# H-alpha velocity fields of spirals falling into a cluster

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What kind of impact would you expect ram pressure stripping / tidal interactions / harassment / interactions with the cluster potential (etc!) to have on the h-alpha velocity fields of infalling spiral galaxies? What I mean is, how easy is it to detect such processes at work, and how are you able to distinguish between these different mechanisms (using also h-alpha intensity maps, rotation curves, optical morphologies if available).

Cheers!

## H-alpha velocity fields of spirals falling into a cluster - Astronomy

A new-developed 3D numerical code is applied to an uniform external (primordial) magnetic field subject to a complex flow pattern representing the situation in a turbulent spiral galaxy. The spiral arms are defined by the radial-azimuthal profiles of density and the turbulent velocity, but they do not yet possess any own large-scale velocity field. No dynamo alpha is assumed to exist, but all the known turbulence effects such as eddy diamagnetism and turbulent pumping are involved. Two different models are followed: The (nonaxisymmetric) external magnetic field is considered as an initial-value and/or as a boundary condition. In the first case the decay of the magnetic field is rather fast. The initial field cannot survive more than 500 Myr. In its early times the magnetic field is concentrated between the spirals but later it is strongly wound up by the differential rotation. Any amplification of the magnetic energy does not appear. The nonlinear diffusivity quenching only plays a role for small eddy diffusivity. If the galaxy is embedded in an external intergalactic magnetic field there is an amplification of the magnetic energy by a factor of 10. But very soon the magnetic spirals have been transformed into rings and after about 1.5 Gyr the galaxy is nearly field-free. Our results confirm the idea that primordial magnetic fields in galaxies are unable to become old. If both the gaseous and the magnetic spirals had a common origin, the gaseous spirals are revealed here as young phenomena. Tuning the pattern speed of the spirals an exceptional amplification of the magnetic field is found in case of resonance' of the pattern speed and a magnetic drift velocity. Our calculations show that the maximal field then remains in the interarm region. We interpret the peak amplification as being due to the fact that the turbulence in the interarm regions is assumed as weak hence the diffusion there is strongly reduced. The differential rotation then amplifies the initial field maximally while the field decay is delayed.

## H-alpha velocity fields of spirals falling into a cluster - Astronomy

Aims: We present spectroscopic observations of 182 disk galaxies (96 in the cluster and 86 in the field environment) in the region of the Abell 901/902 multiple cluster system, which is located at a redshift of z

0.165. We estimate dynamical parameters of the four subclusters and analyse the kinematics of spiral galaxies, searching for indications of ram-pressure stripping. Furthermore, we focus on dusty red galaxies as a possible intermediate stage in the transformation of field galaxies to lenticulars when falling into the cluster.
Methods: We obtained multi-object slit spectroscopy using the VLT instrument VIMOS. We carried out a redshift analysis, determined velocity dispersions using biweight statistics, and detected possible substructures with the Dressler-Shectman test. We exploited rotation curves from emission lines to analyse distortions in the gaseous disk of a galaxy, as well as HST/ACS images to quantify the morphological distortions of the stellar disk.
Results: The presence of substructures and non-Gaussian redshift distributions indicate that the cluster system is dynamically young and not in a virialised state. We find evidence of two important galaxy populations. Morphologically distorted galaxies are probably subject to increased tidal interactions. They show pronounced rotation-curve asymmetries at intermediate cluster-centric radii and low rest-frame peculiar velocities. Morphologically undistorted galaxies show the strongest rotation-curve asymmetries at high rest-frame velocities and low cluster-centric radii. Supposedly, this group is strongly affected by ram-pressure stripping due to interaction with the intra-cluster medium. Among the morphologically undistorted galaxies, dusty red galaxies have particularly strong rotation-curve asymmetries, suggesting that ram pressure is an important factor in these galaxies. Furthermore, dusty red galaxies have on average a bulge-to-total ratio that is higher by a factor of two than cluster blue-cloud and field galaxies. The fraction of kinematically distorted galaxies is 75% higher in the cluster than in the field environment. This difference mainly stems from morphologically undistorted galaxies, indicating a cluster-specific interaction process that only affects the gas kinematics but not the stellar morphology. Also the ratio between gas and stellar scale length is reduced for cluster galaxies compared to the field sample. Both findings could be explained best by ram-pressure effects.
Conclusions: Ram-pressure stripping seems to be an important interaction process in the multiple cluster system A901/902. Dusty red galaxies might be a crucial element in understanding the transformation of field disk galaxies into cluster lenticular galaxies.

Based on observations with the European Southern Observatory Very Large Telescope (ESO-VLT), observing run ID 384.A-0813.

## Journey to the centre of a galaxy cluster

Why do galaxies come in different shapes, sizes and colours? What are the physical processes that determine a galaxy’s observable properties? These questions drive extragalactic astrophysics. One factor in a galaxy’s evolution that is under scrutiny is the environment it is found in. Depending on whether a galaxy is located in a dense cluster or is relatively isolated in the field, the physical processes that shape it will differ. There is an observable difference in colour and morphology of galaxies depending on their environments: in the densest environments, galaxies tend to be red and elliptical, whereas in low-density environments we find many more blue spirals. Why?

In this paper, the authors investigate what happens in and around galaxy clusters, the largest gravitationally-bound structures in the universe. Such structures grow over time as galaxies fall into their gravitational potential, and during this infall a galaxy can be transformed. In the space between galaxies, a cluster isn’t empty — it’s filled with hot gas (the “intra-cluster medium”), which exerts a pressure on a galaxy falling through it. The resulting wind can blow gas out of the galaxy in a process known as ram pressure stripping. In fact, this process is observed spectacularly in jellyfish galaxies, with tentacles of gas trailing behind the galactic disk. Stripping a galaxy of its gas prevents it from forming stars, thus changing the colour from the blue of young stellar populations to the red of older stars — and so ram pressure stripping is a favoured mechanism behind the blue galaxies in the field vs. red galaxies in clusters problem.

Figure 1: The dark matter content of a simulated galaxy cluster from The Three Hundred project. From Figure 1 in the paper.

To study how and when this happens, the authors of this paper use a suite of over three hundred simulations of galaxy clusters (figure 1 shows an example). These simulations deal with haloes — clumps of gravitationally-bound matter — rather than galaxies themselves, but halo properties tell us about what would happen to galaxies residing in them.

The beauty of simulations is that each cluster can be examined in three dimensions, with a complete understanding of where each infalling halo is and how it is moving. The authors are working in six dimensions: the x, y and z positions of each halo, and the velocities in each of those dimensions.

In reality, we can only observe a galaxy cluster from one angle, along the line of sight that it falls upon viewing it from our position in the universe. This gives us two dimensions in position and a third in line-of-sight velocity. We have to deal with projection effects that make it very difficult to understand exactly how a galaxy is moving. A particular observational challenge arises from “backsplash” galaxies: as a galaxy falls into a cluster, it doesn’t immediately settle in the cluster potential, but will pass through with a velocity taking it out the other side to swing back on a second pass. Figure 2 shows a schematic of this in phase-space from the paper. Backsplash galaxies have already passed through the dense cluster core, and therefore experienced dynamical effects such as ram pressure stripping, but they are located outside the cluster in the same space as the freshly infalling galaxies. This makes an observational sample of true, first-time infallers very difficult to select.

Figure 2: Schematic phase-space diagram, showing the path of one infalling halo over time, with distance from the cluster on the x-axis, and velocity relative to the cluster on the y-axis. The galaxy’s velocity increases (becoming more negative) as it approaches the cluster, then as it swings out the other side as a “backsplash” galaxy we see its velocity decrease, making multiple swings until it reaches equilibrium. The left panel shows this in the 6D plane, where the first infall is in the lower part of the diagram and the backsplash in the upper. The right panel shows how this infall and backsplash would look along one line of sight: we find infall and backsplash galaxies in the same space on the diagram, making them very difficult to distinguish in reality. Figure 2 in the paper.

To investigate how their galaxy clusters would look in reality, the authors also show the projected positions and velocities along a single line of sight alongside the 6D data, making a comparison of the simulations with observations. Infalling haloes are plotted in phase space similarly to figure 1, color-coded to show their gas fractions (figure 3). Haloes are found to lose their gas very quickly on their first infall into the cluster: evolutionary effects occur before the galaxy gets even close to the cluster centre.

Figure 3: Median gas fraction of haloes in phase-space (axes as figure 1). Yellow shows high gas fraction, and purple shows very little gas left in the halo. On the left, in 6D we can see the regions of infall (lower part of the diagram) and backsplash (upper part). The infalling galaxies have high gas fractions that rapidly decrease as they fall towards the cluster, depleted before they reach the centre. On the right, along the line of sight, it is impossible to distinguish infallers from backsplashers, but we can still see this tailing off of gas fractions well before galaxies enter the cluster itself. Adapted from Figure 5 in the paper.

The authors investigate a number of other effects, such as the ram pressure at each point on the infall, and the behaviour of “sub-haloes” within haloes which are shown to lose their gas even more rapidly. Their results point to a scenario of complex evolution happening in the early stages of encounters, with galaxies rushing towards massive clusters being efficiently stripped of their gas on the way in.

The line of sight plots show that from observations alone it is very difficult to determine when and how infalling galaxies lose their gas. However, by combining observations with the 6D information from the simulations, a clearer picture emerges of galaxy evolution around massive clusters. The group’s future work entails using the other wonderful aspect of simulations — the ability to look across time — enabling them to follow the evolution of individual haloes on their journeys into clusters. The Three Hundred looks like an exciting project to watch!

### Affiliations

Astrophysics Science Division, X-ray Astrophysics Laboratory, National Aeronautics and Space Administration Goddard Space Flight Center, Greenbelt, MD, USA

Harvard–Smithsonian Center for Astrophysics, Cambridge, MA, USA

Institute of Astronomy, Cambridge, MA, USA

Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

### Contributions

S.A.W. wrote the manuscript with comments from all authors. S.A.W. performed the Chandra and XMM-Newton data analysis and led the Chandra proposal. J.Z. produced the galaxy cluster sloshing simulations.

## NGC 4299

Σ1.3CO. Therefore,dwarf galaxies and large spirals exhibit the same relationship betweenmolecular gas and star formation rate (SFR). We find that this result isrobust to moderate changes in the RC-to-SFR and CO-to-H2conversion factors. Our data appear to be inconsistent with large (orderof magnitude) variations in the CO-to-H2 conversion factor inthe star-forming molecular gas.

16 to 22 Mpc inline-of-sight distance most of this enhancement arises from galaxiesthat belong to the Virgo cluster proper. However, significant gasdeficiencies are also detected outside the main body of the cluster in aprobable group of galaxies at line-of-sight distances

25-30 Mpc, lyingin the region dominated by the southern edge of the M49 subcluster andclouds W' and W, as well as in various foreground galaxies. In the Virgoregion, the H I content of the galaxies then is not a straightforwardindicator of cluster membership.

0.1% withdecreasing SFR which we propose is due to a [CII]-quiet' component of Ifrom dust heated by the general interstellar radiation field (ISRF). Themore quiescent' galaxies in the sample have values of I/I differentfrom those observed in compact' Galactic interstellar regions. Their[CII]-emission is interpreted to be dominated by diffuse regions of theinterstellar medium (ISM). For normal `star-forming' galaxies thediffuse component of the [CII] emission is estimated to account for atleast 50% of the total.

430 deg2 of sky. We present the data from thedetection survey as well as from the follow-up observations to confirmdetections and improve positions and flux measurements. We find 265galaxies, many of which are extremely low surface brightness. Some ofthese previously uncataloged galaxies lie within the zone of avoidance,where they are obscured by the gas and dust in our Galaxy. Eighty-one ofthese sources are not previously cataloged optically, and there are 11galaxies that have no associated optical counterpart or are onlytentatively associated with faint wisps of nebulosity on the DigitizedSky Survey images. We discuss the properties of the survey, and inparticular we make direct determinations of the completeness andreliability of the sample. The behavior of the completeness and itsdependencies is essential for determining the H I mass function. Weleave the discussion of the mass function for a later paper, but do notethat we find many low surface brightness galaxies and seven sources withMHI

60%) are found tobe members of galaxy pairs (

15% of objects)or groups with at least three members (

500 groups for a total of

45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.

## First detection of matter falling into a black hole at 30 percent of the speed of light

Characteristic disc structure from the simulation of a misaligned disc around a spinning black hole. Credit: K. Pounds et al. / University of Leicester

Black holes are objects with such strong gravitational fields that not even light travels quickly enough to escape their grasp, hence the description "black." They are hugely important in astronomy because they offer the most efficient way of extractingenergy from matter. As a direct result, gas in-fall – accretion – onto black holes mustbepowering the most energetic phenomena in the Universe.

The centre of almost every galaxy – like our own Milky Way – contains a so-called supermassive black hole, with masses of millions to billions of times the mass of our Sun. With sufficient matter falling into the hole, these can become extremely luminous, and are seen as a quasar or active galactic nucleus (AGN).

However black holes are so compact that gas is almost always rotating too much to fall in directly. Instead it orbits the hole, approaching gradually through an accretion disc—a sequence of circular orbits of decreasing size. As gas spirals inwards, it moves faster and faster and becomes hot and luminous, turning gravitational energy into the radiation that astronomers observe.

The orbit of the gas around the black hole is often assumed to be aligned with the rotation of the black hole, but there is no compelling reason for this to be the case. In fact, the reason we have summer and winter is that the Earth's daily rotation does not line up with its yearly orbit around the Sun.

Until now it has been unclear how misaligned rotation might affect the in-fall of gas. This is particularly relevant to the feeding of supermassive black holes since matter (interstellar gas clouds or even isolated stars) can fall in from any direction.

The XMM-Newton spacecraft. Credit: ESA

Using data from XMM-Newton, Prof. Pounds and his collaborators looked at X-ray spectra (where X-rays are dispersed by wavelength) from the galaxy PG211+143. This object lies more than one billion light years away in the direction of the constellation Coma Berenices, and is a Seyfert galaxy, characterised by a very bright AGN resulting from the presence of the massive black hole at its nucleus.

The researchers found the spectra to be strongly red-shifted, showing the observed matter to be falling into the black hole at the enormous speed of 30 per cent of the speed of light, or around 100,000 kilometres per second. The gas has almost no rotation aroundthe hole, and is detected extremely close to it in astronomical terms, at a distance of only 20 times the hole's size (its event horizon, the boundary of the region where escape is no longer possible).

The observation agrees closely with recent theoretical work, also at Leicester and using the UK's Dirac supercomputer facility simulating the 'tearing' of misaligned accretion discs. This work has shown that rings of gas can break off and collide with each other, cancelling out their rotation and leaving gas to fall directly towards the black hole.

Prof. Pounds, from the University of Leicester's Department of Physics and Astronomy, said: "The galaxy we were observing with XMM-Newton has a 40 million solar mass black hole which is very bright and evidently well fed. Indeed some 15 years ago we detected a powerful wind indicating the hole was being over-fed. While such winds are now found in many active galaxies, PG1211+143 has now yielded another 'first," with the detection of matter plunging directly into the hole itself."

He continues: "We were able to follow an Earth-sized clump of matter for about a day, as it was pulled towards the black hole, accelerating to a third of the velocity of light before being swallowed up by the hole."

A further implication of the new research is that 'chaotic accretion' from misaligned discs is likely to be common for supermassive black holes. Such black holes would then spin quite slowly, being able to accept far more gas and grow their masses more rapidly than generally believed, providing an explanation for why black holes which formed in the early Universe quickly gained very large masses.

## Why do the ionized gas clouds stream out from galaxies?

Ionized Hydrogen Gas Clouds Ejected from a Galaxy in the Coma Cluster.

Using the Subaru Prime Focus Camera (Suprime-Cam) in their observations of the Coma Cluster, researchers from the National Astronomical Observatory of Japan (NAOJ), Hiroshima University, the University of Tokyo, and other institutes have discovered 14 galaxies accompanied by extended, ionized hydrogen clouds.

The discovery marks the first time that scientists have detected many galaxies with extended ionized hydrogen gas clouds in a cluster and investigated their spatial and velocity distribution as well as the characteristics of their parent galaxies. The observations captured images of this cluster of galaxies during a critical moment of galaxy evolution and contribute to an understanding of how such clouds may have formed.

A cluster of galaxies is an aggregate of a few to hundreds or even thousands of galaxies. Scientists know that more elliptical (E) and lenticular (S0) galaxies exist more often in cores of clusters of galaxies than in less dense environments. The elliptical and lenticular galaxies are called as "quiescent galaxies" because they show no star formation activity. Meanwhile, spiral galaxies such as our Galaxy are still undergoing star formations, and they are likely to reside in less populated regions. These attributes of clusters raise a number of important questions about the evolution of galaxies: "What kind of mechanisms shape these variety of galaxies occurring in different environments?" and "Why do clusters of galaxies contain many galaxies that do not form stars?" The current research provides observational evidence that addresses these issues.

The team focused their observations on the Coma Cluster, a large cluster of more than 3,000 galaxies and one of the nearest (about 300 billion light years away) clusters to our Galaxy. Past observations had found several extended ionized hydrogen clouds associated with galaxies in the cluster. This group of scientists concentrated on examining these clouds and used a special filter in their observations to catch a specific spectral line (the H-alpha line) created by ionized hydrogen at a particular wavelength. Consequently, they detected 14 galaxies with extended ionized hydrogen clouds, examples of which are shown in Figures 1 and 2.

Most of the ionized hydrogen gas appears as if it was ejected from the galaxy. Follow up observations with Subaru's Faint Object Camera and Spectrograph (FOCAS) confirmed that some of the gas clouds have a recession velocity comparable to that of adjacent galaxies. Therefore, the scientists infer that the overlap between the gas and the galaxy did not occur by chance but resulted from the gas streaming out of the galaxy.

Figure 2：Ionized Hydrogen Gas Clouds Ejected from Galaxies in the Coma Cluster. The colors in the image and the scale of the white bar are the same as those noted for Figure 1. The sizes of the fields are 145 x 87, 121 x 83, and 180 x 96 arcsec² for (a), (b) and (c), respectively.

A more detailed investigation of the ionized hydrogen clouds and their "parent galaxies" reveals that most of the parent galaxies are currently or were recently forming stars. In addition, most parent galaxies have a relatively large velocity difference (more than 1000 km/s) when compared with the average recession velocity of the Coma Cluster. These observational results suggest that the extended ionized hydrogen gas was probably stripped from the parent galaxies by either interaction with the hot gas of the cluster or by the tidal force of the cluster produced when the parent galaxies are trapped by the gravity of the cluster and fall onto the cluster. This scenario predicts a difference in star formation between low and high mass galaxies. Low mass galaxies that lose all of their gas from stripping cease star formation, while higher mass galaxies retain their gas and continue to form stars. The correlation between mass and star-forming activity derived from the observations in the team's research confirms the prediction.

In summary, this study has clarified some of the specific conditions under which extended ionized hydrogen clouds were formed as well as the relationship between the conditions and characteristics of the parent galaxies. Nevertheless, questions remain. How is the stripped gas ionized, and how does it retain the H-alpha emission? The most distant ionized gas cloud lies 300,000 light years from the parent galaxy, and it would take 100 million years or more for that cloud to travel this distance. Since the brightness of the H-alpha emission of the distant clouds is comparable to the clouds near the parent galaxy, the energy to maintain the H-alpha emission must have somehow persisted for more than 100 million years. How these H-alpha emitting structures endure this long remains a mystery. What is going on in the cluster!?

The research group will conduct further spectroscopic observations to help solve this puzzle. They plan to estimate the temperature and density of several parts of the ionized hydrogen clouds and to tackle the question of how the galaxy and gas are evolving in the nearby cluster of galaxies.

## H-alpha velocity fields of spirals falling into a cluster - Astronomy

NUTS at MOO (a.k.a. DrDarkMatter) is actually a tenured faculty member in the Physics Department at the University of Oregon who engages mostly in observational astronomy. Dr. DarkMatter is known to work at all hours under a variety of conditions. This is the only known photo of Dr. DarkMatter at work

Dr. DarkMatter's interest are primarily in galaxy evolution and large scale structure, although he is also quite committed to trying to improve the quality of science education not to mention trying to deal with the various aspects of the energy crises on this planet.

He also serves as a UNIX system administrator and is involved in trying to use the Network as an Educational Outreach instrument between the University and local area schools as well as the general public. He is the current director of the Pine Mountain Observatory , a facility that offers weekend public programs on the Electronic Universe in which the general public can use a 32-inch telescope with CCD to acquire images and take them home. Our Wide Field CCD Camera (cowcam) is now in place and taking data and we are doing several resarch and outreach projects with it. Listed below are most of the collected works of Dr. DarkMatter as they represent his "contribution" to Astronomy (and perhaps other areas). These works are arranged by Subject Matter and Year. They are linked to the Journal Title Page Abstract when that is available from the Abstract Service

## Alan Stockton

### Massive Quiescent Compact Galaxies

Emeritus faculty member Alan Stockton and his collaborators are investigating the nature of the massive quiescent compact galaxies that seem to have been common when the universe was only about 20% of its present age. These were apparently the first massive galaxies to form, but they are extremely rare at the present epoch. While it is possible to study the morphologies and estimate the ages of the stars for this high- redshift sample, detailed spectroscopy of them is almost impossible with currently available instruments because of their faintness and lack of emission lines.

One option for learning more about these interesting and important galaxies is to attempt to find the extremely rare examples of those from this population that have survived closer to the present, intact and essentially unscathed. The figure shows an image of one of about a half-dozen cases, from a survey of

2400 square degrees of sky, that closely mimic the properties of the more extreme examples from those observed in the early universe. Close study of these relatively nearby galaxies should give us a better understanding of these very first massive galaxies and how they were formed.

Fitting stellar population models to the spectrum of this galaxy indicate that the great majority of the total stellar mass formed very early in the history of the Universe, about 13 Gigayears ago. The top panel shows a Keck adaptive-optics image of the galaxy SDSS J014355.21+133451.4, at z = 0.487, along with (in the middle panel) the subtraction of the best 2-component model for the galaxy. The bottom image shows the model without convolving with the instrumental and atmospheric point- spread function, which should give the best global indication of the true shape of the galaxy. Each panel is 3 arc-seconds on a side.