LBT observations of compact star-forming galaxies with extremely high [OIII]/[OII] flux ratios: HeI emission-line ratios as diagnostics of Lyman continuum leakage [GA]

We present Large Binocular Telescope spectrophotometric observations of five low-redshift (z<0.070) compact star-forming galaxies (CSFGs) with extremely high emission-line ratios O32 = [OIII]5007/[OII]3727, ranging from 23 to 43. Galaxies with such high O32 are thought to be promising candidates for leaking large amounts of Lyman continuum (LyC) radiation and, at high redshifts, for contributing to the reionization of the Universe. The equivalent widths EW(Hbeta) of the Hbeta emission line in the studied galaxies are very high, ~350-520A, indicating very young ages for the star formation bursts, <3 Myr. All galaxies are characterized by low oxygen abundances 12+logO/H = 7.46 – 7.79 and low masses Mstar~10^6-10^7 Msun, much lower than the Mstar for known low-redshift LyC leaking galaxies, but probably more typical of the hypothetical population of low-luminosity dwarf LyC leakers at high redshifts. A broad Halpha emission line is detected in the spectra of all CSFGs, possibly related to expansion motions of supernova remnants. Such rapid ionized gas motions would facilitate the escape of the resonant Ly$\alpha$ emission from the galaxy. We show that high O32 may not be a sufficient condition for LyC leakage and propose new diagnostics based on the HeI 3889/6678 and 7065/6678 emission-line flux ratios. Using these diagnostics we find that three CSFGs in our sample are likely to have density-bounded HII regions and are thus leaking large amounts of LyC radiation. The amount of leaking LyC radiation is probably much lower in the other two CSFGs.

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Y. Izotov, T. Thuan and N. Guseva
Wed, 28 Jun 17

Comments: 16 pages, 8 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

On the SFR-M$_*$ main sequence archetypal star-formation history and analytical models [GA]

We derive the SFH of MS galaxies showing how the SFH peak of a galaxy depends on its seed mass at e.g. z=5. Following the MS, galaxies undergo a drastic slow down of their stellar mass growth after reaching the peak of their SFH. According to abundance matching, these masses correspond to hot and massive DM halos which state could results in less efficient gas inflows on the galaxies and thus could be at the origin of the limited stellar mass growth. As a result, galaxies on the MS can enter the passive region of the UVJ diagram while still forming stars. The ability of the classical analytical SFHs to retrieve the SFR of galaxies from SED fitting is studied. Due to mathematical limitations, the exp-declining and delayed SFH struggle to model high SFR which starts to be problematic at z>2. The exp-rising and log-normal SFHs exhibit the opposite behavior with the ability to reach very high SFR, and thus model starburst galaxies, but not low values such as those expected at low redshift for massive galaxies. We show that these four analytical forms recover the SFR of MS galaxies with an error dependent on the model and the redshift. They are, however, sensitive enough to probe small variations of SFR within the MS but all the four fail to recover the SFR of rapidly quenched galaxies. However, these SFHs lead to an artificial gradient of age, parallel to the MS which is not exhibited by a simulated sample. This gradient is also produced on real data, using a sample of GOODS-South galaxies at 1.5<z<1.2. We propose a SFH composed of a delayed form to model the bulk of stellar population plus a flexibility in the recent SFH. This SFH provides very good estimates of the SFR of MS, starbursts, and rapidly quenched galaxies at all z. Furthermore, used on the GOODS-South sample, the age gradient disappears, showing its dependency on the SFH assumption made to perform the SED fitting.

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L. Ciesla, D. Elbaz and J. Fensch
Wed, 28 Jun 17

Comments: Submitted to A&A, includes first round of referee’s comments

First Detection of HC$_{5}$$^{15}$N in the Interstellar Medium [GA]

We report the first detection of HC${5}$$^{15}$N with the $J=9-8$ rotational line from the cyanopolyyne peak in Taurus Molecular Cloud-1 (TMC-1 CP) using the 45-m radio telescope of the Nobeyama Radio Observatory. The column density of HC${5}$$^{15}$N is derived to be (1.9 +- 0.5)*$10^{11}$ cm$^{-2}$ (1 sigma). We apply the double isotope method to derive the $^{14}$N/$^{15}$N ratios of HC$_{5}$N and HC$_{3}$N in TMC-1 CP. The $^{14}$N/$^{15}$N ratios are calculated to be 344 +- 53 and 257 +- 54 for HC$_{5}$N and HC$_{3}$N, respectively. The $^{14}$N/$^{15}$N ratio of HC$_{5}$N is lower than the elemental ratio in the local interstellar medium (~440) and slightly higher than that of HC$_{3}$N in TMC-1 CP. Since HC$_{3}$N is formed via the neutral-neutral reaction between C$_{2}$H$_{2}$ and CN, the slightly higher $^{14}$N/$^{15}$N ratio of HC$_{5}$N may support our previous suggestions that the main formation mechanism of HC$_{5}$N is the ion-molecule reactions between hydrocarbon ions (C$_{5}$H$_{n}^{+}$) and nitrogen atoms.

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K. Taniguchi and M. Saito
Wed, 28 Jun 17

Comments: Accepted by PASJ (Publications of the Astronomical Society of Japan) letter, 1 Figure, 2 Tables

Molecular scale height in NGC 7331 [GA]

Assuming a vertical hydrostatic equilibrium between different baryonic disk components, Poisson’s equations were set up and solved numerically to obtain the molecular scale height in the spiral galaxy NGC 7331. The scale height of the molecular disk was found to vary between $\sim 100-200$ pc depending on radius and assumed velocity dispersion of the molecular gas. The solutions of the hydrostatic equation and the rotation curve were used to produce a dynamical model and a total intensity map of the molecular disk. The modelled and the observed molecular disk matches with each other to a large extent. However, the modelled molecular disk falls short in producing observed projected thickness, specially at the central region. The velocity dispersion of the molecular disk is found to have no detectable influence on the projected thickness and hence can not account for the discrepancy in the thickness. A change in the inclination also found to be incapable of eliminating the difference completely. The molecular disk of NGC 7331 was projected to an inclination of 90$^o$ to estimate its observable edge-on thickness, which was found to be $\sim$ 2 kpc. Our result indicates that a simple hydrostatic condition is capable of explaining thick molecular disks observed in external galaxies.

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N. Patra
Wed, 28 Jun 17

Comments: Submitted to MNRAS

Formation of Massive, Dense Cores by Cloud-Cloud Collisions [GA]

We performed sub-parsec ($\sim$ 0.014 pc) scale simulations of cloud-cloud collisions of two idealized turbulent molecular clouds (MCs) with different masses in the range of $0.76 – 2.67 \times 10^4$M$_{\odot}$ and with collision speeds of 5 $-$ 30 km/s. Those parameters are larger than Takahira, Tasker and Habe (2014) (paper I) in which the colliding system showed a partial gaseous arc morphology that supports the NANTEN observations of objects indicated to be colliding MCs by numerical simulations. Gas clumps with density greater than $10^{-20}$ g cm$^{-3}$ were identified as pre-stellar cores and tracked through the simulation to investigate the effect of mass of colliding clouds and collision speeds on the resulting core population. Our results demonstrate that smaller cloud property is more important for results of cloud cloud collisions. The mass function of formed cores can be approximated by a power law relation with index $\gamma$ = -1.6 in slower cloud cloud collisions ($v \sim 5 $ km/s), in good agreement with observation of MCs. A faster relative velocity increases the number of cores formed in the early stage of collisions and shortens gas accretion phase of cores in the shocked region, leading to suppression of core growth. The bending point appears in the high mass part of the core mass function and the bending point mass decreases with increasing of the collision velocity for the same combination of colliding clouds. The high mass part of the core mass function than the bending point mass can be approximated by a power law with $\gamma$ = -2.3 that is very similar to the power index of the massive part of the observed initial stellar mass function. We discuss implication of our results for the massive star formation in our Galaxy.

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K. Takahira, K. Shima, E. Tasker, et. al.
Wed, 28 Jun 17

Comments: 15 pages, 15 figures

3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2 [GA]

The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still under debate. We present three-dimensional hydrodynamical adaptive mesh refinement (AMR) simulations of G2, modeled as an outflow from a “compact source” moving on the observed orbit. The construction of mock position-velocity (PV) diagrams enables a direct comparison with observations and allow us to conclude that the observational properties of the gaseous component of G2 could be matched by a massive ($\dot{M}\mathrm{w}=5\times 10^{-7} \;M{\odot} \mathrm{yr^{-1}}$) and slow ($50 \;\mathrm{km \;s^{-1}}$) outflow, as observed for T Tauri stars. In order for this to be true, only the material at larger ($>100 \;\mathrm{AU}$) distances from the source must be actually emitting, otherwise G2 would appear too compact compared to the observed PV diagrams. On the other hand, the presence of a central dusty source might be able to explain the compactness of G2’s dust component. In the present scenario, 5-10 years after pericentre the compact source should decouple from the previously ejected material, due to the hydrodynamic interaction of the latter with the surrounding hot and dense atmosphere. In this case, a new outflow should form, ahead of the previous one, which would be the smoking gun evidence for an outflow scenario.

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A. Ballone, M. Schartmann, A. Burkert, et. al.
Wed, 28 Jun 17

Comments: resubmitted to MNRAS after referee report, 16 pages, 11 figures

The L1157-B1 astrochemical laboratory: testing the origin of DCN [GA]

L1157-B1 is the brightest shocked region of the large scale molecular outflow, considered the prototype of chemically rich outflows, being the ideal laboratory to study how shocks affect the molecular gas. Several deuterated molecules have been previously detected with the IRAM 30m, most of them formed on grain mantles and then released into the gas phase due to the shock. We aim to investigate observationally the role of the different chemical processes at work that lead to formation the of DCN and test the predictions of the chemical models for its formation. We performed high angular resolution observations with NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuterated fraction, Dfrac(HCN).We detected emission of DCN(2-1) and H13CN(2-1) arising from L1157-B1 shock. Dfrac(HCN) is ~4×10$^{-3}$ and given the uncertainties, we did not find significant variations across the bow-shock. Contrary to HDCO, for which its emission delineates the region of impact between the jet and the ambient material, DCN is more widespread and not limited to the impact region. This is consistent with the idea that gas-phase chemistry is playing a major role in the deuteration of HCN in the head of the bow-shock, where HDCO is undetected as it is a product of grain-surface chemistry. The spectra of DCN and H13CN match the spectral signature of the outflow cavity walls, suggesting that their emission result from shocked gas. The analysis of the time dependent gas-grain chemical model UCL-CHEM coupled with a C-type shock model shows that the observed Dfrac(HCN) is reached during the post-shocked phase, matching the dynamical timescale of the shock. Our results indicate that the presence of DCN in L1157-B1 is a combination of gas-phase chemistry that produces the widespread DCN emission, dominating in the head of the bow-shock, and sputtering from grain mantles toward the jet impact region.

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G. Busquet, F. Fontani, S. Viti, et. al.
Wed, 28 Jun 17

Comments: Accepted for publication in A&A. 7 pages, 5 Figures, 1 Table