arXiv subjects

Kevin Schawinski

Publications and source records attributed to Kevin Schawinski.

At least 127 records · Page 7Linked to original sources

arXiv2012

Black Hole -- Galaxy Co-evolution

The growth of black holes and the formation and evolution of galaxies appear to be linked at such a fundamental level that we think of the two as `co-evolving.' Recent observations show that this co-evolution may be complex and the result of several different pathways. While it is clear that black hole accretion is linked to specific phases of the evolution of the host galaxy, the impact of the energy liberated by the black hole on the evolutionary trajectory of the host by feedback is less clear. In this contribution, I review the motivations for co-evolution, the current state of the observational picture, and some challenges by black hole feedback.

astro-ph.CO↗

arXiv2012

Galaxy Zoo: The Environmental Dependence of Bars and Bulges in Disc Galaxies

We present an analysis of the environmental dependence of bars and bulges in disc galaxies, using a volume-limited catalogue of 15810 galaxies at z<0.06 from the Sloan Digital Sky Survey with visual morphologies from the Galaxy Zoo 2 project. We find that the likelihood of having a bar, or bulge, in disc galaxies increases when the galaxies have redder (optical) colours and larger stellar masses, and observe a transition in the bar and bulge likelihoods, such that massive disc galaxies are more likely to host bars and bulges. We use galaxy clustering methods to demonstrate statistically significant environmental correlations of barred, and bulge-dominated, galaxies, from projected separations of 150 kpc/h to 3 Mpc/h. These environmental correlations appear to be independent of each other: i.e., bulge-dominated disc galaxies exhibit a significant bar-environment correlation, and barred disc galaxies show a bulge-environment correlation. We demonstrate that approximately half (50 +/- 10%) of the bar-environment correlation can be explained by the fact that more massive dark matter haloes host redder disc galaxies, which are then more likely to have bars. Likewise, we show that the environmental dependence of stellar mass can only explain a small fraction (25 +/- 10%) of the bar-environment correlation. Therefore, a significant fraction of our observed environmental dependence of barred galaxies is not due to colour or stellar mass dependences, and hence could be due to another galaxy property. Finally, by analyzing the projected clustering of barred and unbarred disc galaxies with halo occupation models, we argue that barred galaxies are in slightly higher-mass haloes than unbarred ones, and some of them (approximately 25%) are satellite galaxies in groups. We also discuss implications about the effects of minor mergers and interactions on bar formation.

astro-ph.CO↗

arXiv2012

Chandra Observations of Galaxy Zoo Mergers: Frequency of Binary Active Nuclei in Massive Mergers

We present the results from a Chandra pilot study of 12 massive galaxy mergers selected from Galaxy Zoo. The sample includes major mergers down to a host galaxy mass of 10$^{11}$ $M_\odot$ that already have optical AGN signatures in at least one of the progenitors. We find that the coincidences of optically selected active nuclei with mildly obscured ($N_H \lesssim 1.1 \times 10^{22}$ cm$^{-2}$) X-ray nuclei are relatively common (8/12), but the detections are too faint ($< 40$ counts per nucleus; $f_{2-10 keV} \lesssim 1.2 \times 10^{-13}$ erg s$^{-1}$ cm$^{-2}$) to reliably separate starburst and nuclear activity as the origin of the X-ray emission. Only one merger is found to have confirmed binary X-ray nuclei, though the X-ray emission from its southern nucleus could be due solely to star formation. Thus, the occurrences of binary AGN in these mergers are rare (0-8%), unless most merger-induced active nuclei are very heavily obscured or Compton thick.

astro-ph.GA↗

arXiv2012

Planet Hunters: Assessing the Kepler Inventory of Short Period Planets

We present the results from a search of data from the first 33.5 days of the Kepler science mission (Quarter 1) for exoplanet transits by the Planet Hunters citizen science project. Planet Hunters enlists members of the general public to visually identify transits in the publicly released Kepler light curves via the World Wide Web. Over 24,000 volunteers reviewed the Kepler Quarter 1 data set. We examine the abundance of \geq 2 R\oplus planets on short period (< 15 days) orbits based on Planet Hunters detections. We present these results along with an analysis of the detection efficiency of human classifiers to identify planetary transits including a comparison to the Kepler inventory of planet candidates. Although performance drops rapidly for smaller radii, \geq 4 R\oplus Planet Hunters \geq 85% efficient at identifying transit signals for planets with periods less than 15 days for the Kepler sample of target stars. Our high efficiency rate for simulated transits along with recovery of the majority of Kepler \geq 4 R\oplus planets suggest suggests the Kepler inventory of \geq 4 R\oplus short period planets is nearly complete.

astro-ph.EP↗

arXiv2012

The Milky Way Project: A statistical study of massive star formation associated with infrared bubbles

The Milky Way Project citizen science initiative recently increased the number of known infrared bubbles in the inner Galactic plane by an order of magnitude compared to previous studies. We present a detailed statistical analysis of this dataset with the Red MSX Source catalog of massive young stellar sources to investigate the association of these bubbles with massive star formation. We particularly address the question of massive triggered star formation near infrared bubbles. We find a strong positional correlation of massive young stellar objects (MYSOs) and H II regions with Milky Way Project bubbles at separations of < 2 bubble radii. As bubble sizes increase, a statistically significant overdensity of massive young sources emerges in the region of the bubble rims, possibly indicating the occurrence of triggered star formation. Based on numbers of bubble-associated RMS sources we find that 67+/-3% of MYSOs and (ultra)compact H II regions appear associated with a bubble. We estimate that approximately 22+/-2% of massive young stars may have formed as a result of feedback from expanding H II regions. Using MYSO-bubble correlations, we serendipitously recovered the location of the recently discovered massive cluster Mercer 81, suggesting the potential of such analyses for discovery of heavily extincted distant clusters.

astro-ph.GA↗

arXiv2012

Galaxy Zoo: dust and molecular gas in early-type galaxies with prominent dust lanes

We study dust and associated molecular gas in 352 nearby early-type galaxies (ETGs) with prominent dust lanes. 65% of these `dusty ETGs' (D-ETGs) are morphologically disturbed, suggesting a merger origin. This is consistent with the D-ETGs residing in lower density environments compared to the controls drawn from the general ETG population. 80% of D-ETGs inhabit the field (compared to 60% of the controls) and <2% inhabit clusters (compared to 10% of the controls). Compared to the controls, D-ETGs exhibit bluer UV-optical colours (indicating enhanced star formation) and an AGN fraction that is more than an order of magnitude greater (indicating higher incidence of nuclear activity). The clumpy dust mass residing in large-scale features is estimated, using the SDSS r-band images, to be 10^{4.5}-10^{6.5} MSun. A comparison to the total (clumpy + diffuse) dust masses- calculated using the far-IR fluxes of 15% of the D-ETGs that are detected by the IRAS- indicates that only ~20% of the dust resides in these large-scale features. The dust masses are several times larger than the maximum value expected from stellar mass loss, ruling out an internal origin. The dust content shows no correlation with the blue luminosity, indicating that it is not related to a galactic scale cooling flow. No correlation is found with the age of the recent starburst, suggesting that the dust is accreted directly in the merger rather than being produced in situ by the triggered star formation. Using molecular gas-to-dust ratios of ETGs in the literature we estimate that the median current and initial molecular gas fraction are ~1.3% and ~4%, respectively. Recent work suggests that the merger activity in nearby ETGs largely involves minor mergers (mass ratios between 1:10 and 1:4). If the IRAS-detected D-ETGs form via this channel, then the original gas fractions of the accreted satellites are 20%-44%. [Abridged]

astro-ph.CO↗

arXiv2011

Ly-alpha Emitting Galaxies at z = 2.1: Stellar Masses, Dust and Star Formation Histories from Spectral Energy Distribution Fitting

We study the physical properties of 216 z ~ 2.1 LAEs discovered in an ultra-deep narrow-band MUSYC image of the ECDF-S. We fit their stacked Spectral Energy Distribution (SED) using Charlot & Bruzual templates. We consider star formation histories parametrized by the e-folding time parameter tau, allowing for exponentially decreasing (tau>0), exponentially increasing (tau<0), and constant star formation rates. These LAEs are characterized by best fit parameters and 68% confidence intervals of log(M_*/M_sun)=8.6[8.4-9.1], E(B-V)=0.22[0.00-0.31], tau=-0.02[(-4)-18] Gyr, and age_ SF=0.018[0.009-3] Gyr. Thus, we obtain robust measurements of low stellar mass and dust content, but we cannot place meaningful constraints on the age or star formation history of the LAEs. We also calculate the instantaneous SFR to be 35[0.003-170] M_sun/yr, with its average over the last 100 Myr before observation giving _100=4[2-30] M_sun/yr. When we compare the results for the same star formation history, LAEs at z~2.1 are dustier and show higher instantaneous SFRs than z~3.1 LAEs, while the observed stellar masses of the two samples seem consistent. LAEs appear to occupy the low-mass end of the distribution of star forming galaxies at z~2. We perform SED fitting on several sub-samples selected based on photometric properties and find that LAE sub-samples at z~2.1 exhibit heterogeneous properties. The IRAC-bright, UV-bright and red LAEs have the largest stellar mass and dust reddening. The UV-faint, IRAC-faint, and high equivalent width LAE sub-samples appear less massive (<10^9 M_sun) and less dusty, with E(B-V) consistent with zero.

astro-ph.CO↗

arXiv2011

The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black-hole accretion events

Some active galactic nuclei (AGN) are surrounded by extended emission-line regions (EELRs), which trace both the illumination pattern of escaping radiation and its history over the light-travel time from the AGN to the gas. From a new set of such EELRs, we present evidence that the AGN in many Seyfert galaxies undergo luminous episodes 20,000-200,000 years in duration. Motivated by the discovery of the spectacular nebula known as Hanny's Voorwerp, ionized by a powerful AGN which has apparently faded dramatically within ~ 100,000 years, Galaxy Zoo volunteers have carried out both targeted and serendipitous searches for similar emission-line clouds around low-redshift galaxies.We present the resulting list of candidates and describe spectroscopy identifying 19 galaxies with AGN-ionized regions at projected radii > 10 kpc. This search recovered known EELRs and identified additional previously unknown cases, one with detected emission to r = 37 kpc. At least 14/19 are in interacting or merging systems; tidal tails are a prime source of extraplanar ionized gas. We see a mix of one- and two-sided structures, with observed cone angles from 23-112 degrees. We consider the energy balance in the ionized clouds, with lower and upper bounds on ionizing luminosity from recombination and ionization-parameter arguments, and estimate the luminosity of the core from the far-infrared data. The implied ratio of ionizing radiation seen by the clouds to that emitted by the nucleus, for a constant nuclear source, ranges from 0.02 to > 12; 7/19 exceed unity. Small values imply heavily obscured AGN. However, large values may require that the AGN has faded over tens of thousands of years, giving us several examples of systems in which such dramatic long-period variation has occurred; this is the only current technique for addressing these timescales in AGN history. (Abridged)

astro-ph.CO↗

arXiv2011

Evidence for Three Accreting Black Holes in a Galaxy at z~1.35: A Snapshot of Recently Formed Black Hole Seeds?

One of the key open questions in cosmology today pertains to understanding when, where and how super massive black holes form, while it is clear that mergers likely play a significant role in the growth cycles of black holes, how supermassive black holes form, and how galaxies grow around them. Here, we present Hubble Space Telescope WFC3/IR grism observations of a clumpy galaxy at z=1.35, with evidence for 10^6 - 10^7 Msun rapidly growing black holes in separate sub-components of the host galaxy. These black holes could have been brought into close proximity as a consequence of a rare multiple galaxy merger or they could have formed in situ. Such holes would eventually merge into a central black hole as the stellar clumps/components presumably coalesce to form a galaxy bulge. If we are witnessing the in-situ formation of multiple black holes, their properties can inform seed formation models and raise the possibility that massive black holes can continue to emerge in star-forming galaxies as late as z=1.35 (4.8 Gyr after the Big Bang).

astro-ph.CO↗

arXiv2011

Spheroidal post-mergers in the local Universe

Galaxy merging is a fundamental aspect of the standard hierarchical galaxy formation paradigm. Recently, the Galaxy Zoo project has compiled a large, homogeneous catalogue of 3373 mergers, through direct visual inspection of the entire SDSS spectro- scopic sample. We explore a subset of galaxies from this catalogue that are spheroidal 'post-mergers' (SPMs) - where a single remnant is in the final stages of relaxation after the merger and shows evidence for a dominant bulge, making them plausible progenitors of early-type galaxies. Our results indicate that the SPMs have bluer colours than the general early-type galaxy population possibly due to merger-induced star formation. An analysis using optical emission line ratios indicates that 20 of our SPMs exhibit LINER or Seyfert-like activity (68%), while the remaining 10 galaxies are classified as either star forming (16%) or quiescent (16%). A comparison to the emission line activity in the ongoing mergers from Darg et al. indicates that the AGN fraction rises in the post-mergers, suggesting that the AGN phase probably becomes dominant only in the very final stages of the merger process. The optical colours of the SPMs and the plausible mass ratios for their progenitors indicate that, while a minority are consistent with major mergers between two early-type galaxies, the vast majority are remnants of major mergers where at least one progenitor is a late-type galaxy.

astro-ph.CO↗

arXiv2011

Planet Hunters: The First Two Planet Candidates Identified by the Public using the Kepler Public Archive Data

Planet Hunters is a new citizen science project, designed to engage the public in an exoplanet search using NASA Kepler public release data. In the first month after launch, users identified two new planet candidates which survived our checks for false- positives. The follow-up effort included analysis of Keck HIRES spectra of the host stars, analysis of pixel centroid offsets in the Kepler data and adaptive optics imaging at Keck using NIRC2. Spectral synthesis modeling coupled with stellar evolutionary models yields a stellar density distribution, which is used to model the transit orbit. The orbital periods of the planet candidates are 9.8844 \pm0.0087 days (KIC 10905746) and 49.7696 \pm0.00039 (KIC 6185331) days and the modeled planet radii are 2.65 and 8.05 R\oplus. The involvement of citizen scientists as part of Planet Hunters is therefore shown to be a valuable and reliable tool in exoplanet detection.

astro-ph.EP↗

arXiv2011

Tidal dwarf galaxies in the nearby Universe

We present a statistical observational study of the tidal dwarf (TD) population in the nearby Universe, by exploiting a large, homogeneous catalogue of galaxy mergers compiled from the SDSS. 95% of TD-producing mergers involve two spiral progenitors, while most remaining systems have at least one spiral progenitor. The fraction of TD-producing mergers where both parents are early-type galaxies is <2%, suggesting that TDs are unlikely to form in such mergers. The bulk of TD-producing systems inhabit a field environment and have mass ratios greater than 1:7 (the median value is 1:2.5). TDs forming at the tidal-tail tips are ~4 times more massive than those forming at the base of the tails. TDs have stellar masses that are less than 10% of the stellar masses of their parents and typically lie within 15 optical half-light radii of their parent galaxies. The TD population is typically bluer than the parents, with a median offset of ~0.3 mag in the (g-r) colour and the TD colours are not affected by the presence of AGN activity in their parents. An analysis of their star formation histories indicates that TDs contain both newly formed stars (with a median age of ~30 Myr) and old stars drawn from the parent disks, each component probably contributing roughly equally to their stellar mass. Thus, TDs are not formed purely through gas condensation in tidal tails but host a significant component of old stars from the parent disks. Finally, an analysis of the TD contribution to the local dwarf-to-massive galaxy ratio indicates that ~6% of dwarfs in nearby clusters may have a tidal origin, if TD production rates in nearby mergers are representative of those in the high-redshift Universe. Even if TD production rates at high redshift were several factors higher, it seems unlikely that the entire dwarf galaxy population today is a result of merger activity over the lifetime of the Universe.

astro-ph.CO↗

arXiv2011

Chandra Discovery of a Binary Active Galactic Nucleus in Mrk 739

We have discovered a binary AGN in the galaxy Mrk 739 using Chandra and Swift BAT. We find two luminous L_2-10 keV=1.1*10^43 and 1.0*10^42 erg/s, unresolved nuclei with a projected separation of 3.4 kpc (5.8\pm0.1) coincident with two bulge components in the optical image. The western X-ray source (Mrk 739W) is highly variable (2.5x) during the 4-hour Chandra observation and has a very hard spectrum consistent with an AGN. While the eastern component was already known to be an AGN based on the presence of broad optical recombination lines, Mrk 739W shows no evidence of being an AGN in optical, UV, and radio observations, suggesting the critical importance of high spatial resolution hard X-ray observations (>2 keV) in finding these binary AGN. A high level of star formation combined with a very low L_[O III]/L_2-10 keV ratio cause the AGN to be missed in optical observations. CO observations of the (3-2) and (2-1) lines indicate large amounts of molecular gas in the system that could be driven towards the black holes during the violent galaxy collision and be key to fueling the binary AGN. Mrk 739E has a high Eddington ratio of 0.71 and a small black hole (log M_BH=7.05\pm0.3) consistent with an efficiently accreting AGN. Other than NGC 6240, this stands as the nearest case of a binary AGN discovered to date.

astro-ph.CO↗

arXiv2011

Black hole growth in the early Universe is self-regulated and largely hidden from view

The formation of the first massive objects in the infant Universe remains impossible to observe directly and yet it sets the stage for the subsequent evolution of galaxies. While some black holes with masses > billion solar masses? have been detected in luminous quasars less than one billion years after the Big Bang, these individual extreme objects have limited utility in constraining the channels of formation of the earliest black holes. The initial conditions of black hole seed properties are quickly erased during the growth process. From deep, optimally stacked, archival X-ray observations, we measure the amount of black hole growth in z=6-8 galaxies (0.7-1 billion years after the Big Bang). Our results imply that black holes grow in tandem with their hosts throughout cosmic history, starting from the earliest times. We find that most copiously accreting black holes at these epochs are buried in significant amounts of gas and dust that absorb most radiation except for the highest energy X-rays. This suggests that black holes grow significantly more than previously thought during these early bursts, and due to obscuration they do not contribute to the re-ionization of the Universe with their ultraviolet emission.

astro-ph.HE↗

arXiv2011

Improved and Quality-assessed Emission and Absorption Line measurements in Sloan Digital Sky Survey galaxies

We present a new database of absorption and emission-line measurements based on the entire spectral atlas from the Sloan Digital Sky Survey (SDSS) 7th data release of galaxies within a redshift of 0.2. Our work makes use of the publicly available penalized pixel-fitting(pPXF) and gas and absorption line fitting (gandalf) IDL codes, aiming to improve the existing measurements for stellar kinematics, the strength of various absorption-line features, and the flux and width of the emissions from different species of ionised gas. Our fit to the stellar continuum uses both standard stellar population models and empirical templates obtained by combining a large number of stellar spectra in order to fit a subsample of high-quality SDSS spectra for quiescent galaxies. Furthermore, our fit to the nebular spectrum includes an exhaustive list of both recombination and forbidden lines. Foreground Galactic extinction is implicitly treated in our models, whereas reddening in the SDSS galaxies is included in the form of a simple dust screen component affecting the entire spectrum that is accompanied by a second reddening component affecting only the ionised gas emission. In order to check for systematic departures, we provide a quality assessment for our fit to the SDSS spectra in our sample. This quality assessment also allows the identification of objects with either problematic data or peculiar features. For example, based on the quality assessment, approximately 1% of the SDSS spectra classified as "galaxies" by the SDSS pipeline do in fact require additional broad lines to be matched, even though they do not show a strong continuum from an active nucleus, as do the SDSS objects classified as "quasars". Finally, we provide new spectral templates for galaxies of different Hubble types, obtained by combining the results of our spectral fit for a subsample of 452 morphologically selected objects.

astro-ph.CO↗

arXiv2011

A simple model for AGN feedback in nearby early-type galaxies

Recent work indicates that star-forming early-type galaxies (ETGs) residing in the blue cloud migrate rapidly to the red sequence within around a Gyr, passing through several phases of increasingly strong AGN activity in the process (Schawinski et al. 2007, MNRAS, 382, 1415; S07 hereafter). We show that natural depletion of the gas reservoir through star formation (i.e. in the absence of any feedback from the AGN) induces a blue-to-red reddening rate that is several factors lower than that observed in S07. This is because the gas depletion rate due to star formation alone is too slow, implying that another process needs to be invoked to remove gas from the system and accelerate the reddening rate. We develop a simple phenomenological model, in which a fraction of the AGN's luminosity couples to the gas reservoir over a certain 'feedback timescale' and removes part of the gas mass from the galaxy, while the remaining gas continues to contribute to star formation. We use the model to investigate scenarios which yield migration times consistent with the results of S07. We find that acceptable models have feedback timescales <0.2 Gyrs. The mass fraction in young stars in the remnants is <5% and the residual gas fractions are less than 0.6%, in good agreement with the recent literature. At least half of the initial gas reservoir is removed as the galaxies evolve from the blue cloud to the red sequence. If we restrict ourselves to feedback timescales similar to the typical duty cycles of local AGN (a few hundred Myrs) then a few tenths of a percent of the luminosity of an early-type Seyfert (10^11 LSun) must couple to the gas reservoir in order to produce migration times that are consistent with the observations.

astro-ph.CO↗

arXiv2011

Galaxy Zoo: Morphological Classification and Citizen Science

We provide a brief overview of the Galaxy Zoo and Zooniverse projects, including a short discussion of the history of, and motivation for, these projects as well as reviewing the science these innovative internet-based citizen science projects have produced so far. We briefly describe the method of applying en-masse human pattern recognition capabilities to complex data in data-intensive research. We also provide a discussion of the lessons learned from developing and running these community--based projects including thoughts on future applications of this methodology. This review is intended to give the reader a quick and simple introduction to the Zooniverse.

astro-ph.IM↗

arXiv2011

Galaxy Zoo: Bar Lengths in Nearby Disk Galaxies

We present an analysis of bar length measurements of 3150 local galaxies in a volume limited sample of low redshift (z < 0.06) disk galaxies. Barred galaxies were initially selected from the Galaxy Zoo 2 project, and the lengths and widths of the bars were manually drawn by members of the Galaxy Zoo community using a Google Maps interface. Bars were measured independently by different observers, multiple times per galaxy (>=3), and we find that observers were able to reproduce their own bar lengths to 3% and each others' to better than 20%. We find a "color bimodality" in our disk galaxy population with bar length, i.e., longer bars inhabit redder disk galaxies and the bars themselves are redder, and that the bluest galaxies host the smallest galactic bars (< 5 kpc/h). We also find that bar and disk colors are clearly correlated, and for galaxies with small bars, the disk is, on average, redder than the bar colors, while for longer bars the bar then itself is redder, on average, than the disk. We further find that galaxies with a prominent bulge are more likely to host longer bars than those without bulges. We categorise our galaxy populations by how the bar and/or ring are connected to the spiral arms. We find that galaxies whose bars are directly connected to the spiral arms are preferentially bluer and that these galaxies host typically shorter bars. Within the scatter, we find that stronger bars are found in galaxies which host a ring (and only a ring). The bar length and width measurements used herein are made publicly available for others to use (http://data.galaxyzoo.org).

astro-ph.CO↗