Eric D. Miller

The Detection of a Cooling Flow Elliptical Galaxy from O VI Emission

Cooling flow models for the hot gas in elliptical galaxies predict that gas is cooling at a rate of ~1 M☉ yr-1, yet there is little evidence for this phenomenon beyond the X-ray wave band. If hot gas is cooling, it will pass through the 3 × 105 K regime and radiate in the O VI λλ1032, 1038 ultraviolet lines, which can be detected with the Far-Ultraviolet Spectroscopic Explorer (FUSE), and here we report on FUSE observations of the X-ray bright early-type galaxies NGC 1404 and NGC 4636. In NGC 1404, the O VI doublet is not detected, implying a cooling rate less than 0.3 M☉ yr-1, which is below the predicted values from the cooling flow model of 0.4-0.9 M☉ yr-1. In NGC 4636, both O VI lines are clearly detected, indicating a cooling rate of 0.43 ± 0.06 M☉ yr-1, which falls within the range of values from the cooling flow prediction, 0.36-2.3 M☉ yr-1, and is closest to the model where the production of the cooled gas is distributed through the galaxy. The emission-line widths, 44 ± 15 km s-1, are close to the Doppler broadening value (30 km s-1), indicating that the flow is quiescent rather than turbulent and that the flow velocity is less than 30 km s-1.

Cosmic Filaments in Superclusters

Large-scale structure calculations show that modest overdensity filaments will connect clusters of galaxies and these filaments are reservoirs of baryons, mainly in gaseous form. To determine whether such filaments exist, we have examined the UV absorption line properties of three active galactic nuclei (AGNs) projected behind possible filaments in superclusters of galaxies; the AGNs lie within 3 Mpc of the center lines of loci connecting clusters. All three lines of sight show absorption in Lyα, Lyβ, and/or O VI at redshifts within about 1300 km s-1 of the nearby galaxy clusters that would define the closest filaments. For one AGN, the absorption-line redshifts are close to the emission-line redshift of the AGN, so we cannot rule out self-absorption for this object. These absorption-line associations with superclusters are unlikely to have occurred by chance, a result consistent with the presence of cosmic filaments within superclusters.

Constraints on Cool Gas in Rich Clusters of Galaxies

Cool gas should be present in galaxy clusters due to stripping of galactic gas, in-fall onto the cluster, and from cooling flows. We have searched for this gas through metal resonance absorption lines from low-ionization gas toward six background quasars. Both cooling-flow and non-cooling-flow clusters were observed, with lines of sight ranging from the inner to outer parts of the cluster (0.32 Mpc ≤ rproj ≤ 1.40 Mpc). The HST Faint Object Spectrograph observations failed to detect Fe II or Mg II absorption at the cluster redshift, with 1 σ upper limits on the ion column densities of N ≤ 1012-1013 cm-2. From existing X-ray data, we estimate that ram-pressure stripping of galactic gas by the intracluster medium should produce cool gas along these sight lines. The failure to detect absorption lines implies that any gas stripped in such a way has a low covering fraction, possibly due to a short lifetime in this low-ionization state.

The size of the cooling region of hot gas in two elliptical galaxies

Some early-type galaxies show O VI emission, a tracer of gas at 105.5 K, and a predicted product of gas cooling from the X-ray-emitting temperatures. We studied the spatial extent and velocity structure of this cooling gas by obtaining spectra of the O VI doublet in NGC 4636 and NGC 5846 with the Far Ultraviolet Spectroscopic Explorer. For NGC 4636, the central LWRS pointing shows that the O VI lines are double-peaked and symmetrical about the systemic velocity of the galaxy, with a separation of 210 km s-1. An LWRS observation 30 from the center failed to show additional O VI emission. For NGC 5846, three spectra were obtained with 4 × 20 apertures (MDRS) at the center and 4 to the east and west of the center. The O VI line flux seen in the previous LWRS is contained in the sum of the smaller apertures, with most of the flux in a single noncentral MDRS aperture, suggesting a size for the emission ≤0.5 kpc; the emission consists of a blue and red peak. For both galaxies, the O VI velocity structure is similar to that of the optical [N II] emission and is consistent with rotation. The compactness and velocity structure of the O VI emission rules out cooling flow models with broadly distributed mass dropout but is consistent with cooling flow models in which the cooling occurs primarily in the central region. The 104 K gas may be the end state of the O VI emitting gas.

Characterizing particle background of ATHENA WFI for the science products module: swift XRT full frame and XMM-PN small window mode observations

The Wide Field Imager (WFI) is one of two focal plane detector systems of ESA’s Advanced Telescope for High ENergy Astrophysics (ATHENA) X-ray observatory. The Science Products Module (SPM) will have on-board processing algorithms that will reduce the ATHENA WFI particle background level significantly by improving background rejection on board and in post-processing on the ground. To this end, we examine the full frame observations from existing X-ray telescopes to understand and characterize the physics of the particle background. In particular, we determine phenomenological correlations between high energy particle events and X-ray events to improve the rejection of particle background events. We will present our results from the Swift XRT and XMM-Newton PN full frame data analysis in this talk. We will also discuss how these results could be used to reduce the expected background in the ATHENA WFI observations by the SPM processing.

Arcus: the x-ray grating spectrometer explorer (Conference Presentation)

Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50 A bandpass with unprecedented sensitivity: effective areas of >350 cm^2 and spectral resolution >2500 at the energies of O VII and O VIII for z=0-0.3. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Arcus will be launched into an ~ 7 day 4:1 lunar resonance orbit, resulting in high observing efficiency, low particle background and a favorable thermal environment. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned. The baseline science mission will be completed in <2 years, although the margin on all consumables allows for 5+ years of operation.

The Closest Damped Lyman Alpha System

A difficulty of studying damped Lyman alpha systems is that they are distant, so one knows little about the interstellar medium of the galaxy. Here we report upon a damped Lyman alpha system in the nearby galaxy NGC 4203, which is so close (v_helio = 1117 km/s) and bright (B_o = 11.62) that its HI disk has been mapped. The absorption lines are detected against Ton 1480, which lies only 1.9 (12 h_50 kpc) from the center of NGC 4203. Observations were obtained with the Faint Object Spectrograph on HST (G270H grating) over the 2222-3277 Angstrom region with 200 km/s resolution. Low ionization lines of Fe, Mn, and Mg were detected, leading to metallicities of -2.29, -2.4, which are typical of other damped Lyman alpha systems, but well below the stellar metallicity of this type of galaxy. Most notably, the velocity of the lines is 1160 +- 10 km/s, which is identical to the HI rotational velocity of 1170 km/s at that location in NGC 4203, supporting the view that these absorption line systems can be associated with the rotating disks of galaxies. In addition, the line widths of the Mg lines give an upper limit to the velocity dispersion of 167 km/s, to the 99% confidence level.

The Closest Damped Lyα System

A difficulty of studying damped Lyman alpha systems is that they are distant, so one knows little about the interstellar medium of the galaxy. Here we report upon a damped Lyman alpha system in the nearby galaxy NGC 4203, which is so close (v_helio = 1117 km/s) and bright (B_o = 11.62) that its HI disk has been mapped. The absorption lines are detected against Ton 1480, which lies only 1.9 (12 h_50 kpc) from the center of NGC 4203. Observations were obtained with the Faint Object Spectrograph on HST (G270H grating) over the 2222-3277 Angstrom region with 200 km/s resolution. Low ionization lines of Fe, Mn, and Mg were detected, leading to metallicities of -2.29, -2.4, which are typical of other damped Lyman alpha systems, but well below the stellar metallicity of this type of galaxy. Most notably, the velocity of the lines is 1160 +- 10 km/s, which is identical to the HI rotational velocity of 1170 km/s at that location in NGC 4203, supporting the view that these absorption line systems can be associated with the rotating disks of galaxies. In addition, the line widths of the Mg lines give an upper limit to the velocity dispersion of 167 km/s, to the 99% confidence level.