Mid-infrared Colors Research Articles

The Far-Infrared Radio Correlation at low radio frequency with LOFAR/H-ATLAS

The radio and far-infrared luminosities of star-forming galaxies are tightly correlated over several orders of magnitude; this is known as the far-infrared radio correlation (FIRC). Previous studies have shown that a host of factors conspire to maintain a tight and linear FIRC, despite many models predicting deviation. This discrepancy between expectations and observations is concerning since a linear FIRC underpins the use of radio luminosity as a star-formation rate indicator. Using LOFAR 150MHz, FIRST 1.4 GHz, and Herschel infrared luminosities derived from the new LOFAR/H-ATLAS catalogue, we investigate possible variation in the monochromatic (250$\mathrm{\mu m}$) FIRC at low and high radio frequencies. We use statistical techniques to probe the FIRC for an optically-selected sample of 4,082 emission-line classified star-forming galaxies as a function of redshift, effective dust temperature, stellar mass, specific star formation rate, and mid-infrared colour (an empirical proxy for specific star formation rate). Although the average FIRC at high radio frequency is consistent with expectations based on a standard power-law radio spectrum, the average correlation at 150MHz is not. We see evidence for redshift evolution of the FIRC at 150MHz, and find that the FIRC varies with stellar mass, dust temperature and specific star formation rate, whether the latter is probed using MAGPHYS fitting, or using mid-infrared colour as a proxy. We can explain the variation, to within 1$\sigma$, seen in the FIRC over mid-infrared colour by a combination of dust temperature, redshift, and stellar mass using a Bayesian partial correlation technique.

Far-infrared dust properties of highly dust-obscured active galactic nuclei from the AKARI and WISE all-sky surveys

Abstract The combination of the AKARI and WISE infrared all-sky surveys provides a unique opportunity to identify and characterize the most highly dust-obscured active galactic nuclei (AGNs) in the universe. Dust-obscured AGNs are not easily detectable and are potentially underrepresented in extragalactic surveys due to their high optical extinction, but are readily found in the WISE catalog due to their extremely red mid-infrared (IR) colors. Combining these surveys with photometry from Pan-STARRS and Herschel, we use spectral energy distribution (SED) modeling to characterize the extinction and dust properties of these AGNs. From mid-IR WISE colors we are able to compute bolometric corrections to AGN luminosities. Using AKARI’s far-IR wavelength photometry and broadband AGN/galaxy spectral templates we estimate AGN dust mass and temperature using simple analytic models with three or four parameters. Even without spectroscopic data we can determine a number of AGN dust properties only using SED analysis. These methods, combined with the abundance of archival photometric data publicly available, will be valuable for large-scale studies of dusty, IR-luminous AGNs.

A Machine-learning Method for Identifying Multiwavelength Counterparts of Submillimeter Galaxies: Training and Testing Using AS2UDS and ALESS

Abstract We describe the application of supervised machine-learning algorithms to identify the likely multiwavelength counterparts to submillimeter sources detected in panoramic, single-dish submillimeter surveys. As a training set, we employ a sample of 695 (S 870μm ≳ 1 mJy) submillimeter galaxies (SMGs) with precise identifications from the ALMA follow-up of the SCUBA-2 Cosmology Legacy Survey’s UKIDSS-UDS field (AS2UDS). We show that radio emission, near-/mid-infrared colors, photometric redshift, and absolute H-band magnitude are effective predictors that can distinguish SMGs from submillimeter-faint field galaxies. Our combined radio + machine-learning method is able to successfully recover ∼85% of ALMA-identified SMGs that are detected in at least three bands from the ultraviolet to radio. We confirm the robustness of our method by dividing our training set into independent subsets and using these for training and testing, respectively, as well as applying our method to an independent sample of ∼100 ALMA-identified SMGs from the ALMA/LABOCA ECDF-South Survey (ALESS). To further test our methodology, we stack the 870 μm ALMA maps at the positions of those K-band galaxies that are classified as SMG counterparts by the machine learning but do not have a >4.3σ ALMA detection. The median peak flux density of these galaxies is S 870μm = (0.61 ± 0.03) mJy, demonstrating that our method can recover faint and/or diffuse SMGs even when they are below the detection threshold of our ALMA observations. In future, we will apply this method to samples drawn from panoramic single-dish submillimeter surveys that currently lack interferometric follow-up observations to address science questions that can only be tackled with large statistical samples of SMGs.

Discovery of 21 New Changing-look AGNs in the Northern Sky

Abstract The rare case of changing-look (CL) active galactic nuclei (AGNs), with the appearance or disappearance of broad Balmer emission lines within a few years, challenges our understanding of the AGN unified model. We present a sample of 21 new CL AGNs at 0.08 < z < 0.58 , which doubles the number of such objects known to date. These new CL AGNs were discovered in various ways, from (1) repeat spectra in the SDSS, (2) repeat spectra in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and SDSS, and (3) photometric variability and new spectroscopic observations. We use the photometric data from surveys, including the SDSS imaging survey, the Pan-STARRS1, the DESI Legacy imaging survey, the Wide-field Infrared Survey Explorer (WISE), the Catalina Real-time Transient Survey, and the Palomar Transient Factory. The estimated upper limits of the transition timescale of the CL AGNs in this sample spans from 0.9 to 13 years in the rest-frame. The continuum flux in the optical and mid-infrared becomes brighter when the CL AGNs turn on, or vice versa. Variations of more than 0.2 mag in the W1 band were detected in 15 CL AGNs during the transition. The optical and mid-infrared variability is not consistent with the scenario of variable obscuration in 10 CL AGNs at more than the 3σ confidence level. We confirm a bluer-when-brighter trend in the optical. However, the mid-infrared WISE colors W1–W2 become redder when the objects become brighter in the W1 band, possibly due to a stronger hot dust contribution in the W2 band when the AGN activity becomes stronger. The physical mechanism of type transition is important for understanding the evolution of AGNs.

Unidentified quasars among stationary objects from Gaia DR2

Here we apply a technique selecting quasar candidates purely as sources with zero proper motions in the Gaia data release 2 (DR2). We demonstrate that this approach is highly efficient toward high Galactic latitudes with ≲25% contamination from stellar sources. Such a selection technique offers very pure sample completeness, since all cosmological point sources are selected regardless of their intrinsic spectral properties within the limiting magnitude of Gaia. We carry out a pilot-study, defining a sample compiled by including all Gaia-DR2 sources within one degree of the north Galactic pole (NGP) selected to have proper motions consistent with zero within 2σ uncertainty. By cross-matching the sample to the optical Sloan Digital Sky Survey (SDSS) and the mid-infrared (MIR) AllWISE photometric catalogues, we investigate the colours of each of our sources. We determine the efficiency of our selection by comparison with previously spectroscopically confirmed quasars. The majority of the zero-proper-motion sources selected here have optical to MIR colours consistent with known quasars. The remaining population may be contaminating stellar sources, but some may also be quasars with colours similar to stars. Spectroscopic follow-up of the zero-proper-motion sources is needed to unveil such a hitherto hidden quasar population. This approach has the potential to allow substantial progress on many important questions concerning quasars, such as determining the fraction of dust-obscured quasars, the fraction of broad absorption line (BAL) quasars, and the metallicity distribution of damped Lyman-α absorbers. The technique could also potentially reveal new types of quasars or even new classes of cosmological point sources.

Planetary nebulae in the UWISH2 survey

Near-infrared imaging in the 1 - 0 S(1) emission line of molecular hydrogen is able to detect planetary nebulae (PNe) that are hidden from optical emission line surveys. We present images of 307 objects from the UWISH2 survey of the northern Galactic Plane, and with the aid of mid-infrared colour diagnostics draw up a list of 291 PN candidates. The majority, 183, are new detections and 85 per cent of these are not present in H$\alpha$ surveys of the region. We find that more than half (54 per cent) of objects have a bipolar morphology and that some objects previously considered as elliptical or point-source in H$\alpha$ imaging, appear bipolar in UWISH2 images. By considering a small subset of objects for which physical radii are available from the H$\alpha$ surface brightness-radius relation, we find evidence that the H2 surface brightness remains roughly constant over a factor 20 range of radii from 0.03 to 0.6 pc, encompassing most of the visible lifetime of a PN. This leads to the H$\alpha$ surface brightness becoming comparable to that of H2 at large radius (> 0:5 pc). By combining the number of UWISH2 PNe without H$\alpha$ detection with an estimate of the PN detection efficiency in H2 emission, we estimate that PN numbers from H$\alpha$ surveys may underestimate the true PN number by a factor between 1.5 and 2.5 within the UWISH2 survey area.

The power of infrared AGN selection in mergers: a theoretical study

The role of galaxy mergers in fueling active galactic nuclei (AGN) is still debated, owing partly to selection effects inherent to studies of the merger/AGN connection. In particular, luminous AGN are often heavily obscured in late-stage mergers. Mid-infrared (IR) color selection of dust-enshrouded AGN with, e.g., the Wide-field Infrared Survey Explorer (WISE) has uncovered large new populations of obscured AGN. However, this method is sensitive mainly to AGN that dominate emission from the host. To understand how these selection biases affect mid-IR studies of the merger/AGN connection, we simulate the evolution of obscured AGN throughout galaxy mergers. Although mid-IR colors closely trace luminous, obscured AGN, we show that nearly half of merger-triggered AGN are missed with common mid-IR selection criteria, even in late-stage, gas-rich major mergers. At z < 0.5, where merger signatures and dual nuclei can most easily be detected, we find that a more lenient W1-W2 > 0.5 cut greatly improves completeness without significantly decreasing reliability. Extreme nuclear starbursts are briefly able to mimic this AGN signature, but this is largely irrelevant in mergers, where such starbursts are accompanied by AGN. We propose a two-color cut that yields high completeness and reliability even in starbursting systems. Further, we show that mid-IR color selection very effectively identifies dual AGN hosts, with the highest fraction at the smallest separations (< 3 kpc). Thus, many merger hosts of mid-IR AGN should contain unresolved dual AGN; these are ideal targets for high-resolution follow-up, particularly with the James Webb Space Telescope.

Infrared colours and inferred masses of metal-poor giant stars in the Kepler field

Intrinsically luminous giant stars in the Milky Way are the only potential volume-complete tracers of the distant disk, bulge, and halo. The chemical abundances of metal-poor giants also reflect the compositions of the earliest star-forming regions, providing the initial conditions for the chemical evolution of the Galaxy. However, the intrinsic rarity of metal-poor giants combined with the difficulty of efficiently identifying them with broad-band optical photometry has made it difficult to exploit them for studies of the Milky Way. We re-derive a giant star photometric selection using existing public g-band and narrow-band DDO51 photometry obtained in the Kepler field. Our selection is simple and yields a contamination rate of main-sequence stars of $\lesssim$1% and a completeness of about 80% for giant stars with $T_{\rm eff} \lesssim 5250$ K - subject to the selection function of the spectroscopic surveys used to estimate these rates, and the magnitude range considered ($11\lesssim g\lesssim 15$). While the DDO51 filter is known to be sensitive to stellar surface gravity, we further show that the mid-infrared colours of DDO51-selected giants are strongly correlated with spectroscopic metallicity. This extends the infrared selection developed by Schlaufman & Casey, demonstrating that the principal contaminants in their selection can be efficiently removed by the photometric separation of dwarfs and giants. This implies that any similarly efficient dwarf/giant discriminant (e.g., Gaia) can be used with WISE colours to select samples of giant stars with high completeness and low contamination. We employ our photometric selection to identify three metal-poor giant candidates in the Kepler field with global asteroseismic parameters and find that masses inferred for these three stars using standard asteroseismic scaling relations are systematically over-estimated by 20-175%. [abridged]

An L Band Spectrum of the Coldest Brown Dwarf

Abstract The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to have an atmosphere rich in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths and emits almost all its energy in the mid-infrared. Skemer et al. presented a spectrum of WISE 0855 from 4.5–5.1 μm (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in the L band, from 3.4–4.14 μm. We present a set of atmosphere models that include a range of compositions (metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum. The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging the temperature range between the long-studied solar system planets and accessible exoplanets. The James Webb Space Telescope will soon revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared, allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and formation processes.

Long-term decline of the mid-infrared emission of normal galaxies: dust echo of tidal disruption flare?

We report the discovery of a sample of 19 low redshift (z<0.22) spectroscopically non-Seyfert galaxies that show slow declining mid-infrared (MIR) light-curves (LCs), similar to those of tidal disruption event (TDE) candidates with extreme coronal lines. Two sources also showed a relatively fast rising MIR LCs. They consist of 61% sample of the WISE MIR variable non-Seyfert galaxies with SDSS spectra. In a comparison sample of optically selected Seyfert galaxies, the fraction of sources with such a LC is only 15%. After rejecting 5 plausible obscured Seyfert galaxies with red MIR colours, remaining 14 objects are studied in detail in this paper. We fit the declining part of LC with an exponential law, and the decay time is typically one year. The observed peak MIR luminosities ($\nu L_\nu$) after subtracting host galaxies are in the range of a few 10^42 to 10^44 erg~s^-1 with a median of 5x10^43 erg~s^-1 in the W2 band. The black hole masses distribute in a wide range with more than half in between 10^7 to 10^8 ~M_sun, but significantly different from that of optical/UV selected TDEs. Furthermore, MIR luminosities are correlated with black hole masses, the stellar mass or luminosity of their host bulges. Most galaxies in the sample are red and luminous with an absolute magnitude of r between -20 to -23. We estimate the rate of event about 10^-4 gal^-1~yr^-1 among luminous red galaxies. We discuss several possibilities for the variable infrared sources, and conclude that most likely, they are caused by short sporadic fueling to the supermassive black holes via either the instability of accretion flows or tidal disruption of stars.

Star-forming Galaxies as AGN Imposters? A Theoretical Investigation of the Mid-infrared Colors of AGNs and Extreme Starbursts

Abstract We conduct for the first time a theoretical investigation of the mid-infrared spectral energy distribution (SED) produced by dust heated by an active galactic nucleus (AGN) and an extreme starburst. These models employ an integrated modeling approach using photoionization and stellar population synthesis models in which both the line and emergent continuum is predicted from gas exposed to the ionizing radiation from a young starburst and an AGN. In this work, we focus on the infrared colors from the Wide-field Infrared Survey Explorer, predicting the dependence of the colors on the input radiation field, the interstellar medium conditions, the obscuring column, and the metallicity. We find that an extreme starburst can mimic an AGN in two band mid-infrared color cuts employed in the literature. However, the three-band color cuts employed in the literature require starbursts with extremely high ionization parameters or gas densities. We show that the extreme mid-infrared colors seen in some blue compact dwarf galaxies are not due to metallicity but rather a combination of high ionization parameters and high column densities. Based on our theoretical calculations, we present a theoretical mid-infrared color cut that will exclude even the most extreme starburst that we have modeled in this work. The theoretical AGN demarcation region presented here can be used to identify elusive AGN candidates for future follow-up studies with the James Webb Space Telescope. The full suite of simulated SEDs are available online.

Radio and infrared study of southern H II regions G346.056−0.021 and G346.077−0.056

Aim. We present a multiwavelength study of two southern Galactic H II regions G346.056−0.021 and G346.077−0.056 which are located at a distance of 10.9 kpc. The distribution of ionized gas, cold and warm dust, and the stellar population associated with the two H II regions are studied in detail using measurements at near-infrared, mid-infrared, far-infrared, submillimeter and radio wavelengths. Methods. The radio continuum maps at 1280 and 610 MHz were obtained using the Giant Metrewave Radio Telescope to probe the ionized gas. The dust temperature, column density, and dust emissivity maps were generated using modified blackbody fits in the far-infrared wavelength range 160–500 μm. Various near- and mid-infrared color and magnitude criteria were adopted to identify candidate ionizing star(s) and the population of young stellar objects in the associated field. Results. The radio maps reveal the presence of diffuse ionized emission displaying distinct cometary morphologies. The 1280 MHz flux densities translate to zero age main sequence spectral types in the range O7.5V–O7V and O8.5V–O8V for the ionizing stars of G346.056−0.021 and G346.077−0.056, respectively. A few promising candidate ionizing star(s) are identified using near-infrared photometric data. The column density map shows the presence of a large, dense dust clump enveloping G346.077−0.056. The dust temperature map shows peaks towards the two H II regions. The submillimeter image shows the presence of two additional clumps, one being associated with G346.056−0.021. The masses of the clumps are estimated to range between ~1400 and 15250 M⊙. Based on simple analytic calculations and the correlation seen between the ionized gas distribution and the local density structure, the observed cometary morphology in the radio maps is better explained invoking the champagne-flow model.

Is 4C+29.48 a γ-ray source?

Context. The Fermi Large Area Telescope revealed that the extragalactic γ-ray sky is dominated by blazars, active galactic nuclei (AGN) whose jet is seen at very small angle to the line of sight. To associate and then classify the γ-ray sources, data have been collected from lower frequency surveys and observations. Since those have superior angular resolution and positional accuracy compared to the γ-ray observations, some associations are not straightforward. Aims. The γ-ray source 3FGL J1323.0+2942 is associated with the radio source 4C+29.48 and classified as a blazar of unknown type, lacking optical spectrum and redshift. The higher-resolution radio data showed that 4C+29.48 comprises three bright radio-emitting features located within a ~1′-diameter area. We aim to reveal their nature and pinpoint the origin of the γ-ray emission. Methods. We (re-)analyzed archival Very Large Array (VLA) and unpublished very long baseline interferometry (VLBI) observations conducted by the Very Long Baseline Array (VLBA) and the European VLBI Network of 4C+29.48. We also collected data form optical, infrared and X-ray surveys. Results. According to the VLBI data, the northernmost complex of 4C+29.48 contains a blazar with a high brightness temperature compact core and a steep-spectrum jet feature. The blazar is positionally coincident with an optical source at a redshift of 1.142. Its mid-infrared colors also support its association with a γ-ray emitting blazar. The two other radio complexes have steep radio spectra similar to AGN-related lobes and do not have optical or infrared counterparts in currently available surveys. Based on the radio morphology, they are unlikely to be related to the blazar. There is an optical source between the two radio features, also detected in infrared wavebands. We discuss the possibilities whether the two radio features are lobes of a radio galaxy, or gravitationally lensed images of a background source. Conclusions. We propose to associate the γ-ray source 3FGL J1323.0+2942 in subsequent versions of the Fermi catalog with the blazar residing in northernmost complex. We suggest naming this radio source J1323+2941A to avoid misinterpretation arising from the fact that the coordinates of the currently listed radio counterpart 4C+29.48 is closer to a most probably unrelated radio source.

MALS–NOT: Identifying Radio-bright Quasars for the MeerKAT Absorption Line Survey

We present a preparatory spectroscopic survey to identify radio-bright, high-redshift quasars for the MeerKAT Absorption Line Survey (MALS). The candidates have been selected on the basis of a single flux density limit at 1.4 GHz (>200 mJy) together with mid-infrared color criteria from the Wide-field Infrared Survey Explorer (WISE). Through spectroscopic observations using the Nordic Optical Telescope, we identify 72 quasars out of 99 candidates targeted. We measure the spectroscopic redshifts based on characteristic, broad emission lines present in the spectra. Of these 72 quasars, 64 and 48 objects are at sufficiently high redshift (z>0.6 and z>1.4) to be used for the L-band and UHF-band spectroscopic follow-up with the Square Kilometre Array (SKA) precursor in South Africa: the MeerKAT.

Searching for Be Stars in the Open Clusters with PTF/iPTF. I. Cluster Sample and Be Star Candidates

Abstract We conducted a search for Be star candidates in open clusters using Hα imaging photometry of the Palomar Transient Factory Survey to investigate some connections among Be star phenomena, cluster environments, and ages. Stellar members of clusters were identified by spatial distributions, near-infrared magnitudes and colors, and by proper motions. Among 104 open clusters, we identified 96 Be star candidates in 32 clusters; 11 of our candidates have been reported in previous studies. We found that the clusters with age 7.5 &lt; log(t(year)) 8.5 tend to have more Be star candidates; there is about a 40% occurrence rate within this age bin. The clusters in this age bin also tend to have a higher Be fraction N(Be)/N(Be+B-type). These results suggest that the environments of young and intermediate clusters are favorable to the formation of Be phenomena. Spatial distribution of Be star candidates with different ages implies that they do not form preferentially in the central regions. Furthermore, we showed that the mid-infrared (MIR) colors of the Be star candidates are similar to known Be stars, which could be caused by free–free emission or bound-free emission. Some Be star candidates might have no circumstellar dust according to their MIR colors. Finally, among 96 Be candidates, we discovered that one Be star candidate FSR 0904-1 exhibits long-term variability on the timescale of ∼2000 days with an amplitude of 0.2–0.3 mag, indicating a long timescale of disk evolution.

Spatially Resolved Dust, Gas, and Star Formation in the Dwarf Magellanic Irregular NGC 4449*

Abstract We investigate the relation between gas and star formation in subgalactic regions, ∼360 pc to ∼1.5 kpc in size, within the nearby starburst dwarf NGC 4449, in order to separate the underlying relation from the effects of sampling at varying spatial scales. Dust and gas mass surface densities are derived by combining new observations at 1.1 mm, obtained with the AzTEC instrument on the Large Millimeter Telescope, with archival infrared images in the range 8–500 μm from the Spitzer Space Telescope and the Herschel Space Observatory. We extend the dynamic range of our millimeter (and dust) maps at the faint end, using a correlation between the far-infrared/millimeter colors F(70)/F(1100) (and F(160)/F(1100)) and the mid-infrared color F(8)/F(24) that we establish for the first time for this and other galaxies. Supplementing our data with maps of the extinction-corrected star formation rate (SFR) surface density, we measure both the SFR–molecular gas and the SFR–total gas relations in NGC 4449. We find that the SFR–molecular gas relation is described by a power law with an exponent that decreases from ∼1.5 to ∼1.2 for increasing region size, while the exponent of the SFR–total gas relation remains constant with a value of ∼1.5 independent of region size. We attribute the molecular law behavior to the increasingly better sampling of the molecular cloud mass function at larger region sizes; conversely, the total gas law behavior likely results from the balance between the atomic and molecular gas phases achieved in regions of active star formation. Our results indicate a nonlinear relation between SFR and gas surface density in NGC 4449, similar to what is observed for galaxy samples.

Characterizing the WISE-selected heavily obscured quasar population with optical spectroscopy from the Southern African Large Telescope

We present the results of an optical spectroscopic survey of 46 heavily obscured quasar candidates. Objects are selected using their mid-infrared (mid-IR) colours and magnitudes from the Wide-Field Infrared Survey Explorer (WISE) and their optical magnitudes from the Sloan Digital Sky Survey (SDSS). Candidate Active Galactic Nuclei (AGNs) are selected to have mid-IR colours indicative of quasar activity and lie in a region of mid-IR colour space outside previously published X-ray based selection regions. We obtain optical spectra for our sample using the Robert Stobie Spectrograph on the Southern African Large Telescope. Thirty objects (65%) have identifiable emission lines, allowing for the determination of spectroscopic redshifts. Other than one object at $z\sim2.6$, candidates have moderate redshifts ranging from $z=0.1$ to $0.8$ with a median of 0.3. Twenty-one (70%) of our objects with identified redshift (46% of the whole sample) are identified as AGNs through common optical diagnostics. We model the spectral energy distributions of our sample and found that all require a strong AGN component, with an average intrinsic AGN fraction at 8$\,\mu$m of 0.91. Additionally, the fits require large extinction coefficients with an average $E(B-V)_\textrm{AGN} = 17.8$ (average $A(V)_\textrm{AGN} = 53.4$). By focusing on the area outside traditional mid-IR photometric cuts, we are able to capture and characterise a population of deeply buried quasars that were previously unattainable through X-ray surveys alone.

Composite Spectral Energy Distributions and Infrared–Optical Colors of Type 1 and Type 2 Quasars

Abstract 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 μm 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 ( W 1 – W 2 &gt; 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 – W 3 [ AB ] ) &gt; 1.4 ( W 1 – W 2 [ Vega ] ) + 3.2 ). These results illustrate the power of large statistical studies of obscured quasars selected on the basis of MIR and optical photometry.

Buried AGNs in Advanced Mergers: Mid-infrared Color Selection as a Dual AGN Candidate Finder

Abstract A direct consequence of hierarchical galaxy formation is the existence of dual supermassive black holes, which may be preferentially triggered as active galactic nuclei (AGNs) during galaxy mergers. Despite decades of searching, however, dual AGNs are extremely rare, and most have been discovered serendipitously. Using the all-sky WISE survey, we identified a population of over 100 morphologically identified interacting galaxies or mergers that display red mid-infrared colors often associated in extragalactic sources with powerful AGNs. The vast majority of these advanced mergers are optically classified as star-forming galaxies, which suggests that they may represent an obscured population of AGNs that cannot be found through optical studies. In this work, we present Chandra/ACIS observations and near-infrared spectra with the Large Binocular Telescope of six advanced mergers with projected pair separations less than ∼10 kpc. The combined X-ray, near-infrared, and mid-infrared properties of these mergers provide confirmation that four out of the six mergers host at least one AGN, with four of the mergers possibly hosting dual AGNs with projected separations less than ∼10 kpc, despite showing no firm evidence for AGNs based on optical spectroscopic studies. Our results demonstrate that (1) optical studies miss a significant fraction of single and dual AGNs in advanced mergers, and (2) mid-infrared pre-selection is extremely effective in identifying dual AGN candidates in late-stage mergers. Our multi-wavelength observations suggest that the buried AGNs in these mergers are highly absorbed, with intrinsic column densities in excess of cm−2, consistent with hydrodynamic simulations.

The physical properties of galaxies with unusually red mid-infrared colours

The goal of this paper is to investigate the physical nature of galaxies in the redshift range $0.02<z<0.15$ that have strong excess emission at mid-IR wavelengths and to determine whether they host a population of accreting black holes that cannot be identified using optical emission lines. We show that at fixed stellar mass $M_*$ and $D_n(4000)$, the distribution of [3.4]-[4.6] $\mu$m (WISE W1-W2 band) colours is sharply peaked, with a long tail to much redder W1-W2 colours. We introduce a procedure to pull out the red outlier population based on a combination of three stellar population diagnostics. When compared with optically-selected AGN, red outliers are more likely to be found in massive galaxies, and they tend to have lower stellar mass densities, younger stellar ages and higher dust content than optically-selected AGN hosts. They are twice as likely to be detected at radio wavelengths. We examine W1-W2 colour profiles for a subset of the nearest, reddest outliers and find that most are not centrally peaked, indicating that the hot dust emission is spread throughout the galaxy. We find that radio luminosity is the quantity that is most predictive of a redder central W1-W2 colour. Radio-loud galaxies with centrally concentrated hot dust emission are almost always morphologically disturbed, with compact, unresolved emission at 1.4 Ghz. Eighty percent of such systems are identifiable as AGN using optical emission line diagnostics.