[en] We estimate the abundance of Compton-thick (CT) active galactic nuclei (AGNs) based on our joint model of X-ray and infrared backgrounds. At L_rest2-10keV > 10⁴² erg s^–1, the CT AGN density predicted by our model is a few ×10^–4 Mpc^–3 from z = 0 up to z = 3. CT AGNs with higher luminosity cuts (>10⁴³, 10⁴⁴, and 10⁴⁵ erg s^–1) peak at higher redshift and show a rapid increase in number density from z = 0 to z ∼ 2-3. The CT AGN to all AGN ratio appears to be low (2%-5%) at f_2-10keV > 10^–15 erg s^–1 cm^–2 but rises rapidly toward fainter flux levels. The CT AGNs account for ∼38% of the total accreted supermassive black hole mass and contribute ∼25% of the cosmic X-ray background spectrum at 20 keV. Our model predicts that the majority (90%) of luminous and bright CT AGNs (L_rest2-10keV > 10⁴⁴ erg s^–1 or f_2-10keV > 10^–15 erg s^–1 cm^–2) have detectable hot dust 5-10 μm emission, which we associate with a dusty torus. The fraction drops for fainter objects, to around 30% at L_rest2-10keV > 10⁴² erg s^–1 or f_2-10keV > 10^–17 erg s^–1 cm^–2. Our model confirms that heavily obscured AGNs (N_HI > 10²³ cm^–2) can be separated from unobscured and mildly obscured ones (N_HI < 10²³ cm^–2) in the plane of observed frame X-ray hardness versus mid-IR/X-ray ratio

[en] We report the discovery of spiral galaxies that are as optically luminous as elliptical brightest cluster galaxies, with r-band monochromatic luminosity L_r = 8–14L* (4.3–7.5 × 10"4"4 erg s"−"1). These super spiral galaxies are also giant and massive, with diameter D = 57–134 kpc and stellar mass M_s_t_a_r_s = 0.3–3.4 × 10"1"1M_⊙. We find 53 super spirals out of a complete sample of 1616 SDSS galaxies with redshift z < 0.3 and L_r > 8L*. The closest example is found at z = 0.089. We use existing photometry to estimate their stellar masses and star formation rates (SFRs). The SDSS and Wide-field Infrared Survey Explorer colors are consistent with normal star-forming spirals on the blue sequence. However, the extreme masses and rapid SFRs of 5–65 M_⊙ yr"−"1 place super spirals in a sparsely populated region of parameter space, above the star-forming main sequence of disk galaxies. Super spirals occupy a diverse range of environments, from isolation to cluster centers. We find four super spiral galaxy systems that are late-stage major mergers—a possible clue to their formation. We suggest that super spirals are a remnant population of unquenched, massive disk galaxies. They may eventually become massive lenticular galaxies after they are cut off from their gas supply and their disks fade

[en] We study a 24 μm selected sample of 330 galaxies observed with the infrared spectrograph for the 5 mJy Unbiased Spitzer Extragalactic Survey. We estimate accurate total infrared luminosities by combining mid-IR spectroscopy and mid-to-far infrared photometry, and by utilizing new empirical spectral templates from Spitzer data. The infrared luminosities of this sample range mostly from 10⁹ L_sun to 10^13.5 L_sun, with 83% in the range 10¹⁰ L_sun < L_IR < 10¹² L_sun. The redshifts range from 0.008 to 4.27, with a median of 0.144. The equivalent widths of the 6.2 μm aromatic feature have a bimodal distribution, probably related to selection effects. We use the 6.2 μm polycyclic aromatic hydrocarbon equivalent width (PAH EW) to classify our objects as starburst (SB)-dominated (44%), SB-AGN composite (22%), and active galactic nucleus (AGN)-dominated (34%). The high EW objects (SB-dominated) tend to have steeper mid-IR to far-IR spectral slopes and lower L_IR and redshifts. The low EW objects (AGN-dominated) tend to have less steep spectral slopes and higher L_IR and redshifts. This dichotomy leads to a gross correlation between EW and slope, which does not hold within either group. AGN-dominated sources tend to have lower log(L_PAH7.7μm/L_PAH11.3μm) ratios than star-forming galaxies, possibly due to preferential destruction of the smaller aromatics by the AGN. The log(L_PAH7.7μm/L_PAH11.3μm) ratios for star-forming galaxies are lower in our sample than the ratios measured from the nuclear spectra of nearby normal galaxies, most probably indicating a difference in the ionization state or grain size distribution between the nuclear regions and the entire galaxy. Finally, we provide a calibration relating the monochromatic continuum or aromatic feature luminosity to L_IR for different types of objects.

[en] We present an extragalactic population model of the cosmic background light to interpret the rich high-quality survey data in the X-ray and IR bands. The model incorporates star formation and supermassive black hole (SMBH) accretion in a co-evolution scenario to fit simultaneously 617 data points of number counts, redshift distributions, and local luminosity functions (LFs) with 19 free parameters. The model has four main components, the total IR LF, the SMBH accretion energy fraction in the IR band, the star formation spectral energy distribution (SED), and the unobscured SMBH SED extinguished with a H I column density distribution. As a result of the observational uncertainties about the star formation and SMBH SEDs, we present several variants of the model. The best-fit reduced χ² reaches as small as 2.7-2.9 of which a significant amount (>0.8) is contributed by cosmic variances or caveats associated with data. Compared to previous models, the unique result of this model is to constrain the SMBH energy fraction in the IR band that is found to increase with the IR luminosity but decrease with redshift up to z ∼ 1.5; this result is separately verified using aromatic feature equivalent-width data. The joint modeling of X-ray and mid-IR data allows for improved constraints on the obscured active galactic nucleus (AGN), especially the Compton-thick AGN population. All variants of the model require that Compton-thick AGN fractions decrease with the SMBH luminosity but increase with redshift while the type 1 AGN fraction has the reverse trend.

[en] We present an analysis of far-infrared (FIR) dust emission from diffuse cirrus clouds. This study is based on serendipitous observations at 160 μm at high-galactic latitude with the Multiband Imaging Photometer onboard the Spitzer Space Telescope by the Spitzer Infrared Nearby Galaxies Survey. These observations are complemented with IRIS data at 100 and 60 μm and constitute one of the most sensitive and unbiased samples of FIR observations at a small scale of diffuse interstellar clouds. Outside regions dominated by the cosmic infrared background fluctuations, we observe a substantial scatter in the 160/100 colors from cirrus emission. We compared the 160/100 color variations to 60/100 colors in the same fields and find a trend of decreasing 60/100 with increasing 160/100. This trend cannot be accounted for by current dust models by changing solely the interstellar radiation field. It requires a significant change of dust properties such as grain size distribution or emissivity or a mixing of clouds in different physical conditions along the line of sight. These variations are important as a potential confusing foreground for extragalactic studies.

[en] We present Spitzer observations at 3.6 and 4.5 μm of the supernova SN 2011dh (PTF 11eon) in M51 from 18 days to 625 days after explosion. The mid-infrared emission peaks at 24 days after explosion at a few ×10⁷ L _☉, and decays more slowly than the visible-light bolometric luminosity. The infrared color temperature cools for the first 90 days and then is constant. Simple numerical models of a thermal echo can qualitatively reproduce the early behavior. At late times, the mid-IR light curve cannot be explained by a simple thermal echo model, suggesting additional dust heating or line emission mechanisms. We also propose that thermal echoes can serve as effective probes to uncover supernovae in heavily obscured environments, and speculate that under the right conditions, integrating the early epoch of the mid-infrared light curve may constrain the total energy in the shock breakout flash

[en] We present a search for infrared dust emission associated with the Leo cloud, a large intergalactic cloud in the M96 group. Mid-infrared and far-infrared images were obtained with the InfraRed Array Camera and the Multiband Imaging Photometer for Spitzer on the Spitzer Space Telescope. Our analysis of these maps is done at each wavelength relative to the H I spatial distribution. We observe a probable detection at 8 μm and a marginal detection at 24 μm associated with the highest H I column densities in the cloud. At 70 and 160 μm, upper limits on the dust emission are deduced. The level of the detection is low so that the possibility of a fortuitous cirrus clump or of an overdensity of extragalactic sources along the line of sight cannot be excluded. If this detection is confirmed, the quantities of dust inferred imply a dust-to-gas ratio in the intergalactic cloud up to a few times solar but no less than 1/20 solar. A confirmed detection would therefore exclude the possibility that the intergalactic cloud has a primordial origin. Instead, this large intergalactic cloud could therefore have been formed through interactions between galaxies in the group.

[en] We propose an 'extended Schmidt law' with explicit dependence of the star formation efficiency (SFE = SFR/M_gas) on the stellar mass surface density (Σ_star). This relation has a power-law index of 0.48 ± 0.04 and a 1σ observed scatter on the SFE of 0.4 dex, which holds over five orders of magnitude in the stellar density for individual global galaxies, including various types and especially the low-surface-brightness (LSB) galaxies that deviate significantly from the Kennicutt-Schmidt (KS) law. When applying it to regions of a sample of 12 spiral galaxies at sub-kiloparsec resolution, the extended Schmidt law not only holds for LSB regions but also shows significantly smaller scatters both within and across galaxies compared with the KS law. We argue that this new relation points to the role of existing stars in regulating the SFE, thus better encoding the star formation physics. Comparison with physical models of star formation recipes shows that the extended Schmidt law can be reproduced by some models including gas free fall in a stellar-gravitational potential and pressure-supported star formation. By implementing this new law into the analytic model of gas accretion in ΛCDM, we show that it can reproduce the observed main sequence of star-forming galaxies (a relation between the SFR and stellar mass) from z = 0 up to z = 2.

[en] We present new results on the physical nature of infrared-luminous sources at 0.5 < z < 2.8 as revealed by HST/NICMOS imaging and Infrared Spectrograph mid-infrared spectroscopy. Our sample consists of 134 galaxies selected at 24 μm with a flux of S(24 μm)>0.9 mJy. We find many (∼60%) of our sources to possess an important bulge and/or central point source component, most of which reveal additional underlying structures after subtraction of a best-fit Sersic (or Sersic+PSF) profile. Based on visual inspection of the NIC2 images and their residuals, we estimate that ∼80% of all our sources are mergers. We calculate lower and upper limits on the merger fraction to be 62% and 91%, respectively. At z < 1.5, we observe objects in early (pre-coalescence) merging stages to be mostly disk and star formation dominated, while we find mergers to be mainly bulge dominated and active galactic nucleus (AGN)-starburst composites during coalescence and then AGN dominated in late stages. This is analogous to what is observed in local ULIRGs. At z ≥ 1.5, we find a dramatic rise in the number of objects in pre-coalescence phases of merging, despite an increase in the preponderance of AGN signatures in their mid-IR spectra and luminosities above 10^12.5 L_sun. We further find the majority of mergers at those redshifts to retain a disk-dominated profile during coalescence. We conclude that, albeit still driven by mergers, these high-z ULIRGs are substantially different in nature from their local counterparts and speculate that this is likely due to their higher gas content. Finally, we observe obscured (τ_9.7μm>3.36) quasars to live in faint and compact hosts and show that these are likely high-redshift analogs of local dense-core mergers. We find late-stage mergers to possess predominantly unobscured AGN spectra, but do not observe other morphological classes to carry any specific combination of τ_9.7μm and polycyclic aromatic hydrocarbon (PAH) equivalent width. This suggests a high degree of variation in the PAH emission and silicate absorption properties of these mergers, and possibly throughout the merging process itself.

[en] We analyze the role played by shear in regulating star formation in the Galaxy on the scale of individual molecular clouds. The clouds are selected from the ¹³CO J = 1-0 line of the Galactic Ring Survey. For each cloud, we estimate the shear parameter which describes the ability of density perturbations to grow within the cloud. We find that for almost all molecular clouds considered, there is no evidence that shear is playing a significant role in opposing the effects of self-gravity. We also find that the shear parameter of the clouds does not depend on their position in the Galaxy. Furthermore, we find no correlations between the shear parameter of the clouds with several indicators of their star formation activity. No significant correlation is found between the shear parameter and the star formation efficiency of the clouds which is measured using the ratio of the massive young stellar objects luminosities, measured in the Red MSX survey, to the cloud mass. There are also no significant correlations between the shear parameter and the fraction of their mass that is found in denser clumps which is a proxy for their clump formation efficiency, nor with their level of fragmentation expressed in the number of clumps per unit mass. Our results strongly suggest that shear is playing only a minor role in affecting the rates and efficiencies at which molecular clouds convert their gas into dense cores and thereafter into stars.