arXiv subjects

Thomas G. Bisbas

Publications and source records attributed to Thomas G. Bisbas.

At least 37 records · Page 2Linked to original sources

arXiv2024

Ionized Carbon in Galaxies: The [C II] 158 $μ$m Line as a Total Molecular Gas Mass Tracer Revisited

In this paper we present a statistical study of the [C II] 158 $μ$m line and the CO(1-0) emission for a sample of $\sim$200 local and high-$z$ (32 sources with $z>1$) galaxies with much different physical conditions. We explore the correlation between the luminosities of [C II] and CO(1-0) lines, and obtain a strong linear relationship, confirming that [C II] is able to trace total molecular gas mass, with a small difference between (U)LIRGs and less-luminous galaxies. The tight and linear relation between [C II] and CO(1-0) is likely determined by the average value of the observed visual extinction $A_V$ and the range of $G_0/n$ in galaxies. Further investigations into the dependence of $L_{\mathrm{[CII]}}/L_{\mathrm{CO(1-0)}}$ on different physical properties show that $L_{\mathrm{[CII]}}/L_{\mathrm{CO(1-0)}}$ (1) anti-correlates with $Σ_{\mathrm{IR}}$, and the correlation becomes steeper when $Σ_{\mathrm{IR}} \gtrsim 10^{11}$ $L_\odot\,\mathrm{kpc}^{-2}$; (2) correlates positively with the distance from the main sequence $Δ(\mathrm{MS})$ when $Δ(\mathrm{MS})\lesssim 0$; and (3) tends to show a systematically smaller value in systems where the [C II] emission is dominated by ionized gas. Our results imply that caution needs to be taken when applying a constant [[C II]-to-$M_{\mathrm{H_2}}$ conversion factor to estimate the molecular gas content in extreme cases, such as galaxies having low-level star formation activity or high SFR surface density.

astro-ph.GA↗

arXiv2024

A new measurement of the Galactic $^{12}$C/$^{13}$C gradient from sensitive HCO$^+$ absorption observations

We present a new constraint on the Galactic $^{12}$C/$^{13}$C gradient with sensitive HCO$^+$ absorption observations against strong continuum sources. The new measurements suffer less from beam dilution, optical depths, and chemical fractionation, allowing us to derive the isotopic ratios precisely. The measured $^{12}$C/$^{13}$C ratio in the Solar neighborhood (66$\pm$5) is consistent with those obtained from CH$^+$. Two measurements toward the Galactic Center are 42.2$\pm$1.7 and 37.5$\pm$6.5. Though the values are a factor of 2$\sim$3 higher than those derived from dense gas tracers (e.g., H$_2$CO, complex organic molecules) toward Sagittarius (Sgr) B2 regions, our results are consistent with the absorption measurements from c-C$_3$H$_2$ toward Sgr B2 ($\sim$40), and those from CH$^+$ toward Sgr A$^*$ and Sgr B2(N) ($>$30). We calculate a new Galactic $^{12}$C/$^{13}$C gradient of (6.4$\pm$1.9)$R_{\rm GC}$/kpc+(25.9$\pm$10.5), and find an increasing trend of $^{12}$C/$^{13}$C gradient obtained from high-density to low-density gas tracers, suggesting opacity effects and chemical fractionation may have a strong impact on the isotopic ratios observed at high-density regions.

astro-ph.GA↗

arXiv2024

A new analytic approach to infer the cosmic-ray ionization rate in hot molecular cores from HCO$^+$, N$_2$H$^+$, and CO observations

The cosmic-ray ionization rate ($ζ_2$) is one of the key parameters in star formation, since it regulates the chemical and dynamical evolution of molecular clouds by ionizing molecules and determining the coupling between the magnetic field and gas. However, measurements of $ζ_2$ in dense clouds (e.g., $n_{\rm H} \geq 10^4$ cm$^{-3}$) are difficult and sensitive to the model assumptions. The aim is to find a convenient analytic approach that can be used in high-mass star-forming regions (HMSFRs), especially for warm gas environments such as hot molecular cores (HMCs). We propose a new analytic approach to calculate $ζ_2$ through HCO$^+$, N$_2$H$^+$, and CO measurements. Our method gives a good approximation, to within $50$\%, of $ζ_2$ in dense and warm gas (e.g., $n_{\rm H} \geq 10^4$ cm$^{-3}$, $T = 50, 100$ K) for $A_{\rm V} \geq 4$ mag and $t \geq 2\times10^4$ yr at Solar metallicity. The analytic approach gives better results for higher densities. However, it starts to underestimate the CRIR at low metallicity ($Z = 0.1Z_\odot$) and high CRIR ($ζ_2 \geq 3\times10^{-15}$ s$^{-1}$). By applying our method to the OMC-2 FIR4 envelope and the L1157-B1 shock region, we find $ζ_2$ values of $(1.0\pm0.3)\times10^{-14}$ s$^{-1}$ and $(2.2\pm0.4)\times10^{-16}$ s$^{-1}$, consistent with those previously reported. We calculate $ζ_2$ toward a total of 82 samples in HMSFRs, finding that the average value of $ζ_2$ toward all HMC samples ($ζ_2$ = (7.4$\pm$5.0)$\times$10$^{-16}$ s$^{-1}$) is more than an order of magnitude higher than the theoretical prediction of cosmic-ray attenuation models, favoring the scenario that locally accelerated cosmic rays in embedded protostars should be responsible for the observed high $ζ_2$.

astro-ph.GA↗

arXiv2024

The CO-dark molecular gas in the cold HI arc

The CO-dark molecular gas (DMG), which refers to the molecular gas not traced by CO emission, is crucial for the evolution of the interstellar medium (ISM). While the gas properties of DMG have been widely explored in the Solar neighborhood, whether or not they are similar in the outer disk regions of the Milky Way is still not well understood. In this Letter, we confirm the existence of DMG toward a cold HI arc structure at 13 kpc away from the Galactic center with both OH emission and HI narrow self-absorption (HINSA). This is the first detection of HINSA in the outer disk region, in which the HINSA fraction ($N_{\rm HINSA}$/$N_{\rm H_2}$ = 0.022$\pm$0.011) is an order of magnitude higher than the average value observed in nearby evolved dark clouds, but is consistent with that of the early evolutionary stage of dark clouds. The inferred H$_2$ column density from both extinction and OH emission ($N_{\rm H_2} \approx 10^{20}$ cm$^{-2}$) is an order of magnitude higher than previously estimated. Although the ISM environmental parameters are expected to be different between the outer Galactic disk regions and the Solar neighborhood, we find that the visual extinction ($A_{\rm V}$ = 0.19$\pm$0.03 mag), H$_2$-gas density ($n_{\rm H_2} = 91\pm46$ cm$^{-3}$), and molecular fraction (58\%$\pm$28\%) of the DMG are rather similar to those of nearby diffuse molecular clouds. The existence of DMG associated with the expanding HI supershell supports a scenario where the expansion of supershells may trigger the formation of molecular clouds within a crossing timescale of the shock wave ($\sim$10$^6$ yr).

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arXiv2023

$α$-enhanced Astrochemistry: the Carbon cycle in extreme galactic conditions

Astrochemistry has been widely developed as a power tool to probe physical properties of the interstellar medium (ISM) in various conditions of the Milky Way (MW) Galaxy, and in near and distant galaxies. Most current studies conventionally apply linear scaling to all elemental abundances based on the gas-phase metallicity. However, these elements, including carbon and oxygen, are enriched differentially by stellar nucleosynthesis and the overall galactic chemical evolution, evident from $α$-enhancement in multiple galactic observations such as starbursts, high-redshift star-forming galaxies, and low-metallicity dwarfs. We perform astrochemical modeling to simulate the impact of an $α$-enhanced ISM gas cloud on the abundances of the three phases of carbon (C$^+$, C, CO) dubbed as `the carbon cycle'. The ISM environmental parameters considered include two cosmic-ray ionization rates ($ζ_{\rm CR}=10^{-17}$ and $10^{-15}\,{\rm s}^{-1}$), two isotropic FUV radiation field strengths ($χ/χ_0=1$ and $10^2$), and (sub-)linear dust-to-gas relations against metallicity, mimicking the ISM conditions of different galaxy types. In galaxies with [C/O] $<$ 0, CO, C and C$^+$ all decrease in both abundances and emission, though with differential biases. The low-$J$ CO emission is found to be the most stable tracer for the molecular gas, while C and C$^+$ trace H$_2$ gas only under limited conditions, in line with recent discoveries of [CI]-dark galaxies. We call for caution when using [CII]~$158μ$m and [CI](1-0) as alternative H$_2$-gas tracers for both diffuse and dense gas with non-zero [C/O] ratios.

astro-ph.GA↗

arXiv2023

SUNRISE: The rich molecular inventory of high-redshift dusty galaxies revealed by broadband spectral line surveys

Understanding the nature of high-$z$ dusty galaxies requires a comprehensive view of their ISM and molecular complexity. However, the molecular ISM at high-$z$ is commonly studied using only a few species beyond CO, limiting our understanding. In this paper, we present the results of deep 3 mm spectral line surveys using the NOEMA targeting two lensed dusty galaxies: APM 08279+5255 (APM), a quasar at redshift $z=3.911$, and NCv1.143 (NC), a $z=3.565$ starburst galaxy. The spectral line surveys cover rest-frame frequencies from about 330-550 GHz. We report the detection of 38 and 25 emission lines in APM and NC, respectively. The spectra reveal the chemical richness and the complexity of the physical properties of the ISM. By comparing the spectra of the two sources and combining the gas excitation analysis, we find that the physical properties and the chemical imprints of the ISM are different between them: the molecular gas is more excited in APM, exhibiting higher molecular-gas temperatures and densities compared to NC; the chemical abundances in APM are akin to the values of local AGN, showing boosted relative abundances of the dense gas tracers that might be related to high-temperature chemistry and/or XDRs, while NC more closely resembles local starburst galaxies. The most significant differences are found in H2O, where the 448GHz H2O line is significantly brighter in APM, likely linked to the intense far-infrared radiation from the dust powered by AGN. Our astrochemical model suggests that at such high column densities, FUV radiation is less important in regulating the ISM, while CRs (X-rays/shocks) are the key players in shaping the abundance of the molecules and the initial conditions of star formation. Such deep spectral line surveys open a new window to study the physical and chemical properties of the ISM and the radiation field of galaxies in the early Universe. (abridged)

astro-ph.GA↗

arXiv2023

Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy

We study the dynamics of cold molecular gas in two main-sequence galaxies at cosmic noon (zC-488879 at $z\simeq1.47$ and zC-400569 at $z\simeq2.24$) using new high-resolution ALMA observations of multiple $^{12}$CO transitions. For zC-400569 we also re-analyze high-quality H$α$ data from the SINS/zC-SINF survey. We find that (1) Both galaxies have regularly rotating CO disks and their rotation curves are flat out to $\sim$8 kpc contrary to previous results pointing to outer declines in the rotation speed $V_{\rm rot}$; (2) The intrinsic velocity dispersions are low ($σ_{\rm CO}\lesssim15$ km/s for CO and $σ_{\rm Hα}\lesssim37$ km/s for H$α$) and imply $V_{\rm rot}/σ_{\rm CO}\gtrsim17-22$ yielding no significant pressure support; (3) Mass models using HST images display a severe disk-halo degeneracy: models with inner baryon dominance and models with "cuspy" dark matter halos can fit the rotation curves equally well due to the uncertainties on stellar and gas masses; (4) Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale $a_0$ measured at $z\simeq0$. The question of the amount and distribution of dark matter in high-$z$ galaxies remains unsettled due to the limited spatial extent of the available kinematic data; we discuss the suitability of various emission lines to trace extended rotation curves at high $z$. Nevertheless, the properties of these two high-$z$ galaxies (high $V_{\rm rot}/σ_{\rm V}$ ratios, inner rotation curve shapes, bulge-to-total mass ratios) are remarkably similar to those of massive spirals at $z\simeq0$, suggesting weak dynamical evolution over more than 10 Gyr of the Universe's lifetime.

astro-ph.GA↗

arXiv2023

Abundance ratios of OH/CO and HCO+/CO as probes of the cosmic ray ionization rate in diffuse clouds

The cosmic-ray ionization rate (CRIR, $ζ_2$) is one of the key parameters controlling the formation and destruction of various molecules in molecular clouds. However, the current most commonly used CRIR tracers, such as H$_3^+$, OH$^+$, and H$_2$O$^+$, are hard to detect and require the presence of background massive stars for absorption measurements. In this work, we propose an alternative method to infer the CRIR in diffuse clouds using the abundance ratios of OH/CO and HCO$^+$/CO. We have analyzed the response of chemical abundances of CO, OH, and HCO$^+$ on various environmental parameters of the interstellar medium in diffuse clouds and found that their abundances are proportional to $ζ_2$. Our analytic expressions give an excellent calculation of the abundance of OH for $ζ_2$ $\leq$10$^{-15}$ s$^{-1}$, which are potentially useful for modelling chemistry in hydrodynamical simulations. The abundances of OH and HCO$^+$ were found to monotonically decrease with increasing density, while the CO abundance shows the opposite trend. With high-sensitivity absorption transitions of both CO (1--0) and (2--1) lines from ALMA, we have derived the H$_2$ number densities ($n_{\rm H_2}$) toward 4 line-of-sights (LOSs); assuming a kinetic temperature of $T_{\rm k}=50\,{\rm K}$, we find a range of (0.14$\pm$0.03--1.2$\pm$0.1)$\times$10$^2$ cm$^{-3}$}. By comparing the observed and modelled HCO$^+$/CO ratios, we find that $ζ_2$ in our diffuse gas sample is in the { range of $1.0_{-1.0}^{+14.8}$ $\times$10$^{-16}- 2.5_{-2.4}^{+1.4}$ $\times$10$^{-15}$ s$^{-1}$. This is $\sim$2 times higher than the average value measured at higher extinction, supporting an attenuation of CRs as suggested by theoretical models.

astro-ph.GA↗

arXiv2022

Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348

Ions (e.g., H$_3^+$, H$_2$O$^+$) have been used extensively to quantify the cosmic-ray ionization rate (CRIR) in diffuse sightlines. However, measurements of CRIR in low-to-intermediate density gas environments are rare, especially when background stars are absent. In this work, we combine molecular line observations of CO, OH, CH, and HCO$^+$ in the star-forming cloud IC~348, and chemical models to constrain the value of CRIR and study the response of the chemical abundances distribution. The cloud boundary is found to have an $A_{\rm V}$ of approximately 4 mag. From the interior to the exterior of the cloud, the observed $^{13}$CO line intensities drop by an order of magnitude. The calculated average abundance of $^{12}$CO (assuming $^{12}$C/$^{13}$C = 65) is (1.2$\pm$0.9) $\times$10$^{-4}$, which increases by a factor of 6 from the interior to the outside regions. The average abundance of CH (3.3$\pm$0.7 $\times$ 10$^{-8}$) is in good agreement with previous findings in diffuse and translucent clouds ($A_{\rm V}$ $<$ 5 mag). However, we did not find a decline in CH abundance in regions of high extinction ($A_{\rm V}\simeq$8 mag) as previously reported in Taurus. By comparing the observed molecular abundances and chemical models, we find a decreasing trend of CRIR as $A_{\rm V}$ increases. The inferred CRIR of $ζ_{cr}$ = (4.7$\pm$1.5) $\times$ 10$^{-16}$ s$^{-1}$ at low $A_{\rm V}$ is consistent with H$^+_3$ measurements toward two nearby massive stars.

astro-ph.GA↗

arXiv2022

PDFchem: A new fast method to determine ISM properties and infer environmental parameters using probability distributions

Determining the atomic and molecular content of the interstellar medium (ISM) as a function of environmental parameters is of fundamental importance to understand the star-formation process across the epochs. Although there exist various three-dimensional hydro-chemical codes modelling the ISM at different scales and redshifts, they are computationally expensive and inefficient for studies over a large parameter space. Building on our earlier approach, we present PDFchem, a novel algorithm that models the cold ISM at moderate and large scales using functions connecting the quantities of the local ($A_{\rm V,eff}$) and the observed ($A_{\rm V,obs}$) visual extinctions, and the local number density, $n_{\rm H}$, with probability density functions (PDF) of $A_{\rm V,obs}$ on cloud scales typically tens-to-hundreds of pc as an input. For any given $A_{\rm V,obs}$-PDF, sampled with thousands of clouds, the algorithm instantly computes the average abundances of the most important species (HI, H$_2$, CII, CI, CO, OH, OH$^+$, H$_2$O$^+$, CH, HCO$^+$) and performs radiative transfer calculations to estimate the average emission of the most commonly observed lines ([CII]~$158μ$m, both [CI] fine-structure lines and the first five rotational transitions of $^{12}$CO). We examine two $A_{\rm V,obs}$-PDFs corresponding to a non star-forming and a star-forming ISM region, under a variety of environmental parameters combinations. These cover FUV intensities in the range of $χ/χ_0=10^{-1}-10^3$, cosmic-ray ionization rates in the range of $ζ_{\rm CR}=10^{-17}-10^{-13}\,{\rm s}^{-1}$ and metallicities in the range of $Z=0.1-2\,{\rm Z}_{\odot}$. PDFchem is fast, easy to use, reproduces the PDR quantities of the time-consuming hydrodynamical models and can be used directly with observed data to understand the evolution of the cold ISM chemistry.

astro-ph.GA↗

arXiv2022

Cosmic-ray-induced H$_2$ line emission: Astrochemical modeling and implications for JWST observations

Context: It has been proposed that H$_2$ near-infrared lines may be excited by cosmic rays and allow for a determination of the cosmic-ray ionization rate in dense gas. One-dimensional models show that measuring both the H$_2$ gas column density and H$_2$ line intensity enables a constraint on the cosmic-ray ionization rate as well as the spectral slope of low-energy cosmic-ray protons in the interstellar medium (ISM). Aims: We aim to investigate the impact of certain assumptions regarding the H$_2$ chemical models and ISM density distributions on the emission of cosmic-ray induced H$_2$ emission lines. This is of particular importance for utilizing observations of these lines with the James Webb Space Telescope to constrain the cosmic-ray ionization rate. Methods: We compare the predicted emission from cosmic-ray induced, ro-vibrationally excited H$_2$ emission lines for different one- and three-dimensional models with varying assumptions on the gas chemistry and density distribution. Results: We find that the model predictions of the H$_2$ line intensities for the (1-0)S(0), (1-0)Q(2), (1-0)O(2) and (1-0)O(4) transitions at 2.22, 2.41, 2.63 and 3.00 $μ$m, respectively, are relatively independent of the astro-chemical model and the gas density distribution when compared against the H$_2$ column density, making them robust tracer of the cosmic-ray ionization rate. Conclusions: We recommend the use of ro-vibrational H$_2$ line emission in combination with estimation of the cloud's H$_2$ column density, to constrain the ionization rate and the spectrum of low energy cosmic-rays.

astro-ph.GA↗

arXiv2022

The origin of the [CII]-deficit in a simulated dwarf galaxies starburst

We present [CII] synthetic observations of smoothed particle hydrodynamics (SPH) simulations of a dwarf galaxy merger. The merging process varies the star-formation rate by more than three orders of magnitude. Several star clusters are formed, the feedback of which disperses and unbinds the dense gas through expanding HII regions and supernova (SN) explosions. For galaxies with properties similar to the modelled ones, we find that the [CII] emission remains optically thin throughout the merging process. We identify the Warm Neutral Medium ($3<\log T_{\rm gas}<4$ with $χ_{\rm HI}>2χ_{\rm H2}$) to be the primary source of [CII] emission ($\sim58\%$ contribution), although at stages when the HII regions are young and dense (during star cluster formation or SNe in the form of ionized bubbles) they can contribute $\gtrsim50\%$ to the total [CII] emission. We find that the [CII]/FIR ratio decreases due to thermal saturation of the [CII] emission caused by strong FUV radiation fields emitted by the massive star clusters, leading to a [CII]-deficit medium. We investigate the [CII]-SFR relation and find an approximately linear correlation which agrees well with observations, particularly those from the Dwarf Galaxy Survey. Our simulation reproduces the observed trends of [CII]/FIR versus $Σ_{\rm SFR}$ and $Σ_{\rm FIR}$, and it agrees well with the Kennicutt relation of SFR-FIR luminosity. We propose that local peaks of [CII] in resolved observations may provide evidence for ongoing massive cluster formation.

astro-ph.GA↗

arXiv2022

The impact of cosmic-ray attenuation on the carbon cycle emission in molecular clouds

Observations of the emission of the carbon cycle species (C, C+ CO) are commonly used to diagnose gas properties in the interstellar medium but are significantly sensitive to the cosmic-ray ionization rate. The carbon-cycle chemistry is known to be quite sensitive to the cosmic-ray ionization rate, $ζ$, controlled by the flux of low-energy cosmic rays which get attenuated through molecular clouds. However, astrochemical models commonly assume a constant cosmic-ray ionization rate in the clouds. We investigate the effect of cosmic-ray attenuation on the emission of carbon cycle species from molecular clouds, in particular the [CII] 158 $μ$m, [CI] 609 $μ$m and CO (J = 1 - 0) 115.27 GHz lines. We use a post-processed chemical model of diffuse and dense simulated molecular clouds and quantify the variation in both column densities and velocity integrated line emission of the carbon cycle with different cosmic-ray ionization rate models. We find that the abundances and column densities of carbon cycle species is significantly impacted by the chosen cosmic-ray ionization rate model: no single constant ionization rate can reproduce the abundances modelled with an attenuated cosmic-ray model. Further, we show that constant ionization rate models fail to simultaneously reproduce the integrated emission of the lines we consider, and their deviations from a physically derived cosmic-ray attenuation model is too complex to be simply corrected. We demonstrate that the two clouds we model exhibit a similar average $A_{\rm V, eff}$ -- $n_{\rm H}$ relationship, resulting in an average relation between the cosmic-ray ionization rate and density $ζ(n_{\rm H})$. We conclude by providing a number of implementation recommendations for CRs in astrochemical models, but emphasize the necessity for column-dependent cosmic-ray ionization rate prescriptions.

astro-ph.GA↗

arXiv2020

Photodissociation Region Diagnostics Across Galactic Environments

We present three-dimensional astrochemical simulations and synthetic observations of magnetised, turbulent, self-gravitating molecular clouds. We explore various galactic interstellar medium environments, including cosmic-ray ionization rates in the range of $ζ_{\rm CR}=10^{-17}$-$10^{-14}\,{\rm s}^{-1}$, far-UV intensities in the range of $G_0=1$-$10^3$ and metallicities in the range of $Z=0.1$-$2\,{\rm Z}_{\odot}$. The simulations also probe a range of densities and levels of turbulence, including cases where the gas has undergone recent compression due to cloud-cloud collisions. We examine: i) the column densities of carbon species across the cycle of CII, CI and CO, along with OI, in relation to the HI-to-H$_2$ transition; ii) the velocity-integrated emission of [CII]~$158μ$m, [$^{13}$CII]~$158μ$m, [CI]~$609μ$m and $370μ$m, [OI]~$63μ$m and $146μ$m, and of the first ten $^{12}$CO rotational transitions; iii) the corresponding Spectral Line Energy Distributions; iv) the usage of [CII] and [OI]~$63μ$m to describe the dynamical state of the clouds; v) the behavior of the most commonly used ratios between transitions of CO and [CI]; and vi) the conversion factors for using CO and CI as H$_2$-gas tracers. We find that enhanced cosmic-ray energy densities enhance all aforementioned line intensities. At low metallicities, the emission of [CII] is well connected with the H$_2$ column, making it a promising new H$_2$ tracer in metal-poor environments. The conversion factors of $X_{\rm CO}$ and $X_{\rm CI}$ depend on metallicity and the cosmic-ray ionization rate, but not on FUV intensity. In the era of ALMA, SOFIA and the forthcoming CCAT-prime telescope, our results can be used to understand better the behaviour of systems in a wide range of galactic and extragalactic environments.

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arXiv2020

Star Cluster Formation in Orion A

We introduce new analysis methods for studying the star cluster formation processes in Orion A, especially examining the scenario of a cloud-cloud collision. We utilize the CARMA-NRO Orion survey $^{13}$CO (1-0) data to compare molecular gas to the properties of YSOs from the SDSS III IN-SYNC survey. We show that the increase of $v_{\rm 13CO} - v_{\rm YSO}$ and $Σ$ scatter of older YSOs can be signals of cloud-cloud collision. SOFIA-upGREAT 158$μ$m [CII] archival data toward the northern part of Orion A are also compared to the $^{13}$CO data to test whether the position and velocity offsets between the emission from these two transitions resemble those predicted by a cloud-cloud collision model. We find that the northern part of Orion A, including regions ONC-OMC-1, OMC-2, OMC-3 and OMC-4, shows qualitative agreements with the cloud-cloud collision scenario, while in one of the southern regions, NGC1999, there is no indication of such a process in causing the birth of new stars. On the other hand, another southern cluster, L1641N, shows slight tendencies of cloud-cloud collision. Overall, our results support the cloud-cloud collision process as being an important mechanism for star cluster formation in Orion A.

astro-ph.SR↗

arXiv2019

The Astrochemical Impact of Cosmic Rays in Protoclusters II: CI-to-H$_2$ and CO-to-H$_2$ Conversion Factors

We utilize a modified astrochemistry code which includes cosmic ray attenuation in-situ to quantify the impact of different cosmic-ray models on the CO-to-H$_2$ and CI-to-H$_2$ conversion factors, $X_{\rm CO}$ and $X_{\rm CI}$, respectively. We consider the impact of cosmic rays accelerated by accretion shocks, and show that clouds with star formation efficiencies greater than 2\% have $X_{\rm CO} = (2.5 \pm 1)\times10^{20}$ cm$^{-2}$(K km s$^{-1}$)$^{-1}$, consistent with Milky Way observations. We find that changing the cosmic ray ionization rate from external sources from the canonical $ζ\approx 10^{-17}$ to $ζ\approx 10^{-16}$ s$^{-1}$, which better represents observations in diffuse gas, reduces $X_{\rm CO}$ by 0.2 dex for clusters with surface densities below 3 g cm$^{-2}$. We show that embedded sources regulate $X_{\rm CO}$ and decrease its variance across a wide range of surface densities and star formation efficiencies. Our models reproduce the trends of a decreased $X_{\rm CO}$ in extreme cosmic ray environments. $X_{\rm CI}$ has been proposed as an alternative to $X_{\rm CO}$ due to its brightness at high redshifts. The inclusion of internal cosmic ray sources leads to 1.2 dex dispersion in $X_{\rm CI}$ ranging from $2\times10^{20} < X_{\rm CI} < 4\times10^{21}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$. We show that $X_{\rm CI}$ is highly sensitive to the underlying cosmic ray model.

astro-ph.GA↗

arXiv2019

The Astrochemical Impact of Cosmic Rays in Protoclusters I: Molecular Cloud Chemistry

We present astrochemical photo-dissociation region models in which cosmic ray attenuation has been fully coupled to the chemical evolution of the gas. We model the astrochemical impact of cosmic rays, including those accelerated by protostellar accretion shocks, on molecular clouds hosting protoclusters. Our models with embedded protostars reproduce observed ionization rates. We study the imprint of cosmic ray attenuation on ions for models with different surface cosmic ray spectra and different star formation efficiencies. We find that abundances, particularly ions, are sensitive to the treatment of cosmic rays. We show the column densities of ions are under predicted by the `classic' treatment of cosmic rays by an order of magnitude. We also test two common chemistry approximations used to infer ionization rates. We conclude that the approximation based on the H$_3^+$ abundance under predicts the ionization rate except in regions where the cosmic rays dominate the chemistry. Our models suggest the chemistry in dense gas will be significantly impacted by the increased ionization rates, leading to a reduction in molecules such as NH$_3$ and causing H$_2$-rich gas to become [C II] bright.

astro-ph.GA↗

arXiv2019

Simulating the atomic and molecular content of molecular clouds using probability distributions of physical parameters

Modern observations of the interstellar medium (ISM) in galaxies detect a variety of atomic and molecular species. The goal is to connect these observations to the astrochemical properties of the ISM. 3D hydro-chemical simulations attempt this but due to extreme computational cost, they have to rely on simplified chemical networks and are bound to individual case studies. We present an alternative approach which models the ISM at larger scales by an ensemble of pre-calculated 1D thermo-chemical photodissociation region (PDR) calculations that determine the abundance and excitation of atomic and molecular species. We adopt lognormal distributions of column density (\avpdf s) for which each column density is linked to a volume density as derived by hydrodynamical simulations. We consider two lognormal {\avpdf}s: a diffuse, low density medium with average visual extinction of $\overline{{\rm A}_V}=0.75\,{\rm mag}$ and dispersion of $σ=0.5$ and a denser giant molecular cloud with $\overline{{\rm A}_V}=4\,{\rm mag}$ and $σ=0.8$. We treat the UV radiation field, cosmic-ray ionization rate and metallicity as free parameters. We find that the low density medium remains fully HI- and CII-dominated under all explored conditions. The denser cloud remains almost always molecular (i.e. H$_2$-dominated) while its carbon phase (CO, CI and CII) is sensitive to the above free parameters, implying that existing methods of tracing H$_2$-rich gas may require adjustments depending on environment. Our numerical framework can be used to estimate the PDR properties of large ISM regions and quantify trends with different environmental parameters as it is fast, covers wide parameter space, and is flexible for extensions.

astro-ph.GA↗