William C. Keel

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 ob jects, 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⊙) with high star formation rates (� 10 M⊙yr −1 ), low metallicities (log[O/H] + 12 �8.7) and low reddening (E(B V) 6 0.25) and they reside in low density environments. They have some of the highest specific star for mation 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., Lymanbreak galaxies and Lyman-� 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 u nderstanding the build up of stellar mass in the Universe.

The effects of interactions on spiral galaxies. II - Disk star-formation rates

H-alpha emission-line and IRAS far-IR observations of interacting spiral and irregular galaxies are here used to assess the influence of interactions on their global star-formation rates. Two samples of interacting galaxies were observed: a complete sample of close pairs, and an Arp atlas sample of peculiar systems. When compared to a control sample of single galaxies, both samples of interacting systems exhibit systematically higher levels of H-alpha and infrared emission on average, and a larger dispersion in emission properties. Emission levels in the very active system are much more strongly correlated with the properties of the interaction than with the internal properties of the galaxies themselves. Strong disk emission is almost always accompanied by unusually strong nuclear activity. Simple star-formation burst models can reproduce the observed H-alpha equivalent widths and broadband colors of most of the galaxies. The bursts are relatively short (few times 10 million yr) and rarely involve more than 1-2 percent of a galaxys total mass. 56 references.

The Rest-frame Optical Spectra of SCUBA Galaxies

We present near-infrared spectroscopy and narrowband imaging at the wavelength of redshifted Hα for a sample of 30 high-redshift, far-infrared luminous galaxies. This sample is selected from surveys in the submillimeter, millimeter, and radio wave bands and has complete redshift coverage with a median redshift of z ~ 2.4. We use our data to measure the Hα properties of these systems and to gauge the prevalence of active galactic nuclei (AGNs) in these galaxies through their [N II]/Hα ratios and Hα line widths. Removing obvious AGNs, we find that the predicted Hα star formation rates in this diverse population are suppressed (by a factor of ~10) compared to those derived from their far-infrared luminosities. Using the AGN indicators provided by our near-infrared spectra, we estimate that AGNs are present in at least 40% of the galaxies in our sample. To further investigate this, we construct a composite rest-frame spectrum for both the entire sample and those galaxies that individually show no signs of nuclear activity. We find [N II]/Hα ratios for both composite spectra that suggest that the energy output of the galaxies is star formation rather than AGN dominated. However, we also find that the Hα line in the composite non-AGN spectrum is best fitted with an underlying broad-line component with a narrow/broad flux ratio of 0.45 ± 0.20. The median Hα line width for our sample (removing obvious AGNs) is 400 ± 70 km s-1 (FWHM), and the typical spatial extent of the Hα emission in our narrowband observations is 4-8 kpc, which indicates a dynamical mass of (1-2) × 1011 M☉ with corresponding dynamical times of 10-20 Myr. Using both high-resolution imaging and spectroscopically identified velocity offsets, we find that seven of the far-infrared luminous galaxies have companions, suggesting that they are undergoing interactions/mergers, and from their relative velocities we can determine a dynamical mass of (1.5 ± 0.9) × 1011 M☉. These measurements are comparable to millimeter CO estimates for the dynamical masses of these systems on similar scales and larger than recent estimates of the dynamical masses of UV-selected galaxies at similar redshifts derived in an identical manner. Using the [N II]/Hα index to predict abundances, we investigate the luminosity-metallicity relation for these galaxies and find that many have metallicities consistent with UV-selected high-redshift galaxies and slightly lower than local luminous infrared and elliptical galaxies (although we caution that our metallicity estimates have possible systematic uncertainties). We also compared our Hα and far-infrared luminosities with deep Chandra observations of a subset of our survey fields and use these data to further assess their AGN content. We conclude that these high-redshift, far-infrared luminous galaxies represent a population of massive, metal-rich, merging systems with high instantaneous star formation rates, strong dust obscuration, and actively fueled AGNs that are likely to be the progenitors of massive local elliptical galaxies.

The green valley is a red herring: Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies

We use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in low-redshift galaxies. We show that the green valley between the blue cloud of star-forming galaxies and the red sequence of quiescent galaxies in the colour-mass diagram is not a single transitional state through which most blue galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear that only a small population of blue early-type galaxies move rapidly across the green valley after the morphologies are transformed from disc to spheroid and star formation is quenched rapidly. In contrast, the majority of blue star-forming galaxies have significant discs, and they retain their late-type morphologies as their star formation rates decline very slowly. We summarize a range of observations that lead to these conclusions, including UV-optical colours and halo masses, which both show a striking dependence on morphological type. We interpret these results in terms of the evolution of cosmic gas supply and gas reservoirs. We conclude that late-type galaxies are consistent with a scenario where the cosmic supply of gas is shut off, perhaps at a critical halo mass, followed by a slow exhaustion of the remaining gas over several Gyr, driven by secular and/or environmental processes. In contrast, early-type galaxies require a scenario where the gas supply and gas reservoir are destroyed virtually instantaneously, with rapid quenching accompanied by a morphological transformation from disc to spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases transforms galaxies from disc to elliptical morphology, and mergers could play a role in inducing black hole accretion and possibly active galactic nuclei feedback.

Planet Hunters: A Transiting Circumbinary Planet in a Quadruple Star System

We report the discovery and confirmation of a transiting circumbinary planet (PH1b) around KIC 4862625, an eclipsing binary in the Kepler field. The planet was discovered by volunteers searching the first six Quarters of publicly available Kepler data as part of the Planet Hunters citizen science project. Transits of the planet across the larger and brighter of the eclipsing stars are detectable by visual inspection every ~137 days, with seven transits identified in Quarters 1-11. The physical and orbital parameters of both the host stars and planet were obtained via a photometric-dynamical model, simultaneously fitting both the measured radial velocities and the Kepler light curve of KIC 4862625. The 6.18 ± 0.17 R_⊕ planet orbits outside the 20 day orbit of an eclipsing binary consisting of an F dwarf (1.734 ± 0.044 R_☉, 1.528 ± 0.087 M_☉) and M dwarf (0.378 ± 0.023 R_☉, 0.408 ± 0.024 M_☉). For the planet, we find an upper mass limit of 169 M_⊕ (0.531 Jupiter masses) at the 99.7% confidence level. With a radius and mass less than that of Jupiter, PH1b is well within the planetary regime. Outside the planets orbit, at ~1000 AU, a previously unknown visual binary has been identified that is likely bound to the planetary system, making this the first known case of a quadruple star system with a transiting planet.

Galaxy Zoo 2: detailed morphological classifications for 304 122 galaxies from the Sloan Digital Sky Survey

We present the data release for Galaxy Zoo 2 (GZ2), a citizen science project with more than 16 million morphological classifications of 304 122 galaxies drawn from the Sloan Digital Sky Survey (SDSS). Morphology is a powerful probe for quantifying a galaxys dynamical history; however, automatic classifications of morphology (either by computer analysis of images or by using other physical parameters as proxies) still have drawbacks when compared to visual inspection. The large number of images available in current surveys makes visual inspection of each galaxy impractical for individual astronomers. GZ2 uses classifications from volunteer citizen scientists to measure morphologies for all galaxies in the DR7 Legacy survey with mr > 17, in addition to deeper images from SDSS Stripe 82. While the original GZ2 project identified galaxies as early-types, late-types or mergers, GZ2 measures finer morphological features. These include bars, bulges and the shapes of edge-on disks, as well as quantifying the relative strengths of galactic bulges and spiral arms. This paper presents the full public data release for the project, including measures of accuracy and bias. The majority (≳90 per cent) of GZ2 classifications agree with those made by professional astronomers, especially for morphological T-types, strong bars and arm curvature. Both the raw and reduced data products can be obtained in electronic format at http://data.galaxyzoo.org.

Galaxy Zoo:bars in disc galaxies

We present first results from Galaxy Zoo 2, the second phase of the highly successful Galaxy Zoo project (http://www.galaxyzoo.org). Using a volume-limited sample of 13 665 disc galaxies (0.01 < z < 0.06 and Mr < −19.38), we study the fraction of galaxies with bars as a function of global galaxy properties like colour, luminosity and bulge prominence. Overall, 29.4 ± 0.5 per cent of galaxies in our sample have a bar, in excellent agreement with previous visually classified samples of galaxies (although this overall fraction is lower than that measured by automated bar-finding methods). We see a clear increase in the bar fraction with redder (g−r) colours, decreased luminosity and in galaxies with more prominent bulges, to the extent that over half of the red, bulge-dominated disc galaxies in our sample possess a bar. We see evidence for a colour bimodality for our sample of disc galaxies, with a ‘red sequence’ that is both bulge and bar dominated, and a ‘blue cloud’ which has little, or no, evidence for a (classical) bulge or bar. These results are consistent with similar trends for barred galaxies seen recently both locally and at higher redshift, and with early studies using the RC3. We discuss these results in the context of internal (secular) galaxy evolution scenarios and the possible links to the formation of bars and bulges in disc galaxies.

Galaxy Zoo: ‘Hanny's Voorwerp’, a quasar light echo?

We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny’s Voorwerp, is bright in the SDSS g band due to unusually strong [O III]4959, 5007 emission lines. We present the results of the first targeted observations of the object in the optical, ultraviolet and X-ray, which show that the object contains highly ionized gas. Although the line ratios are similar to extended emission-line regions near luminous active galactic nucleus (AGN), the source of this ionization is not apparent. The emission-line properties, and lack of X-ray emission from IC 2497, suggest either a highly obscured AGN with a novel geometry arranged to allow photoionization of the object but not the galaxy’s own circumnuclear gas, or, as we argue, the first detection of a quasar light echo. In this case, either the luminosity of the central source has decreased dramatically or else the obscuration in the system has increased within 10 5 yr. This object may thus represent the first direct probe

The Morphological Mix of Field Galaxies to mI = 24.25 Magnitudes (bJ ~ 26 Magnitudes) from a Deep Hubble Space Telescope WFPC2 Image

We determine the morphological mix of field galaxies down to mI 24.25 mag (mB ~ 26.0 mag) from a single ultradeep Hubble Space Telescope wide field planetary camera (WFPC2) image in both the V606 and the I814 filters. In total, we find 227 objects with mI ? 24.5 mag and classify these into three types: ellipticals (16%), early-type spirals (37%), and late-type spirals/irregulars (47%). The differential number counts for each type are compared with simple models in a standard flat cosmology. We find that both the elliptical and the early-type spiral number counts are well described by little-or-no-evolution models, but only when normalized at bJ = 18.0 mag. Given the uncertainties in the luminosity function (LF) normalization, both populations are consistent with a mild evolutionary scenario based on a normal/low rate of star formation. This constrains the end of the last major star formation epoch in the giant galaxy populations to z ? 0.8. Conversely, the density of the observed late-type/irregular population is found to be a factor of 10 in excess of the conventional no-evolution model. This large population might be explained by a modified local dwarf-rich LF and/or strong evolution acting on the local LF. For the dwarf-rich case, a steep faint-end Schechter slope (? -1.8) is required, plus a fivefold increase in the dwarf normalization. For a purely evolving model based on a flat Loveday et al. LF (? -1.0), a ubiquitous starburst of ?I ~ 2.0 mag is needed at z 0.5 for the entire late-type population. We argue for a combination of these possibilities, and show that for a steep Marzke et al. LF (? -1.5) a starburst of ~1.3 mag is required at z 0.5 in the entire late-type population, or ~2.0 mag in ~20% of the population.

Automated Morphological Classification in Deep Hubble Space Telescope UBVI Fields: Rapidly and Passively Evolving Faint Galaxy Populations

We analyze deep Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images in U, B, V, I using artificial neural network (ANN) classifiers, which are based on galaxy surface brightness and light profile (but not on color nor on scale length, rhl). The ANN distinguishes quite well between E/S0, Sabc, and Sd/Irr+M galaxies (M for merging systems) for BJ 27 mag. We discuss effects from the cosmological surface brightness (SB) dimming and from the redshifted UV morphology on the classifications, and we correct for the latter. We present classifications in UBVI from (a) four independent human classifiers; (b) ANNs trained on V606 and I814 images; and (c) an ANN trained on images in the rest-frame UBV according to the expected redshift distribution as a function of BJ. For each of the three methods, we find that the fraction of galaxy types does not depend significantly on wavelength, and that they produce consistent counts as a function of type. The median scale length at BJ 27 mag is rhl 025-03 (1-2 kpc at z ≈ 1-2). Early- and late-type galaxies are fairly well separated in BVI color-magnitude diagrams for B 27 mag, with E/S0 galaxies being the reddest and Sd/Irr+M galaxies generally blue. We present the B-band galaxy counts for five WFPC2 fields as a function of morphological type for BJ 27 mag. E/S0 galaxies are only marginally above the no-evolution predictions, and Sabc galaxies are at most 0.5 dex above the nonevolving models for BJ 24 mag. The faint blue galaxy counts in the B band are dominated by Sd/Irr+M galaxies and can be explained by a moderately steep local luminosity function (LF) undergoing strong luminosity evolution. We suggest that these faint late-type objects (24 mag BJ 28 mag) are a combination of low-luminosity lower redshift dwarf galaxies, plus compact star-forming galaxies and merging systems at z 1-3, possibly the building blocks of the luminous early-type galaxies seen today.