Infrared Array Camera Photometry Research Articles

Knot a Bad Idea: Testing BLISS Mapping for Spitzer Space Telescope Photometry

Much of transiting exoplanet science relies on high-precision photometry. The current generation of instruments can exhibit sensitivity variations greater than the astrophysical signals. For the InfraRed Array Camera (IRAC) on the Spitzer Space Telescope, a popular way to handle this is BiLinearly-Interpolated Subpixel Sensitivity (BLISS) mapping. As part of a Markov Chain Monte Carlo (MCMC), BLISS mapping estimates the sensitivity at many locations (knots) on the pixel, then interpolates to the target star’s centroids. We show that such embedded optimization schemes can misfit or bias parameters. Thus, we construct a model of Spitzer eclipse light curves to test the accuracy and precision of BLISS mapping. We compare standard BLISS mapping to a variant where the knots are fit during the MCMC, as well as to a polynomial model. Both types of BLISS mapping give similar eclipse depths, and we find that standard knots behave like real parameters. Standard BLISS mapping is therefore a reasonable shortcut to fitting for knots in an MCMC. BLISS maps become inaccurate when the photon noise is low, but typically approximate the real sensitivity well. We also find there is no perfect method for choosing the ideal number of BLISS knots to use on given data. BLISS mapping gives fits that are usually more accurate than precise (i.e., they are overly conservative), and the routine is more precise than polynomial models for significant eclipses or pixels with more varied sensitivities. BLISS mapping has better predictive power for most of these particular synthetic data, depending on how one treats time-correlated residuals. Overall, we conclude that BLISS mapping can be a reasonable sensitivity model for IRAC photometry.

THE SPITZER-HETDEX EXPLORATORY LARGE-AREA SURVEY

ABSTRACT We present post-cryogenic Spitzer imaging at 3.6 and 4.5 μm with the Infrared Array Camera (IRAC) of the Spitzer/HETDEX Exploratory Large-Area (SHELA) survey. SHELA covers ≈24 deg2 of the Sloan Digital Sky Survey “Stripe 82” region, and falls within the footprints of the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) and the Dark Energy Survey. The HETDEX blind R ∼ 800 spectroscopy will produce ∼200,000 redshifts from the Lyα emission for galaxies in the range 1.9 < z < 3.5, and an additional ∼200,000 redshifts from the [O ii] emission for galaxies at z < 0.5. When combined with deep ugriz images from the Dark Energy Camera, K-band images from NEWFIRM, and other ancillary data, the IRAC photometry from Spitzer will enable a broad range of scientific studies of the relationship between structure formation, galaxy stellar mass, halo mass, the presence of active galactic nuclei, and environment over a co-moving volume of ∼0.5 Gpc3 at 1.9 < z < 3.5. Here, we discuss the properties of the SHELA IRAC data set, including the data acquisition, reduction, validation, and source catalogs. Our tests show that the images and catalogs are 80% (50%) complete to limiting magnitudes of 22.0 (22.6) AB mag in the detection image, which is constructed from the weighted sum of the IRAC 3.6 and 4.5 μm images. The catalogs reach limiting sensitivities of 1.1 μJy at both 3.6 and 4.5 μm (1σ, for R = 2″ circular apertures). As a demonstration of the science, we present IRAC number counts, examples of highly temporally variable sources, and galaxy surface density profiles of rich galaxy clusters. In the spirit of the Spitzer Exploratory programs, we provide all of the images and catalogs as part of the publication.

ULTRADEEP IRAC IMAGING OVER THE HUDF AND GOODS-SOUTH: SURVEY DESIGN AND IMAGING DATA RELEASE

The IRAC ultradeep field (IUDF) and IRAC Legacy over GOODS (IGOODS) programs are two ultradeep imaging surveys at 3.6{\mu}m and 4.5{\mu}m with the Spitzer Infrared Array Camera (IRAC). The primary aim is to directly detect the infrared light of reionization epoch galaxies at z > 7 and to constrain their stellar populations. The observations cover the Hubble Ultra Deep Field (HUDF), including the two HUDF parallel fields, and the CANDELS/GOODS-South, and are combined with archival data from all previous deep programs into one ultradeep dataset. The resulting imaging reaches unprecedented coverage in IRAC 3.6{\mu}m and 4.5{\mu}m ranging from > 50 hour over 150 arcmin^2, > 100 hour over 60 sq arcmin2, to 200 hour over 5 - 10 arcmin$^2$. This paper presents the survey description, data reduction, and public release of reduced mosaics on the same astrometric system as the CANDELS/GOODS-South WFC3 data. To facilitate prior-based WFC3+IRAC photometry, we introduce a new method to create high signal-to-noise PSFs from the IRAC data and reconstruct the complex spatial variation due to survey geometry. The PSF maps are included in the release, as are registered maps of subsets of the data to enable reliability and variability studies. Simulations show that the noise in the ultradeep IRAC images decreases approximately as the square root of integration time over the range 20 - 200 hours, well below the classical confusion limit, reaching 1{\sigma} point source sensitivities as faint as of 15 nJy (28.5 AB) at 3.6{\mu}m and 18 nJy (28.3 AB) at 4.5{\mu}m. The value of such ultradeep IRAC data is illustrated by direct detections of z = 7 - 8 galaxies as faint as HAB = 28.

An empirical mass-loss law for Population II giants from theSpitzer-IRAC survey of Galactic globular clusters

The main aim of the present work is to derive an empirical mass-loss (ML) law for Population II stars in first and second ascent red giant branches. We used the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8 micron range of a carefully chosen sample of 15 Galactic globular clusters spanning the entire metallicity range and sampling the vast zoology of horizontal branch (HB) morphologies. We complemented the IRAC photometry with near-infrared data to build suitable color-magnitude and color-color diagrams and identify mass-losing giant stars. We find that while the majority of stars show colors typical of cool giants, some stars show an excess of mid-infrared light that is larger than expected from their photospheric emission and that is plausibly due to dust formation in mass flowing from them. For these stars, we estimate dust and total (gas + dust) ML rates and timescales. We finally calibrate an empirical ML law for Population II red and asymptotic giant branch stars with varying metallicity. We find that at a given red giant branch luminosity only a fraction of the stars are losing mass. From this, we conclude that ML is episodic and is active only a fraction of the time, which we define as the duty cycle. The fraction of mass-losing stars increases by increasing the stellar luminosity and metallicity. The ML rate, as estimated from reasonable assumptions for the gas-to-dust ratio and expansion velocity, depends on metallicity and slowly increases with decreasing metallicity. In contrast, the duty cycle increases with increasing metallicity, with the net result that total ML increases moderately with increasing metallicity, about 0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars, we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.

A study of the massive star-forming region M8 using images from the Spitzer Infrared Array Camera

We present photometry and images (3.6, 4.5, 5.8 and 8.0 μm) from the Spitzer Infrared Array Camera (IRAC) of the star-forming region Messier 8 (M8). The IRAC photometry reveals ongoing star formation in the M8 complex, with 64 class 0/I and 168 class II sources identified in several locations in the vicinity of submm gas cores/clumps. Nearly 60 per cent of these young stellar objects (YSOs) occur in about seven small clusters. The spatial surface density of the clustered YSOs is determined to be about 10-20 YSOs pc -2 . Fresh star formation by the process of 'collect and collapse' might have been triggered by the expanding H II regions and winds from massive stars. IRAC ratio images are generated and studied in order to identify possible diagnostic emission regions in M8. The image of 4.5/8.0 μm reveals a Brα counterpart of the optical Hourglass H II region, while the ratio 8.0/4.5 μm indicates PAH emission in a cavity-like structure to the east of the Hourglass. The ratio maps of 3.6/4.5, 5.8/4.5 and 8.0/4.5 μm seem to identify PAH emission regions in the sharp ridges and filamentary structures seen east to west and north-east to south-west in the M8 complex.

The ‘DODO’ survey — I. Limits on ultra-cool substellar and planetary-mass companions to van Maanen's star (vMa 2)

Abstract We report limits in the planetary-mass regime for companions around the nearest single white dwarf to the Sun, van Maanen's star (vMa 2), from deep J-band imaging with Gemini North and Spitzer Infrared Array Camera (IRAC) mid-IR photometry. We find no resolved common proper motion companions to vMa 2 at separations from 3 to 45 arcsec, at a limiting magnitude of J≈ 23. Assuming a total age for the system of 4.1 ± 1 Gyr, and utilizing the latest evolutionary models for substellar objects, this limit is equivalent to companion masses &amp;gt;7 ± 1 MJup(Teff≈ 300 K). Taking into account the likely orbital evolution of very low mass companions in the post-main-sequence phase, these J-band observations effectively survey orbits around the white dwarf progenitor from 3 to 50 au. There is no flux excess detected in any of the complimentary Spitzer IRAC mid-IR filters. We fit a white dwarf model atmosphere to the optical BVRI, JHK and IRAC photometry. The best solution gives Teff= 6030 ± 240 K, log g= 8.10 ± 0.04 and, hence, M= 0.633 ± 0.022 M⊙. We then place a 3σ upper limit of 10 ± 2 MJup on the mass of any unresolved companion in the 4.5 μm band.

SpitzerObservations of NGC 2362: Primordial Disks at 5 Myr

We present results from a mid-infrared imaging survey of the ~5 Myr old cluster NGC 2362 carried out with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. The archival mid-infrared data were merged with extant Hα emission data, optical and near-infrared photometry, and moderate-resolution optical spectroscopy to identify the remnant disk-bearing population of the cluster and to estimate the fraction of stars that still retain primordial circumstellar disks. The principal sample of 232 suspected cluster members with masses ranging from ~10 to 0.3 M_⊙ (B2-M5 spectral types) was drawn from known Hα emission stars, X-ray-detected stars from a single 100 ks archival Chandra observation, and established lithium-rich stars. A second sample of 153 stars over a similar mass range whose membership status was based on optical photometry alone was also examined. Measured fluxes in the optical and infrared passbands were fitted with synthetic, low-resolution spectra created using the NextGen atmospheric models, permitting the detection of infrared excesses relative to predicted stellar photospheric fluxes. Using the measured slope of the stellar spectral energy distribution through the four IRAC channels to characterize disk emission for the 195 out of 232 activity/lithium-selected stars and the 105 out of 153 photometric membership candidates having complete IRAC photometry, we derive an upper limit for the primordial, optically thick disk fraction of NGC 2362 of ~7% ± 2%, with another ~12% ± 3% of suspected members exhibiting infrared excesses indicative of weak or optically thin disk emission. The presence of circumstellar disks among candidate members of NGC 2362 is strongly mass-dependent, such that no stars more massive than ~1.2 M_⊙ exhibit significant infrared excess shortward of 8 μm. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10.7% ± 4% for stars with masses between 1.05 and 0.6 M_⊙, but the Spitzer IRAC survey is sensitivity-limited below ~0.3 M_⊙. From Hα emission-line strengths, an upper limit for the accretion fraction of the cluster is estimated at ~5%, with most suspected accretors associated with primordial, optically thick disks identified with Spitzer. The presence of primordial disk-bearing stars in NGC 2362, some of which are suspected of still experiencing gaseous accretion, may imply that even within dense cluster environments, sufficient numbers of inner disks survive to ages consistent with core accretion models of giant planet formation to account for the observed frequency of exoplanets within 5 AU of all FGKM-type stars.

SpitzerIRAC Photometry of M, L, and T Dwarfs

We present the results of a program to acquire photometry for 86 late M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We examine the behavior of these cool dwarfs in various color-color and color-magnitude diagrams composed of near-IR and IRAC data. The T dwarfs exhibit the most distinctive positions in these diagrams. In M_(5.8) versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either magnitude or color, giving the clearest indication yet that the T dwarfs are not a one-parameter family in T_(eff). Because metallicity does not vary enough in the solar neighborhood to act as the second parameter, the most likely candidate then is gravity, which in turn translates to mass. Among objects with similar spectral type, the range of mass suggested by our sample is about a factor of 5 (~70M_J to ~15M_J), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 μm fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 μm. This suggests that equilibrium CH_4/CO chemistry underestimates the abundance of CO in T dwarf atmospheres, confirming earlier results based on M-band observations from the ground. In combining IRAC photometry with near-IR JHK photometry and parallax data, we find the combination of K_s, IRAC 3.6 μm, and 4.5 μm bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M_k versus K_s-[4.5] relation, which shows a smooth progression over spectral type, and splits the M, L, and T types cleanly.

Shedding New Light on the 3C 273 Jet with theSpitzer Space Telescope

We have performed infrared imaging of the jet of the quasar 3C 273 at wavelengths of 3.6 and 5.8 μm with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. When combined with the radio, optical, and X-ray measurements, the IRAC photometry of the X-ray-bright jet knots clearly shows that the optical emission is dominated by the high-energy emission component of the jet, not by the radio synchrotron component, as had been assumed to date. The high-energy component, represented by a power law from the optical through X-ray, may be due to a second synchrotron component or to inverse Compton scattering of ambient photons. In the former case, we argue that the acceleration of protons exceeding energies of Ep ~ 1016 eV or possibly even to Ep ~ 1019 eV would be taking place in the jet knots of 3C 273, assuming that the acceleration time is proportional to the particle gyroradius. In contrast, the inverse Compton model, into which highly relativistic Doppler beaming has to be incorporated, requires very low energy electrons of Ee ~ 1 MeV in the jet knots. The present polarization data in the radio and optical would favor the former interpretation in the case of the 3C 273 jet. Sensitive and detailed measurements of optical polarization are important in order to establish the radiation mechanism responsible for the high-energy emission. The present study offers new clues as to the controversial origin of the X-ray emission seen in many quasar jets.

A Survey for New Members of Taurus with theSpitzer Space Telescope

We present the results of a search for new members of the Taurus star-forming region using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope. With IRAC images of 29.7 deg2 of Taurus at 3.6, 4.5, 5.8, and 8.0 μm, we have identified sources with red mid-infrared colors indicative of disk-bearing objects and have obtained optical and infrared spectra of 23 of these candidate members. Through this work, we have discovered 13 new members of Taurus, two of which have spectral types later than M6 and thus are likely to be brown dwarfs, according to the theoretical evolutionary models of Chabrier & Baraffe. This survey indicates that the previous census of Taurus has a completeness of ~80% for members with disks. The new members that we have found do not significantly modify the previously measured distributions of Taurus members as a function of position, mass, and extinction. For instance, we find no evidence for a population of highly reddened brown dwarfs (AK ~ 2) that has been missed by previous optical and near-infrared surveys, which suggests that brown dwarf disks are not significantly more flared than disks around stars. In addition to the new members, we also present IRAC photometry for the 149 previously known members that appear within this survey, which includes 27 objects later than M6.

Absolute Calibration of the Infrared Array Camera on the Spitzer Space Telescope

ABSTRACTThe Infrared Array Camera (IRAC) on the Spitzer Space Telescope is absolutely calibrated by comparing photometry of a set of A stars near the north ecliptic pole to predictions based on ground‐based observations and a stellar atmosphere model. The brightness of point sources is calibrated to an accuracy of 3%, relative to models for A‐star stellar atmospheres, for observations performed and analyzed in the same manner as for the calibration stars. This includes corrections for the location of the star in the array and the location of the centroid within the peak pixel. Long‐term stability of the IRAC photometry was measured by monitoring the brightness of A dwarfs and K giants (near the north ecliptic pole) observed several times per month; the photometry is stable to 1.5% (rms) over a year. Intermediate‐timescale stability of the IRAC photometry was measured by monitoring at least one secondary calibrator (near the ecliptic plane) every 12 hr while IRAC was in nominal operations; the intermediate‐term photometry is stable, with a 1% dispersion (rms). One of the secondary calibrators was found to have significantly time‐variable (5%) mid‐infrared emission, with a period (7.4 days) matching the optical light curve; it is possibly a Cepheid variable.

A Spitzer/IRAC survey of the Orion Molecular Clouds

We present initial results from a survey of the Orion A and B molecular clouds made with the InfraRed Array Camera (IRAC) onboard the Spitzer Space Telescope. This survey encompasses a total of 5.6 square degrees with the sensitivity to detect objects below the hydrogen burning limit at an age of 1 Myr. These observations cover a number of known star forming regions, from the massive star forming clusters in the Orion Nebula and NGC 2024, to small groups of low mass stars in the L1641. We combine the IRAC photometry with photometry from the 2MASS point source catalog and use the resulting seven band data to identify stars with infrared excesses due to dusty disks and envelopes. Using the presence of an infrared excess as an indicator of youth, we show the distribution of young stars and protostars in the two molecular clouds. We find that roughly half of the stars are found in dense clusters surrounding the two regions of recent massive star formation in the Orion clouds, NGC 2024 and the Orion Nebula.

IRAC Mid-Infrared Imaging of the Hubble Deep Field-South: Star Formation Histories and Stellar Masses of Red Galaxies at z &gt; 2

We present deep 3.6-8 μm imaging of the Hubble Deep Field-South with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We study distant red galaxies (DRGs) at z > 2 selected by Js-Ks > 2.3 and compare them with a sample of Lyman break galaxies (LBGs) at z = 2-3. The observed UV-to-8 μm spectral energy distributions are fitted with stellar population models to constrain star formation histories and derive stellar masses. We find that 70% of the DRGs are best described by dust-reddened star-forming models and 30% are very well fitted with old and "dead" models. Using only the I-Ks and Ks-4.5 μm colors, we can effectively separate the two groups. The dead systems are among the most massive at z ~ 2.5 (mean stellar mass ⟨M*⟩ = 0.8 × 1011 M☉) and likely formed most of their stellar mass at z > 5. To a limit of 0.5 × 1011 M☉, their number density is ~10 times lower than that of local early-type galaxies. Furthermore, we use the IRAC photometry to derive rest-frame near-infrared J, H, and K fluxes. The DRGs and LBGs together show a large variation (a factor of 6) in the rest-frame K-band mass-to-light ratios (M/LK), implying that even a Spitzer 8 μm-selected sample would be very different from a mass-selected sample. The average M/LK of the DRGs is about 3 times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/LK values and ages of the two samples appear to correlate with derived stellar mass, with the most massive galaxies being the oldest and having the highest mass-to-light ratios, similar to what is found in the low-redshift universe.

Spitzer Space Telescope Observations of the Aftermath of Microlensing Event MACHO‐LMC‐5

We have carried out photometry of the microlensing event MACHO-LMC-5 with Spitzer Infrared Array Camera (IRAC) 10 years after the magnification of the LMC source star was recorded. This event is unique in the annals of gravitational microlensing: the lensing star itself has been observed using the Hubble Space Telescope (once with WFPC2 and twice with ACS/HRC). Since the separation between the source and lens at the epoch of the Spitzer observations was ~024, the two stars cannot be resolved in the Spitzer images. However, the IRAC photometry clearly establishes that the lens is an M5 dwarf star from its infrared excess, which in turn yields a mass of ~0.2 M⊙. This demonstrates the potential of Spitzer to detect the lenses in other gravitational microlensing events.