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Karin M. Sandstrom,
Alberto D. Bolatto,
Caroline Bot,
B. T. Draine,
James G. Ingalls,
Frank P. Israel,
James M. Jackson,
Adam K. Leroy,
Aigen Li,
Mónica Rubio, Joshua D. Simon,
J. D. T. Smith,
Snežana Stanimirović,
A. G. G. M. Tielens,
Jacco Th. van Loon
[show abstract]
[hide abstract]
ABSTRACT: We present results of mid-infrared spectroscopic mapping observations of six
star-forming regions in the Small Magellanic Cloud from the Spitzer
Spectroscopic Survey of the SMC (S4MC). We detect the mid-IR emission from
polycyclic aromatic hydrocarbons (PAHs) in all of the mapped regions, greatly
increasing the range of environments where PAHs have been spectroscopically
detected in the SMC. We investigate the variations of the mid-IR bands in each
region and compare our results to studies of the PAH bands in the SINGS sample
and in a sample of low-metallicity starburst galaxies. PAH emission in the SMC
is characterized by low ratios of the 6-9 micron features relative to the 11.3
micron feature and weak 8.6 and 17.0 micron features. Interpreting these band
ratios in the light of laboratory and theoretical studies, we find that PAHs in
the SMC tend to be smaller and less ionized than those in higher metallicity
galaxies. Based on studies of PAH destruction, we argue that a size
distribution shifted towards smaller PAHs cannot be the result of processing in
the interstellar medium, but instead reflects differences in the formation of
PAHs at low metallicity. Finally, we discuss the implications of our
observations for our understanding of the PAH life-cycle in low-metallicity
galaxies---namely that the observed deficit of PAHs may be a consequence of
PAHs forming with smaller average sizes and therefore being more susceptible to
destruction under typical interstellar medium conditions.
The Astrophysical Journal 09/2011; 744(1). · 6.02 Impact Factor
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Karl D. Gordon,
Margaret Meixner,
Marilyn Meade,
Barbara A. Whitney,
Charles W. Engelbracht,
Caroline Bot,
Martha L. Boyer,
Brandon Lawton,
Marta Sewilo,
Mr. Brian L. Babler, [......],
Karin Sandstrom,
S Sato,
Hiroshi Shibai, Joshua D. Simon,
Linda J Smith,
Sundar Srinivasan,
Uma P. Vijh,
Schuyler D. Van Dyk,
Jacobus Theodorus van Loon,
Dennis F. Zaritsky
[show abstract]
[hide abstract]
ABSTRACT: 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 sq. deg.) including the Body, Wing, and Tail in
7 bands from 3.6 to 160 micron using the IRAC and MIPS instruments on the
Spitzer Space Telescope. The data were reduced, mosaicked, and the point
sources 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 micron/24 micron ratio is somewhat lower and the 70
micron/160 micron ratio is somewhat higher than the average. The global
infrared spectral energy distribution shows that the SMC has ~3X lower aromatic
emission/PAH (polycyclic aromatic hydrocarbon) abundances compared to 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 spectral energy distribution (SED)
of HII/star formation regions is compared to the equivalent Large Magellanic
Cloud average HII/star formation region SED. These preliminary results are
expanded in detail in companion papers.
07/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We use Spitzer Space Telescope observations from the Spitzer Survey of the Small Magellanic Cloud (S3MC) to study the young stellar content of N66, the largest and brightest H II region in the SMC. In addition to large numbers of normal stars, we detect a significant population of bright, red infrared sources that we identify as likely to be young stellar objects (YSOs). We use spectral energy distribution (SED) fits to classify objects as ordinary (main-sequence or red giant) stars, asymptotic giant branch stars, background galaxies, and YSOs. This represents the first large-scale attempt at blind source classification based on Spitzer SEDs in another galaxy. We firmly identify at least 61 YSOs, with another 50 probable YSOs; only one embedded protostar in the SMC was reported in the literature prior to the S3MC. We present color selection criteria that can be used to identify a relatively clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the infrared-bright YSOs in N66 have stellar masses ranging from 2 to 17 M☉, and that approximately half of the objects are stage II protostars, with the remaining YSOs roughly evenly divided between stage I and stage III sources. We find evidence for primordial mass segregation in the H II region, with the most massive YSOs being preferentially closer to the center than lower mass objects. Despite the low metallicity and dust content of the SMC, the observable properties of the YSOs appear consistent with those in the Milky Way. Although the YSOs are heavily concentrated within the optically bright central region of N66, there is ongoing star formation throughout the complex, and we place a lower limit on the star formation rate of 3.2 × 10-3 M☉ yr-1 over the last ~1 Myr.
The Astrophysical Journal 12/2008; 669(1):327. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present infrared observations of the young, oxygen-rich supernova remnant 1E 0102.2-7219 (E0102) in the Small Magellanic Cloud, obtained with the Spitzer Space Telescope. The remnant is detected at 24 μm but not at 8 or 70 μm and has a filled morphology with two prominent filaments. We find evidence for the existence of up to 8 × 10-4 M☉ of hot dust (Td ~ 120 K) associated with the remnant. Most of the hot dust is located in the central region of E0102, which appears significantly enhanced in infrared and radio continuum emission relative to the X-ray emission. Even if all of the hot dust was formed in the explosion of E0102, the estimated mass of dust is at least 100 times lower than what is predicted by some recent theoretical models.
The Astrophysical Journal 12/2008; 632(2):L103. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We show that there are strong local correlations between metallicity, surface brightness, and dynamical mass-to-light ratio with M33, analogous to the fundamental line of dwarf galaxies identified by Prada & Burkert. Using near-infrared imaging from 2MASS, the published rotation curve of M33, and literature measurements of the metallicities of H II regions and supergiant stars, we demonstrate that these correlations hold for points at radial distances between 140 pc and 6.2 kpc from the center of the galaxy. At a given metallicity or surface brightness, M33 has a mass-to-light ratio approximately 4 times as large as the Local Group dwarf galaxies; other than this constant offset, we see broad agreement between the M33 and dwarf galaxy data. We use analytical arguments to show that at least two of the three fundamental line correlations are basic properties of disk galaxies that can be derived from very general assumptions. We investigate the effect of supernova feedback on the fundamental line with numerical models and conclude that while feedback clearly controls the scatter in the fundamental line, it is not needed to create the fundamental line itself, in agreement with our analytical calculations. We also compare the M33 data with measurements of a simulated disk galaxy, finding that the simulation reproduces the trends in the data correctly and matches the fundamental line, although the metallicity of the simulated galaxy is too high, and the surface brightness is lower than that of M33.
The Astrophysical Journal 12/2008; 649(2):709. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We have used new and archival infrared and radio observations to search for a dwarf galaxy associated with the high-velocity cloud (HVC) known as `complex H.' Complex H is a large (Ω 400 deg2) and probably nearby (d = 27 kpc) HVC whose location in the Galactic plane has hampered previous investigations of its stellar content. The H I mass of the cloud is 2.0 × 107(d/27 kpc)2 M☉, making complex H one of the most massive HVCs if its distance is more than ~20 kpc. Virtually all similar H I clouds in other galaxy groups are associated with low surface brightness dwarf galaxies. We selected mid-infrared sources observed by the MSX satellite in the direction of complex H that appeared likely to be star-forming regions and observed them at the wavelength of the CO J = 1 → 0 rotational transition in order to determine their velocities. Of the 60 observed sources, 59 show emission at Milky Way velocities, and we detected no emission at velocities consistent with that of complex H. We use these observations to set an upper limit on the ongoing star formation rate in the HVC of 5 × 10-4 M☉ yr-1. We also searched the 2MASS database for evidence of any dwarf-galaxy-like stellar population in the direction of the HVC and found no trace of a distant red giant population, with an upper limit on the stellar mass of ~106 M☉. Given the lack of evidence for either current star formation or an evolved population, we conclude that complex H cannot be a dwarf galaxy with properties similar to those of known dwarfs. Complex H is therefore one of the most massive known H I clouds that does not contain any stars. If complex H is self-gravitating, then this object is one of the few known dark galaxy candidates. These findings may offer observational support for the idea that the cold dark matter substructure problem is related to the difficulty of forming stars in low-mass dark matter halos; alternatively, complex H could be an example of a cold accretion flow onto the Milky Way.
The Astrophysical Journal 12/2008; 640(1):270. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We use Spitzer Space Telescope observations from the Spitzer Survey of the Small Magellanic Cloud (S3MC) to study the young stellar content of N66, the largest and brightest HII region in the SMC. In addition to large numbers of normal stars, we detect a significant population of bright, red infrared sources that we identify as likely to be young stellar objects (YSOs). We use spectral energy distribution (SED) fits to classify objects as ordinary (main sequence or red giant) stars, asymptotic giant branch stars, background galaxies, and YSOs. This represents the first large-scale attempt at blind source classification based on Spitzer SEDs in another galaxy. We firmly identify at least 61 YSOs, with another 50 probable YSOs; only one embedded protostar in the SMC was reported in the literature prior to the S3MC. We present color selection criteria that can be used to identify a relatively clean sample of YSOs with IRAC photometry. Our fitted SEDs indicate that the infrared-bright YSOs in N66 have stellar masses ranging from 2 Msun to 17 Msun, and that approximately half of the objects are Stage II protostars, with the remaining YSOs roughly evenly divided between Stage I and Stage III sources. We find evidence for primordial mass segregation in the HII region, with the most massive YSOs being preferentially closer to the center than lower-mass objects. Despite the low metallicity and dust content of the SMC, the observable properties of the YSOs appear consistent with those in the Milky Way. Although the YSOs are heavily concentrated within the optically bright central region of N66, there is ongoing star formation throughout the complex and we place a lower limit on the star formation rate of 3.2 x 10^-3 Msun/yr over the last ~1 Myr.
08/2007;
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Alberto D. Bolatto, Joshua D. Simon,
Snezana Stanimirovic,
Jacco Th. van Loon,
Ronak Y. Shah,
Kim Venn,
Adam K. Leroy,
Karin Sandstrom,
James M. Jackson,
Frank P. Israel,
Aigen Li,
Lister Staveley-Smith,
Caroline Bot,
Francois Boulanger,
Monica Rubio
[show abstract]
[hide abstract]
ABSTRACT: We present the initial results from the Spitzer Survey of the Small
Magellanic Cloud (S3MC), which imaged the star-forming body of the Small
Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the
F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and
takes a wide range of values that are unrelated to metallicity but
anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that
photodestruction is primarily responsible for the low abundance of PAHs
observed in star-forming low-metallicity galaxies. We use the S3MC images to
compile a photometric catalog of ~400,000 mid- and far-infrared point sources
in the SMC. The sources detected at the longest wavelengths fall into four main
categories: 1) bright 5.8 um sources with very faint optical counterparts and
very red mid-infrared colors ([5.8]-[8.0]>1.2), which we identify as YSOs. 2)
Bright mid-infrared sources with mildly red colors (0.16<[5.8]-[8.0]<0.6),
identified as carbon stars. 3) Bright mid-infrared sources with neutral colors
and bright optical counterparts, corresponding to oxygen-rich evolved stars.
And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This
excess is reminiscent of debris disks, and is detected in only a small fraction
of these stars (<5%). The majority of the brightest infrared point sources in
the SMC fall into groups one to three. We use this photometric information to
produce a catalog of 282 bright YSOs in the SMC with a very low level of
contamination (~7%).
08/2006;
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[show abstract]
[hide abstract]
ABSTRACT: We have imaged the entire Small Magellanic Cloud (SMC), one of the two nearest star-forming dwarf galaxies, in all seven IRAC and MIPS bands. The low mass and low metallicity (1/6 solar) of the SMC make it the best local analog for primitive galaxies at high redshift. By studying the properties of dust and star formation in the SMC at high resolution, we can gain understanding of similar distant galaxies that can only be observed in much less detail. In this contribution, we present a preliminary analysis of the properties of point sources detected in the Spitzer Survey of the Small Magellanic Cloud (S^3MC). We find ~400,000 unresolved or marginally resolved sources in our IRAC images, and our MIPS 24 micron mosaic contains ~17,000 point sources. Source counts decline rapidly at the longer MIPS wavelengths. We use color-color and color-magnitude diagrams to investigate the nature of these objects, cross-correlate their positions with those of known sources at other wavelengths, and show examples of how these data can be used to identify interesting classes of objects such as carbon stars and young stellar objects. For additional examples of some of the questions that can be studied with these data, please see the accompanying contributions by the other members of our team. The mosaic images and point source catalogs we have made have been released to the public on our website (http://celestial.berkeley.edu/spitzer).
04/2006;
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[show abstract]
[hide abstract]
ABSTRACT: We present infrared observations of the young, oxygen-rich supernova remnant 1E 0102.2-7219 (E0102) in the Small Magellanic Cloud, obtained with the Spitzer Space Telescope. The remnant is detected at 24 um but not at 8 or 70 um and has a filled morphology with two prominent filaments. We find evidence for the existence of up to 8x10^-4 Msolar of hot dust (T~120 K) associated with the remnant. Most of the hot dust is located in the central region of E0102 which appears significantly enhanced in infrared and radio continuum emission relative to the X--ray emission. Even if all of the hot dust was formed in the explosion of E0102, the estimated mass of dust is at least 100 times lower that what is predicted by some recent theoretical models. Comment: 5 pages, accepted by ApJL
09/2005;
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[show abstract]
[hide abstract]
ABSTRACT: We present a new survey for CO in dwarf galaxies using the Kitt Peak 12m telescope. We observed the central regions of 121 northern dwarfs with IRAS detections and no known CO emission. We detect CO in 28 of these galaxies and marginally detect another 16, increasing by about 50% the number of such galaxies known to have significant CO emission. The galaxies we detect are comparable in mass to the LMC, although somewhat brighter in CO and fainter in the FIR. Within dwarfs, we find that the CO luminosity, L_CO, is most strongly correlated with the K-band and the far infrared luminosities. There are also strong correlations with the radio continuum and B-band luminosities, and linear diameter. We suggest that L_CO and L_K correlate well because the stellar component of a galaxy dominates the midplane gravitational field and thus sets the pressure of the atomic gas, which controls the formation of H_2 from HI. We compare our sample with more massive galaxies and find that dwarfs and large galaxies obey the same relationship between CO and the 1.4 GHz radio continuum (RC) surface brightness. This relationship is well described by a Schmidt Law with Sigma_RC proportional to Sigma_CO^1.3. Therefore, dwarf galaxies and large spirals exhibit the same relationship between molecular gas and star formation rate (SFR). We find that this result is robust to moderate changes in the RC-to-SFR and CO-to-H_2 conversion factors. Our data appear to be inconsistent with large (order of magnitude) variations in the CO-to-H_2 conversion factor in the star forming molecular gas. [abridged] Comment: 28 pages, 14 figures, 5 tables, ApJ accepted
02/2005;
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[show abstract]
[hide abstract]
ABSTRACT: We derive rotation curves for four nearby, low-mass spiral galaxies and use them to constrain the shapes of their dark matter density profiles. This analysis is based on high-resolution two-dimensional Halpha velocity fields of NGC 4605, NGC 5949, NGC 5963, and NGC 6689 and CO velocity fields of NGC 4605 and NGC 5963. In combination with our previous study of NGC 2976, the full sample of five galaxies contains density profiles that span the range from alpha_dm = 0 to alpha_dm = 1.20, where alpha_dm is the power law index describing the central density profile. The scatter in alpha_dm from galaxy to galaxy is 0.44, three times as large as in Cold Dark Matter (CDM) simulations, and the mean density profile slope is alpha_dm = 0.73, shallower than that predicted by the simulations. These results call into question the hypothesis that all galaxies share a universal dark matter density profile. We show that one of the galaxies in our sample, NGC 5963, has a cuspy density profile that closely resembles those seen in CDM simulations, demonstrating that while galaxies with the steep central density cusps predicted by CDM do exist, they are in the minority. In spite of these differences between observations and simulations, the relatively cuspy density profiles we find do not suggest that this problem represents a crisis for CDM. Improving the resolution of the simulations and incorporating additional physics may resolve the remaining discrepancies. We also find that four of the galaxies contain detectable radial motions in the plane of the galaxy. We investigate the hypothesis that these motions are caused by a triaxial dark matter halo, and place lower limits on the ellipticity of the orbits in the plane of the disk of 0.043 - 0.175.
01/2005;
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[show abstract]
[hide abstract]
ABSTRACT: We present observations and analysis of rotation curves and dark matter halo density profiles in the central regions of four nearby dwarf galaxies. This observing program has been designed to overcome some of the limitations of other rotation curve studies that rely mostly on longslit spectra. We find that these objects exhibit the full range of central density profiles between constant density and NFW halos. This result suggests that there is a distribution of central density slopes rather than a unique halo density profile.
12/2003;
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[show abstract]
[hide abstract]
ABSTRACT: We present new HI observations of the high-velocity cloud (HVC) that we resolved near the Local Group dwarf galaxy LGS 3. The cloud is rotating, with an implied mass that makes it dark matter-dominated no matter what its distance from the Milky Way is. Our new, high-sensitivity Arecibo observations demonstrate that the faint HI features that we previously described as tidal tails are indeed real and do connect to the main body of the HVC. Thus, these observations are consistent with our original hypothesis of a tidal interaction between the HVC and LGS 3. We suggest that the HVC may be one of the missing dark matter satellites in the Local Group that are seen in Cold Dark Matter numerical simulations but have not yet been identified observationally.
11/2003;
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[show abstract]
[hide abstract]
ABSTRACT: We present high-resolution two-dimensional velocity fields in Halpha and CO of the nearby dwarf galaxy NGC 2976. Our observations were made at both higher spatial resolution (~75 pc) and higher velocity resolution (13 km/s in Halpha and 2 km/s in CO) than most previous studies. We show that NGC 2976 has a very shallow dark matter density profile, with rho(r) lying between rho ~ r^-0.3 and rho ~ r^0. We carefully test the effects of systematic uncertainties on our results, and demonstrate that well-resolved, two-dimensional velocity data can eliminate many of the systematic problems that beset longslit observations. We also present a preliminary analysis of the velocity field of NGC 5963, which appears to have a nearly NFW density profile.
11/2003;
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[show abstract]
[hide abstract]
ABSTRACT: We have obtained two-dimensional velocity fields of the dwarf spiral galaxy NGC 2976 in Halpha and CO. The high spatial (~75 pc) and spectral (13 km/s and 2 km/s, respectively) resolution of these observations, along with our multicolor optical and near-infrared imaging, allow us to measure the shape of the density profile of the dark matter halo with good precision. We find that the total (baryonic plus dark matter) mass distribution of NGC 2976 follows a rho_tot ~ r^(-0.27 +/- 0.09) power law out to a radius of 1.8 kpc, assuming that the observed radial motions provide no support. The density profile attributed to the dark halo is even shallower, consistent with a nearly constant density of dark matter over the entire observed region. A maximal disk fit yields an upper limit to the K-band stellar mass-to-light ratio (M*/L_K) of 0.09^{+0.15}_{-0.08} M_sun/L_sun,K (including systematic uncertainties), with the caveat that for M*/L_K > 0.19 M_sun/L_sun,K the dark matter density increases with radius, which is unphysical. Assuming 0.10 M_sun/L_sun,K < M*/L_K < 0.19 M_sun/L_sun,K, the dark matter density profile lies between rho_dm ~ r^-0.17 and rho_dm ~ r^-0.01. Therefore, independent of any assumptions about the stellar disk or the functional form of the density profile, NGC 2976 does not contain a cuspy dark matter halo. We also investigate some of the systematic effects that can hamper rotation curve studies, and show that 1) longslit rotation curves are far more vulnerable to systematic errors than two-dimensional velocity fields, 2) NGC 2976 contains large radial motions at small radii, and 3) the Halpha and CO velocity fields of NGC 2976 agree within their uncertainties. [slightly abridged] Comment: 30 pages, 4 tables, 13 figures (7 in color; Figures 1 and 3 are low-resolution to save space). Accepted for publication in ApJ. Version with full-resolution figures available at http://astro.berkeley.edu/~bolatto/ngc2976rotation.ps (46 MB)
07/2003;
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[show abstract]
[hide abstract]
ABSTRACT: We present a 93' by 93' map of the area near the Local Group dwarf galaxy LGS 3, centered on an HI cloud 30' away from the galaxy. Previous authors associated this cloud with LGS 3 but relied on observations made with a 36' beam. Our high-resolution (3.4'), wide-field Arecibo observations of the region reveal that the HI cloud is distinct from the galaxy and suggest an interaction between the two. We point out faint emission features in the map that may be gas that has been tidally removed from the HI cloud by LGS 3. We also derive the rotation curve of the cloud and find that it is in solid-body rotation out to a radius of 10', beyond which the rotation velocity begins to decline. Assuming a spherical geometry for the cloud, the implied mass is 2.8 x 10^7 (d/Mpc) M_{Sun}, where d is the distance in Mpc. The observed HI mass is 5.5 x 10^6 (d/Mpc)^2 M_{Sun}, implying that the cloud is dark-matter dominated unless its distance is at least 1.9 Mpc. We propose that the cloud is a high-velocity cloud that is undergoing a tidal interaction with LGS 3 and therefore is located roughly 700 kpc away from the Milky Way. The cloud then contains a total mass of ~2.0 x 10^7 M_{Sun}, 82% of which consists of dark matter. Comment: 5 pages, 2 color figures. Accepted for publication in ApJ Letters
10/2002;
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[show abstract]
[hide abstract]
ABSTRACT: We present the results of our search for faint Local Group dwarf galaxies in compact high-velocity clouds (HVCs). We used digitized Palomar Observatory Sky Survey (POSS) data to examine 1 square degree of sky around each of ~250 northern hemisphere HVCs. The POSS images were processed to remove foreground stars and large-scale backgrounds, smoothed to enhance arcminute-sized low surface brightness features, and then compared to the original plates. Using this technique, we located 60 candidate dwarf galaxies in the ~250 square degrees that we surveyed. Followup observations of these candidates have revealed several distant clusters of galaxies and a number of possible Galactic cirrus clouds, but no Local Group dwarfs. It appears that many of the low surface brightness features in the sky survey data are plate flaws. The second-generation red POSS plates are sensitive down to surface brightness levels of 25-26 magnitudes per square arcsecond, and our followup images reached 10-sigma limiting magnitudes of R = 21-23 for point sources. Given these limits, all known Local Group galaxies except four of the very diffuse, extended dwarf spheroidals located within 100 kpc of the Milky Way would have been detected had they been in our survey. Therefore, we can rule out the possibility that these HVCs are associated with normal but faint dwarf galaxies. If compact HVCs do contain stars, they must have surface brightnesses > 1 magnitude per square arcsecond fainter than most known Local Group galaxies. Comment: 17 pages, 6 figures, 2 tables, submitted to ApJ
04/2002;
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[show abstract]
[hide abstract]
ABSTRACT: We have obtained ~100 pc resolution CO and ~60 pc resolution Halpha observations of the dwarf spiral galaxy NGC 4605. We use them to derive a high resolution rotation curve and study the central density profile of NGC 4605's dark matter halo. We find that these observations do not agree with the predictions of most high resolution Cold Dark Matter calculations. We investigate two extreme cases: 1) NGC 4605 has a maximal exponential disk, that we model using K-band observations and remove to study the structure of its dark matter halo, and 2) NGC 4605 is dark matter dominated and its disk is dynamically negligible. Because the mass-to-light ratio of the maximal disk is already very low we favor the first solution, which indicates the halo has one component with a density profile rho ~ r^{-0.6} out to R~2.8 kpc. In the second case, the rotation curve requires the presence of two components: a small (~600 pc) core surrounded by a much steeper rho ~ r^{-1.1} halo. Removal of intermediate (submaximal) disks does not ameliorate the discrepancy between the predictions and the observations. Comment: 8 pages, 5 figures. Revised version (minor changes), accepted for publication in The Astrophysical Journal. Also available at http://astron.berkeley.edu/~bolatto/
08/2001;
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Alberto D. Bolatto, Joshua D. Simon,
Snežana Stanimirović,
Jacco Th. van Loon,
Ronak Y. Shah,
Kim Venn,
Adam K. Leroy,
Karin Sandstrom,
James M. Jackson,
Frank P. Israel,
Aigen Li,
Lister Staveley-Smith,
Caroline Bot,
Francois Boulanger,
Mónica Rubio
[show abstract]
[hide abstract]
ABSTRACT: We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S^3MC), which imaged the star-forming body of the SMC in all seven MIPS and IRAC wave bands. We find that the F_8/F_(24) ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 μm brightness and F_(24)/F_(70) ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S3MC images to compile a photometric catalog of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: (1) bright 5.8 μm sources with very faint optical counterparts and very red mid-infrared colors ([5.8] - [8.0] > 1.2), which we identify as YSOs; (2) bright mid-infrared sources with mildly red colors (0.16 ≾ [5.8] - [8.0] < 0.6), identified as carbon stars; (3) bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars; and (4) unreddened early B stars (B3-O9) with a large 24 μm excess. This excess is reminiscent of debris disks and is detected in only a small fraction of these stars (≾5%). The majority of the brightest infrared point sources in the SMC fall into groups 1-3. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (~7%).
