104 research outputs found
Why Are Ring Galaxies Interesting?
Compared with ordinary spirals, the ISM in ring galaxies experiences markedly
different physical conditions and evolution. As a result, ring galaxies provide
interesting perspectives on the triggering/quenching of large scale star
formation and the destructive effects of massive stars on molecular cloud
complexes. We use high resolution radio, sub-millimeter, infrared, and optical
data to investigate the role of gravitational stability in star formation
regulation, factors influencing the ISM's molecular fraction, and evidence of
peculiar star formation laws and efficiencies in two highly evolved ring
galaxies: Cartwheel and the Lindsay-Shapley ring.Comment: 6 pages with 4 figures (2 color). To appear in the conference
proceedings for "Galaxy Wars: Stellar Populations and Star Formation in
Interacting Galaxies"
Wheels of Fire. IV: Star Formation and the Neutral ISM in the Ring Galaxy AM0644-741
We combine data from the Australia Telescope National Facility and Swedish ESO Submillimeter Telescope to investigate the neutral interstellar medium (ISM) in AM0644-741, a large and robustly star-forming ring galaxy. The galaxy\u27s ISM is concentrated in the 42 kpc diameter starburst ring, but appears dominated by atomic gas, with a global molecular fraction (f mol) of only 0.062 ± 0.005. Apart from the starburst peak, the gas ring appears stable against the growth of gravitational instabilities (Q gas = 3-11). Including the stellar component lowers Q overall, but not enough to make Q \u3c 1 everywhere. High star formation efficiencies (SFEs) follow from the ring\u27s low H2 content. AM0644-741\u27s star formation law is highly peculiar: H I obeys a Schmidt law while H2 is uncorrelated with star formation rate density. Photodissociation models yield low volume densities in the ring, especially in the starburst quadrant (n 2 cm–3), implying a warm neutral medium dominated ISM. At the same time, the ring\u27s pressure and ambient far-ultraviolet radiation field lead to the expectation of a predominantly molecular ISM. We argue that the ring\u27s high SFE, low f mol and n, and peculiar star formation law follow from the ISM\u27s 100 Myr confinement time in the starburst ring, which amplifies the destructive effects of embedded massive stars and supernovae. As a result, the ring\u27s molecular ISM becomes dominated by small clouds, causing to be significantly underestimated by 12CO line fluxes: in effect, X CO X Gal despite the ring\u27s ≥solar metallicity. The observed H I is primarily a low-density photodissociation product, i.e., a tracer rather than a precursor of massive star formation. Such an over-cooked ISM may be a general characteristic of evolved starburst ring galaxies
Molecular Gas and Star Formation in the Cartwheel
Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(J=1-0) observations
are used to study the cold molecular ISM of the Cartwheel ring galaxy and its
relation to HI and massive star formation (SF). CO moment maps find
M of H associated with the inner ring
(72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor
(). The spokes and disk are not detected. Analysis of the
inner ring's CO kinematics show it to be expanding ( km
s) implying an Myr age. Stack averaging reveals CO emission
in the starburst outer ring for the first time, but only where HI surface
density () is high, representing M for a metallicity appropriate
, giving small ( M
pc), molecular fraction (), and H depletion
timescales ( Myr). Elsewhere in the outer ring
M pc,
and Myr (all ). The inner ring and
nucleus are H-dominated and are consistent with local spiral SF laws.
in the outer ring appears independent of ,
or . The ISM's long confinement in the
robustly star forming rings of the Cartwheel and AM0644-741 may result in
either a large diffuse H component or an abundance of CO-faint low column
density molecular clouds. The H content of evolved starburst rings may
therefore be substantially larger. Due to its lower and age
the Cartwheel's inner ring has yet to reach this state. Alternately, the outer
ring may trigger efficient SF in an HI-dominated ISM.Comment: 10-pages text; 5-figure
Molecular Gas and Star Formation in the Cartwheel
Atacama Large Millimeter/submillimeter Array 12CO(J = 1–0) observations are used to study the cold molecular ISM of the Cartwheel ring galaxy and its relation to H i and massive star formation (SF). CO moment maps find (2.69 ± 0.05) × 109 M⊙ of H2 associated with the inner ring (72%) and nucleus (28%) for a Galactic ICO-to- conversion factor (αCO). The spokes and disk are not detected. Analysis of the inner ring\u27s CO kinematics shows it to be expanding (Vexp = 68.9 ± 4.9 km s−1), implying an ≈70 Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where H i surface density (ΣH i) is high, representing M⊙ for a metallicity-appropriate αCO, giving small (3.7 M⊙ pc−2), molecular fraction (fmol = 0.10), and H2 depletion timescales (τmol ≈ 50–600 Myr). Elsewhere in the outer ring M⊙ pc−2, fmol 0.1 and τmol 140–540 Myr (all 3σ). The inner ring and nucleus are H2 dominated and are consistent with local spiral SF laws. ΣSFR in the outer ring appears independent of ΣH i, or The ISM\u27s long confinement in the robustly star-forming rings of the Cartwheel and AM0644-741 may result in either a large diffuse H2 component or an abundance of CO-faint low column density molecular clouds. The H2 content of evolved starburst rings may therefore be substantially larger. Due to its lower ΣSFR and age, the Cartwheel\u27s inner ring has yet to reach this state. Alternately, the outer ring may trigger efficient SF in a H i-dominated ISM
Detection of [O III] at z~3: A Galaxy above the Main Sequence, Rapidly Assembling its Stellar Mass
We detect bright emission in the far infrared fine structure [O III] 88m
line from a strong lensing candidate galaxy, H-ATLAS J113526.3-014605,
hereafter G12v2.43, at z=3.127, using the generation Redshift (z)
and Early Universe Spectrometer (ZEUS-2) at the Atacama Pathfinder Experiment
Telescope (APEX). This is only the fifth detection of this far-IR line from a
sub-millimeter galaxy at the epoch of galaxy assembly. The observed [O III]
luminosity of likely
arises from HII regions around massive stars, and the amount of Lyman continuum
photons required to support the ionization indicate the presence of
equivalent O5.5 or higher stars;
where would be the lensing magnification factor. The observed line
luminosity also requires a minimum mass of in ionized gas, that is
of the estimated total molecular gas mass of
. We compile multi-band
photometry tracing rest-frame UV to millimeter continuum emission to further
constrain the properties of this dusty high redshift star-forming galaxy. Via
SED modeling we find G12v2.43 is forming stars at a rate of 916
and already has a stellar
mass of . We also
constrain the age of the current starburst to be 5 million years,
making G12v2.43 a gas rich galaxy lying above the star-forming main sequence at
z3, undergoing a growth spurt and, could be on the main sequence within
the derived gas depletion timescale of 66 million years.Comment: 11 pages, 3 figures, accepted for publication in The Astrophysical
Journa
Radio and Infrared Selected Optically Invisible Sources in the Boötes NDWFS
We have combined data from the NOAO Deep Wide-Field Survey in Bo¨otes and the Spitzer Space Telescope to determine basic properties for sixteen optically “invisible” MIPS 24 µm (OIMS) and thirty-five optically “invisible” radio (OIRS) sources, including their spectral energy distributions (SED) and luminosities. Most OIMSs possess steep power-law SEDs over λrest = 1 − 10 µm, indicating the presence of obscured AGN in agreement with Spitzer spectroscopy. These objects are extremely luminous at rest-frame near and mid-IR (νLν(5 µm) ≈ 1038 − 1039 W), consistent with accretion near the Eddington limit and further implying that they host buried QSOs. The majority of the IRAC detected OIRSs have flat 3.6 to 24 µm SEDs, implying comparable emission from stellar photospheres and hot AGN illuminated dust. This may reflect relatively small amounts of dust close to the central engine or current low mass accretion rates. A small subset of OIRSs appear to be starburst dominated with photometric redshifts from 1.0 to 4.5. The OIMSs and OIRSs with significant starburst components have similar LK and stellar masses (M∗ ≈ 1011 M⊙) assuming minimal AGN contribution. Roughly half of the OIRSs are not detected by Spitzer’s IRAC or MIPS. These are most likely z & 2 radio galaxies. The IRAC detected OIRSs are more likely than OIMSs to appear non point-like in the 3.6 µm and 4.5 µm images, suggesting that interactions play a role in triggering their activity. The AGN powered OIMSs may represent sub-millimeter galaxies making the transition from starburst to accretion dominance in their evolution to current epoch massive ellipticals
Interacting dark matter contribution to the Galactic 511 keV gamma ray emission: constraining the morphology with INTEGRAL/SPI observations
We compare the full-sky morphology of the 511 keV gamma ray excess measured
by the INTEGRAL/SPI experiment to predictions of models based on dark matter
(DM) scatterings that produce low-energy positrons: either MeV-scale DM that
annihilates directly into e+e- pairs, or heavy DM that inelastically scatters
into an excited state (XDM) followed by decay into e+e- and the ground state.By
direct comparison to the data, we find that such explanations are consistent
with dark matter halo profiles predicted by numerical many-body simulations for
a Milky Way-like galaxy. Our results favor an Einasto profile over the cuspier
NFW distribution and exclude decaying dark matter scenarios whose predicted
spatial distribution is too broad. We obtain a good fit to the shape of the
signal using six fewer degrees of freedom than previous empirical fits to the
511 keV data. We find that the ratio of flux at Earth from the galactic bulge
to that of the disk is between 1.9 and 2.4, taking into account that 73% of the
disk contribution may be attributed to the beta decay of radioactive 26Al.Comment: 7 pages, 4 figures. Includes minor corrections, and a discussion of
threshold energies in XDM models. Published in JCA
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