THE CO-TO-H2CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

We simultaneously measured the spatially resolved CO-to-H2 conversion factor (αCO) and dust-to-gas ratio (DGR) in nearby galaxies on a kiloparsec scale. In this study, we used 12CO(J = 1–0) data obtained by the Nobeyama 45 m radio telescope with H i and dust mass surface densities. We obtained the values of global αCO and DGR in 22 nearby spiral galaxies, with averages of 2.66 ± 1.36 M⊙ pc−2 (K km s−1)−1 and 0.0052 ± 0.0026, respectively. Furthermore, the radial variations of αCO and DGR in four barred spiral galaxies (IC 342, NGC 3627, NGC 5236, and NGC 6946) were obtained by dividing them into inner and outer regions with a boundary of 0.2 R25, where R25 is the isophotal radius at 25 mag arcsec−2 in the B band. The averages of αCO and DGR in the inner region (≤0.2 R25) are 0.36 ± 0.08 M⊙ pc−2 (K km s−1)−1 and 0.0199 ± 0.0058, while those in the outer region (>0.2 R25) are 1.49 ± 0.76 M⊙ pc−2 (K km s−1)−1 and 0.0084 ± 0.0037, respectively. The value of αCO in the outer region is 2.3 to 5.3 times larger than that of the inner region. When separated into the inner and outer regions, we find that αCO and DGR correlate with the metallicity and the star formation rate surface density. The value of αCO derived in this study tends to be smaller than those obtained in previous studies for the Milky Way and nearby star-forming galaxies. This fact can be attributed to our measurements being biased toward the inner region; we measured αCO at 0.85 and 0.76 times smaller in radius than the previous works for nearby star-forming galaxies and the Milky Way, respectively.

A comprehensive analysis of 355 high-quality Westerbork Synthesis Radio Telescope (WSRT) HI 21-cm line maps of nearby galaxies shows that the properties and incident rate of damped Lyman alpha absorption systems (DLAs) observed in the spectra of high-redshift QSOs are in good agreement with DLAs originating in gas discs of galaxies like those in the z approximate to 0 population. Comparison of low-z DLA statistics with the HI incidence rate and column density distribution f(N (HI)) for the local galaxy sample shows no evidence for evolution in the integral 'cross-section density' = l(-1) (l = mean free path between absorbers) below z approximate to 1.5, implying that there is no need for a hidden population of galaxies or H I clouds to contribute significantly to the DLA cross-section. Compared with z approximate to 4, our data indicate evolution of a factor of 2 in the comoving density along a line of sight. We find that dN/dz(z = 0) = 0.045 +/- 0.006. The idea that the local galaxy population can explain the DLAs is further strengthened by comparing the properties of DLAs and DLA galaxies with the expectations based on our analysis of local galaxies. The distribution of luminosities of DLA host galaxies, and of impact parameters between QSOs and the centres of DLA galaxies, is in good agreement with what is expected from local galaxies. Approximately 87 per cent of low-z DLA galaxies are expected to be fainter than L-*, and 37 per cent have impact parameters less than 1 arcsec at z = 0.5. The analysis shows that some host galaxies with very low impact parameters and low luminosities are expected to be missed in optical follow-up surveys. The well-known metallicity-luminosity relation in galaxies, in combination with metallicity gradients in galaxy discs, causes the expected median metallicity of low-z DLAs to be low (similar to 1/7 solar), which is also in good agreement with observations of low-z DLAs. We find that f(N H I) can be fitted satisfactorily with a gamma distribution, a single power law is not a good fit at the highest column densities N H I > 10(21) cm(-2). The vast majority (approximate to 81 per cent) of the H I gas in the local Universe resides in column densities above the classical DLA limit (N (H I) > 2 x 10(20) cm(-2)), with N (H I) similar to 10(21) cm(-2) dominating the cosmic H I mass density.

We present a detailed study of an estimator of the HI column density, based on a combination of HI 21cm absorption and HI 21cm emission spectroscopy. This "isothermal" estimate is given by $N_{\rm HI,ISO} = 1.823 \times 10^{18} \int \left[ \tau_{\rm tot} \times {\rm T_B} \right] / \left[ 1 - e^{-\tau_{\rm tot}} \right] {\rm dV}$, where $\tau_{\rm tot}$ is the total HI 21cm optical depth along the sightline and ${\rm T_B}$ is the measured brightness temperature. We have used a Monte Carlo simulation to quantify the accuracy of the isothermal estimate by comparing the derived $N_{\rm HI,ISO}$ with the true HI column density $N_{\rm HI}$. The simulation was carried out for a wide range of sightlines, including gas in different temperature phases and random locations along the path. We find that the results are statistically insensitive to the assumed gas temperature distribution and the positions of different phases along the line of sight. The median value of the ratio of the true H{\sc i} column density to the isothermal estimate, $N_{\rm HI}/{N_{\rm HI, ISO}}$, is within a factor of 2 of unity while the 68.2% confidence intervals are within a factor of $\approx 3$ of unity, out to high HI column densities, $\le 5 \times 10^{23}$\,cm$^{-2}$ per 1 km s$^{-1}$ channel, and high total optical depths, $\le 1000$. The isothermal estimator thus provides a significantly better measure of the HI column density than other methods, within a factor of a few of the true value even at the highest columns, and should allow us to directly probe the existence of high HI column density gas in the Milky Way.

M17 is a well-known massive star-forming region, and its gas-to-dust ratio (GDR) may vary significantly compared to the other areas. The mass of gas can be traced by the ${\rm CO}$ emission observed in the Milky Way Imaging Scroll Painting project. The dust mass can be traced by analysing the interstellar extinction magnitude obtained from the United Kingdom Infrared Telescope. We computed the ratio ${W({\rm CO})/A_\mathit{ V}}$: for ${A_\mathit{ V} \le }$ 10 mag, ${{ W(^{12}{\rm CO})/ A_\mathit{ V}}= (6.27 \pm 0.19)}$${\mathrm{{K \, km\,s^{-1}}\, mag^{-1}}}$ and ${{ W(^{13}{\rm CO})/ A_\mathit{ V}} = (0.75 \pm 0.72)}$${ \mathrm{{K \, km\,s^{-1}} \, mag^{-1}}}$; whereas for ${{A_\mathit{ V}} \ge 10}$ mag, ${{ W(^{12}{\rm CO})/ A_\mathit{ V}} = (15.8 \pm 0.06) }$ ${\mathrm{{K \, km\,s^{-1}} \, mag^{-1}}}$, and ${{ W(^{13}{\rm CO})/ A_\mathit{ V}} = (3.11 \pm 0.25)}$ ${ \mathrm{{K \, km\,s^{-1}} \, mag^{-1}}}$. Then, we converted the ${W({\rm CO})/A_\mathit{ V}}$ into ${N(\rm H)/\mathit{ A}_\mathit{ V}}$. Using the WD01 model, we derived the GDR: for ${A_\mathit{ V} \le }$ 10 mag, the GDRs were ${118 \pm 9}$ for ${^{12}{\rm CO}}$ and ${83 \pm 62}$ for ${^{13}{\rm CO}}$, comparable to those of the Milky Way; however, for ${A_\mathit{ V} \ge }$ 10 mag, the GDRs increased significantly to ${296 \pm 3}$ for ${^{12}{\rm CO}}$ and ${387 \pm 40}$ for ${^{13}{\rm CO}}$, approximately three times higher than those of the Milky Way. In the discussion, we compared the results of this work with previous studies and provided a detailed discussion of the influence of massive stars and other factors on GDR.

We present predictions for the evolution of the galaxy dust-to-gas ratio (DGR) and dust-to-metal ratio (DTM) from z = 0 → 6, using a model for the production, growth, and destruction of dust grains implemented into the simba cosmological hydrodynamic galaxy formation simulation. In our model, dust forms in stellar ejecta, grows by the accretion of metals, and is destroyed by thermal sputtering and supernovae. Our simulation reproduces the observed dust mass function at z = 0, but modestly underpredicts the mass function by ∼×3 at z ∼ 1–2. The z = 0 DGR versus metallicity relationship shows a tight positive correlation for star-forming galaxies, while it is uncorrelated for quenched systems. There is little evolution in the DGR–metallicity relationship between z = 0 and 6. We use machine learning techniques to search for the galaxy physical properties that best correlate with the DGR and DTM. We find that the DGR is primarily correlated with the gas-phase metallicity, though correlations with the depletion time-scale, stellar mass, and gas fraction are non-negligible. We provide a crude fitting relationship for DGR and DTM versus the gas-phase metallicity, along with a public code package that estimates the DGR and DTM given a set of galaxy physical properties.

We present a catalogue of Giant Molecular Clouds (GMCs) in M33, extracted from cold dust continuum emission. Our GMCs are identified by computing dendrograms. We measure the spatial distribution of these clouds, and characterise their dust properties. Combining these measured properties with CO(J=2–1) and 21cm H i data, we calculate the gas-to-dust ratio (GDR) of these clouds, and from this compute a total cloud mass. In total, we find 165 GMCs with cloud masses in the range of 104-107 M⊙. We find that radially, log10(GDR) = −0.043( ± 0.038) R[kpc] + 1.88( ± 0.15), a much lower GDR than found in the Milky Way, and a correspondingly higher αCO factor. The mass function of these clouds follows a slope proportional to M−2.84, steeper than many previous studies of GMCs in local galaxies, implying that M33 is poorer at forming massive clouds than other nearby spirals. Whilst we can rule out interstellar pressure as the major contributing factor, we are unable to disentangle the relative effects of metallicity and H i velocity dispersion. We find a reasonably featureless number density profile with galactocentric radius, and weak correlations between galactocentric radius and dust temperature/mass. These clouds are reasonably consistent with Larson’s scaling relationships, and many of our sources are co-spatial with earlier CO studies. Massive clouds are identified at large galactocentric radius, unlike in these earlier studies, perhaps indicating a population of CO-dark gas dominated clouds at these larger distances.

We study the high column density regime of the HI column density distribution function and argue that there are two distinct features: a turnover at NHI ~ 10^21 cm^-2 which is present at both z=0 and z ~ 3, and a lack of systems above NHI ~ 10^22 cm^-2 at z=0. Using observations of the column density distribution, we argue that the HI-H2 transition does not cause the turnover at NHI ~ 10^21 cm^-2, but can plausibly explain the turnover at NHI > 10^22 cm^-2. We compute the HI column density distribution of individual galaxies in the THINGS sample and show that the turnover column density depends only weakly on metallicity. Furthermore, we show that the column density distribution of galaxies, corrected for inclination, is insensitive to the resolution of the HI map or to averaging in radial shells. Our results indicate that the similarity of HI column density distributions at z=3 and z=0 is due to the similarity of the maximum HI surface densities of high-z and low-z disks, set presumably by universal processes that shape properties of the gaseous disks of galaxies. Using fully cosmological simulations, we explore other candidate physical mechanisms that could produce a turnover in the column density distribution. We show that while turbulence within GMCs cannot affect the DLA column density distribution, stellar feedback can affect it significantly if the feedback is sufficiently effective in removing gas from the central 2-3 kpc of high-redshift galaxies. Finally, we argue that it is meaningful to compare column densities averaged over ~ kpc scales with those estimated from quasar spectra which probe sub-pc scales due to the steep power spectrum of HI column density fluctuations observed in nearby galaxies.

We present the results from our 21-cm absorption survey of a sample of 5 quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range 0.03<zg<0.18, selected from the SDSS. The HI 21-cm absorption was searched towards the 9 sight lines with impact parameters ranging from 10 to 55 kpc using GMRT. 21-cm absorption was detected only in one case i.e. towards the Quasar (zq=2.625 SDSS J124157.54+633241.6)-galaxy (zg=0.143 SDSS J124157.26+633237.6) pair with the impact parameter 11 kpc. The quasar sight line in this case pierces through the stellar disk of a galaxy having near solar metallicity (i.e (O/H)+12=8.7) and star formation rate uncorrected for dust attenuation of 0.1 M_odot/yr. The quasar spectrum reddened by the foreground galaxy is well fitted with the Milky Way extinction curve (with an Av of 0.44) and the estimated HI column density is similar to the value obtained from 21-cm absorption assuming spin temperature of 100K. Combining our sample with the z<0.1 data available in the literature, we find the detectability of 21-cm absorption with integrated optical depth greater than 0.1 km\s to be 50% for the impact parameter less than 20 kpc. Using the surface brightness profiles and relationship between the optical size and extent of the HI disk known for nearby galaxies, we conclude that in most of the cases of 21-cm absorption non-detection, the sight lines may not be passing through the HI gas. We also find that in comparison to the absorption systems associated with these QGPs, z<1 DLAs with 21-cm absorption detections have lower CaII equivalent widths despite having higher 21-cm optical depths and smaller impact parameters. This suggests that the current sample of DLAs may be a biased population that avoids sight lines through dusty star-forming galaxies. A systematic survey of QGPs is needed to confirm these findings and understand the nature of 21-cm absorbers.

To study the atomic, molecular and ionized emission of Giant Molecular Clouds (GMCs), we have initiated a Large Program with the VLA: 'THOR - The HI, OH, Recombination Line survey of the Milky Way'. We map the 21cm HI line, 4 OH lines, 19 H_alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way (l=15-67deg, |b|&lt;1deg) at ~20" resolution. In this paper, we focus on the HI emission from the W43 star-formation complex. Classically, the HI 21cm line is treated as optically thin with column densities calculated under this assumption. This might give reasonable results for regions of low-mass star-formation, however, it is not sufficient to describe GMCs. We analyzed strong continuum sources to measure the optical depth, and thus correct the HI 21cm emission for optical depth effects and weak diffuse continuum emission. Hence, we are able to measure the HI mass of W43 more accurately and our analysis reveals a lower limit of M=6.6x10^6 M_sun, which is a factor of 2.4 larger than the mass estimated with the assumption of optically thin emission. The HI column densities are as high as N(HI)~150 M_sun/pc^2 ~ 1.9x10^22 cm^-2, which is an order of magnitude higher than for low mass star formation regions. This result challenges theoretical models that predict a threshold for the HI column density of ~10 M_sun/pc^2, at which the formation of molecular hydrogen should set in. By assuming an elliptical layered structure for W43, we estimate the particle density profiles. While at the cloud edge atomic and molecular hydrogen are well mixed, the center of the cloud is strongly dominated by molecular hydrogen. We do not identify a sharp transition between hydrogen in atomic and molecular form. Our results are an important characterization of the atomic to molecular hydrogen transition in an extreme environment and challenges current theoretical models.

We report the detection of HI 21-cm absorption in a member of the rare and recently discovered class of compact radio sources: extremely inverted spectrum extragalactic radio sources (EISERS). The EISERS conceivably form a special subclass of the inverted spectrum radio galaxies since the spectral index of the optically thick part of the spectrum for these sources crosses the synchrotron self-absorption limit ofα = +2.5 (S(ν) ∝να). We searched for HI absorption in two EISERS using the recently upgraded Giant Metrewave Radio Telescope (uGMRT) and detected an absorption feature in one of them. The strong associated HI absorption detected against the source J1209−2032 (z= 0.4040) implies an optical depth of 0.178 ± 0.02, corresponding to an HI column density of 34.8 ± 2.9 × 1020cm−2, for an assumed HI spin temperature of 100 K and covering factor of 1. This is among the highest known optical depth and HI column densities found for compact radio sources of peaked spectrum type and supports the free-free absorption model for the steeply inverted radio spectrum of this source. For the other source, J1549+5038 (z= 2.171), no HI absorption was detected in our observations.

The Effelsberg-Bonn HI Survey (EBHIS) is a new 21-cm survey performed with the 100-m telescope at Effelsberg. It covers the whole northern sky out to a redshift of z~0.07 and comprises HI line emission from the Milky Way and the Local Volume. We aim to substitute the northern-hemisphere part of the Leiden/Argentine/Bonn Milky Way HI survey (LAB) with this first EBHIS data release, which presents the HI gas in the Milky Way regime. The use of a seven-beam L-band array made it feasible to perform this all-sky survey with a 100-m class telescope in a reasonable amount of observing time. State-of-the-art fast-Fourier-transform spectrometers provide the necessary data read-out speed, dynamic range, and spectral resolution to apply software radio-frequency interference mitigation. EBHIS is corrected for stray radiation and employs frequency-dependent flux-density calibration and sophisticated baseline-removal techniques to ensure the highest possible data quality. Detailed analyses of the resulting data products show that EBHIS is not only outperforming LAB in terms of sensitivity and angular resolution, but also matches the intensity-scale of LAB extremely well, allowing EBHIS to be used as a drop-in replacement for LAB. Data products are made available to the public in a variety of forms. Most important, we provide a properly gridded Milky Way HI column density map in HEALPix representation. To maximize the usefulness of EBHIS data, we estimate uncertainties in the HI column density and brightness temperature distributions, accounting for systematic effects.

We investigate the correlation between CO and HI emission in 18 nearby galaxies from the CARMA Survey Toward IR-Bright Nearby Galaxies (STING) at sub-kpc and kpc scales. Our sample, spanning a wide range in stellar mass and metallicity, reveals evidence for a metallicity dependence of the HI column density measured in regions exhibiting CO emission. Such a dependence is predicted by the equilibrium model of McKee & Krumholz, which balances H_2 formation and dissociation. The observed HI column density is often smaller than predicted by the model, an effect we attribute to unresolved clumping, although values close to the model prediction are also seen. We do not observe HI column densities much larger than predicted, as might be expected were there a diffuse HI component that did not contribute to H_2 shielding. We also find that the H_2 column density inferred from CO correlates strongly with the stellar surface density, suggesting that the local supply of molecular gas is tightly regulated by the stellar disk.

We use observations of the neutral atomic hydrogen (HI) 21-cm emission line to study the spatial distribution of the HI gas in a 80° × 90° region of the Galaxy halo. The HI column densities in the range of 3–11 × 10 20 cm − 2 have been estimated for some of the studied regions. In our map—obtained with a spectral sensitivity of ∼2 K—we do not detect any HI 21-cm emission line above 2 σ at Galactic latitudes higher than ∼46°. This report summarizes our contribution presented at the conference on the origin and evolution of barionic Galaxy halos.

We investigate the properties of HI-rich galaxies detected in blind radio surveys within the hierarchical structure formation scenario using a semi-analytic model of galaxy formation. By drawing a detailed comparison between the properties of HI-selected galaxies and HI absorption systems, we argue a link between the local galaxy population and quasar absorption systems, particularly for Damped Ly-alpha absorption (DLA) systems and sub-DLA systems. First, we evaluate how many HI-selected galaxies exhibit HI column densities as high as those of DLA systems. We find that HI-selected galaxies with HI masses M(HI) > 10^8 solar masses have gaseous disks that produce HI column densities comparable to those of DLA systems. We conclude that DLA galaxies where the HI column densities are as high as those of DLA systems, contribute significantly to the population of HI-selected galaxies at M(HI) > 10^8 solar masses. Second, we find that star formation rates (SFRs) correlate tightly with HI masses rather than B- (and J-) band luminosities. In the low-mass range M(HI) < 10^8 solar masses, sub-DLA galaxies replace DLA galaxies as the dominant population. The number fraction of sub-DLA galaxies relative to galaxies reaches 40%-60% at HI masses 10^8 solar masses and 30%-80% at 10^7 solar masses. The HI-selected galaxies at 10^7 solar masses are a strong probe of sub-DLA systems that place stringent constraints on galaxy formation and evolution.

Using a sample of face-on star-forming galaxies selected from the Sloan Digital Sky Survey, we statistically derive the typical optical depth τ cl of individual H ii regions based on the “Chocolate Chip Cookie” model of Lu et al. (2022). By binning galaxies into stellar mass and gas-phase metallicity bins and interpreting τ cl as the dust-to-gas ratio (DGR) of H ii regions, we further investigate the correlations between DGR and stellar mass and between DGR and gas-phase metallicity, respectively. We find that DGR increases monotonically with the stellar mass of galaxies. At a given stellar mass, DGR shows a linear correlation with the gas-phase metallicity, which implies a constant dust-to-metal ratio (DTM) of galaxies at a given stellar mass. These results adequately indicate that the DTM of galaxies is simply a function of their stellar masses. In terms of gas-phase metallicity, because of the mass–metallicity relation, DTM increases with increasing metallicity with a power-law index 1.45 in the low-metallicity region, while it remains constant at the high-metallicity end.