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"

Higdon, James L.

Higdon, Sarah J. U.

English

arXiv

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\u27s molecular fraction, and evidence of peculiar star formation laws and efficiencies in two highly evolved ring galaxies: Cartwheel and the Lindsay-Shapley ring

Higdon, Sarah J.U.

Georgia Southern University: Digital Commons@Georgia Southern

Georgia Southern UniversityDigital Commons@Georgia SouthernPhysics Faculty Publications Physics & Astronomy, Department of3-24-2010Why Are Ring Galaxies Interesting?James L. HigdonGeorgia Southern University, jhigdon@georgiasouthern.eduSarah J.U. HigdonPhysics, shigdon@georgiasouthern.eduFollow this and additional works at: https://digitalcommons.georgiasouthern.edu/physics-facpubsPart of the Physics CommonsThis article is brought to you for free and open access by the Physics & Astronomy, Department of at Digital Commons@Georgia Southern. It has beenaccepted for inclusion in Physics Faculty Publications by an authorized administrator of Digital Commons@Georgia Southern. For more information,please contact digitalcommons@georgiasouthern.edu.Recommended CitationHigdon, James L., Sarah J.U. Higdon. 2010. "Why Are Ring Galaxies Interesting?." Galaxy Wars: Stellar Populations and Star Formationin Interacting Galaxies, Beverly Smith, Nate Bastian, Sarach J.U. Higdon, and James L. Higdon (Ed.), 423: 1-6 San Francisco. doi:1003.4691 source: http://arxiv.org/abs/1003.4691https://digitalcommons.georgiasouthern.edu/physics-facpubs/110arXiv:1003.4691v1  [astro-ph.CO]  24 Mar 2010**FULL TITLE**ASP Conference Series, Vol. **VOLUME**, c© **YEAR OF PUBLICATION****NAMES OF EDITORS**Why Are Ring Galaxies Interesting?James L. Higdon1 and Sarah J. U. Higdon1Abstract. Compared with ordinary spirals, the ISM in ring galaxies experi-ences markedly different physical conditions and evolution. As a result, ringgalaxies provide interesting perspectives on the triggering/quenching of largescale star formation and the destructive effects of massive stars on molecularcloud complexes. We use high resolution radio, sub-mm, infrared, and opticaldata to investigate the role of gravitational stability in star formation regulation,factors influencing the ISM’s molecular fraction, and evidence of peculiar starformation laws and efficiencies in two highly evolved ring galaxies: Cartwheeland the Lindsay-Shapley ring.1 IntroductionRing galaxies are dramatic examples of galaxy transformation caused by a re-markably simple interaction. Observations and numerical models (Lynds & Toomre1976) argue persuasively that these objects are formed by the near central pas-sage of a companion through a spiral along the rotation axis. The brief additionalgravitational force induces epicyclic motions throughout the disk, which act toform radially propagating orbit-crowded rings of gas and stars. The concentra-tion of the ISM into the expanding ring (at the disk’s expense) is nearly totaland can last for ≈ 400 Myrs. It is this radical rearrangement of the spiral’s ISMthat is responsible for their interesting star forming properties.2 Star Formation Rates in Ring GalaxiesStar formation rates (SFR) in ring galaxies are typically ≈ 5 M⊙ yr−1, i.e., some-what enhanced over large spirals but far below the SFRs inferred in LIRGs (cf.Appleton & Struck 1987; Higdon 1995; Higdon & Wallin 1997; Sanders & Mirabel1996). However, the distribution of star formation is unique, being completelyrestricted to the expanding rings while simultaneously extinguished over the in-terior disk. Both effects are evident in the Lindsay-Shapley ring (L-S, hereafter)and Cartwheel, shown in Figure 1. A weak nuclear source is responsible for∼< 5% of the star formation in both. The star forming rings are narrow, withslices showing very sharp radial cutoffs in Hα emission. This implies that OBstars remain in the rings for their Main Sequence lifetimes, which constrains thestellar velocity dispersion of the rings: σ∗ < ∆rring/τOB ≈ 45 km s−1. The weakline emission that is sometimes found in ring galaxy disks is post-starburst inorigin, i.e., arising from aging HII complexes powered by A-stars.1 Georgia Southern University, Department of Physics, Statesboro, GA 3045812 Higdon and HigdonFigure 1. A polychromatic view of two ring galaxies: (top) the L-S ringgalaxy (SFR = 8 M⊙ yr−1, Dring = 42 kpc) and (bottom) the Cartwheel(SFR = 21 M⊙ yr−1, Dring = 40 kpc). Massive star formation is restricted tothe rings, which dominate emission at Hα, far-infrared, and radio continuum.The right-most column shows the neutral atomic ISM to be likewise confinedto the rings, with low density gas filling the interior (Higdon et al. 2010).3 The Interstellar Medium in Ring Galaxies3.1 Atomic HydrogenL-S and Cartwheel have been mapped in HI (Figure 1). In such large systems≈ 95% of the atomic ISM is concentrated in the rings, resulting in high HIsurface densities: ΣHI = 30 − 120 M⊙ pc−2. At the same time, their interiorsare very gas poor, with ΣHI ∼< 2 M⊙ pc−2. HI line-widths are typically narrow(σHI < 10 km s−1). Kinematic analysis of the HI yields the ring’s expansionspeed (vexp), and thus an estimate of the ring galaxy’s age (Rring/vexp). Fromtheir measured radii and vexp (53±9 and 154±10 km s−1, respectively), the ringsin Cartwheel and L-S are ≈ 250 and 140 Myrs old (Higdon 1996;Higdon et al.2010). The SFR in ring galaxies correlates with their peak ΣHI, which explainswhy young systems (e.g., NGC 2793 with age ≈ 50 Myr) have such low SFR:they are still organizing their ISM into a dense ring.3.2 Molecular Gas in the L-S Ring GalaxyStars form in cold molecular gas, so HI data can only tell part of the story. Ringgalaxies are not very luminous in the rotational transitions of 12CO, a fact oftenattributed to reduced metallicities from snow-plowing outer disk gas into thering (cf. Horrelou et al. 1995). However L-S’s ring possesses ≈solar abundances(Few et al. 1982), which together with its large angular size, made it an idealtarget for the SEST (Higdon et al. 2010). We observed 16 positions in L-S in12CO(J=1-0) and 12CO(J=2-1) transitions: 14 on the ring and one each centeredon the nucleus and enclosed disk. Figure 2 shows CO detections in 9/14 ringpositions, defining two molecular arcs in the ring’s north and southwest. TheRing Galaxies 3latter coincides with the galaxy’s peak ΣHI and ΣHα. L-S’s ring is dominated by12CO(2-1)P26400 6800 720020-2P16400 6800 720020-2P36400 6800 7200 7600420-2P46400 6800 7200 76006420P56400 6800 7200420-2P66400 6800 720020-2P86000 6400 68008420-2P76400 6800 720020-2P136000 6400 680020-2P126000 6400 6800420-2P106000 6400 68006420-2P116000 6400 68001086420-2P96000 6400 680086420-2-4T A*( mK )Velocity ( km/s )Disk6400 6800 72002-2Nucleus6400 6800 720020-2P146000 6400 6800 720020-2Figure 2. H2 in L-S as traced by12CO(J=2-1) emission using the SwedishESO Sub-millimeter Telescope (SEST ). The circles represent the beam at230 GHz (22′′ FWHM). Hα emission is shown in grey-scale (linear stretch),while contours show HI surface density. Molecular gas is clearly detectedat P3-P7 (north) and P9-P12 (southwest), with broad and complicated lineprofiles as a rule. The disk and nucleus are not detected (Higdon et al. 2010).atomic rather than molecular gas. For a Galactic ICO-NH2 , we find MH2/MHI =0.06 ± 0.01. Astonishingly, a typical dwarf galaxy has nearly as much H2 asL-S’s ring (Leroy et al. 2005). The molecular gas fraction (fmol =MH2/(MHI +MH2)) varies considerably around the ring, and is lowest in the ring’s southwestquadrant (fmol ∼< 0.03 at P9-P11), where both ΣHI and ΣHα peak.The 12CO and HI line profiles in L-S’s ring can be extremely broad (σgas =250− 400 km s−1), with multiple velocity components or broad tails evident. Itis not clear if this represents out-of-plane gas motions or caustics, though prelim-inary numerical models suggest the latter (J. Wallin, private communication).4 Star Formation Processes in the Ring4.1 The Role of Gravitational InstabilitiesThe onset of robust star formation in gas disks can be described in terms of localgravitational stability parameters, e.g., Qgas = σgasκ/(piGΣgas), where σgas isthe gas velocity dispersion and κ is the disk’s epicyclic frequency (Quirk 1972).4 Higdon and HigdonRegions of the disk where Qgas < 1 are Jeans unstable and prone to collapse,leading to the formation of molecular cloud complexes and eventually stars. The“bead on a string” morphology evident in Hα (Figure 1) suggests that the ringsare gravitationally unstable. Are they?The Cartwheel’s ring is, even ignoring its (unknown) molecular component.However, Qgas > 1 essentially everywhere in L-S’a ring (Figure 3), due to thelarge σgas. This ignores, however, the stellar component’s contribution to Q.Using IRAC 4.5 µm data we find that the stellar mass surface density (Σ∗)everywhere exceeds that of gas (i.e., Σ∗ > ΣHI+ΣH2). A gravitational stabilityparameter combining stars and gas can be written Qtot =κpiG(Σgasσgas+ Σ∗σ∗)−1(Wang & Silk 1994), where the ring’s radial Hα profile constrains σ∗ ∼< 45 kms−1. Figure 3 shows that when the stellar component is included, Qtot < 1 andL-S’s ring is everywhere Jeans unstable. Conversely, the interior disks in bothFigure 3. Azimuthal variations in Q for Cartwheel (left) and L-S (right).The Cartwheel’s HI ring is largely sub-critical (Qgas < 1), but not L-S evenafter adding molecular gas. Only when stellar mass is included does the ringbecome gravitationally unstable (i.e., Qtot < 1) everywhere.satisfy Qtot > 1, i.e., these regions are stable against the growth of gravitationalinstabilities and star formation is effectively quenched.4.2 Evidence for Peculiar Star Formation LawsA strong correlation exists between SFR/area (ΣSFR) and the surface densityof cold gas in galaxies, i.e., the “Schmidt Law”, written ΣSFR = βΣNgas. InM 51 for example, H2 and SFR/area obey ΣSFR ∝ Σ1.37±0.03H2(Figure 4). HIis uncorrelated with ΣSFR, implying that it is a photo-dissociation product andnot directly involved in the star formation process. We show star formation lawsderived for the Cartwheel and L-S in the same figure. Both are peculiar. UnlikeM 51, atomic gas in the Cartwheel correlates with ΣSFR in most of the ring,though with a small N . The exponent becomes negative (i.e., anti-correlated)where ΣSFR peaks. Would this peculiarity disappear if the molecular componentwere available? Not if L-S’s ring is any guide: atomic gas obeys M 51’s molecularSchmidt Law, but its cold molecular ISM is uncorrelated with ΣSFR, which isRing Galaxies 5completely the opposite from M 51. How can H2 be so apparently disconnectedfrom star formation?Figure 4. Star formation laws in the rings of L-S (left) and Cartwheel (right)relative to M 51 (Kennicutt et al. 2007). H2 in M 51 (black & green triangles)obeys a Schmidt Law, but HI (blue) is uncorrelated with ΣSFR. In L-S theopposite is true: HI (red squares) obeys a Schmidt Law but H2 is uncorrelated(red circles & arrows). In Cartwheel atomic gas can be correlated (filled purplesquares) or anti-correlated (empty squares) with SFR/area.4.3 Enhanced Star Formation EfficienciesStar formation efficiency (SFE) is the yield of massive stars per unit H2 mass.Young et al. (1996) find nearly constant SFE (≡ log(LHα/MH2)) from S0 to Scd(≈ −1.8 L⊙/M⊙). Later types show higher SFE, peaking at −0.8 L⊙/M⊙ forIrr. Detecting molecular gas in L-S’s ring allows the first estimate of SFE in aring galaxy. We find SFE = −0.7 ± 0.1 L⊙/M⊙, i.e., similar to an Irr and anorder of magnitude higher than the (presumably ∼Sa) progenitor. This resultdepends, of course, on our ability to reliably measure H2 in the ring using12COemission.5 Why is the Molecular Gas Fraction So Low?The rings are gas rich but seemingly H2 poor. Since HI rapidly converts intoH2, the low fmol cannot signal the consumption of the molecular gas reservoir.Nor can metallicity effects by themselves be responsible, at least in L-S. Thegas phase pressure (PISM) might be a factor, as it directly affects the HI to H2conversion rate. Elmegreen (1993) finds fmol ≈ (PISM/P⊙)2.2(χ/χ⊙)−1, whereχ is the ambient UV-field. For L-S’s ring we estimated χ with Spitzer andGALEX images, and PISM ≈ (piG/2)(Σ2gas +σgasσ∗ΣgasΣ∗), with the ring’s stellarsurface mass density derived with IRAC 4.5 µm data. We find very high gasphase pressures, with PISM/PISM,local ≈ 30− 400, leading us to expect fmol ≈ 1everywhere. L-S’s ring should be dominated by molecular gas. Why isn’t it?6 Higdon and HigdonWe used photo-dissociation models in Allen et al. (2004) to estimate averagegas volume densities (n) in L-S’s ring given its ΣHI and UV-field. In the northernhalf of the ring, n = 100 − 300 cm−3, implying an ISM dominated by the ColdNeutral Medium (CNM, T = 50 − 100 K), i.e., the precursor of cold molecularclouds. In the southwest, where ΣSFR and ΣHI are both much higher, the modelsgive n ≈ 2 cm−3, which taken at face value, points to an ISM dominated by theWarm Neutral Medium (WNM, T ≈ 7000 K). How can you form stars out ofthis?We believe the answer lies in fundamental differences in the environmentsof rings and spiral arms. Consider that molecular clouds spend ≈ 20 Myrs inthe arms of grand design spirals like M 51, whereas the ISM is confined in rings,equally as dense and actively forming stars, for ≈ 200 Myr. While molecularcloud growth is enhanced in the high Σgas rings, the destructive effects of SNeand OB stars are also amplified. A dominant WNM might be expected as themolecular clouds become fragmented and “over-cooked” by shocks and sustainedUV-fields. CO might in this case retreat to the inner-most cloud cores resultingin weak ICO and underestimates of ΣH2 . This might explain the peculiar SchmidtLaws and enhanced SFE. At the same time, higher cloud collision rates mightfavor the formation of unusually large molecular cloud complexes and moreefficient star formation. More work remains though results from Cartwheel andL-S are intriguing.6 Summary and Future ProspectsThe distribution and gravitational stability of a ring galaxy’s ISM changes dra-matically as it evolves. Further, the long confinement in the dense star formingring is expected result in a fragmented and largely atomic ISM, which may ex-plain the peculiar star formation laws, efficiencies, and fmol observed in the twolargest and most evolved ring galaxies, Cartwheel and L-S. Future progress willrequire sensitive and high resolution assays of the molecular ISM in these andother ring galaxies, which will be possible with ALMA and CARMA.ReferencesAppleton, P. & Struck, C. J. 1987, ApJ, 312, 103Allen, R. J., Heaton, H., & Kaufmann, M. J. 2004, ApJ, 608, 314Elmegreen, B. E. 1993, ApJ, 411, 170Few, J. M., Madore, B., & Arp, H. 1982, MNRAS, 199, 633Higdon, J. L. 1995, ApJ, 455, 524Higdon, J. L. 1996, ApJ, 467, 241Higdon, J. L. & Wallin, J. F. 1997, ApJ, 474, 686Higdon, J. L., Higdon, S. J. U., Rand, R. J., & Nord, M. 2010, submitted to ApJHorrelou, C., Casoli, F., Combes, F., & Dupraz, C. 1995, A&A, 298, 743Kennicutt, R. C. et al. 2007, ApJ, 671, 333Leroy, A. et al. 2005, ApJ, 625, 763Lynds, R. & Toomre, A. 1976, ApJ, 209, 382Quirk, W. 1972, ApJ, 176, L9Sanders, D. & Mirabel, I. 1996, ARA&A, 34, 749Wang, B. & Silk, J. 1994, ApJ, 427, 759Young, J. et al. 1996, AJ, 112, 1903

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