Dr Selcuk Topal

Abstract:

We present ALMA observations of CO isotopologues and high-density molecular tracers (HCN, HCO+, CN, etc.) in NGC 7465, an unusually gas-rich early-type galaxy that acquired its cold gas recently. In the inner 300 pc, the molecular gas kinematics are misaligned with respect to all other galaxy components; as the gas works its way inward it is torqued into polar orbits about the stellar kinematically-decoupled core (KDC), indicating that the stellar KDC is not related to the current gas accretion event. The galaxy also exhibits unusually high 12CO/13CO line ratios in its nucleus but typical 13CO/C18O ratios. Our calculations show that this result does not necessarily indicate an unusual [12CO/13CO] abundance ratio but rather that 12CO (1-0) is optically thin due to high temperatures and/or large linewidths associated with the inner decoupled, misaligned molecular structure. Line ratios of the higher-density tracers suggest that the densest phase of molecular gas in NGC 7465 has a lower density than is typical for nearby galaxies, possibly as a result of the recent gas accretion. All of the observed molecular properties of NGC 7465 are consistent with it having acquired its molecular (and atomic) gas from a spiral galaxy. Further detailed studies of the CO isotopologues in other early-type galaxies would be valuable for investigating the histories of those that may have acquired their gas from dwarfs. Finally, these ALMA data also show an unidentified line source that is probably a background galaxy similar to those found at z=1-3 in blind CO surveys.

Abstract:

We present interferometric observations of CO lines (12CO(1-0, 2-1) and 13CO(1-0, 2-1)) and dense gas tracers (HCN(1-0), HCO+ (1-0), HNC(1-0) and HNCO(4-3)) in two nearby edgeon barred lenticular galaxies, NGC 4710 and NGC 5866, with most of the gas concentrated in a nuclear disc and an inner ring in each galaxy. We probe the physical conditions of a two-component molecular interstellar medium in each galaxy and each kinematic component by using molecular line ratio diagnostics in three complementary ways. First, we measure the ratios of the position-velocity diagrams of different lines, second we measure the ratios of each kinematic component’s integrated line intensities as a function of projected position, and third we model these line ratios using a non-local thermodynamic equilibrium radiative transfer code. Overall, the nuclear discs appear to have a tenuous molecular gas component that is hotter, optically thinner and with a larger dense gas fraction than that in the inner rings, suggesting more dense clumps immersed in a hotter more diffuse molecular medium. This is consistent with evidence that the physical conditions in the nuclear discs are similar to those in photo-dissociation regions. A similar picture emerges when comparing the observed molecular line ratios with those of other galaxy types. The physical conditions of the molecular gas in the nuclear discs of NGC 4710 and NGC 5866 thus appear intermediate between those of spiral galaxies and starbursts, while the star formation in their inner rings is even milder.

Abstract:

We present the stellar mass (M*) and Wide-Field Infrared Survey Explorer (WISE) absolute Band 1 magnitude (MW1) Tully-Fisher relations (TFRs) of subsets of galaxies from the CO Legacy Database for the Galex Arecibo SDSS Survey (COLD GASS). We examine the benefits and drawbacks of several commonly used fitting functions in the context of measuring CO(1-0) line widths (and thus rotation velocities), favouring the Gaussian Double Peak function. We find the MW1 and M* TFR, for a carefully selected sub-sample, to be MW1 = (-7.1 ± 0.6) [log(W50/sin i / km s^-1) - 23.83 ± 0.09 and log (M*/M⊙) = (3.3 ± 0.3) [log(W50/sin i / km s^-1) -2.58] + 10:51 ± 0.04, respectively, where W50 is the width of a galaxy's CO(1-0) integrated profile at 50% of its maximum and the inclination i is derived from the galaxy axial ratio measured on the SDSS r-band image. We find no evidence for any significant offset between the TFRs of COLD GASS galaxies and those of comparison samples of similar redshifts and morphologies. The slope of the COLD GASS M* TFR agrees with the relation of Pizagno et al. (2005). However, we measure a comparitively shallower slope for the COLD GASS MW1 TFR as compared to the relation of Tully and Pierce (2000). We attribute this to the fact that the COLD GASS sample comprises galaxies of various (late-type) morphologies. Nevertheless, our work provides a robust reference point with which to compare future CO TFR studies.

Abstract:

In this paper we use observations of molecular tracers inmetal-rich and a-enhanced galaxies to study the effect of abundance changes on molecular chemistry. We selected a sample of metalrich spiral and star-bursting objects from the literature, and present here new data for a sample of early-type galaxies (ETGs) previously studied by Crocker et al. We conducted the first survey of carbon monosulphide (CS) and methanol emission in ETGs, detecting seven objects in at least one CS transition, and methanol emission in five ETGs. We find that ETGs whose gas is dominated by ionization from star formation have enhanced CS emission, compared to their hydrogen cyanide (HCN) emission, supporting the hypothesis that CS is a better tracer of dense star-forming gas than HCN. We suggest that the methanol emission in these sources is driven by dust mantle destruction due to ionization from high-mass star formation in dense molecular clouds, but cannot rule out a component due to shocks dominating in some sources. We construct rotation diagrams for each early-type source where at least two transitions of a given species were detected. The rotational temperatures we derive for linear molecules vary between 3 and 9 K, with the majority of sources having rotational temperatures around 5 K. Despite the large uncertainty inherent in this method, the derived source-averaged CS and methanol column densities are similar to those found by other authors for normal spiral and starburst galaxies. This may suggest dense clouds are little affected by the differences between early-and late-type galaxies. Finally, we used the total column density ratios for both our ETG and literature galaxy sample to show for the first time that some molecular tracers do seem to show systematic variations that appear to correlate with metallicity, and that these variations roughly match those predicted by chemical models. Using this fact, the chemical models of Bayet et al. and assumptions about the optical depth we are able to roughly predict the metallicity of our spiral and ETG sample, with a scatter of ~0.3 dex. We provide the community with linear approximations to the relationship between the HCN and CS column density ratio and metallicity. Further study will clearly be required to determine if this, or any, molecular tracer can be used to robustly determine gas-phase metallically, but that a relationship exists at all suggests that in the future it may be possible to calibrate a metallicity indicator for the molecular interstellar medium. © 2013 The Authors.

Abstract:

Combining observations of multiple CO lines with radiative transfer modelling is a verypowerful tool to investigate the physical properties of the molecular gas in galaxies. Usingnew observations and literature data, we provide the most complete CO ladders ever generatedfor eight star-forming regions in the spiral arms and inter-arms of the spiral galaxyNGC 6946, with observations of the CO(1-0), CO(2-1), CO(3-2), CO(4-3), CO(6-5),13CO(1-0) and 13CO(2-1) transitions. For each region, we use the large velocity gradientassumption to derive beam-averaged molecular gas physical properties, namely the gas kinetictemperature (TK), H2 number volume density (n(H2)) and CO number column density(N(CO)). Two complementary approaches are used to compare the observations with themodel predictions: x2 minimization and likelihood. The physical conditions derived varygreatly from one region to the next: TK = 10-250 K, n(H2) = 102.3-107.0 cm-3 and N(CO) =1015.0-1019.3 cm-2. The spectral line energy distribution (SLED) in some of these extranuclearregions indicate a star formation activity that is more intense than that at the centre of ourown Milky Way. The molecular gas in regions with a large SLED turnover transition (Jmax > 4) is hot but tenuous with a high CO column density, while that in regions with a low SLEDturnover transition (Jmax ≤ 4) is cold but dense with a low CO column density. We finallydiscuss and find some correlations between the physical properties of the molecular gas ineach region and the presence of young stellar population indicators (supernova remnants, HIIregions, HI holes, etc.). © 2013 The Authors.