R. D. Blum

Spitzer survey of the large magellanic cloud: Surveying the agents of a Galaxy's evolution (SAGE). I. Overview and initial results

We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; ~7° × 7°) using the IRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70, and 160 μm) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxys Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1.2 × 10^(21) H cm^(-2) permits detailed studies of dust processes in the ISM. SAGEs point-source sensitivity enables a complete census of newly formed stars with masses >3 M_☉ that will determine the current star formation rate in the LMC. SAGEs detection of evolved stars with mass-loss rates >1 × 10^(-8) M_☉ yr^(-1) will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 μm images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H I 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 μm band and decrease dramatically toward longer wavelengths, consistent with the fact that stars dominate the point-source catalogs and the dusty objects detected at the longer wavelengths are rare in comparison. The SAGE epoch 1 point-source catalog has ~4 × 10^6 sources, and more are anticipated when the epoch 1 and 2 data are combined. Using Milky Way (MW) templates as a guide, we adopt a simplified point-source classification to identify three candidate groups—stars without dust, dusty evolved stars, and young stellar objects—that offer a starting point for this work. We outline a strategy for identifying foreground MW stars, which may comprise as much as 18% of the source list, and background galaxies, which may comprise ~12% of the source list.

Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). I. Overview

The Small Magellanic Cloud (SMC) provides a unique laboratory for the study of the lifecycle of dust given its low metallicity (~1/5 solar) and relative proximity (~60 kpc). This motivated the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud) Spitzer Legacy program with the specific goals of studying the amount and type of dust in the present interstellar medium, the sources of dust in the winds of evolved stars, and how much dust is consumed in star formation. This program mapped the full SMC (30 deg^2) including the body, wing, and tail in seven bands from 3.6 to 160 μm using IRAC and MIPS on the Spitzer Space Telescope. The data were reduced and mosaicked, and the point sources were measured using customized routines specific for large surveys. We have made the resulting mosaics and point-source catalogs available to the community. The infrared colors of the SMC are compared to those of other nearby galaxies and the 8 μm/24 μm ratio is somewhat lower than the average and the 70 μm/160 μm ratio is somewhat higher than the average. The global infrared spectral energy distribution (SED) shows that the SMC has approximately 1/3 the aromatic emission/polycyclic aromatic hydrocarbon abundance of most nearby galaxies. Infrared color-magnitude diagrams are given illustrating the distribution of different asymptotic giant branch stars and the locations of young stellar objects. Finally, the average SED of H II/star formation regions is compared to the equivalent Large Magellanic Cloud average H II/star formation region SED. These preliminary results will be expanded in detail in subsequent papers.

The NGC 404 Nucleus: Star Cluster and Possible Intermediate-mass Black Hole

We examine the nuclear morphology, kinematics, and stellar populations in nearby S0 galaxy NGC 404 using a combination of adaptive optics assisted near-IR integral-field spectroscopy, optical spectroscopy, and Hubble Space Telescope imaging. These observations enable study of the NGC 404 nucleus at a level of detail possible only in the nearest galaxies. The surface brightness profile suggests the presence of three components: a bulge, a nuclear star cluster (NSC), and a central light excess within the cluster at radii < 3 pc. These components have distinct kinematics with modest rotation seen in the NSC and counter-rotation seen in the central excess. Molecular hydrogen emission traces a disk with rotation nearly orthogonal to that of the stars. The stellar populations of the three components are also distinct, with half of the mass of the NSC having ages of ~1 Gyr (perhaps resulting from a galaxy merger), while the bulge is dominated by much older stars. Dynamical modeling of the stellar kinematics gives a total NSC mass of 1.1 × 107 M ☉. Dynamical detection of a possible intermediate-mass black hole (BH) is hindered by uncertainties in the central stellar mass profile. Assuming a constant mass-to-light ratio, the stellar dynamical modeling suggests a BH mass of <1 × 105 M ☉, while the molecular hydrogen gas kinematics are best fitted by a BH with a mass of 4.5+3.5 –2.0 × 105 M ☉. Unresolved and possibly variable dust emission in the near-infrared and active galactic nucleus-like molecular hydrogen emission-line ratios do suggest the presence of an accreting BH in this nearby LINER galaxy.

Spitzer survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) IV: dust properties in the interstellar medium

The goal of this paper is to present the results of a preliminary analysis of the extended infrared (IR) emission by dust in the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). We combine Spitzer Surveying the Agents of Galaxy Evolution (SAGE) and Infrared Astronomical Satellite (IRAS) data and correlate the infrared emission with gas tracers of H I, CO, and Hα. We present a global analysis of the infrared emission as well as detailed modeling of the spectral energy distribution (SED) of a few selected regions. Extended emission by dust associated with the neutral, molecular, and diffuse ionized phases of the ISM is detected at all IR bands from 3.6 μm to 160 μm. The relative abundance of the various dust species appears quite similar to that in the Milky Way (MW) in all the regions we have modeled. We construct maps of the temperature of large dust grains. The temperature map shows variations in the range 12.1-34.7 K, with a systematic gradient from the inner to outer regions, tracing the general distribution of massive stars and individual H II regions as well as showing warmer dust in the stellar bar. This map is used to derive the far-infrared (FIR) optical depth of large dust grains. We find two main departures in the LMC with respect to expectations based on the MW: (1) excess mid-infrared (MIR) emission near 70 μm, referred to as the 70 μm excess, and (2) departures from linear correlation between the FIR optical depth and the gas column density, which we refer to as FIR excess emission. The 70 μm excess increases gradually from the MW to the LMC to the Small Magellanic Cloud (SMC), suggesting evolution with decreasing metallicity. The excess is associated with the neutral and diffuse ionized gas, with the strongest excess region located in a loop structure next to 30 Dor. We show that the 70 μm excess can be explained by a modification of the size distribution of very small grains with respect to that in the MW, and a corresponding mass increase of ≃13% of the total dust mass in selected regions. The most likely explanation is that the 70 μm excess is due to the production of large very small grains (VSG) through erosion of larger grains in the diffuse medium. This FIR excess could be due to intrinsic variations of the dust/gas ratio, which would then vary from 4.6 to 2.3 times lower than the MW values across the LMC, but X_(CO) values derived from the IR emission would then be about three times lower than those derived from the Virial analysis of the CO data. We also investigate the possibility that the FIR excess is associated with an additional gas component undetected in the available gas tracers. Assuming a constant dust abundance in all ISM phases, the additional gas component would have twice the known H I mass. We show that it is plausible that the FIR excess is due to cold atomic gas that is optically thick in the 21 cm line, while the contribution by a pure H_2 phase with no CO emission remains a possible explanation.

Optical and Infrared Spectroscopy of SN 1999ee and SN 1999ex

We report optical and infrared spectroscopic observations of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex, both of which were hosted by the galaxy IC 5179. For SN 1999ee we obtained a continuous sequence with an unprecedented wavelength and temporal coverage beginning 9 days before maximum light and extending through day 42. Before maximum light SN 1999ee displayed a normal spectrum with a strong Si II λ6355 absorption, thus showing that not all slow-declining supernovae (SNe) are spectroscopically peculiar at these evolutionary phases. A comparative study of the infrared spectra of SN 1999ee and other Type Ia SNe shows that there is a remarkable homogeneity among the Branch-normal SNe Ia during their first 60 days of evolution. SN 1991bg–like objects, on the other hand, display spectroscopic peculiarities at infrared wavelengths. SN 1999ex was characterized by the lack of hydrogen lines, weak optical He I lines, and strong He I λλ10830, 20581, thus providing an example of an intermediate case between pure Ib and Ic supernovae. We conclude, therefore, that SN 1999ex provides the first clear evidence for a link between the Ib and Ic classes and that there is a continuous spectroscopic sequence ranging from the He-deficient SNe Ic to the SNe Ib, which are characterized by strong optical He I lines.

Rate coefficients for the excitation of infrared and ultraviolet lines in C II, N III and O IV

New and improved collision strengths and Maxwellian-averaged rate coefficients for temperatures between 1000 and 40,000 K are presented for all the prominent transitions in the spectra of C II, N III, and O IV. The collision strengths show extensive autoionization structures that are delineated in detail and which enhance the rate coefficients for several transitions by a considerable amount. Maxwellian-averaged collision strengths are calculated for all possible fine-structure transitions along the states included in the eigenfunction expansion of the target ion. All calculations are carried out in the close-coupling approximation using the R-matrix method as adapted for the Opacity Project.

CHEMICAL ABUNDANCES OF LUMINOUS COOL STARS IN THE GALACTIC CENTER FROM HIGH-RESOLUTION INFRARED SPECTROSCOPY

We present chemical abundances in a sample of luminous cool stars located within 30 pc of the Galactic center. Abundances of carbon, nitrogen, oxygen, calcium, and iron were derived from high-resolution infrared spectra in the H and K bands. The abundance results indicate that both [O/Fe] and [Ca/Fe] are enhanced, respectively, by averages of +0.2 and +0.3 dex, relative to either the Sun or the Milky Way disk at near-solar Fe abundances. The Galactic center stars show a nearly uniform and nearly solar iron abundance. The mean value of A(Fe) = 7.59 ± 0.06 agrees well with previous work. The total range in Fe abundance among Galactic center stars, 0.16 dex, is significantly narrower than the iron abundance distributions found in the literature for the older bulge population. Our snapshot of the current-day Fe abundance within 30 pc of the Galactic center samples stars with an age less than 1 Gyr; a larger sample in time (or space) may find a wider spread in abundances.

Really cool stars and the star formation history at the Galactic Center

We present ?/?? = 550-1200 near-infrared H and K spectra for a magnitude-limited sample of 79 asymptotic giant branch and cool supergiant stars in the central ?5 pc (diameter) of the Galaxy. We use a set of similar spectra obtained for solar neighborhood stars with known Teff and Mbol that is in the same range as the Galactic center (GC) sample to derive Teff and Mbol for the GC sample. We then construct the H-R diagram for the GC sample. Using an automated maximum likelihood routine, we derive a coarse star formation history of the GC. We find that (1) roughly 75% of the stars formed in the central few parsecs are older than 5 Gyr; (2) the star formation rate (SFR) is variable over time, with a roughly 4 times higher SFR in the last 100 Myr compared to the average SFR; (3) our model can match dynamical limits on the total mass of stars formed only by limiting the initial mass function to masses above 0.7 M? (this could be a signature of mass segregation or of the bias toward massive star formation from the unique star formation conditions in the GC); (4) blue supergiants account for 12% of the total sample observed, and the ratio of red to blue supergiants is roughly 1.5; and (5) models with isochrones with [Fe/H] = 0.0 over all ages fit the stars in our H-R diagram better than models with lower [Fe/H] in the oldest age bins, consistent with the finding of Ram?rez et al. that stars with ages between 10 Myr and 1 Gyr have solar [Fe/H].

The Stellar Content of Obscured Galactic Giant H ii Regions. II. W42

We present near-infrared J, H, and K images and K-band spectroscopy in the giant H II region W42. A massive star cluster is revealed; the color-color plot and K-band spectroscopic emission features of two of the brighter objects suggest the presence of young stellar objects. The spectrum of the bright central star is similar to unobscured stars with MK spectral types of O5–O6.5. If this star is on the zero-age main sequence, then the derived spectrophotometric distance is considerably smaller than previous estimates. The Lyman continuum luminosity of the cluster is a few times that of the Trapezium. The slope of the K-band luminosity function is similar to that for the Trapezium cluster and significantly steeper than that for the massive star cluster in M17 or in the Arches cluster near the Galactic center.

A Population of Accreted Small Magellanic Cloud Stars in the Large Magellanic Cloud

We present an analysis of the stellar kinematics of the Large Magellanic Cloud based on ~5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich AGB stars, and other giants. After correcting the line-of-sight velocities for the LMCs space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published HI data. The amplitude of the rotation curve is v_0=87+/-5 km s^-1 beyond a radius R_0=2.4+/-0.1 kpc, and has a position angle of the kinematic line of nodes of theta=142 degrees +/-5 degrees. By examining the outliers from our fits, we identify a population of 376 stars, or >~5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of the LMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with the LMC disk, or rotating in the same sense as the LMC disk, but in a plane that is inclined by 54 degrees +/- 2 degrees to the LMC. Their kinematics clearly link them to two known HI arms, which have previously been interpreted as being pulled out from the LMC. We measure metallicities from the Ca triplet lines of ~1000 LMC field stars and 30 stars in the kinematically distinct population. For the LMC field, we find a median [Fe/H]=-0.56 +/- 0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 +/- 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the SMC. This interpretation has the consequence that the HI arms kinematically associated with the stars are likely falling into the LMC, instead of being pulled out.