The sporadic accretion following the tidal disruption of a star by a super-massive black hole (TDE) leads to a bright. UV and soft X-ray flare in the galactic nucleus. The gas and dust surrounding the black hole responses to such a flare with an echo in emission lines and infrared emission. In this paper, we report the detection of long fading mid-IR emission lasting up to 14 years after the flare in four TDE candidates with transient coronal lines using the WISE public data release. We estimate that the reprocessed mid-IR luminosities are in the range between 4 x 10(42) and 2 x 10(43) erg s(-1) and dust temperature in the range of 570-800 K when WISE first detected these sources three to five years after the flare. Both luminosity and dust temperature decrease with time. We interpret the mid-IR emission as the infrared echo of the tidal disruption flare. We estimate the UV luminosity at the peak flare to be 1 to 30 times 10(44) erg s(-1) and that for. warm dust masses to be. in the range of 0.05-1.3 M-circle dot within a few parsecs. Our results suggest that the. mid-infrared echo is a general signature of TDE in the gas-rich environment.

We present a complete census of all Herschel-detected sources within the six massive lensing clusters of the HST Frontier Fields (HFF). We provide a robust legacy catalogue of 263 sources with Herschel fluxes, primarily based on imaging from the Herschel Lensing Survey and PEP/HerMES Key Programmes. We optimally combine Herschel, Spitzer and WISE infrared (IR) photometry with data from HST, VLA and ground-based observatories, identifying counterparts to gain source redshifts. For each Herschel-detected source we also present magnification factor (mu), intrinsic IR luminosity and characteristic dust temperature, providing a comprehensive view of dust-obscured star formation within the HFF. We demonstrate the utility of our catalogues through an exploratory overview of the magnified population, including more than 20 background sub-LIRGs unreachable by Herschel without the assistance gravitational lensing.

To elucidate the intrinsic broadband infrared (IR) emission properties of active galactic nuclei (AGNs), we analyze the spectral energy distributions (SEDs) of 87 z less than or similar to 0.5 Palomar-Green (PG) quasars. While the Elvis AGN template with a moderate far-IR correction can reasonably match the SEDs of the AGN components in similar to 60% of the sample (and is superior to alternatives such as that by Assef), it fails on two quasar populations: (1) hot-dustdeficient (HDD) quasars that show very weak emission thoroughly from the near-IR to the far-IR, and (2) warm-dust- deficient (WDD) quasars that have similar hot dust emission as normal quasars but are relatively faint in the mid-and far-IR. After building composite AGN templates for these dust-deficient quasars, we successfully fit the 0.3-500 mu m SEDs of the PG sample with the appropriate AGN template, an infrared template of a star-forming galaxy, and a host galaxy stellar template. 20 HDD and 12 WDD quasars are identified from the SED decomposition, including seven ambiguous cases. Compared with normal quasars, the HDD quasars have AGNs with relatively low Eddington ratios and the fraction of WDD quasars increases with AGN luminosity. Moreover, both the HDD and WDD quasar populations show relatively stronger mid-IR silicate emission. Virtually identical SED properties are also found in some quasars from z = 0.5 to 6. We propose a conceptual model to demonstrate that the observed dust deficiency of quasars can result from a change of structures of the circumnuclear tori that can occur at any cosmic epoch.

The range of currently proposed active galactic nucleus (AGN) far-infrared templates results in uncertainties in retrieving host galaxy information from infrared observations and also undermines constraints on the outer part of the AGN torus. We discuss how to test and reconcile these templates. Physically, the fraction of the intrinsic AGN IR-processed luminosity compared with that from the central engine should be consistent with the dust-covering factor. In addition, besides reproducing the composite spectral energy distributions (SEDs) of quasars, a correct AGN IR template combined with an accurate library of star-forming galaxy templates should be able to reproduce the IR properties of the host galaxies, such as the luminosity-dependent SED shapes and aromatic feature strengths. We develop tests based on these expected behaviors and find that the shape of the AGN intrinsic far-IR emission drops off rapidly starting at similar to 20 mu m and can be matched by an Elvis et al.-like template with a. minor modification. Despite the variations in the near- to mid-IR bands, AGNs in quasars and Seyfert galaxies have remarkably similar intrinsic far-IR SEDs at lambda similar to 20-100 mu m, suggesting a similar emission character of the outermost region of the circumnuclear torus. The variations of the intrinsic AGN IR SEDs among the type-1 quasar population can be explained by the changing relative strengths of four major dust components with similar characteristic temperatures, and there is evidence for compact AGN-heated dusty structures at sub-kiloparsec scales in the far-IR.

We present observed mid-infrared and optical colors and composite spectral energy distributions (SEDs) of type 1 (broad-line) and 2 (narrow-line) quasars selected from Sloan Digital Sky Survey (SDSS) spectroscopy. A significant fraction of powerful quasars are obscured by dust and are difficult to detect in optical photometric or spectroscopic surveys. However, these may be more easily identified on the basis of mid-infrared (MIR) colors and SEDs. Using samples of SDSS type 1 and 2 matched in redshift and [O III] luminosity, we produce composite rest-frame 0.2-15 mu m SEDs based on SDSS, UKIDSS, and Wide-field Infrared Survey Explorer photometry and perform model fits using simple galaxy and quasar SED templates. The SEDs of type 1 and 2 quasars are remarkably similar, with the differences explained primarily by the extinction of the quasar component in the type 2 systems. For both types of quasar, the flux of the active galactic nucleus (AGN) relative to the host galaxy increases with AGN luminosity (L-[O III]) and redder observed MIR color, but we find only weak dependencies of the composite SEDs on mechanical jet power as determined through radio luminosity. We conclude that luminous quasars can be effectively selected using simple MIR color criteria similar to those identified previously (W1-W2 > 0.7; Vega), although these criteria miss many heavily obscured objects. Obscured quasars can be further identified based on optical-IR colors (for example, (u-W3[AB])> 1.4(W1-W2[Vega])+ 3.2). These results illustrate the power of large statistical studies of obscured quasars selected on the basis of MIR and optical photometry.

We present and analyze the possibility of using optical u-band luminosities to estimate star-formation rates (SFRs) of galaxies based on the data from the South Galactic Cap u. band Sky Survey (SCUSS), which provides a deep u-band photometric survey covering about 5000 deg(2) of the South Galactic Cap. Based on two samples of normal star-forming galaxies selected by the. BPT diagram, we explore the correlations between u-band, H alpha, and IR luminosities by combing SCUSS data with the Sloan Digital Sky Survey and Wide-field Infrared Survey Explorer (WISE). The attenuation-corrected u-band luminosities are tightly correlated with the Balmer decrement-corrected Ha luminosities with an rms scatter of similar to 0.17 dex. The IR-corrected u luminosities are derived based on the correlations between the attenuation of u-band luminosities and WISE. 12 (or 22) mu m luminosities, and then calibrated with the Balmer-corrected Ha luminosities. The systematic residuals of these calibrations are tested against the physical properties over the ranges covered by our sample objects. We find that the best-fitting nonlinear relations are better than the linear ones and recommended to be applied in the measurement of SFRs. The systematic deviations mainly come from the pollution of old stellar population and the effect of dust extinction; therefore,. a. more detailed analysis is needed in future work.

We conduct a deep mid-infrared census of nine massive galaxy clusters at (0 < z < 1.3) with a total of ~ 1500 spectroscopically confirmed member galaxies using Spitzer /IRAC photometry and established mid-infrared color selection techniques. Of the 949 cluster galaxies that are detected in at least three of the four IRAC channels at the >= 3 sigma level, we identify 12 that host mid-infrared selected active galactic nuclei (IR-AGN). To compare the IR-AGN across our redshift range, we define two complete samples of cluster galaxies: (1) optically-selected members with rest-frame VAB magnitude < -21.5 and (2) mid-IR selected members brighter than (M*_3.6 +0.5), i.e. essentially a stellar mass cut. In both samples, we measure f_IR-AGN ~ 1% with a strong upper limit of ~3% at z < 1. This uniformly low IR-AGN fraction at z < 1 is surprising given the fraction of 24 micrometer sources in the same galaxy clusters is observed to increase by about a factor of four from z ~ 0 to z ~ 1; this indicates that most of the detected 24 micrometer flux is due to star formation. Only in our single galaxy cluster at z = 1.24 is the IR-AGN fraction measurably higher at ~15% (all members; ~70% for late-types only). In agreement with recent studies, we find the cluster IR-AGN are predominantly hosted by late-type galaxies with blue optical colors, i.e. members with recent/ongoing star formation. The four brightest IR-AGN are also X-ray sources; these IR+X-ray AGN all lie outside the cluster core (R_proj > 0.5 Mpc) and are hosted by highly morphologically disturbed members. Although our sample is limited, our results suggest that f_IR-AGN in massive galaxy clusters is not strongly correlated with star formation at z < 1, and that IR-AGN have a more prominent role at z &gt; 1.

In this thesis, I explore both obscured and unobscured star formation over a large fraction of cosmic time. I use the HAWK-I Y -band science verification data over GOODS-South, in conjunction with optical and infrared data to search for Lyman-break galaxies at z >∼ 6.5 (i.e. within the first billion years of the Universe). I find four possible (two robust) z′-drop candidates (z >∼ 6.5) and four possible (but no robust) Y -drop candidates (z >∼ 7). I use my results to place constraints on the luminosity function at z ∼ 6.5 and find significant evolution in the population of Lyman-break galaxies between 3 < z <∼ 6.5. I also explore obscured star formation with a population of 70μm selected galaxies over the COSMOS field. I use AAT spectroscopy in conjunction with other available spectroscopic redshifts for my sample, and photometric redshifts otherwise, to calculate the total infrared luminosity of each galaxy. Two libraries of spectral energy distributions are considered; Siebenmorgen & Krügel (2007) templates and Chary & Elbaz (2001) models. We have supplemented our data with that of Huynh et al. (2007) collected over the GOODS-North field and adapted it to directly compare with the results of this work. The far-infrared luminosity function is then determined using the 1/Vmax technique. A double power law parameterisation is found to provide the best fit to the data. The far-infrared luminosity function was fitted for all parameters and the evolution was measured out to z ∼ 1. Three different types of evolution were allowed, pure luminosity, pure density and luminosity dependent density evolution. In all cases strong positive evolution was evident with the best-fit case being pure luminosity evolution where p = 2.4+0.6 −0.7. Due to the larger volume surveyed compared to previous studies, this work provides better constraints on the bright end of the far-infrared luminosity function displaying a shallower bright end slope (α2 ∼ −1.6) than previously determined, implying a higher number density of the most luminous objects and thereby a greater contribution from these objects to the total infrared energy density. However the shallower slope determined here can be reconciled with other work if the Chary & Elbaz (2001) models are used instead of the Siebenmorgen & Krügel templates; demonstrating that spectral energy distribution model selection is a key component in determining luminosity functions at far-infrared wavelengths. The far-infrared–radio correlation (FIRC; qIR) was determined for the sample of 70μm selected star-forming galaxies using 1.4GHz radio data over the COSMOS field, and no evolution was found out to z ∼ 2. The 70μm monochromatic evolution in the FIRC was also examined (q70) and no evolution was found in this parameter with redshift.

We have observed the nearest ultraluminous infrared galaxy Arp 220 with a 13 month near-infrared observing program using the Canada France Hawaii Telescope to search for obscured supernovae in this extreme star forming environment. This monitoring program was aimed as a feasibility study to determine the practicality of a large scale near-IR LIRG/ULIRG imaging survey. Establishing the supernova rate in these dusty galaxies is an important step toward confirming theorized star formation rates and settling the debate between the dominant energy source in LIRGs: star formation or AGN activity. Both the deduced high star formation rate and far-IR luminosity of Arp 220 suggest an atypically high supernova rate of 1-4 per year, two orders of magnitude greater than that of the Milky Way. We attempt the first direct observation of this rate which to date has been probed primarily through radio measurements of supernovae and remnants. Through a point-spread function matching and image subtraction procedure we find no supernovae outside the galactic nucleus, consistent with the paradigm of a strong nuclear-contained starburst. Image subtraction residuals prevent the discovery of supernovae in the central regions of the galaxy. Using differential photometry we find evidence for a statistically significant brightening in the Arp 220 nucleus with a K-band peak of approximately $\Delta m_K=0.16$ magnitudes. To find the true peak magnitude we use Hubble Space Telescope archival data to subtract off the nuclear background and find an absolute magnitude of $M_K = -22.19 \pm 0.16$ (non-absorbed). This exceeds the luminosity of a typical core collapse supernova by roughly 3.5 magnitudes; rather, the observed variations in nuclear brightness are most likely the signature of an active galactic nucleus embedded in the dusty nuclei of Arp 220 or the superposition of light from several supernovae. This method is not sensitive to the detection of individual supernovae and we cannot rule out the occurrence of any nuclear SNe during the observing period. The brightening event is dimmer in the H and J bands, appearing to be affected by extinction. Interpreting this as a supernova-related event we estimate the extinction in the nuclear regions of Arp 220 to lie between $2.01 \le A_K \le 3.40$ or $17.95 \le A_V \le 30.36$ in the optical, in agreement with several other estimates. Improved resolution is required in order to detect supernovae in the extremely bright nuclear environments of LIRGs. Alternatively, infrared spectroscopy would reveal the telltale spectral features of nuclear supernovae. Spectroscopic observations of the Arp 220 nuclei were conducted using Keck in July 2013 for this very purpose; results are pending. We also explore the hypothesis that type Ia supernovae are produced primarily from young stellar populations. We model elliptical galaxies as two component stellar systems using PEGASE stellar templates: a fixed older underlying population coupled with a younger, less massive population. Varying the age and mass ratio of the young component, we examine its effect on I) the colours and II) the supernova rate of the single underlying population. We explore the effect with redshift and employ both theoretical and observational forms of the type Ia delay-time distribution. We then apply our models to the MENeaCS supernova survey and find that the number and distribution of red sequence SN Ia hosts agrees with theoretical expectations. The lack of evidence for a type Ia rate cutoff argues for a continuous delay-time distribution in support of the double degenerate model as the primary SN Ia progenitor channel. We conclude that it is not possible for all type Ia events in ellipticals to originate from a young frosting of stars. / Graduate / 0606 / jkezwer@uvic.ca

This paper provides an update of our previous scaling relations between galaxy-integrated molecular gas masses, stellar masses, and star formation rates (SFRs), in the framework of the star formation main sequence (MS), with the main goal of testing for possible systematic effects. For this purpose our new study combines three independent methods of determining molecular gas masses from CO line fluxes, far-infrared dust spectral energy distributions, and similar to 1 mm dust photometry, in a large sample of 1444 star-forming galaxies between z = 0 and 4. The sample covers the stellar mass range log(M-*/M-circle dot) = 9.0-11.8, and SFRs relative to that on the MS, delta MS = SFR/SFR (MS), from 10(-1.3) to 10(2.2). Our most important finding is that all data sets, despite the different techniques and analysis methods used, follow the same scaling trends, once method-to-method zero-point offsets are minimized and uncertainties are properly taken into account. The molecular gas depletion time t(depl), defined as the ratio of molecular gas mass to SFR, scales as (1 + z)(-0.6) x (delta MS)(-0.44) and is only weakly dependent on stellar mass. The ratio of molecular to stellar mass mu(gas) depends on (1+ z)(2.5) x (delta MS)(0.52) x (M-*)(-0.36), which tracks the evolution of the specific SFR. The redshift dependence of mu(gas) requires a curvature term, as may the mass dependences of t(depl) and mu(gas). We find no or only weak correlations of t(depl) and mu(gas) with optical size R or surface density once one removes the above scalings, but we caution that optical sizes may not be appropriate for the high gas and dust columns at high z.