Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userConstraining cloud parameters using high density gas tracers in galaxies
63
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Far-infrared molecular emission is an important tool used to understand the excitation mechanisms of the gas in the inter-stellar medium of star-forming galaxies. In the present work, we model the emission from rotational transitions with critical densities n >~ 10^4 cm-3. We include 4-3 < J <= 15-14 transitions of CO and 13CO, in addition to J <= 7-6 transitions of HCN, HNC, and HCO+ on galactic scales. We do this by re-sampling high density gas in a hydrodynamic model of a gas-rich disk galaxy, assuming that the density field of the interstellar medium of the model galaxy follows the probability density function (PDF) inferred from the resolved low density scales. We find that in a narrow gas density PDF, with a mean density of ~10 cm-3 and a dispersion sigma = 2.1 in the log of the density, most of the emission of molecular lines, emanates from the 10-1000 cm-3 part of the PDF. We construct synthetic emission maps for the central 2 kpc of the galaxy and fit the line ratios of CO and 13CO up to J = 15-14, as well as HCN, HNC, and HCO+ up to J = 7-6, using one photo-dissociation region (PDR) model. We attribute the goodness of the one component fits for our model galaxy to the fact that the distribution of the luminosity, as a function of density, is peaked at gas densities between 10 and 1000 cm-3. We explore the impact of different log-normal density PDFs on the distribution of the line-luminosity as a function of density, and we show that it is necessary to have a broad dispersion, corresponding to Mach numbers >~ 30 in order to obtain significant emission from n > 10^4 cm-3 gas. Such Mach numbers are expected in star-forming galaxies, LIRGS, and ULIRGS. By fitting line ratios of HCN(1-0), HNC(1-0), and HCO+(1-0) for a sample of LIRGS and ULIRGS using mechanically heated PDRs, we constrain the Mach number of these galaxies to 29 < M < 77.
rate research
Read More
We compare the magnetic field orientation for the young giant molecular cloud Vela C inferred from 500-$mu$m polarization maps made with the BLASTPol balloon-borne polarimeter to the orientation of structures in the integrated line emission maps from Mopra observations. Averaging over the entire cloud we find that elongated structures in integrated line-intensity, or zeroth-moment maps, for low density tracers such as $^{12}$CO and $^{13}$CO $J$ $rightarrow$ 1 - 0 are statistically more likely to align parallel to the magnetic field, while intermediate or high density tracers show (on average) a tendency for alignment perpendicular to the magnetic field. This observation agrees with previous studies of the change in relative orientation with column density in Vela C, and supports a model where the magnetic field is strong enough to have influenced the formation of dense gas structures within Vela C. The transition from parallel to no preferred/perpendicular orientation appears to happen between the densities traced by $^{13}$CO and by C$^{18}$O $J$ $rightarrow$ 1 - 0. Using RADEX radiative transfer models to estimate the characteristic number density traced by each molecular line we find that the transition occurs at a molecular hydrogen number density of approximately $10^3$ cm$^{-3}$. We also see that the Centre-Ridge (the highest column density and most active star-forming region within Vela C) appears to have a transition at a lower number density, suggesting that this may depend on the evolutionary state of the cloud.
We present the results of our ALMA observations of eleven (ultra)luminous infrared galaxies ((U)LIRGs) at J=4-3 of HCN, HCO+, HNC and J=3-2 of HNC. This is an extension of our previously published HCN and HCO+ J=3-2 observations to multiple rotational J-transitions of multiple molecules, to investigate how molecular emission line flux ratios vary at different J-transitions. We confirm that ULIRGs that contain or may contain luminous obscured AGNs tend to show higher HCN-to-HCO+ flux ratios than starburst galaxies, both at J=4-3 and J=3-2. For selected HCN-flux-enhanced AGN-important ULIRGs, our isotopologue H13CN, H13CO+, and HN13C J=3-2 line observations suggest a higher abundance of HCN than HCO+ and HNC, which is interpreted to be primarily responsible for the elevated HCN flux in AGN-important galaxies. For such sources, the intrinsic HCN-to-HCO+ flux ratios after line opacity correction will be higher than the observed ratios, making the separation between AGNs and starbursts even larger. The signature of the vibrationally excited (v2=1f) HCN J=4-3 emission line is seen in one ULIRG, IRAS 12112-0305 NE. P Cygni profiles are detected in the HCO+ J=4-3 and J=3-2 lines toward IRAS 15250+3609, with an estimated molecular outflow rate of ~250-750 Mo/year. The SiO J=6-5 line also exhibits a P Cygni profile in IRAS 12112+0305 NE, suggesting the presence of shocked outflow activity. Shock tracers are detected in many sources, suggesting ubiquitous shock activity in the nearby ULIRG population.
[Abridged] We combine new CO(1-0) line observations of 24 intermediate redshift galaxies (0.03 < z < 0.28) along with literature data of galaxies at 0<z<4 to explore scaling relations between the dust and gas content using PAH 6.2 $mu$m ($L_{6.2}$), CO ($L_{rm CO}$), and infrared ($L_{rm IR}$) luminosities for a wide range of redshifts and physical environments. Our analysis confirms the existence of a universal $L_{6.2}-L_{rm CO}$ correlation followed by normal star-forming galaxies (SFGs) and starbursts (SBs) at all redshifts. This relation is also followed by local ULIRGs that appear as outliers in the $L_{6.2}-L_{rm IR}$ and $L_{rm IR}-L_{rm CO}$ relations from the sequence defined by normal SFGs. The emerging tight ($sigma approx 0.26$ dex) and linear ($alpha = 1.03$) relation between $L_{6.2}$ and $L_{rm CO}$ indicates a $L_{6.2}$ to molecular gas ($M_{rm H_2}$) conversion factor of $alpha_{6.2} = M_{rm H2}/L_{6.2} = (2.7pm1.3) times alpha_{rm CO}$, where $alpha_{rm CO}$ is the $L_{rm CO}$ to $M_{rm H_2}$ conversion factor. We also find that on galaxy integrated scales, PAH emission is better correlated with cold rather than with warm dust emission, suggesting that PAHs are associated with the diffuse cold dust, which is another proxy for $M_{rm H_2}$. Focusing on normal SFGs among our sample, we employ the dust continuum emission to derive $M_{rm H_2}$ estimates and find a constant $M_{rm H_2}/L_{6.2}$ ratio of $alpha_{6.2} = 12.3 M_{rm H_2}/{rm L}_{odot}$ ($sigmaapprox 0.3$ dex). We propose that the presented $L_{6.2}-L_{rm CO}$ and $L_{6.2}-M_{rm H_2}$ relations will serve as useful tools for the determination of the physical properties of high-$z$ SFGs, for which PAH emission will be routinely detected by the James Webb Space Telescope.
In this paper we examine the factors that shape the distribution of molecular gas surface densities on the 150 pc scale across 67 morphologically diverse star-forming galaxies in the PHANGS-ALMA CO (2-1) survey. Dividing each galaxy into radial bins, we measure molecular gas surface density contrasts, defined here as the ratio between a fixed high percentile of the CO distribution and a fixed reference level in each bin. This reference level captures the level of the faint CO floor that extends between bright filamentary features, while the intensity level of the higher percentile probes the structures visually associated with bright, dense ISM features like spiral arms, bars, and filaments. We compare these contrasts to matched percentile-based measurements of the 3.6 $mu$m emission measured using Spitzer/IRAC imaging, which trace the underlying stellar mass density. We find that the logarithms of CO contrasts on 150 pc scales are 3-4 times larger than, and positively correlated with, the logarithms of 3.6 $mu$m contrasts probing smooth non-axisymmetric stellar bar and spiral structures. The correlation appears steeper than linear, consistent with the compression of gas as it flows supersonically in response to large-scale stellar structures, even in the presence of weak or flocculent spiral arms. Stellar dynamical features appear to play an important role in setting the cloud-scale gas density in our galaxies, with gas self-gravity perhaps playing a weaker role in setting the 150 pc-scale distribution of gas densities.
We present the results of our ALMA Cycle 0 observations, using HCN/HCO+/HNC J=4-3 lines, of six nearby luminous infrared galaxies with various energetic contributions from active galactic nuclei (AGNs) estimated from previous infrared spectroscopy. These lines are very effective for probing the physical properties of high-density molecular gas around the hidden energy sources in the nuclear regions of these galaxies. We find that HCN to HCO+ J=4-3 flux ratios tend to be higher in AGN-important galaxies than in starburst-dominated regions, as was seen at the J=1-0 transition, while there is no clear difference in the HCN-to-HNC J=4-3 flux ratios among observed sources. A galaxy with a starburst-type infrared spectral shape and very large molecular line widths shows a high HCN-to-HCO+ J=4-3 flux ratio, which could be due to turbulence-induced heating. We propose that enhanced HCN J=4-3 emission relative to HCO+ J=4-3 could be used to detect more energetic activity than normal starbursts, including deeply buried AGNs, in dusty galaxy populations.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
