arXiv subjects

M. A. Pahre

Publications and source records attributed to M. A. Pahre.

At least 19 recordsLinked to original sources

arXiv2008

The Mid-Infrared Properties of X-ray Sources

We combine the results of the Spitzer IRAC Shallow Survey and the Chandra XBootes Survey of the 8.5 square degrees Bootes field of the NOAO Deep Wide- Field Survey to produce the largest comparison of mid-IR and X-ray sources to date. The comparison is limited to sources with X-ray fluxes >8x10-15 erg cm-2s-1 in the 0.5-7.0 keV range and mid-IR sources with 3.6 um fluxes brighter than 18.4 mag (12.3 uJy). In this most sensitive IRAC band, 85% of the 3086 X-ray sources have mid-IR counterparts at an 80% confidence level based on a Bayesian matching technique. Only 2.5% of the sample have no IRAC counterpart at all based on visual inspection. Even for a smaller but a significantly deeper Chandra survey in the same field, the IRAC Shallow Survey recovers most of the X-ray sources. A majority (65%) of the Chandra sources detected in all four IRAC bands occupy a well-defined region of IRAC [3.6] - [4.5] vs [5.8] - [8.0] color-color space. These X-ray sources are likely infrared luminous, unobscured type I AGN with little mid-infrared flux contributed by the AGN host galaxy. Of the remaining Chandra sources, most are lower luminosity type I and type II AGN whose mid-IR emission is dominated by the host galaxy, while approximately 5% are either Galactic stars or very local galaxies.

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arXiv2007

The Local Galaxy 8 micron Luminosity Function

A SST survey in the NOAO Deep-Wide Field in Boötes provides a complete, 8-micron-selected sample of galaxies to a limiting (Vega) magnitude of 13.5. In the 6.88 deg$^2$ field sampled, 79% of the 4867 galaxies have spectroscopic redshifts, allowing an accurate determination of the local (z<0.3) galaxy luminosity function. Stellar and dust emission can be separated on the basis of observed galaxy colors. Dust emission (mostly PAH) accounts for 80% of the 8 micron luminosity, stellar photospheres account for 19%, and AGN emission accounts for roughly 1 %. A sub-sample of the 8 micron-selected galaxies have blue, early-type colors, but even most of these have significant PAH emission. The luminosity functions for the total 8 micron luminosity and for the dust emission alone are both well fit by Schechter functions. For the 8 micron luminosity function, the characteristic luminosity is νL_ν^*(8.0 \micron) = 1.8 \times 10^{10}$ \Lsun while for the dust emission alone it is 1.6 x 10^{10}$ \Lsun\null. The average 8 \micron luminosity density at z<0.3 is 3.1 x 10^7 \Lsun Mpc^{-3}, and the average luminosity density from dust alone is 2.5 x 10^7 \Lsun Mpc^{-3}. This luminos ity arises predominantly from galaxies with 8 \micron luminosities ($νL_ν$) between $2\times 10^9$ and $2 x 10^{10}$ \Lsun, i.e., normal galaxies, not LIRGs or ULIRGs.

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arXiv2006

Dusty waves on a starry sea: the mid-infrared view of M31

Mid-infrared observations of the Andromeda galaxy, M31, obtained with the Infrared Array Camera on board the Spitzer Space Telescope, are presented. The image mosaics cover areas of approximate 3.7deg x 1.6deg and include the satellite galaxies M32 and NGC 205. The appearance of M31 varies dramatically in the different mid-infrared bands, from the smooth bulge and disk of the old stellar population seen at 3.6um to the well-known '10 kpc ring' dominating the 8um image. The similarity of the 3.6um and optical isophotes and nearly constant optical-mid-infrared color over the inner 400 arcsec confirms that there is no significant extinction at optical wavelengths in M31's bulge. The nuclear colors indicate the presence of dust but not an infrared-bright nucleus. The integrated 8um non-stellar luminosity implies a star formation rate of 0.4 Msun/yr, consistent with other indicators that show M31 to be a quiescent galaxy.

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arXiv2006

An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy

The unusual morphology of the Andromeda Spiral (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M~31 has a well-known outer ring of star formation at a radius of 10 kpc whose center is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped as seen at both optical and radio wavelengths. The halo contains numerous loops and ripples. Here we report the discovery, based on analysis of previously-obtained data, of a second, inner dust ring with projected dimensions 1.5 by 1 kpc and offset by ~0.5kpc from the center of the galaxy. The two rings appear to be density waves propagating in the disk. Numerical simulations offer a completely new interpretation for the morphology of M31: both rings result from a companion galaxy plunging head-on through the center of the disk of M31. The most likely interloper is M32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.

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arXiv2006

Photometric Redshifts in the IRAC Shallow Survey

Accurate photometric redshifts are calculated for nearly 200,000 galaxies to a 4.5 micron flux limit of ~13 uJy in the 8.5 deg^2 Spitzer/IRAC Shallow survey. Using a hybrid photometric redshift algorithm incorporating both neural-net and template-fitting techniques, calibrated with over 15,000 spectroscopic redshifts, a redshift accuracy of σ= 0.06(1+z) is achieved for 95% of galaxies at 0 1) galaxy clusters. We present one such spectroscopically confirmed cluster at =1.24, ISCS J1434.5+3427. Finally, we present a measurement of the 4.5 micron-selected galaxy redshift distribution.

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arXiv2005

Understanding Radio-Selected Thermal Sources in M 33: Ultraviolet, Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations

We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and radio images of 14 radio-selected objects in M 33. These objects are thought to represent the youngest phase of star cluster formation. We have detected the majority of cluster candidates in M 33 at all wavelengths. From the near-IR images, we derived ages 2-10 Myr, K_S-band extinctions (A_K_S) of 0-1 mag, and stellar masses of 10^3-10^4 M_solar. We have generated spectral energy distributions (SEDs) of each cluster from 0.1 micron to 160 microns. From these SEDs, we have modeled the dust emission around these star clusters to determine the dust masses (1-10^3 M_solar) and temperatures (40-90 K) of the clusters' local interstellar medium. Extinctions derived from the JHK_S, Halpha, and UV images are similar to within a factor of 2 or 3. These results suggest that eleven of the fourteen radio-selected objects are optically-visible young star clusters with a surrounding H II region, that two are background objects, possibly AGN, and that one is a Wolf-Rayet star with a surrounding H II region.

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arXiv2004

Spatial Distribution of Warm Dust in Early-Type Galaxies

Images taken with the IRAC instrument on the Spitzer Space Telescope show that the spatial distribution of warm dust emission in lenticular galaxies is often organized into dynamically-stable structures strongly resembling spiral arms. These galaxies have bulge-to-disk-ratios and colors for their stellar content appropriate for their morphological classification. Two of the three galaxies with warm dust detected at 8.0 um also show far-IR emission expected from that dust. More importantly, the [5.8]-[8.0] color of the dust emission matches the colors found for late-type, star-forming galaxies, as well as theoretical predictions for PAH emission from dust grains. The spatially resolved dust structures may be powerful indicators of the evolutionary history of the lenticular class of galaxies, either as a tracer of on-going quiescent star formation or as a fossil record of a previous episode of more active star formation.

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arXiv2004

The Off-nuclear Starbursts In NGC 4038/4039 (The Antennae Galaxies)

Imaging of the Antennae galaxies (NGC 4038/4039) with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope reveals large concentrations of star forming activity away from both nuclei of the two merging galaxies. These images confirm earlier findings based on ISO data with lower angular resolution. The short wavelength emission shows numerous compact sources identified as stellar clusters. At the longer wavelengths, bright, more amorphous and filamentary features correlate well with the known distributions of denser gas, warm dust, and HII regions. There are also fainter, more diffuse components at all wavelengths that permeate the entire region and extend into the two tidal tails. Non-stellar dust emission dominates the 5.8 and 8.0 micron images, accounting for as much as 79% of the light at 5.8 micron and 95% at 8 micron, averaged over the entire galaxy. Assuming that the non-stellar emission traces star formation, the IRAC data provide a view into the total underlying star forming activities unaffected by obscuration. Using the flux ratio of non-stellar to stellar emission as a guide, we map the local star formation rate in the Antennae and compare that to similar measurements in both normal and infrared-luminous galaxies. This rate in the active regions is found to be as high as those seen in starburst and some ultra-luminous infrared galaxies on ``per unit mass'' basis. The two galactic centers actually have lower star forming rates than the off-nuclear regions despite the presence of abundant dense gas and dust, suggesting that the latter is a necessary but not sufficient condition for on-going star formation.

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arXiv2004

The IRAC Shallow Survey

The IRAC shallow survey covers 8.5 square degrees in the NOAO Deep Wide-Field Survey in Bootes with 3 or more 30 second exposures per position. An overview of the survey design, reduction, calibration, star-galaxy separation, and initial results is provided. The survey includes approximately 370,000, 280,000, 38,000, and 34,000 sources brighter than the 5 sigma limits of 6.4, 8.8, 51, and 50 microJy at 3.6, 4.5, 5.8, and 8 microns respectively, including some with unusual spectral energy distributions.

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arXiv2004

Deep mid-infrared observations of Lyman-break galaxies

As part of the In-Orbit Checkout activities for the Spitzer Space Telescope, the IRAC team carried out a deep observation (average integration time ~8 hours) of a field surrounding the bright QSO HS 1700+6416. This field contains several hundred z~3 Lyman-break galaxy candidates, and we report here on their mid-infrared properties, including the IRAC detection rate, flux densities and colors, and the results of fitting population synthesis models to the optical, near-infrared, and IRAC magnitudes. The results of the model-fitting show that previous optical/near-infrared studies of LBGs were not missing large, hidden old stellar populations. The LBG candidates' properties are consistent with those of massive, star-forming galaxies at z~3. Other IRAC sources in the same field have similar properties, so IRAC selection may prove a promising method of finding additional high-redshift galaxies.

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arXiv2004

Number Counts At 3 < lambda < 10 um From the Spitzer Space Telescope

Infrared source counts at wavelengths 3 < lambda < 10 um cover more than 10 magnitudes in source brightness, four orders of magnitude in surface density, and reach an integrated surface density of 10^5 sources/deg^2. At m<14 mag, most of the sources are Galactic stars, in agreement with models. After removal of Galactic stars, galaxy counts are consistent with what few measurements exist at nearby wavelengths. At 3.6 and 4.5 um, the galaxy counts follow the expectations of a Euclidean world model down to ~16 mag and drop below the Euclidean curve for fainter magnitudes. Counts at these wavelengths begin to show decreasing completeness around magnitude 19.5. At 5.8 and 8 um, the counts relative to a Euclidean world model show a large excess at bright magnitudes. This is probably because local galaxies emit strongly in the aromatic dust (``PAH'') features. The counts at 3.6 um resolve <50% of the Cosmic Infrared Background at that wavelength.

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arXiv2004

Mid-Infrared Galaxy Morphology Along the Hubble Sequence

The mid-infrared emission from 18 nearby galaxies imaged with the IRAC instrument on Spitzer Space Telescope samples the spatial distributions of the reddening-free stellar photospheric emission and the warm dust in the ISM. These two components provide a new framework for galaxy morphological classification, in which the presence of spiral arms and their emission strength relative to the starlight can be measured directly and with high contrast. Four mid-infrared classification methods are explored, three of which are based on quantitative global parameters (colors, bulge-to-disk ratio) similar to those used in the past for optical studies; in this limited sample, all correlate well with traditional B-band classification. We suggest reasons why infrared classification may be superior to optical classification.

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arXiv2000

Evidence for Inconsistencies in Galaxy Luminosity Functions Defined by Spectral Type

Galaxy morphological and spectroscopic types should be nearly independent of apparent magnitude in a local, magnitude-limited sample. Recent luminosity function surveys based on morphological classification of galaxies are substantially more successful at passing this test than surveys based on spectroscopic classifications. Among spectroscopic classifiers, those defined by small aperture fibers (ESP, LCRS) show far stronger systematic classification biases than those defined by large apertures (APM). This effect can be easily explained as an aperture bias, whereby galaxies with globally late-type spectra are assigned earlier spectral types which depend on the redshift and luminosity of the galaxy. The effect is demonstrated by extracting successively larger aperture spectra from long-slit spectroscopy of nearby galaxies. This systematic classification bias is generic, and the 2dFGRS and SDSS surveys will show similar systematic biases in their spectral types. We suggest several methods for correcting the problem or avoiding it altogether. If not corrected, this aperture bias can mimic galaxy evolutionary effects and distort estimates of the luminosity function.

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arXiv2000

The K-Band Galaxy Luminosity Function

We measured the K-band luminosity function using a complete sample of 4192 morphologically-typed 2MASS galaxies with 7 < K < 11.25 mag spread over 2.12 str. Early-type (T < -0.5) and late-type (T > -0.5) galaxies have similarly shaped luminosity functions, alpha_e=-0.92+/-0.10 and alpha_l=-0.87+/-0.09. The early-type galaxies are brighter, M_*e=-23.53+/-0.06 mag compared to M_*l=-22.98\pm0.06 mag, but less numerous, n_*e=(0.0045+/-0.0006)h^3/Mpc^3 compared to n_*l=(0.0101+/-0.0013)h^3/Mpc^3 for H_0=100h km/s Mpc, such that the late-type galaxies slightly dominate the K-band luminosity density, j_late/j_early=1.17+/-0.12. Our morphological classifications are internally consistent, consistent with previous classifications and lead to luminosity functions unaffected by the estimated uncertainties in the classifications. These luminosity functions accurately predict the K-band number counts and redshift distributions for K < 18 mag, beyond which the results depend on galaxy evolution and merger histories.

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arXiv2000

RJK Observations of the Optical Afterglow of GRB 991216

We present near-infrared and optical observations of the afterglow to the Gamma-Ray Burst (GRB) 991216 obtained with the F. L. Whipple Observatory 1.2-m telescope and the University of Hawaii 2.2-m telescope. The observations range from 15 hours to 3.8 days after the burst. The temporal behavior of the data is well described by a single power-law decay with index -1.36 +/-0.04, independent of wavelength. The optical spectral energy distribution, corrected for significant Galactic reddening of E(B-V)=0.626, is well fitted by a single power-law with index -0.58 +/- 0.08. Combining the IR/optical observations with a Chandra X-ray measurement gives a spectral index of -0.8 +/- 0.1 in the synchrotron cooling regime. A comparison between the spectral and temporal power-law indices suggest that a jet is a better match to the observations than a simple spherical shock.

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arXiv1998

Detection of Surface Brightness Fluctuations in NGC 4373 Using the Hubble Space Telescope

Surface brightness fluctuations (SBF) have been detected for three elliptical galaxies-NGC 3379 in the Leo group, NGC 4406 in the Virgo cluster, and NGC 4373 in the Hydra-Centaurus supercluster-using marginally-sampled, deep images taken with the Planetary Camera of the WFPC-2 instrument on the Hubble Space Telescope (HST). The power spectrum of the fluctuations image is well-fit by an empirical model of the point-spread function (PSF) constructed using point sources identified in the field. The previous ground-based SBF measurements for NGC 3379 and NGC 4406 are recovered, thereby demonstrating the capability of the Planetary Camera of WFPC-2 to measure distances using the SBF technique despite the marginal sampling of the images. The residual variance due to unresolved sources in all three galaxies is only 2-5% of the detected fluctuations signal, which confirms the advantage of HST imaging in minimizing the uncertainty of this SBF correction. Extensive consistency checks, including an independent SBF analysis using an alternate software package, suggest that our internal uncertainties are < 0.02 mag. The fluctuations magnitude for NGC 4373 is I814bar = 31.31 +/- 0.05 mag, corresponding to a distance modulus of DM = 32.99 +/- 0.11. This implies a peculiar velocity for this galaxy of 415 +/- 330 km/s, which is smaller than derived from the D_n-sigma relation. These results demonstrate the power of the post-repair HST to measure distances to elliptical galaxies at significant redshifts using the SBF technique.

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arXiv1998

Systematic Variations in Age and Metallicity Along the Early-Type Galaxy Sequence

The form of the early-type galaxy scaling relation (the Fundamental Plane or FP) is a direct indicator of the underlying physical origins for the galaxy sequence. Observed properties of the FP include: (1) the slope increases with wavelength; (2) the slope deviates from the virial expectation (assuming homology and constant M/L) at all wavelengths; (3) the intercept evolves passively with redshift; and (4) the slope decreases slowly with redshift. The first property implies that stellar populations contribute to the slope of the FP, the second and fourth properties exclude metallicity effects as the sole cause of the slope, and the third implies that the stellar content of the "average" early-type galaxy formed at high redshift. A composite model--including variations in age and metallicity, as well as a wavelength-independent effect such as homology breaking--is presented which can fit all four observed properties. This model implies that the most luminous early-type galaxies contain the oldest and most metal-rich stars, while the lowest luminosity galaxies formed the bulk of their stars as recently at z_f~1.

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arXiv1998

Near-Infrared Imaging of Early-Type Galaxies IV. The Physical Origins of the Fundamental Plane Scaling Relations

The physical origins of the Fundamental Plane (FP) scaling relations are investigated for early-type galaxies observed at optical and near-infrared wavelengths. The slope for the FP is shown to increase systematically with wavelength from the U-band through the K-band. A distance-independent construction of the observables is described which provides an accurate measurement of the change in the FP slope between any pair of bandpasses. The variation of the FP slope with wavelength is strong evidence of systematic variations in stellar content along the elliptical galaxy sequence. The intercept of the diagnostic relationship between log(D_K/D_V) and log(sigma_0) shows no significant dependence on environment within the uncertainties of the Galactic extinction corrections, demonstrating the universality of the stellar populations contributions at the level of Delta(V-K)=0.03 mag to the zero-point of the global scaling relations. Several other constraints on the properties of early-type galaxies --- the slope of the Mg_2-sigma_0 relation, the effects of stellar populations gradients, and deviations of early-type galaxies from a dynamically homologous family --- are included to construct an empirical, self-consistent model which provides a complete picture of the underlying physical properties which are varying along the early-type galaxy sequence. This empirical approach demonstrates that there are significant systematic variations in both age and metallicity along the elliptical galaxy sequence, and that a small, but systematic, breaking of dynamical homology (or a similar, wavelength independent effect) is required. Predictions for the evolution of the slope of the FP with redshift are described. [abriged]

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