpha blobs - luminous, spatially extended emission-line nebulae, often lacking bright continuum counterparts - are common in dense environments at high redshift. Until recently, atmospheric absorption and filter technology have limited our knowledge of any similar objects at z<2. We use GALEX slitless spectroscopy to search for similar objects in the rich environments of two known cluster and supercluster fields at z=0.8, where the instrumental sensitivity peaks. The regions around Cl 1054-0321 and Cl 0023+0423 were each observed in slitless-spectrum mode for 10-19 ksec, with accompanying direct images of 3-6 ksec to assist in recognizing continuum sources. Using several detection techniques, we find no resolved Lyman alpha emitters to a flux limit of(1.5-9) x 10^{-15} erg/ cm^2 s, on size scales of 5-30 arcseconds. This corresponds to line luminosities of (0.5-3) x 10^43 erg/s for linear scales 35-200 kpc. Comparison with both blind and targeted surveys at higher redshifts indicates that the population must have evolved in comoving density at least as strongly as (1+z)^3. These results suggest that the population of Lyman alpha blobs is specific to the the high-redshift Universe.

astro-ph.GA↗

arXiv2006

The Spiral Host Galaxy of the Double Radio Source 0313-192

We present new Hubble, Gemini-S, and Chandra observations of the radio galaxy 0313-192, which hosts a 350-kpc double source and jets, even though previous data have suggested that it is a spiral galaxy. We measure the bulge scale and luminosity, radial and vertical profiles of disk starlight, and consider the distributions of H II regions and absorbing dust. In each case, the HST data confirm its classification as an edge-on spiral galaxy, the only such system known to produce such an extended radio source of this kind. The Gemini near-IR images and Chandra spectral fit reveal a strongly obscured central AGN, seen through the entire ISM path length of the disk and showing X-ray evidence of additional absorption from warm or dense material close to the central object. We consider several possible mechanisms for producing such a rare combination of AGN and host properties, some combination of which may be at work. These include an unusually luminous bulge (suggesting a black hole of mass 0.5-0.9 billion solar masses), orientation of the jets near the pole of the gas-rich disk, and some evidence of a weak gravitational interaction which has warped the disk and could have enhanced fuelling of the central engine. An X-ray counterpart of the kiloparsec-scale radio jet emerges to the south; jet/counterjet limits in both radio and X-ray allow them to be symmetric if seen more than 15 degrees from the plane of the sky, still consistent with the jet axes being within ~30 degrees of the poles of the gas-rich galaxy disk. A linear or disklike emission-line structure is seen around thenucleus, inclined by ~20 degrees to the stellar disk but nearly perpendicular to the jets; this may represent the aftermath of a galaxy encounter, where gas is photoionized by the nuclear continuum.

astro-ph↗

arXiv2006

Stellar abundances and molecular hydrogen in high-redshift galaxies -the far-ultraviolet view

FUSE spectra of star-forming regions in nearby galaxies are compared to composite spectra of Lyman-break galaxies (LBGs), binned by strength of Lyman alpha emission and by mid-UV luminosity. Several far-UV spectral features, including lines dominated by stellar wind and by photospheric components, are very sensitive to stellar abundances. Their measurement in Lyman-break galaxies is compromised by the strong interstellar absorption features, allowing in some cases only upper limits. The derived C and N abundances in the LBGs are no higher than half solar (scaled to oxygen abundance for comparison with emission-line analyses), independent of the strength of Lyman alpha emission. P V absorption indicates abundances as low as 0.1 solar, with an upper limit near 0.4 solar in the reddest and weakest-emission galaxies. Unresolved interstellar absorption components would further lower the derived abundances. Trends of line strength, and derived abundances, are stronger with mid-UV luminosity than with Lyman-alpha strength. H2 absorption in the Lyman and Werner bands is very weak in the LBGs. Template H2 absorption spectra convolved to appropriate resolution show that strict upper limits N(H2)< 10^18 cm^-2 apply in all cases, with more stringent values appropriate for the stronger-emission composites and for mixes of H2 level populations like those on Milky Way sight lines. Since the UV-bright regions are likely to be widespread in these galaxies, these results rule out massive diffuse reservoirs of H2, and suggest that the dust/gas ratio is already fairly large at z~3.

astro-ph↗

arXiv2005

An Atlas of Warm AGN and Starbursts from the IRAS Deep Fields

We present 180 AGN candidates based on color selection from the IRAS slow-scan deep observations, with color criteria broadened from the initial Point-Source Catalog samples to include similar objects with redshifts up to z=1 and allowing for two-band detections. Spectroscopic identifications have been obtained for 80 (44%); some additional ones are secure based on radio detections or optical morphology, although yet unobserved spectroscopically. These spectroscopic identifications include 13 Sy 1 galaxies, 17 Sy 2 Seyferts, 29 starbursts, 7 LINER systems, and 13 emission-line galaxies so heavily reddened as to remain of ambiguous classification. The optical magnitudes range from R=12.0-20.5; counts suggest that incompleteness is important fainter than R=15.5. Redshifts extend to z=0.51, with a significant part of the sample at z>0.2. The sample includes slightly more AGN than star-forming systems among those where the spectra contain enough diagnostic feature to make the distinction. The active nuclei include several broad-line objects with strong Fe II emission, and composite objects with the absorption-line signatures of fading starbursts. These AGN with warm far-IR colors have little overlap with the "red AGN" identified with 2MASS; only a single Sy 1 was detected by 2MASS with J-K > 2. Some reliable IRAS detections have either very faint optical counterparts or only absorption-line galaxies, potentially being deeply obscured AGN. The IRAS detections include a newly identified symbiotic star, and several possible examples of the "Vega phenomenon", including dwarfs as cool as type K. Appendices detail these candidate stars, and the optical-identification content of a particularly deep set of high-latitude IRAS scans (probing the limits of optical identification from IRAS data alone).

astro-ph↗

arXiv2004

Escape of Lyman alpha Emission in the Starburst Galaxy Mkn 357 - a Wind's Far Side

HST imaging and slitless spectroscopy are used to examine where the strong Lyman alpha emission escapes from the interstellar medium in the starburst galaxy Mkn 357. An H-alpha image shows that the ionized gas is mostly in a global wind, rather than associated with the individual star-forming regions seen in the optical and UV continuum. The Lyman alpha emission comes predominantly from the northwest side of the wind structure spatially, and shows a significant redshift relative to the optical lines. Both of these properties are signatures of seeing the line photons backscattered from the far side of a prolate or bipolar starburst wind, fitting both with escape calculations and evidence for winds in high-redshift galaxies with net Lyman alpha emission. Scattering is most important within this wind itself, rather than involving a surrounding neutral medium, as shown by the decreasing relative redshift of the line peak from 250 to 30 km/s between the center and edge of the detected emission. The Lyman alpha emission exhibits strong asymmetry in comparison with both the starlight and H-alpha structures. These results add to the evidence that kinematics, rather than gas metallicity or dust content, are the dominant effect in determining which galaxies have strong Lyman alpha emission, and that powerful (and perhaps episodic) starbursts are common among Lyman-break galaxies as well as those discovered from Lyman alpha line emission.

astro-ph↗

arXiv2004

The Dynamics of Abell 2125

We present 371 galaxy velocities in the field of the very rich cluster Abell 2125 (z~0.25). These were determined using optical spectroscopy collected over several years from both the WIYN 3.5m telescope and NOAO Mayall 4m telescope. Prior studies at a variety of wavelengths (radio, optical, and X-ray) have indicated that A2125 is a likely cluster-cluster merger, a scenario which we are able to test using our large velocity database. We identified 224 cluster galaxies, which were subjected to a broad range of statistical tests using both positional and velocity information to evaluate the cluster dynamics and substructure. The tests confirmed the presence of substructures within the Abell 2125 system at high significance, demonstrating that A2125 is a complex dynamical system. Comparison of the test results with existing simulations strengthens the merger hypothesis, and provides clues about the merger geometry and stage. The merger model for the system can reconcile A2125's low X-ray temperature and luminosity with its apparently high richness, and might also explain A2125's high fraction of active galaxies identified in prior radio and optical studies.

astro-ph↗

arXiv2004

Far-Ultraviolet Spectroscopy of Star-Forming Regions in Nearby Galaxies: Stellar Populations and Abundance Indicators

We present FUSE spectroscopy and supporting data for star-forming regions in nearby galaxies, to examine their massive-star content and explore the use of abundance and population indicators in this spectral range for high-redshift galaxies. New far-ultraviolet spectra are shown for four bright H II regions in M33 (NGC 588, 592, 595, and 604), the H II region NGC 5461 in M101, and the starburst nucleus of NGC 7714, supplemented by the very-low-metallicity galaxy I Zw 18. In each case, we see strong Milky Way absorption systems from H2, but intrinsic absorption within each galaxy is weak or undetectable, perhaps because of the "UV bias" in which reddened stars which lie behind molecular-rich areas are also heavily reddened. We see striking changes in the stellar-wind lines from these populations with metallicity, suggesting that C II, C III, C IV, N II, N III, and P V lines are potential tracers of stellar metallicity in star-forming galaxies. Three of these relations - involving N IV, C III, and P V - are nearly linear over the range from O/H=0.05--0.8 solar. The major difference in continuum shapes among these systems is that the giant H II complex NGC 604 has a stronger continuum shortward of 950 A than any other object in this sample. Small-number statistics would likely go in the other direction; we favor this as the result of a discrete star-forming event ~3 Myr ago, as suggested by previous studies of its stellar population. (Supported by NASA grant NAG5-8959)

astro-ph↗

arXiv2003

Massive star clusters in ongoing galaxy interactions -- clues to cluster formation

We present HST WFPC2 observations, supplemented by ground-based H-alpha data, of the star-cluster populations in two pairs of interacting galaxies, selected for being in very different kinds of encounters seen at different stages. Dynamical information and n-body simulations provide the details of encounter geometry, mass ratio, and timing. In NGC 5752/4, we see a weak encounter well past closest approach, after about 2.5x10^8 years. The large spiral NGC 5754 has a normal population of disk clusters, while the companion NGC 5752 exhibits a rich population of luminous clusters with a flatter luminosity function. The strong, ongoing encounter in NGC 6621/2, seen about 10^8 years past closest approach between roughly equal-mass galaxies, has produced a rich population of luminous clusters, particularly young and bright in a small region between the nuclei. This region is dynamically interesting, so strongly perturbed that the rotation curve reverses sign. Cluster formation requires a threshold level of perturbation, with stage of the interaction less important. The location of the most active star formation in NGC 6621/2 draws attention to a possible role for the Toomre stability threshold in shaping star formation in interacting galaxies. The rich cluster populations in NGC 5752 and 6621 show that direct contact between gas-rich galaxy disks is not a requirement to form luminous clusters, and that they can be triggered by processes happening within a single galaxy disk (albeit triggered by external perturbations). (Abridged)

astro-ph↗

arXiv2002

The Infrared Counterparts of the Optically Unidentified CDF-S 1Ms Sources

The Chandra Deep Field South (CDF-S) 1Ms exposure produced a catalog of 346 X-ray sources, of which 59 were not visible on the VLT/FORS1 and the ESO-MPI/WFI deep R-band images to a limit of R_{vega}=26.1--26.7 mag. Using the first release of the ESO VLT/ISAAC JHK_s data on the CDF-S, we identified six of the twelve such objects that were within the coverage of these IR observations. The VLT/FORS1 I-band data further confirms that five of these six objects are undetected in the optical. The photometric properties of these six counterparts are compared against those of the optically brighter counterparts of Chandra sources in the same field. We found that the location of these optically brighter Chandra sources in the near-IR color space was bifurcated, with the color of one branch being consistent with that of E/S0 galaxies at 0 <= z <= 1.5 and the other branch being consistent with that of unreddened AGN/QSOs at 0 <= z <= 3.5. The six counterparts that we identified seemed to lie on the E/S0 branch and its extension, suggesting that these X-ray source hosts are mostly luminous E/S0 galaxies (M_V ~ -20 mag in AB system) at 1 <= z <= 2.5. On the other hand, some of them can also be explained by AGN/QSOs over a wide redshift range (0 <= z <= 5) if a range of internal extinction (A_V=0--1 mag) is allowed. However, the later interpretation requires fine-tuning extinction together with redshift for these objects individually. If they are indeed AGN/QSOs, the most luminous of them is just barely qualified for being a QSO. Finally, we point out that neither high-redshift (z > 5) star-forming galaxies nor irregular galaxies at lower redshift can be a viable explanation to the nature of these six counterparts.

astro-ph↗

arXiv2002

A Simple Prediction on the Surface Density of Galaxies at z ~ 6

Systematic surveys are being proposed to discover a significant number of galaxies at z ~ 6, which is now suggested as the epoch when the reionization era of the Universe ends. To plan such surveys, we need a reasonable expectation of the surface density of high redshift galaxies at different flux limits. Here we present a simple prediction of the surface density of 5.5 < z < 6.5 galaxies in the optical regime, extrapolating from what is already known about galaxies at z ~ 3. This prediction is consistent with the results of nearly all known searches for objects at z ~ 6, giving confidence that we may use it to plan optimal combination of survey depth and sky coverage in searching for such objects. We suggest that the most efficient strategy with existing ground-based facilities is to do medium-depth (m_{AB}= 24.0 -- 24.5 mag), wide-field (a couple of square degrees) survey using a wide-field camera at a 4m-class telescope. As the predicted surface density at this brightness level is very sensitive to the value of L*, the result of such a survey can be easily used to constrain the luminosity evolution from z ~ 3 to 6.

astro-ph↗

arXiv2001

Seeing Galaxies though Thick and Thin. IV. The Superimposed Spiral Galaxies of NGC 3314

The superimposed pair of spiral galaxies NGC 3314 offers a unique opportunity to trace the dust properties in a spiral galaxy. We analyze multicolor HST imaging, supported by ground-based near-IR imaging and fiber-array spectroscopy to measure dust extinction in the foreground Sc galaxy NGC 3314A, which is backlit by the Sb system NGC 3314B. We can measure extinctions over a wide range of galactocentric radii in the foreground galaxy, from 0.4-4.5 kpc. In the outer disk, the extinction is strongly localized in discrete dust lanes. These dust features show an extinction curve with a slope close to the Galactic mean (R = 3.5+/-0.3) from 1.6 to 3.8 kpc, with no radial trend. Using the I-K color of the background nucleus, we derive an extinction A(I) = 3.3 through the disk at a projected distance 400 pc from the nucleus of NGC 3314A. The extinction in even the inner disk of NGC 3314A is quite patchy, since background H-alpha emission is detected from all parts of the system. Local anticorrelations between foreground and background line emission demonstrate that the dust is concentrated to star-forming regions, as has been found for the blue light in several systems. Colors of dust lanes in NGC 3314A which are projected only partially against the background disk indicate that the dust scale height in the foreground disk is substantially smaller than that of the stars.

astro-ph↗

arXiv2000

Seeing Galaxies Through Thick & Thin. III. HST Imaging of the Dust in Backlit Spiral Galaxies

We present analysis of WFPC2 imaging of two spiral galaxies partially backlit by E/S0 systems in the pairs AM1316-241 and AM0500-620, and the spiral foreground system in NGC 1275. Images in B and I are used to determine the reddening curve of in these systems. The spiral component of AM1316-241 shows dust strongly concentrated in discrete arms, with a reddening law very close to the Milky Way mean. The dust distribution is scale-free between about 100 pc and the arm scale. The spiral in AM0500-620 shows dust concentrated in arms and interarm spurs, with measurable interarm extinction as well. Although its dust properties are less well-determined, we find evidence for a steeper extinction law here. The shape of the reddening law suggests that, at least in AM1316-241, we have resolved most of the dust structure. In AM0500-620, the slope of the fractal perimeter-scale relation steepens systematically from low to high extinction. In AM1316-241, we cannot determine a unique fractal dimension from the defining area-perimeter relation, so the projected dust distribution is best defined as fractal-like. In neither galaxy do we see regions even on single-pixel scales in spiral arms with AB > 2.5. The measurements in NGC 1275 are compromised by our lack of independent knowledge of the foreground system's light distribution, but masked sampling of the absorption suggests an effective reddening curve much flatter than the Milky Way mean (perhaps indicating that the foreground system has been affected by immersion in the hot intracluster gas).

astro-ph↗

arXiv2000

Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)

M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk (R = 48 arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20 arcsec. long ``mini-bar'' at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates an approximately 10^7 or 10^8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the ``core-dominated'' galaxies that have been observed at high redshift. The gravitationally-lensed ``Pretzel Galaxy'' (0024+1654) at a redshift of approximately 1.5 provides an important precedent in this regard.

astro-ph↗

arXiv2000

Seeing Galaxies Through Thick and Thin: II. Direct Measures of Extinction in Spiral Disks Through Spectroscopy of Overlapping Galaxies

We use slit spectroscopy of overlapping pairs of galaxies to directly determine the extinction in disks of foreground spiral galaxies. The Doppler shifts of pair members are determined via cross-correlation and their relative correlation amplitudes are used to separate their contributions to the combined spectra in regions of overlap. This spectroscopic approach is less subject to stringent symmetry constraints than our previous purely photometric analyses. Extinctions of foreground members were obtained for 6 of the candidates in our sample of 18 mostly spiral/spiral pairs, when the signal to noise and velocity difference were suitable. In agreement with our previous imaging results, we find that the extinction in interarm regions is very modest, typically A_B=0.1 mag (corrected to face on), while spiral arms exhibit higher extinctions of 0.3 mag.

astro-ph↗

arXiv1999

Evidence for Large-Scale Structure at z=2.4 From Lyman-alpha Imaging

We present deep wide-field medium-band imaging in redshifted Lyman alpha of fields surrounding HST detections of faint emitters, to characterize their larger-scale environment. The radio galaxy 53W002 was previously found to be part of a rich grouping at z=2.39, including 5 confirmed, compact, star-forming objects. This is now shown to be part of a larger structure traced by bright AGN, all contained within a projected span of 6.8 arcmin (3.2 Mpc). Of the 14 candidate emitters, six have been spectroscopically confirmed as active nuclei in the range z=2.390 +/- 0.008. Various statistical tests give a significance of 95-99% for the reality of this structure on the sky. This grouping is more extended than a relaxed King model, at the 90% confidence level. Either this configuration has yet to decouple fully from the Hubble expansion, or it consists of subgroups which will themselves form a more compact, relaxed structure. The redshift range for measured members is comparable to the Hubble flow across the structure, which may imply that the structure is seen near turnaround. We surveyed two additional fields at z=2.4, and three contiguous fields at z=2.6. Only a single candidate at z=2.4 was found, with six at z=2.6. Comparison with the WFPC2 surveys suggests that the field-to-field contrast is larger for brighter objects. From this survey alone, groupings such as the 53W002 "cluster" must have an area covering fraction <0.04 in this redshift range.

astro-ph↗