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Register a new userInfrared Spectroscopy of the Diffuse Ionized Halo of NGC 891
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We present infrared spectroscopy from the Spitzer Space Telescope at one disk position and two positions at a height of 1 kpc from the disk in the edge-on spiral NGC 891, with the primary goal of studying halo ionization. Our main result is that the [Ne III]/[Ne II] ratio, which provides a measure of the hardness of the ionizing spectrum free from the major problems plaguing optical line ratios, is enhanced in the extraplanar pointings relative to the disk pointing. Using a 2D Monte Carlo-based photo-ionization code which accounts for the effects of radiation field hardening, we find that this trend cannot be reproduced by any plausible photo-ionization model, and that a secondary source of ionization must therefore operate in gaseous halos. We also present the first spectroscopic detections of extraplanar PAH features in an external normal galaxy. If they are in an exponential layer, very rough emission scale-heights of 330-530 pc are implied for the various features. Extinction may be non-negligible in the midplane and reduce these scale-heights significantly. There is little significant variation in the relative emission from the various features between disk and extraplanar environment. Only the 17.4 micron feature is significantly enhanced in the extraplanar gas compared to the other features, possibly indicating a preference for larger PAHs in the halo.
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We present WIYN SparsePak observations of the diffuse ionized gas (DIG) halo of NGC 891. Preliminary results of an analysis of the halo velocity field reveal a clear gradient of the azimuthal velocity with z which agrees with results for the neutral gas. The magnitude of the gradient has been determined, using two independent methods, to be approximately 15 km/s/kpc.
The observed scale heights of extraplanar diffuse ionized gas (eDIG) layers exceed their thermal scale heights by a factor of a few in the Milky Way and other nearby edge-on disk galaxies. Here, we test a dynamical equilibrium model of the extraplanar diffuse ionized gas layer in NGC 891, where we ask whether the thermal, turbulent, magnetic field, and cosmic ray pressure gradients are sufficient to support the layer. In optical emission line spectroscopy from the SparsePak integral field unit on the WIYN 3.5-meter telescope, the H-alpha emission in position-velocity space suggests that the eDIG is found in a ring between galactocentric radii of R_min <= R <= 8 kpc, where R_min >= 2 kpc. We find that the thermal (sigma_th = 11 km/s) and turbulent (sigma_turb = 25 km/s) velocity dispersions are insufficient to satisfy the hydrostatic equilibrium equation given an exponential electron scale height of h_z = 1.0 kpc. Using a literature analysis of radio continuum observations from the CHANG-ES survey, we demonstrate that the magnetic field and cosmic ray pressure gradients are sufficient to stably support the gas at R >= 8 kpc if the cosmic rays are sufficiently coupled to the system (gamma_cr = 1.45). Thus, a stable dynamical equilibrium model is viable only if the extraplanar diffuse ionized gas is found in a thin ring around R = 8 kpc, and non-equilibrium models such as a galactic fountain flow are of interest for further study.
Galaxies are surrounded by halos of hot gas whose mass and origin remain unknown. One of the most challenging properties to measure is the metallicity, which constrains both of these. We present a measurement of the metallicity around NGC 891, a nearby, edge-on, Milky Way analog. We find that the hot gas is dominated by low metallicity gas near the virial temperature at $kT=0.20pm0.01$ keV and $Z/Z_{odot} = 0.14pm0.03$(stat)$^{+0.08}_{-0.02}$(sys), and that this gas co-exists with hotter ($kT=0.71pm0.04$ keV) gas that is concentrated near the star-forming regions in the disk. Model choices lead to differences of $Delta Z/Z_{odot} sim 0.05$, and higher $S/N$ observations would be limited by systematic error and plasma emission model or abundance ratio choices. The low metallicity gas is consistent with the inner part of an extended halo accreted from the intergalactic medium, which has been modulated by star formation. However, there is much more cold gas than hot gas around NGC 891, which is difficult to explain in either the accretion or supernova-driven outflow scenarios. We also find a diffuse nonthermal excess centered on the galactic center and extending to 5 kpc above the disk with a 0.3-10 keV $L_X = 3.1times 10^{39}$ erg s$^{-1}$. This emission is inconsistent with inverse Compton scattering or single-population synchrotron emission, and its origin remains unclear.
We present detections of 21-cm emission from neutral hydrogen (HI) in the circumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565 using the Robert C. Byrd Green Bank Telescope (GBT). With our 5$sigma$ sensitivity of $8.2 times 10^{16}$ cm$^{-2}$ calculated over a 20 km s$^{-1}$ channel, we achieve $>5sigma$ detections out to $90-120$ kpc along the minor axes. The velocity width of the CGM emission is as large as that of the disk $approx 500$ km s$^{-1}$, indicating the existence of a diffuse component permeating the halo. We compare our GBT measurements with interferometric data from the Westerbork Synthesis Radio Telescope (WSRT). The WSRT maps the HI emission from the disk at high S/N but has limited surface brightness sensitivity at the angular scales probed with the GBT. After convolving the WSRT data to the spatial resolution of the GBT (FWHM = 9.1$$), we find that the emission detected by the WSRT accounts for $48^{+15}_{-25}$% ($58^{+4}_{-18}$%) of the total flux recovered by the GBT from the CGM of NGC 891(NGC 4565). The existence of significant GBT-only flux suggests the presence of a large amount of diffuse, low column density HI emission in the CGM. For reasonable assumptions, the extended diffuse HI could account for $5.2pm0.9$% and $2.0pm0.8$% of the total HI emission of NGC 891 and NGC 4565.
Low-frequency radio continuum observations of edge-on galaxies are ideal to study cosmic-ray electrons (CREs) in halos via radio synchrotron emission and to measure magnetic field strengths. We obtained new observations of the edge-on spiral galaxy NGC 891 at 129-163 MHz with the LOw Frequency ARray (LOFAR) and at 13-18 GHz with the Arcminute Microkelvin Imager (AMI) and combine them with recent high-resolution Very Large Array (VLA) observations at 1-2 GHz, enabling us to study the radio continuum emission over two orders of magnitude in frequency. The spectrum of the integrated nonthermal flux density can be fitted by a power law with a spectral steepening towards higher frequencies or by a curved polynomial. Spectral flattening at low frequencies due to free-free absorption is detected in star-forming regions of the disk. The mean magnetic field strength in the halo is 7 +- 2 $mu$G. The scale heights of the nonthermal halo emission at 146 MHz are larger than those at 1.5 GHz everywhere, with a mean ratio of 1.7 +- 0.3, indicating that spectral ageing of CREs is important and that diffusive propagation dominates. The halo scale heights at 146 MHz decrease with increasing magnetic field strengths which is a signature of dominating synchrotron losses of CREs. On the other hand, the spectral index between 146 MHz and 1.5 GHz linearly steepens from the disk to the halo, indicating that advection rather than diffusion is the dominating CRE transport process. This issue calls for refined modelling of CRE propagation.
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