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Register a new userExtraplanar gas in Edge-on Galaxies traced by SOFIA observations of [C II]
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Bursts of localized star formation in galaxies can levitate material from their midplanes. Spiral galaxies that are edge-on allow clear distinction of material that is levitated off the galaxies midplanes. We used SOFIA to measure the vertical distribution of [C II] 157.7 micron line emission for two nearby, edge-on galaxies, NGC 891 and NGC 5907. We find that for the central region and actively-star-forming regions in the northern portion of NGC 891, and for NGC 5907, a thin (0.3 kpc) disk is supplemented by a thick disk with an exponential scale height of about 2 kpc. The [C II] is far more extended than mid-infrared emission (0.1 kpc, tracing present-day massive star formation) but not as extended as the H I (100 kpc, tracing low-metallicity circum/inter-galactic matter). The extraplanar [C II] may arise in walls of chimneys that connect the disk to the halo.
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We present a study of the kinematics of the extraplanar ionized gas around several dozen galaxies observed by the Mapping of Nearby Galaxies at the Apache Point Observatory (MaNGA) survey. We considered a sample of 67 edge-on galaxies out of more than 1400 extragalactic targets observed by MaNGA, in which we found 25 galaxies (or 37%) with regular lagging of the rotation curve at large distances from the galactic midplane. We model the observed $Halpha$ emission velocity fields in the galaxies, taking projection effects and a simple model for the dust extinction into the account. We show that the vertical lag of the rotation curve is necessary in the modeling, and estimate the lag amplitude in the galaxies. We find no correlation between the lag and the star formation rate in the galaxies. At the same time, we report a correlation between the lag and the galactic stellar mass, central stellar velocity dispersion, and axial ratio of the light distribution. These correlations suggest a possible higher ratio of infalling-to-local gas in early-type disk galaxies or a connection between lags and the possible presence of hot gaseous halos, which may be more prevalent in more massive galaxies. These results again demonstrate that observations of extraplanar gas can serve as a potential probe for accretion of gas.
We investigate the prevalence, properties, and kinematics of extraplanar diffuse ionized gas (eDIG) in a sample of 25 edge-on galaxies selected from the CALIFA survey. We measure ionized gas scale heights from ${rm Halpha}$ and find that 90% have measurable scale heights with a median of $0.8^{+0.7}_{-0.4}$ kpc. From the ${rm Halpha}$ kinematics, we find that 60% of galaxies show a decrease in the rotation velocity as a function of height above the midplane. This lag is characteristic of eDIG, and we measure a median lag of 21 km s$^{-1}$ kpc$^{-1}$ which is comparable to lags measured in the literature. We also investigate variations in the lag with radius. $rm H{small I}$ lags have been reported to systematically decrease with galactocentric radius. We find both increasing and decreasing ionized gas lags with radius, as well as a large number of galaxies consistent with no radial lag variation, and investigate these results in the context of internal and external origins for the lagging ionized gas. We confirm that the ${rm [S{small II}]}$/${rm Halpha}$ and ${rm [N{small II}]}$/${rm Halpha}$ line ratios increase with height above the midplane as is characteristic of eDIG. The ionization of the eDIG is dominated by star-forming complexes (leaky ${rm H{small II}}$ regions). We conclude that the lagging ionized gas is turbulent ejected gas likely resulting from star formation activity in the disk as opposed to gas in the stellar thick disk or bulge. This is further evidence for the eDIG being a product of stellar feedback and for the pervasiveness of this WIM-like phase in many local star-forming galaxies.
Neither HI nor CO emission can reveal a significant quantity of so-called dark gas in the interstellar medium (ISM). It is considered that CO-dark molecular gas (DMG), the molecular gas with no or weak CO emission, dominates dark gas. We identified 36 DMG clouds with C$^+$ emission (data from Galactic Observations of Terahertz C+ (GOT C+) project) and HINSA features. Based on uncertainty analysis, optical depth of HI $taurm_{HI}$ of 1 is a reasonable value for most clouds. With the assumption of $taurm_{HI}=1$, these clouds were characterized by excitation temperatures in a range of 20 K to 92 K with a median value of 55 K and volume densities in the range of $6.2times10^1$ cm$^{-3}$ to $1.2times 10^3$ cm$^{-3}$ with a median value of $2.3times 10^2$ cm$^{-3}$. The fraction of DMG column density in the cloud ($frm_{DMG}$) decreases with increasing excitation temperature following an empirical relation $frm_{DMG}=-2.1times 10^{-3}T_(ex,tau_{HI}=1)$+1.0. The relation between $frm_{DMG}$ and total hydrogen column density $N_H$ is given by $frm_{DMG}$=$1.0-3.7times 10^{20}/N_H$. The values of $frm_{DMG}$ in the clouds of low extinction group ($Arm_V le 2.7$ mag) are consistent with the results of the time-dependent, chemical evolutionary model at the age of ~ 10 Myr. Our empirical relation cannot be explained by the chemical evolutionary model for clouds in the high extinction group ($Arm_V > 2.7$ mag). Compared to clouds in the low extinction group ($Arm_V le 2.7$ mag), clouds in the high extinction group ($Arm_V > 2.7$ mag) have comparable volume densities but excitation temperatures that are 1.5 times lower. Moreover, CO abundances in clouds of the high extinction group ($Arm_V > 2.7$ mag) are $6.6times 10^2$ times smaller than the canonical value in the Milky Way. #[Full version of abstract is shown in the text.]#
With new THz maps that cover an area of ~3.3x2.1 pc^2 we probe the spatial distribution and association of the ionized, neutral and molecular gas components in the M17 SW nebula. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain a 5.7x3.7 map of the 12CO J=13-12 transition and the [C II] 158 um fine-structure line in M17 SW and compare the spectroscopically resolved maps with corresponding ground-based data for low- and mid-J CO and [C I] emission. For the first time SOFIA/GREAT allow us to compare velocity-resolved [C II] emission maps with molecular tracers. We see a large part of the [C II] emission, both spatially and in velocity, that is completely non-associated with the other tracers of photon-dominated regions (PDR). Only particular narrow channel maps of the velocity-resolved [C II] spectra show a correlation between the different gas components, which is not seen at all in the integrated intensity maps. These show different morphology in all lines but give hardly any information on the origin of the emission. The [C II] 158 um emission extends for more than 2 pc into the M17 SW molecular cloud and its line profile covers a broader velocity range than the 12CO J=13-12 and [C I] emissions, which we interpret as several clumps and layers of ionized carbon gas within the telescope beam. The high-J CO emission emerges from a dense region between the ionized and neutral carbon emissions, indicating the presence of high-density clumps that allow the fast formation of hot CO in the irradiated complex structure of M17 SW. The [C II] observations in the southern PDR cannot be explained with stratified nor clumpy PDR models.
Gas infall and outflow are critical for determining the star formation rate and chemical evolution of galaxies but direct measurements of gas flows are diffcult to make. Young massive stars and HII regions in the halos of galaxies are potential tracers for accretion and/or outflows of gas. Gas phase abundances of three HII regions in the lower halos of the edge-on galaxies NGC 3628 and NGC 4522 are determined by analysing optical long-slit spectra. The observed regions have projected distances to the midplane of their host from 1.4 to 3 kpc. With the measured flux densities of the optical nebular emission lines, we derive the oxygen abundance 12 + log(O/H) for the three extraplanar HII regions. The analysis is based on one theoretical and two empirical strong-line calibration methods. The resulting oxygen abundances of the extraplanar HII regions are comparable to the disk HII regions in one case and a little lower in the other case. Since our results depend on the accuracy of the metallicity determinations, we critically discuss the difference of the calibration methods we applied and confirm previously noted offsets. From our measurements, we argue that these three extraplanar HII regions were formed in the disk or at least from disk material. We discuss the processes that could transport disk material into the lower halo of these systems and conclude that gravitational interaction with a companion galaxy is most likely for NGC 3628 while ram pressure is favoured in the case of NGC 4522.
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