John S. Mulchaey

The Properties of Poor Groups of Galaxies. II. X-Ray and Optical Comparisons

We use ROSAT PSPC data to study the X-ray properties of a sample of 12 poor groups that have extensive membership information. Diffuse X-ray emission is detected in nine of these groups. In all but one of the X-ray-detected groups, the X-ray emission is centered on a luminous elliptical galaxy. Fits to the surface brightness profiles of the X-ray emission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy and is extended on scales of 20-40 h-1 kpc. The location and extent of this component, combined with its X-ray temperature (~0.7- 0.9 keV) and luminosity (~1041-42 h-2 ergs s-1), favor an origin in the interstellar medium of the central galaxy. Alternatively, the central component may be the result of a large-scale cooling flow. The second X-ray component is detected out to a radius of at least ~100-300 h-1 kpc. This component follows the same relationships found among the X-ray temperature (T), X-ray luminosity (LX), and optical velocity dispersion (σr) of rich clusters. This result suggests that the X-ray-detected groups are low-mass versions of clusters and that the extended gas component can properly be called the intragroup medium, by analogy to the intracluster medium in clusters. The failure to detect an intragroup medium in the three groups with very low velocity dispersions is consistent with their predicted X-ray luminosities and temperatures based on the relationships derived for clusters and X-ray-detected groups. The best-fit value of β derived from the σr-T relationship for groups and clusters is ~0.99 ± 0.08, implying that the galaxies and the hot gas trace the same potential with equal energy per unit mass and that the groups are dynamically relaxed. We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the position angle of the large-scale X-ray emission. This trend is consistent with that found for some rich clusters containing cD galaxies. The alignment of the central galaxy with the extended X-ray emission suggests that the formation and/or evolution of the central galaxy is linked to the shape of the global group potential. One possible scenario is that the central galaxy formed via galaxy-galaxy mergers early in the lifetime of the group and has not been subject to significant dynamical evolution recently.

The Properties of Poor Groups of Galaxies. III. The Galaxy Luminosity Function

The form of the galaxy luminosity function (GLF) in poor groups—regions of intermediate galaxy density that are common environments for galaxies—is not well understood. Multiobject spectroscopy and wide-field CCD imaging now allow us to measure the GLF of bound group members directly (i.e., without statistical background subtraction) and to compare the group GLF with the GLFs of the field and of rich clusters. We use R-band images in 1.5 × 1.5 degree2 mosaics to obtain photometry for galaxies in the fields of six nearby (2800 -19 + 5 log h) to giants (MR ≤ -19 + 5 log h) is significantly larger for the five groups with luminous X-ray halos than for the one marginally X-ray-detected group; (2) the composite GLF for the luminous X-ray groups is consistent in shape with two measures of the composite R-band GLF for rich clusters (Trentham; Driver et al.) and flatter at the faint end than another (α ≈ -1.5; Smith et al.); (3) the composite group GLF rises more steeply at the faint end than the R-band GLF of the Las Campanas Redshift Survey (LCRS; α = -0.7 from Lin et al.), a large volume survey dominated by galaxies in environments more rarefied than luminous X-ray groups; (4) the shape difference between the LCRS field and composite group GLFs results mostly from the population of non-emission line galaxies (EW [O II] < 5 A), whose dwarf-to-giant ratio is larger in the denser group environment than in the field (cf. Ferguson & Sandage; Bromley et al.); and (5) the non-emission line dwarfs are more concentrated about the group center than the non-emission line giants, except for the central, brightest (MR < M) group elliptical (BGG). This last result indicates that the dwarfs, giants, and BGGs occupy different orbits (i.e., have not mixed completely) and suggests that at least one of these populations formed at a different time. Our results show that the shape of the GLF varies with environment and that this variation is due primarily to an increase in the dwarf-to-giant ratio of quiescent galaxies in higher density regions, at least up to the densities characteristic of X-ray luminous poor groups. This behavior suggests that, in some environments, dwarfs are more biased than giants with respect to dark matter. This trend conflicts with the prediction of standard biased galaxy formation models.

Multiwavelength tests of the dusty torus model for Seyfert galaxies

We present a compilation of emission properties for a sample of 116 Seyfert galaxies based on both previously unpublished data and measurements available in the literature. These measurements include fluxes in the emission lines (O III) lambda(5007) and H-beta, as well as the infrared (25-60 microns), ultraviolet (1450 A), soft (0.2-4 keV), and hard (2-10 keV) X-ray continua. These are used to try to distinguish between isotropic and anisotropic emission properties of Seyfert galaxies. The distribution functions of (O III) lambda 5007 infrared, and hard X-ray continuum are similar for Seyfert 1s and Seyfert 2s, consistent with these properties being isotropic. The ultraviolet and soft X-ray continua of Seyfert 2s are underluminous relative to the type 1s suggesting photons at these energies escape from the central source anisotropically. There is a correlation between the ultraviolet continuum and emission-line fluxes in Seyfert 1s consistent with the idea that the central engine is responsible for powering the line emission. No such correlation is found for the Seyfert 2s. Instead, the scatter in the plot of ultraviolet continuum versus line emission suggests the true nuclear continuum luminosity is not seen at Earth in these objects. These properties are consistent with those expected in the dusty torus model.

X-RAY PROPERTIES OF GROUPS OF GALAXIES

▪ Abstract ROSAT observations indicate that approximately half of all nearby groups of galaxies contain spatially extended X-ray emission. The radial extent of the X-ray emission is typically 50–500 h−1100 kpc or approximately 10–50% of the virial radius of the group. Diffuse X-ray emission is generally restricted to groups that contain at least one early-type galaxy. X-ray spectroscopy suggests the emission mechanism is most likely a combination of thermal bremsstrahlung and line emission. This interpretation requires that the entire volume of groups be filled with a hot, low-density gas known as the intragroup medium. ROSAT and ASCA observations indicate that the temperature of the diffuse gas in groups ranges from approximately 0.3 keV to 2 keV. Higher temperature groups tend to follow the correlations found for rich clusters between X-ray luminosity, temperature, and velocity dispersion. However, groups with temperatures below approximately 1 keV appear to fall off the cluster LX-T relationship (and p...

PROBING THE INTERGALACTIC MEDIUM/GALAXY CONNECTION. V. ON THE ORIGIN OF Lyα AND O VI ABSORPTION AT z < 0.2

We analyze the association of galaxies with Lyα and O VI absorption, the most commonly detected transitions of the low-z intergalactic medium (IGM), in the fields of 14 quasars with z em = 0.06-0.57. Confirming previous studies, we observe a high covering fraction for Lyα absorption to impact parameter ρ = 300 h –1 72 kpc: 33/37 of our L > 0.01 L* galaxies show Lyα equivalent width W Lyα ≥ 50 mA. Galaxies of all luminosity L > 0.01 L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ~1010.5 ± 0.3 M ☉ for a constant N H = 1019 cm–2. The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lyα absorbers (W Lyα > 300 mA) but cannot reproduce the majority of observed lines in the Lyα forest. We conclude that the majority of Lyα absorption with W Lyα = 30-300 mA occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ≈400 h –1 72 kpc. Regarding O VI, we observe a near unity covering fraction to ρ = 200 h –1 72 kpc for L > 0.1 L* galaxies and to ρ = 300 h –1 72 kpc for sub-L* (0.1 L* 70 mA) arise in the virialized halos of L > 0.1 L* galaxies. Unlike Lyα, the weaker O VI systems (W 1031 ≈ 30 mA) arise in the extended CGM of sub-L* galaxies. The majority of O VI gas observed in the low-z IGM is associated with a diffuse medium surrounding individual galaxies with L ≈ 0.3 L* and rarely originates in the so-called warm-hot IGM (predicted by cosmological simulations.

Strangulation in galaxy groups

We use a cosmological chemodynamical simulation to study how the group environment impacts the star formation ( SF) properties of disk galaxies. The simulated group has a total mass of M similar to 8 x 10(12) M-circle dot and a total X-ray, luminosity of L-x similar to 10(41) erg s(-1). Our simulation suggests that ram pressure is not sufficient in this group to remove the cold disk gas from a V-rot similar to 150 km s galaxy. However, the majority of the hot gas in the galaxy is stripped over a timescale of approximately 1 Gyr. Since the cooling of the hot-gas component provides a source for new cold gas, the stripping of the hot component effectively cuts off the supply of cold gas. This in turn leads to a quenching of SF. The galaxy maintains the disk component after the cold gas is consumed, which may lead to a galaxy similar to an S0. Our self-consistent simulation suggests that this strangulation mechanism works even in low-mass groups, providing an explanation for the lower SF rates in group galaxies relative to galaxies in the field.

The Fueling of Nuclear Activity: The Bar Properties of Seyfert and Normal Galaxies

We use a recent near-infrared imaging survey of samples of Seyfert and normal galaxies to study the role of bars in the fueling of nuclear activity. The active galaxy sample includes Seyfert galaxies in the Revised Shapely-Ames (RSA) catalog and Sandage & Tammanns extension of this catalog. The normal galaxies were selected to match the Seyfert sample in Hubble type, redshift, inclination, and blue luminosity. All the galaxies in both samples classified as barred in the RSA catalog are also barred in the near-infrared. In addition, ~55% of the galaxies classified as nonbarred in the RSA show evidence for bars at 2.1 μm. Overall, ~70% of the galaxies observed show evidence for bar structures. The incidence of bars in the Seyfert and normal galaxies is similar, suggesting that Seyfert nuclei do not occur preferentially in barred systems. Furthermore, a slightly higher percentage of normal galaxies have multiple-bar structures. A significant percentage of the Seyfert galaxies in our sample show no evidence for the presence of a bar even in the near-infrared. This suggests either that large-scale kiloparsec bars are not a universal fueling mechanism in Seyfert galaxies or that the bars in the nonbarred Seyferts were recently destroyed, possibly by the formation of the central black hole.

MORPHOLOGICAL COMPOSITION OF z ∼ 0.4 GROUPS: THE SITE OF S0 FORMATION

The low-redshift universe (z 0.5) is not a dull place. Processes leading to the suppression of star formation and morphological transformation are prevalent: this is particularly evident in the dramatic upturn in the fraction of S0-type galaxies in clusters. However, until now, the process and environment of formation remained unidentified. We present a morphological analysis of galaxies in the optically-selected (spectroscopic friends-of-friends) group and field environments at z ~ 0.4. Groups contain a much higher fraction of S0s at fixed luminosity than the lower density field, with >99.999% confidence. Indeed, the S0 fraction in groups is at least as high as in z ~ 0.4 clusters and X-ray-selected groups, which have more luminous intragroup medium (IGM). An excess of S0s at ≥0.3h –1 75 Mpc from the group center with respect to the inner regions, existing with 97% confidence at fixed luminosity, tells us that formation is not restricted to, and possibly even avoids, the group cores. Interactions with a bright X-ray-emitting IGM cannot be important for the formation of the majority of S0s in the universe. In contrast to S0s, the fraction of elliptical galaxies in groups at fixed luminosity is similar to the field, while the brightest ellipticals are strongly enhanced toward the group centers (greater than 99.999% confidence within 0.3h –1 75 Mpc). Interestingly, while spirals are altogether less common in groups than in the field, there is also an excess of faint, Sc+ type spirals within 0.3h –1 75 Mpc of the group centers (99.953% confidence). We conclude that the group and subgroup environments must be dominant for the formation of S0 galaxies, and that minor mergers, galaxy harassment, and tidal interactions are the most likely responsible mechanisms. This has implications not only for the inferred preprocessing of cluster galaxies, but also for the global morphological and star formation budget of galaxies: as hierarchical clustering progresses, more galaxies will be subject to these transformations as they enter the group environment.

An X-Ray Atlas of Groups of Galaxies

A search was conducted for a hot intragroup medium in 109 low-redshift galaxy groups observed with the ROSAT PSPC. Evidence for diffuse, extended X-ray emission is found in at least 61 groups. Approximately one-third of these detections have not been previously reported in the literature. Most of the groups are detected out to less than half of the virial radius with ROSAT. Although some spiral-rich groups do contain an intragroup medium, diffuse emission is restricted to groups that contain at least one early-type galaxy.

A real-time fast radio burst: polarization detection and multiwavelength follow-up

Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progenitors and origins remain unknown and until now no rapid multiwavelength follow-up of an FRB has been possible. New instrumentation has decreased the time between observation and discovery from years to seconds, and enables polarimetry to be performed on FRBs for thefirst time. We have discovered an FRB (FRB 140514) in real-time on 2014 May 14 at 17:14:11.06 UTCattheParkesradiotelescopeandtriggeredfollow-upatotherwavelengthswithinhoursof theevent.FRB140514wasfoundwithadispersionmeasure(DM)of562.7(6)cm −3 pc,giving an upper limit on source redshift of z 0.5. FRB 140514 was found to be 21 ± 7 per cent (3σ) circularly polarized on the leading edge with a 1σ upper limit on linear polarization <10 per cent. We conclude that this polarization is intrinsic to the FRB. If there was any intrinsic linear polarization, as might be expected from coherent emission, then it may have been depolarized by Faraday rotation caused by passing through strong magnetic fields and/or high-density environments. FRB 140514 was discovered during a campaign to re-observe known FRB fields, and lies close to a previous discovery, FRB 110220; based on the difference in DMs of these bursts and time-on-sky arguments, we attribute the proximity to sampling bias and conclude that they are distinct objects. Follow-up conducted by 12 telescopes observing from X-ray to radio wavelengths was unable to identify a variable multiwavelength counterpart, allowing us to rule out models in which FRBs originate from nearby ( z< 0.3) supernovae and long duration gamma-ray bursts.