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"

Spitzer Observations of Tidal Dwarf Galaxies

We present Spitzer observations of Tidal Dwarf Galaxies (TDGs) in three interacting systems: NGC 5291, Arp 105 & Stephan’s Quintet. The spectra show bright emission from polyaromatic hydrocarbons (PAHs), nebular lines and warm molecular hydrogen, characteristic of recent episodes of star formation. The PAH emission that falls in the IRAC 8.0 µm band leads to the TDGs having an extremely red IRAC color, with [4.5] − [8.0] \u3e 3. The emission from PAHs is characterized by a model with mainly neutral 100-C PAH atoms

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 $(2.69\pm0.05)\times10^{9}$ M$_{\odot}$ of H$_2$ associated with the inner ring (72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor ($\alpha_{\rm CO}$). The spokes and disk are not detected. Analysis of the inner ring's CO kinematics show it to be expanding ($V_{\rm exp}=68.9\pm4.9$ km s$^{-1}$) implying an $\approx70$ Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where HI surface density ($\Sigma_{\rm HI}$) is high, representing $M_{\rm H_2}=(7.5\pm0.8)\times10^{8}$ M$_{\odot}$ for a metallicity appropriate $\alpha_{\rm CO}$, giving small $\Sigma_{\rm H_2}$ ($3.7$ M$_{\odot}$ pc$^{-2}$), molecular fraction ($f_{\rm mol}=0.10$), and H$_2$ depletion timescales ($\tau_{\rm mol} \approx50-600$ Myr). Elsewhere in the outer ring $\Sigma_{\rm H_2}\lesssim 2$ M$_{\odot}$ pc$^{-2}$, $f_{\rm mol}\lesssim 0.1$ and $\tau_{\rm mol}\lesssim 140-540$ Myr (all $3\sigma$). The inner ring and nucleus are H$_2$-dominated and are consistent with local spiral SF laws. $\Sigma_{\rm SFR}$ in the outer ring appears independent of $\Sigma_{\rm H_2}$, $\Sigma_{\rm HI}$ or $\Sigma_{\rm HI+H_2}$. 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$_2$ component or an abundance of CO-faint low column density molecular clouds. The H$_2$ content of evolved starburst rings may therefore be substantially larger. Due to its lower $\Sigma_{\rm SFR}$ 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] 88$\mu$m line from a strong lensing candidate galaxy, H-ATLAS J113526.3-014605, hereafter G12v2.43, at z=3.127, using the $\rm 2^{nd}$ 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 $7.1\times10^{9}\,\rm(\frac{10}{\mu})\,\rm{L_{\odot}}\,$ likely arises from HII regions around massive stars, and the amount of Lyman continuum photons required to support the ionization indicate the presence of $(1.2-5.2)\times10^{6}\,\rm(\frac{10}{\mu})$ equivalent O5.5 or higher stars; where $\mu$ would be the lensing magnification factor. The observed line luminosity also requires a minimum mass of $\sim 2\times 10^{8}\,\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,$ in ionized gas, that is $0.33\%$ of the estimated total molecular gas mass of $6\times10^{10}\,\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,$. 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 $\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,\rm{yr^{-1}}$ and already has a stellar mass of $8\times 10^{10}\,\rm(\frac{10}{\mu})\,\rm{M_{\odot}}\,$. We also constrain the age of the current starburst to be $\leqslant$ 5 million years, making G12v2.43 a gas rich galaxy lying above the star-forming main sequence at z$\sim$3, undergoing a growth spurt and, could be on the main sequence within the derived gas depletion timescale of $\sim$66 million years.Comment: 11 pages, 3 figures, accepted for publication in The Astrophysical Journa

VLT Observations of NGC 1097\u27s “Dog-Leg” Tidal Stream: Dwarf Spheroidals and Tidal Streams

Aims. We investigate the structure and stellar population of two large stellar condensations (knots A & B) along one of the faint optical “jet-like” tidal streams associated with the spiral NGC 1097, with the goal of establishing their physical association with the galaxy and their origin. Methods. We use the VLT/FORS2 to get deep V-band imaging and low-resolution optical spectra of two knots along NGC 1097\u27s northeast “dog-leg” tidal stream. With this data, we explore their morphology and stellar populations. Results. Spectra were obtained for eleven sources in the field surrounding the tidal stream. The great majority of them turned out to be background or foreground sources, but the redshift of knot A (and perhaps of knot B) is consistent with that of NGC 1097. Using the V-band image of the “dog-leg” tidal feature we find that the two knots match the photometric scaling relations of canonical dwarf spheroidal galaxies (dSph) very well. Spectral analysis shows that knot A is mainly composed of stars near G-type, with no signs of ongoing star formation. Comparing its spectrum with a library of high resolution spectra of galactic globular clusters (GCs), we find that the stellar population of this dSph-like object is most similar to intermediate to metal rich galactic GCs. We find moreover, that the tidal stream shows an “S” shaped inflection as well as a pronounced stellar overdensity at knot A\u27s position. This suggests that knot A is being tidally stripped, and populating the stellar stream with its stars. Conclusions. We have discovered that two knots along NGC 1097\u27s northeast tidal stream share most of their spectral and photometric properties with ordinary dwarf spheroidal galaxies (dSph). Moreover, we find strong indications that the “dog-leg” tidal stream arises from the tidal disruption of knot A. Since it has been demonstrated that tidally stripping dSph galaxies need to loose most of their dark matter before starting to loose stars, we suggest that knot A is at present a CDM-poor object

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