We present a new survey of HCN(1-0) emission, a tracer of dense molecular gas, focused on the little-explored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) ...emission, we investigate the role of dense gas in star formation, finding systematic variations in both the apparent dense gas fraction (traced by the HCN-to-CO ratio) and the apparent star formation efficiency of dense gas. The latter may be unexpected, given the recent popularity of gas density threshold models to explain star formation scaling relations. Our survey used the IRAM 30 m telescope to observe HCN(1-0), CO(1-0), and several other emission lines across 29 nearby disk galaxies whose CO(2-1) emission has previously been mapped by the HERACLES survey. We detected HCN in 48 out of 62 observed positions. We explore one such model in which variations in the Mach number drive many of the trends within galaxy disks, while density contrasts drive the differences between disk and merging galaxies.
ABSTRACT We analyze the velocity dispersions of individual H i and CO profiles in a number of nearby galaxies from the high-resolution HERACLES CO and THINGS H i surveys. Focusing on regions with ...bright CO emission, we find a CO dispersion value = 7.3 1.7 km s−1. The corresponding H i dispersion = 11.7 2.3 km s−1, yielding a mean dispersion ratio = 1.4 0.2, independent of radius. We find that the CO velocity dispersion increases toward lower peak fluxes. This is consistent with previous work where we showed that when using spectra averaged ("stacked") over large areas, larger values for the CO dispersion are found, and a lower ratio = 1.0 0.2. The stacking method is more sensitive to low-level diffuse emission, whereas individual profiles trace narrow-line, GMC-dominated, bright emission. These results provide further evidence that disk galaxies contain not only a thin, low velocity dispersion, high density CO disk that is dominated by GMCs, but also a fainter, higher dispersion, diffuse disk component.
We present the Heterodyne Receiver Array CO Line Extragalactic Survey, an atlas of CO emission from 18 nearby galaxies that are also part of The H I Nearby Galaxy Survey and the Spitzer Infrared ...Nearby Galaxies Survey. We used the HERA multipixel receiver on the IRAM 30-m telescope to map the CO J = 2 -> 1 line over the full optical disk (defined by the isophotal radius r 25) of each target, at 13'' angular resolution and 2.6 km s-1 velocity resolution. Here we describe the observations and reduction of the data and show channel maps, azimuthally averaged profiles, integrated intensity maps, and peak intensity maps. The implied H2 masses range from 7 X 106 to 6 X 109 M , with four low metallicity dwarf irregular galaxies yielding only upper limits. In the cases where CO is detected, the integrated H2-to-H I ratios range from 0.02 to 1.13 and H2-to-stellar mass ratios from 0.01 to 0.25. Exponential scale lengths of the CO emission for our targets are in the range 0.8-3.2 kpc, or 0.2 ± 0.05r 25. The intensity-weighted mean velocity of CO matches that of H I very well, with a 1s scatter of only 6 km s-1. The CO J = 2 -> 1/J = 1 -> 0 line ratio varies over a range similar to that found in the Milky Way and other nearby galaxies, ~0.6-1.0, with higher values found in the centers of galaxies. The typical line ratio, ~0.8, could be produced by optically thick gas with an excitation temperature of ~10 K.
We combine data from The H I Nearby Galaxy Survey and the GALEX Nearby Galaxy Survey to study the relationship between atomic hydrogen (H I) and far-ultraviolet (FUV) emission outside the optical ...radius (r 25) in 17 spiral and 5 dwarf galaxies. In this regime, H I is likely to represent most of the interstellar medium (ISM) and FUV emission to trace recent star formation with little bias due to extinction, so that the two quantities closely trace the underlying relationship between gas and star formation rate (SFR). The azimuthally averaged H I and FUV intensities both decline with increasing radius in this regime, with the scale length of the FUV profile typically half that of the H I profile. Despite the mismatch in profiles, there is a significant spatial correlation (at 15'' resolution) between local FUV and H I intensities; near r 25 this correlation is quite strong, in fact stronger than anywhere inside r 25 (where H I is not a good tracer for the bulk of the ISM), and shows a decline toward larger radii. The star formation efficiency (SFE)--defined as the ratio of FUV/H I and thus the inverse of the gas depletion time--decreases with galactocentric radius across the outer disks, though much shallower than across the optical disks. On average, we find the gas depletion times to be well above a Hubble time (~1011 yr). We observe a clear relationship between FUV/H I and H I column in the outer disks, with the SFE increasing with increasing H I column. Despite observing systematic variations in FUV/H I, we find no clear evidence for step-function-type star formation thresholds, though we emphasize that it may not be realistic to expect them. When compared with results from inside r 25, we find outer disk star formation to be distinct in several ways: it is extremely inefficient (depletion times of many Hubble times which are also long compared to either the free fall or orbital timescale) with column densities and SFRs lower than found anywhere inside the optical disks. It appears that the H I column is one of the key environmental factors--perhaps the key factor--in setting the SFR in outer galaxy disks.
The Core-Cusp Problem de Blok, W. J. G.
Advances in Astronomy,
01/2010, Volume:
2010, Issue:
1
Journal Article
Peer reviewed
Open access
This paper gives an overview of the attempts to determine the distribution of dark matter in low surface brightness disk and gas-rich dwarf galaxies, both through observations and computer ...simulations. Observations seem to indicate an approximately constant dark matter density in the inner parts of galaxies, while cosmological computer simulations indicate a steep power-law-like behaviour. This difference has become known as the “core/cusp problem,” and it remains one of the unsolved problems in small-scale cosmology.
We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a Delta *LCDM cosmology and the observed dwarf galaxies sample ...from the THINGS survey in terms of (1) the rotation curve shape and (2) the logarithmic inner density slope Delta *a of mass density profiles. The simulations, which include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating, and most importantly, physically motivated gas outflows driven by supernovae, form bulgeless galaxies with DM cores. We show that the stellar and baryonic mass is similar to that inferred from photometric and kinematic methods for galaxies of similar circular velocity. Analyzing the simulations in exactly the same way as the observational sample allows us to address directly the so-called cusp/core problem in the Delta *LCDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than cold dark matter rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes Delta *a of the simulated galaxies' DM density profiles is ~--0.4 ? 0.1, which shows good agreement with Delta *a = --0.29 ? 0.07 of the THINGS dwarf galaxies. The effect of non-circular motions is not significant enough to affect the results. This confirms that the baryonic feedback processes included in the simulations are efficiently able to make the initial cusps with Delta *a ~--1.0 to --1.5 predicted by DM-only simulations shallower and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.
We present a systematic study of the extraplanar gas (EPG) in a sample of 15 nearby late-type galaxies at intermediate inclinations using publicly available, deep interferometric H I data from the ...Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. For each system we masked the H I emission coming from the regularly rotating disc and used synthetic datacubes to model the leftover “anomalous” H I flux. Our model consists of a smooth, axisymmetric thick component described by three structural and four kinematical parameters, which are fit to the data via a Markov chain Monte Carlo (MCMC) based Bayesian method. We find that extraplanar H I is nearly ubiquitous in disc galaxies as we fail to detect it in only two of the systems with the poorest spatial resolution. The EPG component encloses ∼5−25% of the total H I mass with a mean value of 14%, and has a typical thickness of a few kpc which is incompatible with expectations based on hydrostatic equilibrium models. The EPG kinematics is remarkably similar throughout the sample, and consists of a lagging rotation with typical vertical gradients of ∼ − 10 km s−1 kpc−1, a velocity dispersion of 15−30 km s−1, and, for most galaxies, a global inflow in both the vertical and radial directions with speeds of 20−30 km s−1. The EPG H I masses are in excellent agreement with predictions from simple models of the galactic fountain that are powered by stellar feedback. The combined effect of photo-ionisation and interaction of the fountain material with the circumgalactic medium can qualitatively explain the kinematics of the EPG, but dynamical models of the galactic fountain are required to fully test this framework.
ABSTRACT
We report the detection of a bright fast radio burst, FRB 191108, with Apertif on the Westerbork Synthesis Radio Telescope. The interferometer allows us to localize the FRB to a narrow 5 ...arcsec × 7 arcmin ellipse by employing both multibeam information within the Apertif phased-array feed beam pattern, and across different tied-array beams. The resulting sightline passes close to Local Group galaxy M33, with an impact parameter of only 18 kpc with respect to the core. It also traverses the much larger circumgalactic medium (CGM) of M31, the Andromeda Galaxy. We find that the shared plasma of the Local Group galaxies could contribute ∼10 per cent of its dispersion measure of 588 pc cm−3. FRB 191108 has a Faraday rotation measure (RM) of +474 $\pm \, 3$ rad m−2, which is too large to be explained by either the Milky Way or the intergalactic medium. Based on the more moderate RMs of other extragalactic sources that traverse the halo of M33, we conclude that the dense magnetized plasma resides in the host galaxy. The FRB exhibits frequency structure on two scales, one that is consistent with quenched Galactic scintillation and broader spectral structure with Δν ≈ 40 MHz. If the latter is due to scattering in the shared M33/M31 CGM, our results constrain the Local Group plasma environment. We found no accompanying persistent radio sources in the Apertif imaging survey data.
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