Yale Center for Astronomy and Astrophysics

New Haven, CT, United States

Yale Center for Astronomy and Astrophysics

New Haven, CT, United States
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Mao P.,Yale Center for Astronomy and Astrophysics | Urry C.M.,Yale Center for Astronomy and Astrophysics
Astrophysical Journal | Year: 2017

We investigate a sample of 622 blazars with measured fluxes at 12 wavebands across the radio-to-gamma-ray spectrum but without spectroscopic or photometric redshifts. This sample includes hundreds of sources with newly analyzed X-ray spectra reported here. From the synchrotron peak frequencies, estimated by fitting the broadband spectral energy distributions (SEDs), we find that the fraction of high-synchrotron-peaked blazars in these 622 sources is roughly the same as in larger samples of blazars that do have redshifts. We characterize the no-redshift blazars using their infrared colors, which lie in the distinct locus called the WISE blazar strip, then estimate their redshifts using a KNN regression based on the redshifts of the closest blazars in the WISE color-color plot. Finally, using randomly drawn values from plausible redshift distributions, we simulate the SEDs of these blazars and compare them to known blazar SEDs. Based on all these considerations, we conclude that blazars without redshift estimates are unlikely to be high-luminosity, high-synchrotron-peaked objects, which had been suggested in order to explain the "blazar sequence" - an observed trend of SED shape with luminosity - as a selection effect. Instead, the observed properties of no-redshift blazars are compatible with a causal connection between jet power and electron cooling, i.e., a true blazar sequence. © 2017. The American Astronomical Society. All rights reserved..

Oesch P.A.,Yale Center for Astronomy and Astrophysics
Proceedings of the International Astronomical Union | Year: 2015

Understanding when and how the first galaxies formed and what sources reionized the universe are key goals of extragalactic astronomy. Thanks to deep surveys with the powerful WFC3/IR camera on the HST, the observational frontier of galaxy build-up now lies at only ∼450 Myr after the Big Bang, at redshifts z ∼10-12. In combination with deep data from Spitzer/IRAC we can now probe the evolution of the stellar mass density over 96% of cosmic history. However, detecting and characterizing galaxies at these early epochs is challenging even for HST and the sample sizes at the earliest redshifts are still very small. The Hubble Frontier Fields provide a prime new dataset to improve upon our current, sparse sampling of the UV luminosity function at z>8 from blank fields to answer some of the most pressing open questions. For instance, even the evolution of the cosmic star-formation rate density at z>8 is still debated. While our measurements based on blank field data indicate that galaxies with SFR>0.7 Msol/yr disappear quickly from the cosmic record between z∼8 and z∼10, other previous results, e.g., from the CLASH survey favor a more moderate decline. Here, we briefly review the recent progress in studying galaxy build-up out to z∼10 from the combined blank field and existing Frontier Field datasets and discuss their implications for primordial galaxy formation and cosmic reionization. © International Astronomical Union 2016.

Cisternas M.,Institute of Astrophysics of Canarias | Cisternas M.,University of La Laguna | Sheth K.,U.S. National Radio Astronomy Observatory | Salvato M.,Max Planck Institute for Extraterrestrial Physics | And 5 more authors.
Astrophysical Journal | Year: 2015

We present empirical constraints on the influence of stellar bars on the fueling of active galactic nuclei (AGNs) out to z = 0.84 using a sample of X-ray-selected AGNs hosted in luminous non-interacting face-on and moderately inclined disk galaxies from the Chandra COSMOS survey. Using high-resolution Hubble Space Telescope imaging to identify bars, we find that the fraction of barred active galaxies displays a similar behavior as that of inactive spirals, declining with redshift from 71% at z ∼ 0.3, to 35% at z ∼ 0.8. With active galaxies being typically massive, we compare them against a mass-matched sample of inactive spirals and show that, while at face value the AGN bar fraction is slightly higher at all redshifts, we cannot rule out that the bar fractions of active and inactive galaxies are the same. The presence of a bar has no influence on the AGN strength, with barred and unbarred active galaxies showing equivalent X-ray luminosity distributions. From our results, we conclude that the occurrence and the efficiency of the fueling process is independent of the large scale structure of a galaxy. The role of bars, if any, may be restricted to providing the suitable conditions for black hole fueling to occur, i.e., bring a fresh supply of gas to the central 100 pc. At the high-redshift end, we find that roughly 60% of active disk galaxies are unbarred. We speculate this to be related with the known dynamical state of disks at higher redshifts - more gas-rich and prone to instabilities than local spirals - which could also lead to gas inflows without the need of bars. © 2015. The American Astronomical Society. All rights reserved.

Oesch P.A.,Yale Center for Astronomy and Astrophysics | Bouwens R.J.,Leiden University | Illingworth G.D.,University of California at Santa Cruz | Franx M.,Leiden University | And 4 more authors.
Astrophysical Journal | Year: 2015

We search the complete Hubble Frontier Field data set of Abell 2744 and its parallel field for z ∼ 10 sources to further refine the evolution of the cosmic star formation rate density (SFRD) between z ∼ 8 and z ∼ 10. We independently confirm two images of the recently discovered triply imaged z ∼ 9.8 source by Zitrin et al. and set an upper limit for similar z ∼ 10 galaxies with red colors of J125 - H160 1.2 in the parallel field of Abell 2744. We utilize extensive simulations to derive the effective selection volume of Lyman-break galaxies at z ∼ 10, both in the lensed cluster field and in the adjacent parallel field. Particular care is taken to include position-dependent lensing shear to accurately account for the expected sizes and morphologies of highly magnified sources. We show that both source blending and shear reduce the completeness at a given observed magnitude in the cluster, particularly near the critical curves. These effects have a significant, but largely overlooked, impact on the detectability of high-redshift sources behind clusters, and substantially reduce the expected number of highly magnified sources. The detections and limits from both pointings result in an SFRD which is consistent within the uncertainties with previous estimates at z ∼ 10 from blank fields. The combination of these new results with all other estimates is also consistent with a rapidly declining SFRD in the 170 Myr from z ∼ 8 to z ∼ 10 as predicted by cosmological simulations and dark-matter halo evolution in as predicted by cosmological simulations and dark-matter halo evolution in ΛCDM. Once biases introduced by magnification-dependent completeness are accounted for, the full six cluster and parallel Frontier Field program will be an extremely powerful new data set to probe the evolution of the galaxy population at z > 8 before the advent of the James Webb Space Telescope. © 2015. The American Astronomical Society. All rights reserved.

Treister E.,University of Concepción | Schawinski K.,Yale Center for Astronomy and Astrophysics | Schawinski K.,Yale University | Schawinski K.,ETH Zurich | And 5 more authors.
Astrophysical Journal Letters | Year: 2012

Using multiwavelength surveys of active galactic nuclei (AGNs) across a wide range of bolometric luminosities (1043 < L bol (erg s-1) <5 × 1046) and redshifts (0 < z < 3), we find a strong, redshift-independent correlation between the AGN luminosity and the fraction of host galaxies undergoing a major merger. That is, only the most luminous AGN phases are connected to major mergers, while less luminous AGNs appear to be driven by secular processes. Combining this trend with AGN luminosity functions to assess the overall cosmic growth of black holes, we find that 50% by mass is associated with major mergers, while only 10% of AGNs by number, the most luminous, are connected to these violent events. Our results suggest that to reach the highest AGN luminosities - where the most massive black holes accreted the bulk of their mass - a major merger appears to be required. The luminosity dependence of the fraction of AGNs triggered by major mergers can successfully explain why the observed scatter in the M-σ relation for elliptical galaxies is significantly lower than in spirals. The lack of a significant redshift dependence of the L bol-f merger relation suggests that downsizing, i.e., the general decline in AGN and star formation activity with decreasing redshift, is driven by a decline in the frequency of major mergers combined with a decrease in the availability of gas at lower redshifts. © © 2012. The American Astronomical Society. All rights reserved.

Treister E.,University of Hawaii at Manoa | Urry C.M.,Yale University | Urry C.M.,Yale Center for Astronomy and Astrophysics | Schawinski K.,Yale Center for Astronomy and Astrophysics | And 4 more authors.
Astrophysical Journal Letters | Year: 2010

We take advantage of the rich multiwavelength data available in the Chandra Deep Field South (CDF-S), including the 4 Ms Chandra observations (the deepest X-ray data to date), in order to search for heavily obscured lowluminosity active galactic nuclei (AGNs) among infrared-luminous galaxies. In particular, we obtained a stacked rest-frame X-ray spectrum for samples of galaxies binned in terms of their IR luminosity or stellar mass.We detect a significant signal at E ∼ 1-8 keV, which we interpret as originating from a combination of emission associated with star formation processes at low energies combined with a heavily obscured AGN at E > 5 keV. We further find that the relative strength of this AGN signal decays with decreasing IR luminosity, indicating a higher AGN fraction for more luminous IR sources. Together, these results strongly suggest the presence of a large number of obscured AGNs in IR-luminous galaxies. Using samples binned in terms of stellar mass in the host galaxy, we find a significant excess at E = 6-7 keV for sources with M > 10 11M⊙, consistent with a large obscured AGN population in high mass galaxies. In contrast, no strong evidence of AGN activity was found for less-massive galaxies. The integrated intensity at high energies indicates that a significant fraction of the total black hole growth, ∼22%, occurs in heavily obscured systems that are not individually detected in even the deepest X-ray observations. There are also indications that the number of low-luminosity, heavily obscured AGNs does not evolve significantly with redshift, in contrast to the strong evolution seen in higher luminosity sources. © 2010 The American Astronomical Society. All rights reserved.

Van Der Marel R.P.,US Space Telescope Science Institute | Kallivayalil N.,Yale Center for Astronomy and Astrophysics | Kallivayalil N.,University of Virginia
Astrophysical Journal | Year: 2014

We present the first detailed assessment of the large-scale rotation of any galaxy based on full three-dimensional velocity measurements. We do this for the LMC by combining our Hubble Space Telescope average proper motion (PM) measurements for stars in 22 fields, with existing line-of-sight (LOS) velocity measurements for 6790 individual stars. We interpret these data with a model of circular rotation in a flat disk. The PM and LOS data paint a consistent picture of the LMC rotation, and their combination yields several new insights. The PM data imply a stellar dynamical center that coincides with the H I dynamical center, and a rotation curve amplitude consistent with that inferred from LOS velocity studies. The implied disk viewing angles agree with the range of values found in the literature, but continue to indicate variations with stellar population and/or radius. Young (red supergiant) stars rotate faster than old (red and asymptotic giant branch) stars due to asymmetric drift. Outside the central region, the circular velocity is approximately flat at V circ = 91.7 ± 18.8 km s-1. This is consistent with the baryonic Tully-Fisher relation and implies an enclosed mass M(8.7 kpc) = (1.7 ± 0.7) × 1010 M·. The virial mass is larger, depending on the full extent of the LMC's dark halo. The tidal radius is 22.3 ± 5.2 kpc (24.°0 ± 5.°6). Combination of the PM and LOS data yields kinematic distance estimates for the LMC, but these are not yet competitive with other methods. © 2014. The American Astronomical Society. All rights reserved.

Kallivayalil N.,Yale Center for Astronomy and Astrophysics | Kallivayalil N.,University of Virginia | Van Der Marel R.P.,US Space Telescope Science Institute | Besla G.,Columbia Astrophysics Laboratory | And 2 more authors.
Astrophysical Journal | Year: 2013

We present proper motions for the Large and Small Magellanic Clouds (LMC and SMC) based on three epochs of Hubble Space Telescope data, spanning a ∼7 yr baseline, and centered on fields with background QSOs. The first two epochs, the subject of past analyses, were obtained with ACS/HRC, and have been reanalyzed here. The new third epoch with WFC3/UVIS increases the time baseline and provides better control of systematics. The three-epoch data yield proper-motion random errors of only 1%-2% per field. For the LMC this is sufficient to constrain the internal proper-motion dynamics, as will be discussed in a separate paper. Here we focus on the implied center-of-mass proper motions: μW, LMC = -1.910 ± 0.020 mas yr -1, μN, LMC = 0.229 ± 0.047 mas yr-1, and μW, SMC = -0.772 ± 0.063 mas yr-1, μN, SMC = -1.117 ± 0.061 mas yr-1. We combine the results with a revised understanding of the solar motion in the Milky Way to derive Galactocentric velocities: vtot, LMC = 321 ± 24 km s-1 and vtot, SMC = 217 ± 26 km s-1. Our proper-motion uncertainties are now dominated by limitations in our understanding of the internal kinematics and geometry of the Clouds, and our velocity uncertainties are dominated by distance errors. Orbit calculations for the Clouds around the Milky Way allow a range of orbital periods, depending on the uncertain masses of the Milky Way and LMC. Periods ≲ 4 Gyr are ruled out, which poses a challenge for traditional Magellanic Stream models. First-infall orbits are preferred (as supported by other arguments as well) if one imposes the requirement that the LMC and SMC must have been a bound pair for at least several Gyr. © 2013. The American Astronomical Society. All rights reserved.

Treister E.,University of Concepción | Schawinski K.,ETH Zurich | Volonteri M.,Paris Institute of Astrophysics | Natarajan P.,Yale Center for Astronomy and Astrophysics | Natarajan P.,Yale University
Astrophysical Journal | Year: 2013

We constrain the total accreted mass density in supermassive black holes at z > 6, inferred via the upper limit derived from the integrated X-ray emission from a sample of photometrically selected galaxy candidates. Studying galaxies obtained from the deepest Hubble Space Telescope images combined with the Chandra 4 Ms observations of the Chandra Deep Field-South, we achieve the most restrictive constraints on total black hole growth in the early universe. We estimate an accreted mass density <1000 M ⊙ Mpc -3 at z ∼ 6, significantly lower than the previous predictions from some existing models of early black hole growth and earlier prior observations. These results place interesting constraints on early black hole growth and mass assembly by accretion and imply one or more of the following: (1) only a fraction of the luminous galaxies at this epoch contain active black holes; (2) most black hole growth at early epochs happens in dusty and/or less massive - as yet undetected - host galaxies; (3) there is a significant fraction of low-z interlopers in the galaxy sample; (4) early black hole growth is radiatively inefficient, heavily obscured, and/or due to black hole mergers as opposed to accretion; or (5) the bulk of the black hole growth occurs at late times. All of these possibilities have important implications for our understanding of high-redshift seed formation models. © 2013. The American Astronomical Society. All rights reserved.

Massey R.,University of Edinburgh | Kitching T.,University of Edinburgh | Nagai D.,Yale University | Nagai D.,Yale Center for Astronomy and Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2011

The unique properties of dark matter are revealed during collisions between clusters of galaxies, such as the bullet cluster (1E0657-56) and baby bullet (MACSJ0025-12). These systems provide evidence for an additional, invisible mass in the separation between the distributions of their total mass, measured via gravitational lensing, and their ordinary 'baryonic' matter, measured via its X-ray emission. Unfortunately, the information available from these systems is limited by their rarity. Constraints on the properties of dark matter, such as its interaction cross-section, are therefore restricted by uncertainties in the individual systems' impact velocity, impact parameter and orientation with respect to the line of sight. Here we develop a complementary, statistical measurement in which every piece of substructure falling into every massive cluster is treated as a bullet. We define 'bulleticity' as the mean separation between dark matter and ordinary matter, and we measure the signal in hydrodynamical simulations. The phase space of substructure orbits also exhibits symmetries that provide an equivalent control test. Any detection of bulleticity in real data would indicate a difference in the interaction cross-sections of baryonic and dark matter that may rule out hypotheses of non-particulate dark matter that are otherwise able to model individual systems. A subsequent measurement of bulleticity could constrain the dark matter cross-section. Even with conservative estimates, the existing Hubble Space Telescope archive should yield an independent constraint tighter than that from the bullet cluster. This technique is then trivially extendable to and benefits enormously from larger, future surveys. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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