arXiv subjects

Giovanni G. Fazio

Publications and source records attributed to Giovanni G. Fazio.

69 records · Page 4Linked to original sources

arXiv2006

What do we learn from IRAC observations of galaxies at 2 < z < 3.5?

We analyze very deep HST, VLT and Spitzer photometry of galaxies at 2 2 galaxies. The estimated distributions of these properties do not change significantly when adding IRAC data to the UBVIJHK photometry. However, for individual galaxies the addition of IRAC can improve the constraints on the stellar populations, especially for red galaxies: uncertainties in stellar mass decrease by a factor of 2.7 for red (U-V > 1) galaxies, but only by a factor of 1.3 for blue (U-V < 1) galaxies. We find a similar color-dependence of the improvement for estimates of age and dust extinction. In addition, the improvement from adding IRAC depends on the availability of full near-infrared JHK coverage; if only K-band were available, the mass uncertainties of blue galaxies would decrease by a more substantial factor 1.9. Finally, we find that a trend of galaxy color with stellar mass is already present at z>2. The most massive galaxies at high redshift have red rest-frame U-V colors compared to lower mass galaxies even when allowing for complex star formation histories.

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arXiv2006

The effects of cosmic rays and solar flares on the IRAC detectors: the first two years of in-flight operation

The Infrared Array Camera (IRAC) is a four-channel camera on the Spitzer Space Telescope, one of three focal plane science instruments. IRAC uses two pairs of 256x256 pixel InSb and Si:As IBC detectors to provide simultaneous imaging at 3.6, 4.5, 5.8, and 8 microns. IRAC experiences a flux of cosmic rays and solar protons that produce transient effects in science images from each of the arrays, with 4-6 pixels per second being affected during each integration. During extreme solar flares, IRAC experiences a much higher rate of transients which affects the science data quality. We present cosmic ray rates and observed detector characteristics for IRAC during the first two years of science operation, and rates observed in a period of elevated solar proton flux during an intense solar flare in January 2005. We show the changes to the IRAC detectors observed since launch, and assess their impacts to the science data quality.

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arXiv2006

Spitzer/IRAC Photometry of M, L, and T Dwarfs

We present the results of a program to acquire photometry for eighty-six 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 Teff. 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 five (~70 M_Jup to ~15 M_Jup), with the less massive objects making up the younger members of the sample. We also find the IRAC 4.5 micron fluxes to be lower than expected, from which we infer a stronger CO fundamental band at ~4.67 microns. 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 Ks, IRAC 3.6 micron, and 4.5 micron bands to provide the best color-color discrimination for a wide range of M, L, and T dwarfs. Also noteworthy is the M_Ks versus Ks-[4.5] relation, which shows a smooth progression over spectral type and splits the M, L, and T types cleanly.

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arXiv2005

The Chandra XBootes Survey - III: Optical and Near-IR Counterparts

The XBootes Survey is a 5-ks Chandra survey of the Bootes Field of the NOAO Deep Wide-Field Survey (NDWFS). This survey is unique in that it is the largest (9.3 deg^2), contiguous region imaged in X-ray with complementary deep optical and near-IR observations. We present a catalog of the optical counterparts to the 3,213 X-ray point sources detected in the XBootes survey. Using a Bayesian identification scheme, we successfully identified optical counterparts for 98% of the X-ray point sources. The optical colors suggest that the optically detected galaxies are a combination of z<1 massive early-type galaxies and bluer star-forming galaxies whose optical AGN emission is faint or obscured, whereas the majority of the optically detected point sources are likely quasars over a large redshift range. Our large area, X-ray bright, optically deep survey enables us to select a large sub-sample of sources (773) with high X-ray to optical flux ratios (f_x/f_o>10). These objects are likely high redshift and/or dust obscured AGN. These sources have generally harder X-ray spectra than sources with 0.1<f_x/f_o<10. Of the 73 X-ray sources with no optical counterpart in the NDWFS catalog, 47 are truly optically blank down to R~25.5 (the average 50% completeness limit of the NDWFS R-band catalogs). These sources are also likely to be high redshift and/or dust obscured AGN.

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arXiv2005

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

The Infrared Array Camera (IRAC) on the Spitzer Space Telescope is absolutely calibrated by comparing photometry on 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 the calibration stars. This includes corrections for location of the star in the array and the location if 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-time-scale stability of the IRAC photometry was measured by monitoring at least one secondary calibrator (near the ecliptic plane) every 12 hr while IRAC is 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 period (7.4 days) matching the optical light curve; it is possibly a Cepheid variable.

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arXiv2005

Spitzer view on the evolution of star-forming galaxies from z=0 to z~3

We use a 24 micron selected sample containing more than 8,000 sources to study the evolution of star-forming galaxies in the redshift range from z=0 to z~3. We obtain photometric redshifts for most of the sources in our survey using a method based on empirically-built templates spanning from ultraviolet to mid-infrared wavelengths. The accuracy of these redshifts is better than 10% for 80% of the sample. The derived redshift distribution of the sources detected by our survey peaks at around z=0.6-1.0 (the location of the peak being affected by cosmic variance), and decays monotonically from z~1 to z~3. We have fitted infrared luminosity functions in several redshift bins in the range 0 1.e11 L_sun) to the total SFR density increases steadily from z~0 up to z~2.5, forming at least half of the newly-born stars by z~1.5. Ultraluminous infrared galaxies (L_TIR>1.e12 L_sun) play a rapidly increasing role for z>~1.3.

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arXiv2004

IRAC Colors of Young Stellar Objects

We compare the infrared colors predicted by theoretical models of protostellar envelopes and protoplanetary disks with initial observations of young stellar objects made with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (Werner et al. 2004, Fazio et al. 2004). Disk and envelope models characterized by infall and/or accretion rates found in previous studies can quantitatively account for the range of IRAC colors found in four young embedded clusters: S 140, S 171, NGC 7129, and Cep C. The IRAC color-color diagram ([3.6]-[4.5] vs. [5.8]-[8.0]) can be used to help to distinguish between young stars with only disk emission and protostars with circumstellar envelopes.

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arXiv2004

Warm molecular gas in galaxy-galaxy merger NGC6090

We present observations of the CO 2-1 and 3-2 transitions toward the merging galaxies of NGC6090 with the Submillimeter Array (SMA) (The Submillimeter Array (SMA) is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and the Academia Sinica.). The high resolution CO data reveal three gas concentrations. The main component is peaking in the overlap region between the two galaxies, where the near-IR a nd radio continuum emission are weak. The CO 2-1 emission from the face-on galaxy NGC6090E is somewhat stronger than that from the edge-o n galaxy NGC6090W. The CO 3-2 emission peaks in the overlap region, similar to the CO 2-1 emission . More than 50% of the CO 3-2 emission arises from the 2$''$ (1.2 kpc) area of the overlap region. There appears to be CO 3-2 emission toward the nuclear region and the north-west arm of NGC6090E, while no CO 3-2 emission is detected toward NGC6090W. Unlike the CO gas, most of the radio continuum emission comes from NGC6090E. The strong CO emission, together with the weak radio continuum emission, suggests that star formation in the overlap region has not proceeded long enoug h to produce significant numbers of supernovae which would be detectable due to their radio continuum emission.

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arXiv2001

Mid-IR Observations of Mira Circumstellar Environment

This paper presents results from high-angular resolution mid-IR imaging of the Mira AB circumbinary environment using the MIRAC3 camera at the NASA Infrared Telescope Facility (IRTF). We resolved the dusty circumstellar envelope at 9.8, 11.7 and 18 micron around Mira A (o Ceti), and measured the size of the extended emission. Strong deviations from spherical symmetry are detected in the images of Mira AB system, including possible dust clumps in the direction of the companion (Mira B). These observations suggest that Mira B plays an active role in shaping the morphology of the circumstellar environment of Mira A as it evolves toward the Planetary Nebula phase.

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arXiv2001

Mid-Infrared Imaging of Candidate Vega-Like Systems

We have conducted deep mid-infrared imaging of a relatively nearby sample of candidate Vega-like stars using the OSCIR instrument on the CTIO 4-meter and Keck II 10-meter telescopes. Our discovery of a spatially-resolved disk around HR 4796A has already been reported (Jayawardhana et al. 1998). Here we present imaging observations of the other members of the sample, including the discovery that only the primary in the HD 35187 binary system appears to harbor a substantial circumstellar disk and the possible detection of extended disk emission around 49 Ceti. We derive global properties of the dust disks, place constraints on their sizes, and discuss several interesting cases in detail. Although our targets are believed to be main sequence stars, we note that several have large infrared excesses compared to prototype Vega-like systems, and may therefore be somewhat younger. The disk size constraints we derive, in many cases, imply emission from relatively large ($\gtrsim$ 10$μ$m) particles at mid-infrared wavelengths.

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arXiv2000

Spatial Distributions of Multiple Dust Components in the PPN/PN Dust Shells

We investigate spatial distributions of specific dust components in the circumstellar shells of a proto-planetary nebula candidate, HD 179821, and a planetary nebula, BD$+30^{\circ}$3639, by means of spectral imaging. With high-resolution ground-based images and ISO spectra in the mid-infrared, we can derive ``dust feature only'' maps by subtracting synthesized continuum maps from the observed images at the feature wavelength. Such spatially detailed information will help to develop models for the evolution of dust grains around evolved stars.

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arXiv2000

Mid-infrared imaging of the massive young star AFGL 2591: Probing the circumstellar environment of an outflow source

Most, if not all, stars are now believed to produce energetic outflows during their formation. Yet, almost 20 years after the discovery of bipolar outflows from young stars, the origins of this violent phenomenon are not well understood. One of the difficulties of probing the outflow process, particularly in the case of massive embedded stars, is a deficit of high spatial resolution observations. Here, we present sub-arcsecond-resolution mid-infrared images of one massive young stellar object, AFGL 2591, and its immediate surroundings. Our images, at 11.7, 12.5 and 18.0 microns, reveal a knot of emission ~6'' SW of the star, which may be evidence for a recent ejection event or an embedded companion star. This knot is roughly coincident with a previously seen near-infrared reflection nebula and a radio source, and lies within the known large-scale CO outflow. We also find a new faint NW source which may be another embedded lower-luminosity star. The IRAS mid-infrared spectrum of AFGL 2591 shows a large silicate absorption feature at 10 microns, implying that the primary source is surrounded by an optically thick dusty envelope. We discuss the interrelationship of these phenomena and suggest that mid-infrared imaging and spectroscopy provide powerful tools for probing massive star birth.

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arXiv2000

Dust in the 55 Cancri planetary system

The presence of debris disks around $\sim$ 1-Gyr-old main sequence stars suggests that an appreciable amount of dust may persist even in mature planetary systems. Here we report the detection of dust emission from 55 Cancri, a star with one, or possibly two, planetary companions detected through radial velocity measurements. Our observations at 850$μ$m and 450$μ$m imply a dust mass of 0.0008-0.005 Earth masses, somewhat higher than that in the the Kuiper Belt of our solar system. The estimated temperature of the dust grains and a simple model fit both indicate a central disk hole of at least 10 AU in radius. Thus, the region where the planets are detected is likely to be significantly depleted of dust. Our results suggest that far-infrared and sub-millimeter observations are powerful tools for probing the outer regions of extrasolar planetary systems.

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arXiv1996

Time Evolution and the Nature of the Near-Infrared Jets in GRS1915+105

We observed the galactic microquasar GRS1915+105 in the K ($2.2 μ$m) band on October 16 and 17, 1995 UTC using the COB infrared (IR) imager on the Kitt Peak National Observatory 2.1-m telescope with a 0.2-arcsec/pixel plate scale and under good ($\sim 0.7$-arcsec) seeing conditions. Using a neighboring star in the image frames to determine the point spread function (PSF), we PSF-subtract the images of GRS1915+105. We find no evidence of extended emission such as the apparent near-IR jets seen by Sams et al. (1996) in July, 1995. Simple modelling of the star + jet structure allows us to place an upper limit on any similar emission at that position of $K>16.4$ at the 95% confidence level, as compared to $K=13.9$ as seen by Sams et al. (1996). This lack of extended IR flux during continued hard X-ray flaring activity confirms the hypothesis that the extended IR emission arises from the superluminal radio-emitting jets rather than reprocessing of the X-ray emission on other structures around the compact central object. Given the large apparent velocity of the radio-emitting jets, by the time of our observations the Sams et al. feature would have moved $>1$ arcsec from GRS1915+105, and we can place a limit of $K>17.7$ (95% confidence level) on any infrared emission in this region. We can thus place an upper limit on the radiative timescale of the feature of $τ< 25$ days, which is consistent with synchrotron jet emission.

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arXiv1996

The Infrared to Gamma-Ray Pulse Shape of the Crab Nebula Pulsar

We analyze the pulse shape of the Crab Nebula pulsar in the near-infrared, optical, ultraviolet, X-ray, and gamma-ray bands, including previously unpublished ROSAT HRI observations. We show that, in addition to the previously known trend for the fluences of the Bridge and Peak 2 to increase with energy relative to the fluence of Peak 1, there is a small but statistically significant trend for both to decrease with energy relative to Peak 1 over the near-infrared range. We find that the phase separation between the two peaks of the pulse profile decreases nearly continuously as a function of energy over 7 decades of energy. We show that the peaks' full-width half-maxima are significantly variable over this energy range, but without any clear pattern to the variability. We find that the differences between the energy dependences of the leading and trailing edge half-width half-maxima of both peaks found by Eikenberry et al. (1996a) also continue over 7 decades of energy. We show that the cusped shape of Peak 2 reverses direction between the infrared/optical and X-ray/gamma-ray bands, while the cusped shape of Peak 1 shows weak evidence of reversing direction between the X-ray and gamma-ray bands. Finally, we find that many of the pulse shape parameters show maxima or minima at energies of 0.5-1 eV, implying that an important change in the pulsar emission is occuring near this energy. Many of these complex phenomena are not predicted by current pulsar emission models, and offer new challenges for the development of such models.

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