Jesper Rasmussen

The formation of fossil galaxy groups in the hierarchical universe

We use a set of 12 high-resolution N-body/hydrodynamical simulations in the ΛCDM cosmology to investigate the origin and formation rate of fossil groups (FGs), which are X-ray-bright galaxy groups dominated by a large elliptical galaxy, with the second brightest galaxy being at least 2 mag fainter. The simulations invoke star formation, chemical evolution with noninstantaneous recycling, metal-dependent radiative cooling, strong starburst-driven galactic superwinds, effects of a metagalactic UV field, and full stellar population synthesis. We find an interesting correlation between the magnitude gap between the brightest and second-brightest galaxy and the formation time of the group. It is found that FGs have already assembled half of their final dark matter mass at z 1, and subsequently they typically grow by minor merging only, whereas non-FGs on average form later. The early assembly of FGs leaves sufficient time for galaxies of L ~ L* to merge into the central one by dynamical friction, resulting in the large magnitude gap at z = 0. About 33% ± 16% of the groups simulated are found to be FGs, whereas the observational estimate is ~10%-20%. The FGs are found to be overluminous in the X-ray relative to non-FGs of the same optical luminosity, in qualitative agreement with observations. Finally, from a dynamical friction analysis, we find that FGs exist at all only because infall of L ~ L* galaxies happens along filaments with small impact parameters.

Temperature and abundance profiles of hot gas in galaxy groups – I. Results and statistical analysis

The distribution of metals in groups of galaxies holds important information about the chemical enrichment history of the Universe. Here we present radial profiles of temperature and the abundance of iron and silicon of the hot intragroup medium for a sample of 15 nearby groups of galaxies observed by Chandra, selected for their regular X-ray morphology. All but one group display a cool core, the size of which is found to correlate with the mean temperature of the group derived outside this core. When scaled to this mean temperature, the temperature profiles are remarkably similar, being analogous to those of more massive clusters at large radii but significantly flatter inwards of the temperature peak. The Fe abundance generally shows a central excess followed by a radial decline, reaching a typical value of 0.1 Z ⊙ within r 500 , a factor of 2 lower than corresponding results for clusters. Si shows less systematic radial variation, on average displaying a less pronounced decline than Fe and showing evidence for a flattening at large radii. Off-centre abundance peaks are seen both for Fe and Si in a number of groups with well-resolved cores. Derived abundance ratios indicate that supemovae type Ia are responsible for 80 per cent of the Fe in the group core, but the type II contribution increases with radius and completely dominates at r 500 . We present fitting formulae for the radial dependence of temperature and abundances, to facilitate comparison to results of numerical simulations of group formation and evolution. In a companion paper, we discuss the implications of these results for feedback and enrichment in galaxy groups.

HOT GAS HALOS AROUND DISK GALAXIES: CONFRONTING COSMOLOGICAL SIMULATIONS WITH OBSERVATIONS

Models of disk galaxy formation commonly predict the existence of an extended reservoir of accreted hot gas surrounding massive spirals at low redshift. As a test of these models, we use X-ray and Hα data of the two massive, quiescent edge-on spirals NGCxa05746 and NGCxa05170 to investigate the amount and origin of any hot gas in their halos. Contrary to our earlier claim, the Chandra analysis of NGCxa05746, employing more recent calibration data, does not reveal any significant evidence for diffuse X-ray emission outside the optical disk, with a 3σ upper limit to the halo X-ray luminosity of 4 × 1039 erg s–1. An identical study of the less massive NGCxa05170 also fails to detect any extraplanar X-ray emission. By extracting hot halo properties of disk galaxies formed in cosmological hydrodynamical simulations, we compare these results to expectations for cosmological accretion of hot gas by spirals. For Milky-Way-sized galaxies, these high-resolution simulations predict hot halo X-ray luminosities which are lower by a factor of ~2 compared to our earlier results reported by Toft etxa0al. We find the new simulation predictions to be consistent with our observational constraints for both NGCxa05746 and NGCxa05170, while also confirming that the hot gas detected so far around more actively star-forming spirals is in general probably associated with stellar activity in the disk. Observational results on quiescent disk galaxies at the high-mass end are nevertheless providing powerful constraints on theoretical predictions, and hence on the assumed input physics in numerical studies of disk galaxy formation and evolution.

Powerful H

We present results from the mid-infrared spectral mapping of Stephans Quintet using the Spitzer Space Telescope. A 1000 km s^(-1) collision (t_(col) = 5 × 10^6 yr) has produced a group-wide shock, and for the first time the large-scale distribution of warm molecular hydrogen emission is revealed, as well as its close association with known shock structures. In the main shock region alone we find 5.0 × 10^8 M_☉ of warm H_2 spread over ~480 kpc^2 and additionally report the discovery of a second major shock-excited H_2 feature, likely a remnant of previous tidal interactions. This brings the total H2 line luminosity of the group in excess of 10^(42) erg s^(-1). In the main shock, the H_2 line luminosity exceeds, by a factor of 3, the X-ray luminosity from the hot shocked gas, confirming that the H_2-cooling pathway dominates over the X-ray. [Si II]34.82 μm emission, detected at a luminosity of 1/10th of that of the H_2, appears to trace the group-wide shock closely, and in addition, we detect weak [Fe II]25.99 μm emission from the most X-ray luminous part of the shock. Comparison with shock models reveals that this emission is consistent with regions of fast shocks (100 km s^(-1) < V_s < 300 km s^(-1)) experiencing depletion of iron and silicon onto dust grains. Star formation in the shock (as traced via ionic lines, polycyclic aromatic hydrocarbon and dust emission) appears in the intruder galaxy, but most strikingly at either end of the radio shock. The shock ridge itself shows little star formation, consistent with a model in which the tremendous H_2 power is driven by turbulent energy transfer from motions in a post-shocked layer which suppresses star formation. The significance of the molecular hydrogen lines over other measured sources of cooling in fast galaxy-scale shocks may have crucial implications for the cooling of gas in the assembly of the first galaxies.

_2

Ram pressure stripping of galactic gas is generally assumed to be inefficient in galaxy groups due to the relatively low density of the intragroup medium and the small velocity dispersions of groups. To test this assumption, we obtained Chandra X-ray data of the starbursting spiral NGC 2276 in the NGC 2300 group of galaxies, a candidate for a strong galaxy interaction with hot intragroup gas. The data reveal a shock-like feature along the western edge of the galaxy and a low–surface-brightness tail extending to the east, similar to the morphology seen in other wavebands. Spatially resolved spectroscopy shows that the data are consistent with intragroup gas being pressurized at the leading western edge of NGC 2276 due to the galaxy moving supersonically through the intragroup medium at a velocity � 850 km s −1 . Detailed modelling of the gravitational potential of NGC 2276 shows that the resulting ram-pressure could significantly affect the morphology of the outer gas disc but is probably insufficient to strip large amounts of cold gas from the disc. We estimate the mass loss rates due to turbulent viscous stripping and starburst outflows being swept back by ram pressure, showing that both mechanisms could plausibly explain the presence of the X-ray tail. Comparison to existing Hi measurements shows that most of the gas escaping the galaxy is in a hot phase. With a total mass loss rate of � 5 M⊙ yr −1 , the galaxy could be losing its entire present Hi supply within a Gyr. This demonstrates that the removal of galactic gas through interactions with a hot intragroup medium can occur rapidly enough to transform the morphology of galaxies in groups. Implications of this for galaxy evolution in groups and clusters are briefly discussed.

Line-cooling in Stephan's Quintet : I - Mapping the Significant Cooling Pathways in Group-wide Shocks

Galaxies in compact groups tend to be deficient in neutral hydrogen compared to isolated galaxies of similar optical properties. In order to investigate the role played by a hot intragroup medium (IGM) for the removal and destruction of Hxa0i in these systems, we have performed a Chandra and XMM–Newton study of eight of the most Hxa0i deficient Hickson compact groups. Diffuse X-ray emission associated with an IGM is detected in four of the groups, suggesting that galaxy–IGM interactions are not the dominant mechanism driving cold gas out of the group members. No clear evidence is seen for any of the members being currently stripped of any hot gas, nor for galaxies to show enhanced nuclear X-ray activity in the X-ray bright or most Hxa0i deficient groups. Combining the inferred IGM distributions with analytical models of representative disc galaxies orbiting within each group, we estimate the Hxa0i mass-loss due to ram-pressure and viscous stripping. While these processes are generally insufficient to explain observed Hxa0i deficiencies, they could still be important for Hxa0i removal in the X-ray bright groups, potentially removing more than half of the interstellar medium in the X-ray bright HCGxa097. Ram pressure may also have facilitated strangulation through the removal of galactic coronal gas. In X-ray undetected groups, tidal interactions could be playing a prominent role, but it remains an open question whether they can fully account for the observed Hxa0i deficiencies.

Gas stripping in galaxy groups – the case of the starburst spiral NGC 2276

We have performed a systematic search for X-ray cavities in the hot gas of 51 galaxy groups with Chandra archival data. The cavities are identified based on two methods: subtracting an elliptical β-model fitted to the X-ray surface brightness, and performing unsharp masking. Thirteen groups in the sample (~25%) are identified as clearly containing cavities, with another 13 systems showing tentative evidence for such structures. We find tight correlations between the radial and tangential radii of the cavities, and between their size and projected distance from the group center, in quantitative agreement with the case for more massive clusters. This suggests that similar physical processes are responsible for cavity evolution and disruption in systems covering a large range in total mass. We see no clear association between the detection of cavities and the current 1.4 GHz radio luminosity of the central brightest group galaxy, but there is a clear tendency for systems with a cool core to be more likely to harbor detectable cavities. To test the efficiency of the adopted cavity detection procedures, we employ a set of mock images designed to mimic typical Chandra data of our sample, and find that the model-fitting approach is generally more reliable than unsharp masking for recovering cavity properties. Finally, we find that the detectability of cavities is strongly influenced by a few factors, particularly the signal-to-noise ratio of the data, and that the real fraction of X-ray groups with prominent cavities could be substantially larger than the 25%-50% suggested by our analysis.

Galaxy evolution in Hickson compact groups: the role of ram‐pressure stripping and strangulation

We investigate the history of galactic feedback and chemical enrichment within a sample of 15 X-ray bright groups of galaxies, on the basis of the inferred Fe and Si distributions in the hot gas and the associated metal masses produced by core-collapse and Type Ia supernovae (SNe). Most of these cool-core groups show a central Fe and Si excess, which can be explained by prolonged enrichment by SN Ia and stellar winds in the central early-type galaxy alone, but with tentative evidence for additional processes contributing to core enrichment in hotter groups. Inferred metal mass-to-light ratios inside r 500 show a positive correlation with total group mass but are generally significantly lower than in clusters, due to a combination of lower global intracluster medium (ICM) abundances and gas-to-light ratios in groups. This metal deficiency is present for products from both SN Ia and SN II, and suggests that metals were either synthesized, released from galaxies or retained within the ICM less efficiently in lower mass systems. We explore possible causes, including variations in galaxy formation and metal release efficiency, cooling out of metals, and gas and metal loss via active galactic nuclei (AGN) - or starburst-driven galactic winds from groups or their precursor filaments. Loss of enriched material from filaments coupled with post-collapse AGN feedback emerges as viable explanations, but we also find evidence for metals to have been released less efficiently from galaxies in cooler groups and for the ICM in these to appear chemically less evolved, possibly reflecting more extended star formation histories in less massive systems. Some implications for the hierarchical growth of clusters from groups are briefly discussed.

A SYSTEMATIC SEARCH FOR X-RAY CAVITIES IN THE HOT GAS OF GALAXY GROUPS

X-ray observations of hot, intergalactic gas in galaxy groups provide a useful means of characterizing the global properties of groups. However, X-ray studies of large group samples have typically involved very shallow X-ray exposures or have been based on rather heterogeneous samples. Here we present the first results of the XI (XMM/IMACS) Groups Project, a study targeting, for the first time, a redshift-selected, statistically unbiased sample of galaxy groups using deep X-ray data. Combining this with radio observations of cold gas and optical imaging and spectroscopy of the galaxy population, the project aims to advance the understanding of how the properties and dynamics of group galaxies relate to global group properties. Here, X-ray and optical data of the first four galaxy groups observed as part of the project are presented. In two of the groups we detect diffuse emission with a luminosity of L X ≈ 10 41 erg s -1 , among the lowest found for any X-ray detected group thus far, with a comparable upper limit for the other two. Compared to typical X-ray selected groups of similar velocity dispersion, these four systems are all surprisingly X-ray faint. We discuss possible explanations for the lack of significant X-ray emission in the groups, concluding that these systems are most likely collapsing for the first time. Our results strongly suggest that, unlike our current optically selected sample, previous X-ray selected group samples represented a biased picture of the group population. This underlines the necessity of a study of this kind, if one is to reach an unbiased census of the properties of galaxy groups and the distribution of baryons in the Universe.

Temperature and abundance profiles of hot gas in galaxy groups - II. Implications for feedback and ICM enrichment

Abstract Hot gaseous haloes surrounding galaxies and extending well beyond the distribution of stars are a ubiquitous prediction of galaxy formation scenarios. The haloes are believed to consist of gravitationally trapped gas with a temperature of millions of Kelvin. The existence of such hot haloes around massive elliptical galaxies has been established through their X-ray emission. While gas out-flowing from starburst spiral galaxies has been detected, searches for hot haloes around normal, quiescent spiral galaxies have so far failed, casting doubts on the fundamental physics in galaxy formation models. Here we present the first detection of a hot, large-scale gaseous halo surrounding a normal, quiescent spiral galaxy, NGC 5746 , alleviating a long-standing problem for galaxy formation models. In contrast to starburst galaxies, where the X-ray halo can be powered by the supernova energy, there is no such power source in NGC 5746 . The only compelling explanation is that we are here witnessing a galaxy forming from gradually in-flowing hot and dilute halo gas.