38 research outputs found
Warm Dust and Spatially Variable PAH Emission in the Dwarf Starburst Galaxy NGC 1705
We present Spitzer observations of the dwarf starburst galaxy NGC 1705
obtained as part of SINGS. The galaxy morphology is very different shortward
and longward of ~5 microns: short-wavelength imaging shows an underlying red
stellar population, with the central super star cluster (SSC) dominating the
luminosity; longer-wavelength data reveals warm dust emission arising from two
off-nuclear regions offset by ~250 pc from the SSC. These regions show little
extinction at optical wavelengths. The galaxy has a relatively low global dust
mass (~2E5 solar masses, implying a global dust-to-gas mass ratio ~2--4 times
lower than the Milky Way average). The off-nuclear dust emission appears to be
powered by photons from the same stellar population responsible for the
excitation of the observed H Alpha emission; these photons are unassociated
with the SSC (though a contribution from embedded sources to the IR luminosity
of the off-nuclear regions cannot be ruled out). Low-resolution IRS
spectroscopy shows moderate-strength PAH emission in the 11.3 micron band in
the eastern peak; no PAH emission is detected in the SSC or the western dust
emission complex. There is significant diffuse 8 micron emission after scaling
and subtracting shorter wavelength data; the spatially variable PAH emission
strengths revealed by the IRS data suggest caution in the interpretation of
diffuse 8 micron emission as arising from PAH carriers alone. The metallicity
of NGC 1705 falls at the transition level of 35% solar found by Engelbracht and
collaborators; the fact that a system at this metallicity shows spatially
variable PAH emission demonstrates the complexity of interpreting diffuse 8
micron emission. A radio continuum non-detection, NGC 1705 deviates
significantly from the canonical far-IR vs. radio correlation. (Abridged)Comment: ApJ, in press; please retrieve full-resolution version from
http://www.astro.wesleyan.edu/~cannon/pubs.htm
The Nature of Infrared Emission in the Local Group Dwarf Galaxy NGC 6822 As Revealed by Spitzer
We present Spitzer imaging of the metal-deficient (Z ~30% Z_sun) Local Group
dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR,
H alpha, HI, and radio continuum emission. Nebular emission strength correlates
with IR surface brightness; however, roughly half of the IR emission is
associated with diffuse regions not luminous at H alpha (as found in previous
studies). The global ratio of dust to HI gas in the ISM, while uncertain at the
factor of ~2 level, is ~25 times lower than the global values derived for
spiral galaxies using similar modeling techniques; localized ratios of dust to
HI gas are about a factor of five higher than the global value in NGC 6822.
There are strong variations (factors of ~10) in the relative ratios of H alpha
and IR flux throughout the central disk; the low dust content of NGC 6822 is
likely responsible for the different H alpha/IR ratios compared to those found
in more metal-rich environments. The H alpha and IR emission is associated with
high-column density (> ~1E21 cm^-2) neutral gas. Increases in IR surface
brightness appear to be affected by both increased radiation field strength and
increased local gas density. Individual regions and the galaxy as a whole fall
within the observed scatter of recent high-resolution studies of the radio-far
IR correlation in nearby spiral galaxies; this is likely the result of depleted
radio and far-IR emission strengths in the ISM of this dwarf galaxy.Comment: ApJ, in press; please retrieve full-resolution version from
http://www.astro.wesleyan.edu/~cannon/pubs.htm
On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 micron luminosities for 27 nearby (D < 5 Mpc) dwarf irregular
galaxies measured with the Spitzer Infrared Array Camera. We have constructed
the 4.5 micron luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with
secure distance and interstellar medium oxygen abundance measurements. The 4.5
micron L-Z relation is 12+log(O/H) = (5.78 +/- 0.21) + (-0.122 +/- 0.012)
M_[4.5], where M_[4.5] is the absolute magnitude at 4.5 micron. The dispersion
in the near-infrared L-Z relation is smaller than the corresponding dispersion
in the optical L-Z relation. The subsequently derived stellar mass-metallicity
M-Z relation is 12+log(O/H) = (5.65 +/- 0.23) + (0.298 +/- 0.030) log Mstar.
and extends the SDSS M-Z relation to lower mass by about 2.5 dex. We find that
the dispersion in the M-Z relation is similar over five orders of magnitude in
stellar mass, and that the relationship between stellar mass and interstellar
medium metallicity is similarly tight from high-mass to low-mass systems. We
find a larger scatter at low mass in the relation between effective yield and
total baryonic mass. In fact, there are a few dwarf galaxies with large yields,
which is difficult to explain if galactic winds are ubiquitous in dwarf
galaxies. The low scatter in the L-Z and M-Z relationships are difficult to
understand if galactic superwinds or blowout are responsible for the low
metallicities at low mass or luminosity. Naively, one would expect an ever
increasing scatter at lower masses, which is not observed.Comment: Accepted, Ap.J.; 18 pages (AASTeX 5.2; emulateapj.cls) with 12
figures. Full paper with figures at http://www.astro.umn.edu/~hlee/papers
Hot Dust and PAH Emission at Low Metallicity: A Spitzer Survey of Local Group and Other Nearby Dwarf Galaxies
We present Spitzer Space Telescope 4.5 and 8.0 micron imaging of 15 Local
Group and nearby dwarf galaxies. Our sample spans a range of more than one dex
in nebular metallicity and over three orders of magnitude in current star
formation rate, allowing us to examine the dependence of the diffuse 8 micron
emission, originating from hot dust and PAHs, on these parameters. We detect
prominent diffuse 8 micron emission in four of the most luminous galaxies in
the sample (IC 1613, IC 5152, NGC 55, and NGC 3109), low surface brightness
emission from four others (DDO 216, Sextans A, Sextans B, WLM), and no diffuse
emission from the remaining objects. We observe general correlations of the
diffuse 8 micron emission with both the current star formation rate and the
nebular metallicity of the galaxies in our sample. However, we also see
exceptions to these correlations that suggest other processes may also have a
significant effect on the generation of hot dust/PAH emission. These systems
all have evidence for old and intermediate age star formation, thus the lack of
diffuse 8 micron emission cannot be attributed to young galaxy ages. Also, we
find that winds are unlikely to explain the paucity of diffuse 8 micron
emission, since high resolution imaging of the neutral gas in these objects
show no evidence of blowout. Additionally, we propose the lack of diffuse 8
micron emission in low-metallicity systems may be due to the destruction of
dust grains by supernova shocks, assuming the timescale to regrow dust grains
and PAH molecules is long compared to the destruction timescale. The most
likely explanation for the observed weak diffuse 8 micron emission is at least
partly due to a general absence of dust (including PAHs), in agreement with
their low metallicities.Comment: Accepted by ApJ, 26 pages, 8 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)
We present Spitzer 3.6-160 micron images, Spitzer mid-infrared spectra, and
JCMT SCUBA 850 micron images of the Sombrero Galaxy (NGC 4594), an Sa galaxy
with a 10^9 M_solar low luminosity active galactic nucleus (AGN). The brightest
infrared sources in the galaxy are the nucleus and the dust ring. The spectral
energy distribution of the AGN demonstrates that, while the environment around
the AGN is a prominent source of mid-infrared emission, it is a relatively weak
source of far-infrared emission, as had been inferred for AGN in previous
research. The weak nuclear 160 micron emission and the negligible polycyclic
aromatic hydrocarbon emission from the nucleus also implies that the nucleus is
a site of only weak star formation activity and the nucleus contains relatively
little cool interstellar gas needed to fuel such activity. We propose that this
galaxy may be representative of a subset of low ionization nuclear emission
region galaxies that are in a quiescent AGN phase because of the lack of gas
needed to fuel circumnuclear star formation and Seyfert-like AGN activity.
Surprisingly, the AGN is the predominant source of 850 micron emission. We
examine the possible emission mechanisms that could give rise to the 850 micron
emission and find that neither thermal dust emission, CO line emission,
bremsstrahlung emission, nor the synchrotron emission observed at radio
wavelengths can adequately explain the measured 850 micron flux density by
themselves. The remaining possibilities for the source of the 850 micron
emission include a combination of known emission mechanisms, synchrotron
emission that is self-absorbed at wavelengths longer than 850 microns, or
unidentified spectral lines in the 850 micron band.Comment: Accepted to ApJ, 200
Warm Dust and Spatially Variable Polycyclic Aromatic Hydrocarbon Emission in the Dwarf Starburst Galaxy NGC 1705
We present Spitzer observations of the nearby dwarf starburst galaxy NGC 1705 obtained as part of the Spitzer Infrared Nearby Galaxies Survey. The galaxy morphology is very different shortward and longward of ~5 ÎŒm: optical and short-wavelength IRAC imaging shows an underlying red stellar population, with the central super star cluster (SSC) dominating the luminosity; longer wavelength IRAC and MIPS imaging reveals warm dust emission arising from two off-nuclear regions that are offset by ~250 pc from the SSC and that dominate the far-IR flux of the system. These regions show little extinction at optical wavelengths. The galaxy has a relatively low global dust mass (~2 Ă 10^5 M_â, implying a global dust-to-gas mass ratio ~2-4 times lower than the Milky Way average, roughly consistent with the metallicity decrease). The off-nuclear dust emission appears to be powered by photons from the same stellar population responsible for the excitation of the observed Hα emission; these photons are unassociated with the SSC (although a contribution from embedded sources to the IR luminosity of the off-nuclear regions cannot be ruled out). Low-resolution IRS spectroscopy shows moderate-strength PAH emission in the 11.3 ÎŒm band in the more luminous eastern peak; no PAH emission is detected in the SSC or the western dust emission complex. There is significant diffuse emission in the IRAC 8 ÎŒm band after starlight has been removed by scaling shorter wavelength data; the fact that IRS spectroscopy shows spatially variable PAH emission strengths compared to the local continuum within this diffuse gas suggests caution in the interpretation of IRAC diffuse 8 ÎŒm emission as arising from PAH carriers alone. The nebular metallicity of NGC 1705 falls at the transition level of ~0.35 Z_â found by Engelbracht and collaborators, below which PAH emission is difficult to detect; the fact that a system at this metallicity shows spatially variable PAH emission demonstrates the complexity of interpreting diffuse 8 ÎŒm emission in galaxies. NGC 1705 deviates significantly from the canonical far-infrared versus radio correlation, having significant far-infrared emission but no detected radio continuum
Novel genetic loci associated with hippocampal volume
The hippocampal formation is a brain structure integrally involved in episodic memory, spatial navigation, cognition and stress responsiveness. Structural abnormalities in hippocampal volume and shape are found in several common neuropsychiatric disorders. To identify the genetic underpinnings of hippocampal structure here we perform a genome-wide association study (GWAS) of 33,536 individuals and discover six independent loci significantly associated with hippocampal volume, four of them novel. Of the novel loci, three lie within genes (ASTN2, DPP4 and MAST4) and one is found 200 kb upstream of SHH. A hippocampal subfield analysis shows that a locus within the MSRB3 gene shows evidence of a localized effect along the dentate gyrus, subiculum, CA1 and fissure. Further, we show that genetic variants associated with decreased hippocampal volume are also associated with increased risk for Alzheimer's disease (rg =-0.155). Our findings suggest novel biological pathways through which human genetic variation influences hippocampal volume and risk for neuropsychiatric illness
Novel genetic loci underlying human intracranial volume identified through genome-wide association
Intracranial volume reflects the maximally attained brain size during development, and remains stable with loss of tissue in late life. It is highly heritable, but the underlying genes remain largely undetermined. In a genome-wide association study of 32,438 adults, we discovered five novel loci for intracranial volume and confirmed two known signals. Four of the loci are also associated with adult human stature, but these remained associated with intracranial volume after adjusting for height. We found a high genetic correlation with child head circumference (Ïgenetic=0.748), which indicated a similar genetic background and allowed for the identification of four additional loci through meta-analysis (Ncombined = 37,345). Variants for intracranial volume were also related to childhood and adult cognitive function, Parkinsonâs disease, and enriched near genes involved in growth pathways including PI3KâAKT signaling. These findings identify biological underpinnings of intracranial volume and provide genetic support for theories on brain reserve and brain overgrowth