Brant O. Nelson

The Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 � m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 � m; 4.5 and 8 � m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.

Dwarfs Cooler than “M”: The Definition of Spectral Type “L” Using Discoveries from the 2-Micron All-Sky Survey (2MASS)*

Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the first 371 sq. deg. of actual 2MASS survey data, we have identified another twenty such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W.M. Keck Observatory.

67 Additional L Dwarfs Discovered by the Two Micron All Sky Survey

We present JHKs photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey. One of the goals of this new search was to locate more examples of the latest L dwarfs. Of the 67 new discoveries, 17 have types of L6 or later. Analysis of these new discoveries shows that Hα emission has yet to be convincingly detected in any L dwarf later than type L4.5, indicating a decline or absence of chromospheric activity in the latest L dwarfs. Further analysis shows that 16 (and possibly four more) of the new L dwarfs are lithium brown dwarfs and that the average line strength for those L dwarfs showing lithium increases until type ~L6.5 V, then declines for later types. This disappearance may be the first sign of depletion of atomic lithium as it begins to form into lithium-bearing molecules. Another goal of the search was to locate nearer, brighter L dwarfs of all subtypes. Using absolute magnitudes for 17 L dwarf systems with trigonometric parallax measurements, we develop spectrophotometric relations to estimate distances to the other L dwarfs. Of the 67 new discoveries, 21 have photometric distances placing them within 25 pc of the Sun. A table of all known L and T dwarfs believed to lie within 25 pc—53 in total — is also presented. Using the distance measurement of the coolest L dwarf known, we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350 K and probably much less. If the transition region between the two classes spans a very small temperature interval, this would explain why no transition objects have yet been uncovered. This evidence, combined with model fits to low-resolution spectra of late M and early L dwarfs, indicates that L-class objects span the range 1300 K Teff 2000 K. The near-infrared color-color diagram shows that L dwarfs fall along a natural, redder extension of the well-known M dwarf track. These near-infrared colors get progressively redder for later spectral types, with the L dwarf sequence abruptly ending near (J-H, H-Ks, J-Ks) ≈ (1.3, 0.8, 2.1).

The peculiar Type IA SN 1991T - Detonation of a white dwarf?

SN 1991T was a peculiar object whose premaximum optical spectrum did not resemble that of any known supernova; it appears to have been dominated by lines of iron-group elements. Near maximum brightness, however, lines of intermediate-mass elements slowly appeared, and the spectrum began to resemble that of Type Ia supernovae (SNs Ia). With time, the spectral similarity to classical SNs Ia grew progressively stronger. Two months after the explosion, the spectrum was once again dominated by iron-group elements and appeared almost identical to that of typical SNs Ia

The X-Ray Properties of 2MASS Red Active Galactic Nuclei

The Two Micron All Sky Survey is finding previously unidentified, luminous, red, active galactic nuclei (AGNs). This new sample has a space density similar to, or greater than, previously known AGNs, suggesting that a large fraction of the overall population has been missed. Chandra observations of a well-defined subset of these objects reveal that all are X-ray-faint, with the reddest sources being the faintest in X-rays. The X-ray hardness ratios cover a wide range, generally indicating NH ~ 1021-1023 cm-2, but the softest sources show no spectral evidence for intrinsic absorption. These characteristics suggest that a mix of absorbed, direct emission and unabsorbed, scattered, and/or extended emission contributes to the X-ray flux, although we cannot rule out the possibility that they are intrinsically X-ray-weak. This population of X-ray-faint, predominantly broad-line objects could provide the missing population of X-ray-absorbed AGNs required by current models of the cosmic X-ray background. The existence of AGNs that display both broad emission lines and absorbed X-rays has important implications for unification schemes and emphasizes the need for care in assigning classifications to individual AGNs.

In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on board the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 μm in two nearly adjacent fields of view. We summarize here the in-flight scientific, technical, and operational performance of IRAC.

XMM-Newton Observations of Red AGNs

XMM-Newton spectra of five red 2MASS active galactic nuclei (AGNs), selected from a sample observed by Chandra to be relatively X-ray bright and to cover a range of hardness ratios, confirm the presence of substantial absorbing material in three sources with optical classifications ranging from type 1 to type 2. A flat (hard) power-law continuum is observed in the other two. The combination of X-ray absorption and broad optical emission lines suggests either a small (nuclear) absorber or a favored viewing angle so as to cover the X-ray source but not the broad emission-line region (BELR). A soft excess is detected in all three type 1 sources. We speculate that this may arise in an extended region of ionized gas, perhaps linked to the polarized (scattered) optical light present in these sources. The spectral complexity revealed by XMM-Newton emphasizes the limitations of the low-S/N Chandra data. The new results strengthen our earlier conclusions that the observed X-ray continua of red AGNs are unusually hard at energies 2 keV. Their observed spectra are consistent with contributing significantly to the missing hard or absorbed population of the cosmic X-ray background (CXRB), although their intrinsic power-law slopes are typical of broad-line (type 1) AGNs (Γ ~ 1.7-1.9). This suggests that the missing X-ray-absorbed CXRB population may include type 1 AGNs or QSOs in addition to the type 2 AGNs generally assumed.

Morphology of the 12 micron Seyfert galaxies. II. Optical and near-infrared image atlas

We present 263 optical and near-infrared (NIR) images for 42 1s and 48 Seyfert 2s, selected from the Extended 12 μm Galaxy Sample. Elliptically averaged profiles are derived from the images, and isophotal radii and magnitudes are calculated from these. We also report virtual aperture photometry that, judging from comparison with previous work, is accurate to roughly 0.05 mag in the optical, and 0.07 mag in the NIR. Our B-band isophotal magnitude and radii, obtained from ellipse fitting, are in good agreement with those of Third Reference Catalogue of Bright Galaxies. When compared with the B band, V, I, J, and K isophotal diameters show that the colors in the outer regions of Seyfert galaxies are consistent with the colors of normal spirals. Differences in the integrated isophotal colors and comparison with a simple model show that the active nucleus + bulge are stronger and redder in the NIR than in the optical. Finally, roughly estimated Seyfert disk surface brightnesses are significantly brighter in B and K than those in normal spirals of similar morphological type.

An Unbiased Census of Active Galactic Nuclei in the Two Micron All Sky Survey

We present an unbiased near-IR–selected AGN sample, covering 12.56 deg2 down to Ks ~ 15.5, selected from the Two Micron All Sky Survey (2MASS). Our only selection effect is a moderate color cut (J-Ks > 1.2) designed to reduce contamination from Galactic stars. We observed both pointlike and extended sources. Using the brute-force capabilities of the Two Degree Field multifiber spectrograph on the Anglo-Australian Telescope, we obtained spectra of 65% of the target list: an unbiased subsample of 1526 sources. Eighty percent of the 2MASS sources in our fields are galaxies, with a median redshift of 0.15. The remainder are K and M dwarf stars. We find tentative evidence that Seyfert 2 nuclei are more common in our IR-selected survey than in blue-selected galaxy surveys. We estimate that 5.1 ± 0.7% of the galaxies have Seyfert 2 nuclei with Hα equivalent widths greater than 0.4 nm, measured over a spectroscopic aperture of radius ~2.5 kpc. Blue-selected galaxy samples only find Seyfert 2 nuclei meeting these criteria in ~1.5% of galaxies. We find that 1.2 ± 0.3% of our sources are broadline (type 1) AGNs, giving a surface density of 1.0 ± 0.3 deg2, down to Ks < 15.0. This is the same surface density of type 1 AGNs as optical samples down to B < 18.5. Our type 1 AGNs, however, mostly lie at low redshifts, and host galaxy light contamination would make ~50% of them hard to find in optical QSO samples. We conclude that the type 1 AGN population found in the near-IR is not dramatically different from that found in optical samples. There is no evidence for a large population of AGNs that could not be found at optical wavelengths, although we can only place very weak constraints on any population of dusty high-redshift QSOs. In contrast, the incidence of type 2 (narrow-line) AGNs in a near-IR–selected galaxy sample seems to be higher than in a blue-selected galaxy sample.

2MASS Discoveries of Cool, Dust‐enshrouded Carbon Stars out of the Galactic Plane1

ABSTRACT Several very cool carbon stars have been found in or beyond the Galactic halo in the Two Micron All Sky Survey (2MASS). All appear to be N (C‐N) type asymptotic branch giants, much redder than most carbon stars found at high Galactic latitude at optical wavelengths. Three are heavily dust enshrouded, radiate primarily in the thermal infrared at temperatures of order 1000 K, and are IRAS sources. Low‐resolution red spectrophotometry with the Keck II and Palomar 5 m telescopes shows red to extremely red continua with CN absorption bands, similar to IRAS SSC 08546+1732, which is detected here (2MASSI J0857258+172052). One object, 2MASSI J0401097+182807, featured an overlying low‐excitation emission‐line spectrum with neutral alkalis (K i, Rb i, and Cs i), which disappeared after 19 months. This is argued to be due to vigorous mass loss probably associated with Mira‐like pulsations. These initial results suggest that 2MASS can be used to define a useful sample at high Galactic latitude as tracers of ...