Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSmooth HI Low Column Density Outskirts In Nearby Galaxies
69
0
0.0
(
0
)
Added by
Roger Ianjamasimanana
Publication date
2018
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
The low column density gas at the outskirts of galaxies as traced by the 21 cm hydrogen line emission (HI) represents the interface between galaxies and the intergalactic medium, i.e., where galaxies are believed to get their supply of gas to fuel future episodes of star formation. Photoionization models predict a break in the radial profiles of HI at a column density of 5x10E+19 cm^-2 due to the lack of self-shielding against extragalactic ionizing photons. To investigate the prevalence of such breaks in galactic disks and to characterize what determines the potential edge of the HI disks, we study the azimuthally-averaged HI column density profiles of 17 nearby galaxies from The HI Nearby Galaxy Survey (THINGS) and supplemented in two cases with published Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) data. To detect potential faint HI emission that would otherwise be undetected using conventional moment map analysis, we line up individual profiles to the same reference velocity and average them azimuthally to derive stacked radial profiles. To do so, we use model velocity fields created from a simple extrapolation of the rotation curves to align the profiles in velocity at radii beyond the extent probed with the sensitivity of traditional integrated HI maps. With this method, we improve our sensitivity to outer-disk HI emission by up to an order of magnitude. Except for a few disturbed galaxies, none show evidence for a sudden change in the slope of the HI radial profiles, the alleged signature of ionization by the extragalactic background.
rate research
Read More
The HI in disk galaxies frequently extends beyond the optical image, and can trace the dark matter there. I briefly highlight the history of high spatial resolution HI imaging, the contribution it made to the dark matter problem, and the current tension between several dynamical methods to break the disk-halo degeneracy. I then turn to the flaring problem, which could in principle probe the shape of the dark halo. Instead, however, a lot of attention is now devoted to understanding the role of gas accretion via galactic fountains. The current $rm Lambda$ cold dark matter theory has problems on galactic scales, such as the core-cusp problem, which can be addressed with HI observations of dwarf galaxies. For a similar range in rotation velocities, galaxies of type Sd have thin disks, while those of type Im are much thicker. After a few comments on modified Newtonian dynamics and on irregular galaxies, I close with statistics on the HI extent of galaxies.
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.
The HIDEEP survey (Minchin et al. 2003) was done in an attempt to find objects having low inferred neutral hydrogen column densities, yet they found a distribution which was strongly peaked at 10^20.65 cm^-2. In an attempt to understand this distribution and similar survey results, we model HI profiles of gas discs and use simple simulations of objects having a wide range of HI properties in the presence of an ionizing background. We find that inferred column density (N_HI^o) values, which are found by averaging total HI masses over some disc area, do not vary strongly with central column density (N_max) for detectable objects, so that even a population having a wide range of N_max values will give rise to a strongly peaked distribution of N_HI^o values. We find that populations of objects, having a wide range of model parameters, give rise to inferred column density distributions around 10^20.6+/-0.3 cm^-2. However, populations of fairly massive objects having a wide range of central column densities work best in reproducing the HIDEEP data, and these populations are also consistent with observed Lyman limit absorber counts. It may be necessary to look two orders of magnitude fainter than HIDEEP limits to detect ionized objects having central column densities <10^20 cm^-2, but the inferred column densities of already detected objects might be lower if their radii could be estimated more accurately.
Using archival data from the HI Parkes All Sky Survey (HIPASS) we have searched for 21 cm line absorption in 204 nearby radio and star-forming galaxies with continuum flux densities greater than $S_{1.4} approx 250$ mJy within the redshift range $0 < cz < 12000$ km s$^{-1}$. By applying a detection method based on Bayesian model comparison, we successfully detect and model absorption against the radio-loud nuclei of four galaxies, of which the Seyfert 2 galaxy 2MASX J130804201-2422581 was previously unknown. All four detections were achieved against compact radio sources, which include three active galactic nuclei (AGNs) and a nuclear starburst, exhibiting high dust and molecular gas content. Our results are consistent with the detection rate achieved by the recent ALFALFA (Arecibo Legacy Fast Arecibo L-band Feed Array) HI absorption pilot survey by Darling et al. and we predict that the full ALFALFA survey should yield more than three to four times as many detections as we have achieved here. Furthermore, we predict that future all-sky surveys on the Square Kilometre Array precursor telescopes will be able to detect such strong absorption systems associated with type 2 AGNs at much higher redshifts, providing potential targets for detection of H$_{2}$O megamaser emission at cosmological redshifts.
Observed HI accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion happens in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To contrain the presence and properties of such flows, we present deep HI observations obtained with the NRAO Green Bank Telescope of an area measuring 4 by 4 degrees around NGC 2403. These observations, with a 5 sigma detection limit of 2.4 x 10^18 cm^-2 over a 20 km/s linewidth, reveal the presence of a low-column density, extended cloud outside the main HI disk, about 17 (~16 kpc or ~2R25) to the NW of the center of the galaxy. The total HI mass of the cloud is 6.3 x 10^6 Msun, or 0.15 percent of the total HI mass of NGC 2403. The cloud is associated with an 8-kpc anomalous-velocity HI filament in the inner disk, previously observed in deep VLA observations by Fraternali et al. (2001, 2002). We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigbouring dwarf galaxy.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
