arXiv subjects

G. H. Rieke

Publications and source records attributed to G. H. Rieke.

At least 37 records · Page 2Linked to original sources

arXiv2015

The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System

We describe the layout and unique features of the focal plane system for MIRI. We begin with the detector array and its readout integrated circuit (combining the amplifier unit cells and the multiplexer), the electronics, and the steps by which the data collection is controlled and the output signals are digitized and delivered to the JWST spacecraft electronics system. We then discuss the operation of this MIRI data system, including detector readout patterns, operation of subarrays, and data formats. Finally, we summarize the performance of the system, including remaining anomalies that need to be corrected in the data pipeline.

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arXiv2015

The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity

We present an estimate of the performance that will be achieved during on orbit operations of the JWST Mid Infrared Instrument, MIRI. The efficiency of the main imager and spectrometer systems in detecting photons from an astronomical target are presented, based on measurements at sub-system and instrument level testing, with the end-to-end transmission budget discussed in some detail. The brightest target fluxes that can be measured without saturating the detectors are provided. The sensitivity for long duration observations of faint sources is presented in terms of the target flux required to achieve a signal to noise ratio of 10 after a 10,000 second observation. The algorithms used in the sensitivity model are presented, including the understanding gained during testing of the MIRI Flight Model and flight-like detectors.

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arXiv2015

The Mid-Infrared Instrument for the James Webb Space Telescope, X. Operations and Data Reduction

We describe the operations concept and data reduction plan for the Mid- Infrared Instrument (MIRI) for the James Webb Space Telescope (JWST). The overall JWST operations concept is to use Observation Templates (OTs) to provide a straightforward and intuitive way for users to specify observations. MIRI has four OTs that correspond to the four observing modes: 1.) Imaging, 2.) Coronagraphy, 3.) Low Resolution Spectroscopy, and 4.) Medium Resolution Spectroscopy. We outline the user choices and expansion of these choices into detailed instrument operations. The data reduction plans for MIRI are split into three stages, where the specificity of the reduction steps to the observation type increases with stage. The reduction starts with integration ramps: stage 1 yields uncalibrated slope images; stage 2 calibrates the slope images; and then stage 3 combines multiple calibrated slope images into high level data products (e.g. mosaics, spectral cubes, and extracted source information). Finally, we give examples of the data and data products that will be derived from each of the four different OTs.

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arXiv2015

The Relation between Luminous AGNs and Star Formation in Their Host Galaxies

We study the relation of active galactic nuclei (AGNs) to star formation in their host galaxies. Our sample includes 205 Type-1 and 85 Type-2 AGNs, 162 detected with Herschel, from fields surrounding 30 galaxy clusters in the Local Cluster Substructure Survey (LoCuSS). The sample is identified by optical line widths and ratios after selection to be brighter than 1 mJy at 24 microns. We show that Type-2 AGN [OIII]5007 line fluxes at high z can be contaminated by their host galaxies with typical spectrograph entrance apertures (but our sample is not compromised in this way). We use spectral energy distribution (SED) templates to decompose the galaxy SEDs and estimate star formation rates, AGN luminosities, and host galaxy stellar masses (described in an accompanying paper). The AGNs arise from massive black holes (~ 3 X 10^8 Msun) accreting at ~ 10% of the Eddington rate and residing in galaxies with stellar mass > 3 X 10^{10} Msun; those detected with Herschel have IR luminosity from star formation in the range of 10^{10} -- 10^{12} Lsun. We find that: 1.) the specific star formation rates in the host galaxies are generally consistent with those of normal star-forming (main sequence) galaxies; 2.) there is a strong correlation between the luminosities from star formation and the AGN; and 3.) however, the correlation may not result from a causal connection, but could arise because the black hole mass (and hence AGN Eddington luminosity) and star formation are both correlated with the galaxy mass.

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arXiv2014

Time-monitoring Observations of Br$γ$ Emission from Young Stars

We present multiple epochs of near-IR spectroscopy for a sample of 25 young stars, including T Tauri, Herbig Ae/Be, and FU Ori objects. Using the FSPEC instrument on the Bok 90-inch telescope, we obtained K-band spectra of the BrGamma transition of hydrogen, with a resolution of ~3500. Epochs were taken over a span of >1 year, sampling time-spacings of roughly one day, one month, and one year. The majority of our targets show BrGamma emission, and in some cases these are the first published detections. Time-variability is seen in approximately half of the targets showing BrGamma emission. We compare the observed variability with expectations for rotationally-modulated accretion onto the central stars and time-variable continuum emission or extinction from matter in the inner disk. Our observations are not entirely consistent with models of rotationally-modulated magnetospheric accretion. Further monitoring, over a larger number of epochs, will facilitate more quantitative constraints on variability timescales and amplitudes, and a more conclusive comparison with theoretical models.

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arXiv2014

The Decay of Debris Disks around Solar-Type Stars

We present a Spitzer MIPS study of the decay of debris disk excesses at 24 and 70 $μ$m for 255 stars of types F4 - K2. We have used multiple tests, including consistency between chromospheric and X-ray activity and placement on the HR diagram, to assign accurate stellar ages. Within this spectral type range, at 24 $μ$m, $13.6 \pm 2.8 \%$ of the stars younger than 5 Gyr have excesses at the 3$σ$ level or more, while none of the older stars do, confirming previous work. At 70 $μ$m, $22.5 \pm 3.6\%$ of the younger stars have excesses at $ \ge$ 3 $σ$ significance, while only $4.7^{+3.7}_{-2.2}$% of the older stars do. To characterize the far infrared behavior of debris disks more robustly, we double the sample by including stars from the DEBRIS and DUNES surveys. For the F4 - K4 stars in this combined sample, there is only a weak (statistically not significant) trend in the incidence of far infrared excess with spectral type (detected fractions of 21.9$^{+4.8}_{-4.3}\%$, late F; 16.5$^{+3.9}_{-3.3}\%$, G; and 16.9$^{+6.3}_{-5.0}\%$, early K). Taking this spectral type range together, there is a significant decline between 3 and 4.5 Gyr in the incidence of excesses with fractional luminosities just under $10^{-5}$. There is an indication that the timescale for decay of infrared excesses varies roughly inversely with the fractional luminosity. This behavior is consistent with theoretical expectations for passive evolution. However, more excesses are detected around the oldest stars than is expected from passive evolution, suggesting that there is late-phase dynamical activity around these stars.

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arXiv2013

Time-monitoring Observations of the Ro-Vibrational Overtone CO bands in Young Stars

We present near-IR spectra of a sample of T Tauri, Herbig Ae/Be, and FU Ori objects. Using the FSPEC instrument on the Bok 90-inch telescope, we obtained K-band spectra with a resolution of ~3500. Here we present spectra of the v=2->0 and v=3->1 bandheads of ro-vibrational transitions of carbon monoxide. We observed these spectra over multiple epochs spaced by a few days and approximately one month. Several of our targets show CO emission or absorption features. However we see little evidence of variability in these features across multiple epochs. We compare our results with previous observations, and discuss the physical implications of non-variable CO emission across the sampled timescales.

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arXiv2013

Mid-Infrared Determination of Total Infrared Luminosity and Star Formation Rates of Local and High-Redshift Galaxies

We demonstrate estimating the total infrared luminosity, LIR, and star formation rates (SFRs) of star-forming galaxies at redshift 0 < z < 2.8 from single-band 24 micron observations, using local spectral energy distributions (SED) templates without introducing additional free parameters. Our method is based on characterizing the SEDs of galaxies as a function of their LIR surface density, which is motivated by the indications that the majority of IR luminous star-forming galaxies at 1 < z < 3 have extended star-forming regions, in contrast to the strongly nuclear concentrated, merger-induced starbursts in local luminous and ultraluminous IR galaxies. We validate our procedure for estimating LIR by comparing the resulting LIRs with those measured from far-IR observations at 0 < z < 2.8. AGNs were excluded using X-ray and 3.6-8.0 micron observations, which are generally available in deep cosmological survey fields. The Gaussian fits to the distribution of the discrepancies between the LIR measurements from single-band 24 micron and Herschel observations have sigma < 0.1 dex, with ~10% of objects disagreeing by more than 0.2 dex. Since the 24 micron estimates are based on SEDs for extended galaxies, this agreement suggests that ~90% of IR galaxies at high z are indeed much more physically extended than local counterparts of similar LIR, consistent with recent independent studies of the fractions of galaxies forming stars in the main-sequence and starburst modes, respectively. Because we have not introduced empirical corrections to enhance these estimates, in principle, our method should be applicable to lower luminosity galaxies. This will enable use of the 21 micron band of the Mid-Infrared Instrument (MIRI) on board the JWST to provide an extremely sensitive tracer of obscured SFR in individual star-forming galaxies across the peak of the cosmic star formation history.

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arXiv2013

Asteroid Belts in Debris Disk Twins: VEGA and FOMALHAUT

Vega and Fomalhaut, are similar in terms of mass, ages, and global debris disk properties; therefore, they are often referred as "debris disk twins". We present Spitzer 10-35 um spectroscopic data centered at both stars, and identify warm, unresolved excess emission in the close vicinity of Vega for the first time. The properties of the warm excess in Vega are further characterized with ancillary photometry in the mid infrared and resolved images in the far-infrared and submillimeter wavelengths. The Vega warm excess shares many similar properties with the one found around Fomalhaut. The emission shortward of ~30 um from both warm components is well described as a blackbody emission of ~170 K. Interestingly, two other systems, eps Eri and HR 8799, also show such an unresolved warm dust using the same approach. These warm components may be analogous to the solar system's zodiacal dust cloud, but of far greater. The dust temperature and tentative detections in the submillimeter suggest the warm excess arises from dust associated with a planetesimal ring located near the water-frost line and presumably created by processes occurring at similar locations in other debris systems as well. We also review the properties of the 2 um hot excess around Vega and Fomalhaut, showing that the dust responsible for the hot excess is not spatially associated with the dust we detected in the warm belt. We suggest it may arise from hot nano grains trapped in the magnetic field of the star. Finally, the separation between the warm and cold belt is rather large with an orbital ratio >~10 in all four systems. In light of the current upper limits on the masses of planetary objects and the large gap, we discuss the possible implications for their underlying planetary architecture, and suggest that multiple, low-mass planets likely reside between the two belts in Vega and Fomalhaut.

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arXiv2012

LBT and Spitzer Spectroscopy of Star-Forming Galaxies at 1 < z < 3: Extinction and Star Formation Rate Indicators

We present spectroscopic observations in the rest-frame optical and near- to mid-infrared wavelengths of four gravitationally lensed infrared (IR) luminous star-forming galaxies at redshift 1 < z < 3 from the LUCIFER instrument on the Large Binocular Telescope and the Infrared Spectrograph on Spitzer. The sample was selected to represent pure, actively star-forming systems, absent of active galactic nuclei. The large lensing magnifications result in high signal-to-noise spectra that can probe faint IR recombination lines, including Pa-alpha and Br-alpha at high redshifts. The sample was augmented by three lensed galaxies with similar suites of unpublished data and observations from the literature, resulting in the final sample of seven galaxies. We use the IR recombination lines in conjunction with H-alpha observations to probe the extinction, Av, of these systems, as well as testing star formation rate (SFR) indicators against the SFR measured by fitting spectral energy distributions to far-IR photometry. Our galaxies occupy a range of Av from ~0 to 5.9 mag, larger than previously known for a similar range of IR luminosities at these redshifts. Thus, estimates of SFR even at z ~ 2 must take careful count of extinction in the most IR luminous galaxies. We also measure extinction by comparing SFR estimates from optical emission lines with those from far-IR measurements. The comparison of results from these two independent methods indicates a large variety of dust distribution scenarios at 1 < z < 3. Without correcting for dust extinction, the H-alpha SFR indicator underestimates the SFR; the size of the necessary correction depends on the IR luminosity and dust distribution scenario. Individual SFR estimates based on the 6.2 micron PAH emission line luminosity do not show a systematic discrepancy with extinction, although a considerable, ~0.2 dex scatter is observed.

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arXiv2012

Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems

This paper describes Herschel observations of the nearby (8.5pc) G5V multi-exoplanet host star 61 Vir at 70-500micron carried out by the DEBRIS survey. These reveal emission that is extended out to >15arcsec with a morphology that can be fitted by a nearly edge-on (77deg inclination) radially broad (from 30AU to >100AU) debris disk of fractional luminosity 2.7x10^-5, with two unrelated sources nearby that are more prominent at longer wavelengths. Chance alignment with a background object seen at 1.4GHz provides potential for confusion, but the star's 1.4"/yr proper motion allows Spitzer 70micron images to confirm that what we are interpreting as disk emission really is circumstellar. Although the exact shape of the disk's inner edge is not well constrained, the region inside 30AU must be significantly depleted in planetesimals. This is readily explained if there are additional planets in the 0.5-30AU region, but is also consistent with collisional erosion. We also find tentative evidence that the presence of detectable debris around nearby stars correlates with the presence of the lowest mass planets that are detectable in current radial velocity surveys. Out of an unbiased sample of the nearest 60 G stars, 11 are known to have planets, of which 6 (including 61 Vir) have planets that are all less massive than Saturn, and 4 of these have evidence for debris. The debris toward one of these planet-hosts (HD20794) is reported here for the first time. This fraction (4/6) is higher than that expected for nearby field stars (15%), and implies that systems that form low-mass planets are also able to retain bright debris disks. We suggest that this correlation could arise because such planetary systems are dynamically stable and include regions that are populated with planetesimals in the formation process where the planetesimals can remain unperturbed over Gyr timescales.

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arXiv2011

The Nature of Star Formation at 24 microns in the Group Environment at 0.3 < z < 0.55

Galaxy star formation rates (SFRs) are sensitive to the local environment; for example, the high-density regions at the cores of dense clusters are known to suppress star formation. It has been suggested that galaxy transformation occurs largely in groups, which are the intermediate step in density between field and cluster environments. In this paper, we use deep MIPS 24 micron observations of intermediate-redshift (0.3 < z < 0.55) group and field galaxies from the Group Environment and Evolution Collaboration (GEEC) subset of the Second Canadian Network for Observational Cosmology (CNOC2) survey to probe the moderate-density environment of groups, wherein the majority of galaxies are found. The completeness limit of our study is log(L_TIR (L_sun)) > 10.5, corresponding to SFR > 2.7 M_sun/yr. We find that the group and field galaxies have different distributions of morphologies and mass. However, individual group galaxies have star-forming properties comparable to those of field galaxies of similar mass and morphology; that is, the group environment does not appear to modify the properties of these galaxies directly. There is a relatively large number of massive early-type group spirals, along with E/S0 galaxies, that are forming stars above our detection limit. These galaxies account for the nearly comparable level of star-forming activity in groups as compared with the field, despite the differences in mass and morphology distributions between the two environments. The distribution of specific SFRs (SFR/M_*) is shifted to lower values in the groups, reflecting the fact that groups contain a higher proportion of massive and less active galaxies. Considering the distributions of morphology, mass, and SFR, the group members appear to lie between field and cluster galaxies in overall properties.

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arXiv2011

Hubble and Spitzer Space Telescope Observations of the Debris Disk around the Nearby K Dwarf HD 92945

[ABRIDGED] We present the first resolved images of the debris disk around the nearby K dwarf HD 92945. Our F606W (V) and F814W (I) HST/ACS coronagraphic images reveal an inclined, axisymmetric disk consisting of an inner ring 2".0-3".0 (43-65 AU) from the star and an extended outer disk whose surface brightness declines slowly with increasing radius 3".0-5".1 (65-110 AU) from the star. A precipitous drop in the surface brightness beyond 110 AU suggests that the outer disk is truncated at that distance. The radial surface-density profile is peaked at both the inner ring and the outer edge of the disk. The dust in the outer disk scatters neutrally but isotropically, and it has a low V-band albedo of 0.1. We also present new Spitzer MIPS photometry and IRS spectra of HD 92945. These data reveal no infrared excess from the disk shortward of 30 micron and constrain the width of the 70 micron source to < 180 AU. Assuming the dust comprises compact grains of astronomical silicate with a surface-density profile described by our scattered-light model of the disk, we successfully model the 24-350 micron emission with a minimum grain size of a_min = 4.5 micron and a size distribution proportional to a^-3.7 throughout the disk, but with a maximum grain size of 900 micron in the inner ring and 50 micron in the outer disk. Our observations indicate a total dust mass of ~0.001 M_earth. However, they provide contradictory evidence of the dust's physical characteristics: its neutral V-I color and lack of 24 micron emission imply grains larger than a few microns, but its isotropic scattering and low albedo suggest a large population of submicron-sized grains. The dynamical causes of the disk's morphology are unclear, but recent models of dust creation and transport in the presence of migrating planets indicate an advanced state of planet formation around HD 92945.

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arXiv2011

Absolute Flux Calibration of the IRAC Instrument on the Spitzer Space Telescope using Hubble Space Telescope Flux Standards

The absolute flux calibration of the James Webb Space Telescope will be based on a set of stars observed by the Hubble and Spitzer Space Telescopes. In order to cross-calibrate the two facilities, several A, G, and white dwarf (WD) stars are observed with both Spitzer and Hubble and are the prototypes for a set of JWST calibration standards. The flux calibration constants for the four Spitzer IRAC bands 1-4 are derived from these stars and are 2.3, 1.9, 2.0, and 0.5% lower than the official cold-mission IRAC calibration of Reach et al. (2005), i.e. in agreement within their estimated errors of ~2%. The causes of these differences lie primarily in the IRAC data reduction and secondarily in the SEDs of our standard stars. The independent IRAC 8 micron band-4 fluxes of Rieke et al. (2008) are about 1.5 +/- 2% higher than those of Reach et al. and are also in agreement with our 8 micron result.

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arXiv2010

Forming the first planetary systems: debris around Galactic thick disc stars

The thick disc contains stars formed within the first Gyr of Galactic history, and little is known about their planetary systems. The Spitzer MIPS instrument was used to search 11 of the closest of these old low-metal stars for circumstellar debris, as a signpost that bodies at least as large as planetesimals were formed. A total of 22 thick disc stars has now been observed, after including archival data, but dust is not found in any of the systems. The data rule out a high incidence of debris among star systems from early in the Galaxy's formation. However, some stars of this very old population do host giant planets, at possibly more than the general incidence among low-metal Sun-like stars. As the Solar System contains gas giants but little cometary dust, the thick disc could host analogue systems that formed many Gyr before the Sun.

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arXiv2010

The AGN, Star-Forming, and Morphological Properties of Luminous IR-Bright/Optically-Faint Galaxies

We present the AGN, star-forming, and morphological properties of a sample of 13 MIR-luminous (f(24) > 700uJy) IR-bright/optically-faint galaxies (IRBGs, f(24)/f(R) > 1000). While these z~2 sources were drawn from deep Chandra fields with >200 ks X-ray coverage, only 7 are formally detected in the X-ray and four lack X-ray emission at even the 2 sigma level. Spitzer IRS spectra, however, confirm that all of the sources are AGN-dominated in the mid-IR, although half have detectable PAH emission responsible for ~25% of their mid-infrared flux density. When combined with other samples, this indicates that at least 30-40% of luminous IRBGs have star-formation rates in the ULIRG range (~100-2000 Msun/yr). X-ray hardness ratios and MIR to X-ray luminosity ratios indicate that all members of the sample contain heavily X-ray obscured AGN, 80% of which are candidates to be Compton-thick. Furthermore, the mean X-ray luminosity of the sample, log L(2-10 keV)(ergs/s)=44.6, indicates that these IRBGs are Type 2 QSOs, at least from the X-ray perspective. While those sources most heavily obscured in the X-ray are also those most likely to display strong silicate absorption in the mid-IR, silicate absorption does not always accompany X-ray obscuration. Finally, ~70% of the IRBGs are merger candidates, a rate consistent with that of sub-mm galaxies (SMGs), although SMGs appear to be physically larger than IRBGs. These characteristics are consistent with the proposal that these objects represent a later, AGN-dominated, and more relaxed evolutionary stage following soon after the star-formation-dominated one represented by the SMGs.

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arXiv2010

Improving the identification of high-z Herschel sources with position priors and optical/NIR and FIR/mm photometric redshifts

We present preliminary results about the detection of high redshift (U)LIRGs in the Bullet cluster field by the PACS and SPIRE instruments within the Herschel Lensing Survey (HLS) Program. We describe in detail a photometric procedure designed to recover robust fluxes and deblend faint Herschel sources near the confusion noise. The method is based on the use of the positions of Spitzer/MIPS 24 um sources as priors. Our catalogs are able to reliably (5 sigma) recover galaxies with fluxes above 6 and 10 mJy in the PACS 100 and 160 um channels, respectively, and 12 to 18 mJy in the SPIRE bands. We also obtain spectral energy distributions covering the optical through the far-infrared/millimeter spectral ranges of all the Herschel detected sources, and analyze them to obtain independent estimations of the photometric redshift based on either stellar population or dust emission models. We exemplify the potential of the combined use of Spitzer position priors plus independent optical and IR photometric redshifts to robustly assign optical/NIR counterparts to the sources detected by Herschel and other (sub-)mm instruments.

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arXiv2010

Deep Herschel view of obscured star formation in the Bullet cluster

We use deep, five band (100-500um) data from the Herschel Lensing Survey (HLS) to fully constrain the obscured star formation rate, SFR_FIR, of galaxies in the Bullet cluster (z=0.296), and a smaller background system (z=0.35) in the same field. Herschel detects 23 Bullet cluster members with a total SFR_FIR = 144 +/- 14 M_sun yr^-1. On average, the background system contains brighter far-infrared (FIR) galaxies, with ~50% higher SFR_FIR (21 galaxies; 207 +/- 9 M_sun yr^-1). SFRs extrapolated from 24um flux via recent templates (SFR_24) agree well with SFR_FIR for ~60% of the cluster galaxies. In the remaining ~40%, SFR_24 underestimates SFR_FIR due to a significant excess in observed S_100/S_24 (rest frame S_75/S_18) compared to templates of the same FIR luminosity.

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