Observed Properties Of Galaxies Research Articles

DO BARYONS ALTER THE HALOS OF LOW SURFACE BRIGHTNESS GALAXIES?

High-quality observations of dark matter-dominated low surface brightness (LSB) galaxies indicate that, in contrast to the triaxial, centrally-concentrated cuspy halos formed in collisionless simulations of halo assembly, these galaxies reside in round, roughly constant density cored halos. In order to reconcile these data with galaxy formation in the context of LCDM, processes that alter the shape and density structure of the inner halo are required. We compile observational properties of LSB galaxies to evaluate the plausibility that a previously higher baryonic mass content and feedback from star formation can modify the dark matter halos of these galaxies. We also compare the properties of bulgeless disk galaxies formed in recent simulations to the LSB galaxy sample. We find that observational constraints on LSB galaxy star formation histories, structure, and kinematics make it difficult for baryonic physics to sphericalize and decrease the central density of the dark matter halos of LSB galaxies.

Connection between Newtonian simulations and general relativity

On large scales, comparable to the horizon, the observable clustering properties of galaxies are affected by various general relativistic effects. To calculate these effects one needs to consistently solve for the metric, densities, and velocities in a specific coordinate system or gauge. The method of choice for simulating large-scale structure is numerical N-body simulations which are performed in the Newtonian limit. Even though one might worry that the use of the Newtonian approximation would make it impossible to use these simulations to compute properties on very large scales, we show that the simulations are still solving the dynamics correctly even for long modes and we give formulas to obtain the position of particles in the conformal Newtonian gauge given the positions computed in the simulation. We also give formulas to convert from the output coordinates of N-body simulations to the observable coordinates of the particles.

The dependence of AGN activity on stellar and halo mass in semi-analytic models

AGN feedback is believed to play an important role in shaping a variety of observed galaxy properties, as well as the evolution of their stellar masses and star formation rates. In particular, in the current theoretical paradigm of galaxy formation, AGN feedback is believed to play a crucial role in regulating the levels of activity in galaxies, in relatively massive halos at low redshift. Only in recent years, however, has detailed statistical information on the dependence of galaxy activity on stellar mass, parent halo mass and hierarchy has become available. In this paper, we compare the fractions of galaxies belonging to different activity classes (star-forming, AGN and radio active) with predictions from four different and independently developed semi-analytical models. We adopt empirical relations to convert physical properties into observables (H_alpha emission lines, OIII line strength and radio power). We demonstrate that all models used in this study reproduce the overall distributions of galaxies belonging to different activity classes as a function of stellar mass and halo mass: star forming galaxies and the strongest radio sources are preferentially associated with low-mass and high-mass galaxies/halos respectively. However, model predictions differ from observational measurements in a number of ways. All models used in our study predict that almost every >1.e12 Msun dark matter halo and/or >1.e11 Msun galaxy should host a bright radio source, while only a small fraction of galaxies belong to this class in the data. In addition, radio brightness is expected to depend strongly on the mass of the parent halo mass in the models, while strong and weak radio galaxies are found in similar environments in data. Our results highlight that the distribution of AGN as a function of stellar mass provides one of the most promising discriminants between different gas accretion schemes.

THE ATOMIC-TO-MOLECULAR TRANSITION IN GALAXIES. II: H I AND H2COLUMN DENSITIES

Gas in galactic disks is collected by gravitational instabilities into giant atomic-molecular complexes, but only the inner, molecular parts of these structures are able to collapse to form stars. Determining what controls the ratio of atomic-to-molecular hydrogen in complexes is, therefore, a significant problem in star formation and galactic evolution. In this paper, we use the model of H2 formation, dissociation, and shielding developed in the previous paper in this series to make theoretical predictions for atomic-to-molecular ratios as a function of galactic properties. We find that the molecular fraction in a galaxy is determined primarily by its column density and secondarily by its metallicity, and is to a good approximation independent of the strength of the interstellar radiation field. We show that the column of atomic hydrogen required to shield a molecular region against dissociation is ~10 M ☉ pc–2 at solar metallicity. We compare our model to data from recent surveys of the Milky Way and of nearby galaxies, and show that the both the primary dependence of molecular fraction on column density and the secondary dependence on metallicity that we predict are in good agreement with observed galaxy properties.

The flip side of galaxy formation: a combined model of galaxy formation and cluster heating

Only ~10 per cent of baryons in the Universe are in the form of stars, yet most models of luminous structure formation have concentrated on the properties of the luminous stellar matter. Such models are now largely successful at reproducing the observed properties of galaxies, including the galaxy luminosity function and the star formation history of the universe. In this paper we focus on the 'flip side' of galaxy formation and investigate the properties of the material that is not presently locked up in galaxies. This 'by-product' of galaxy formation can be observed as an X-ray emitting plasma [the intracluster medium (ICM)] in groups and clusters. Since much of this material has been processed through galaxies, observations of the ICM represent an orthogonal set of constraints on galaxy formation models. In this paper, we attempt to self-consistently model the formation of galaxies and the heating of the ICM. We set out the challenges for such a combined model and demonstrate a possible means of bringing the model into line with both sets of constraints. In this paper, we present a version of the Durham semi-analytic galaxy formation model GALFORM that allows us to investigate the properties of the ICM. As we would expect on the basis of gravitational scaling arguments, the previous model fails to reproduce even the most basic observed properties of the ICM. We present a simple modification to the model to allow for heat input into the ICM from the active galactic nucleus (AGN) 'radio-mode' feedback. This heating acts to expel gas from the X-ray luminous central regions of the host halo. With this modification, the model reproduces the observed gas mass fractions and luminosity–temperature (L–T) relation of groups and clusters. In contrast to simple 'pre-heating' models of the ICM, the model predicts mildly positive evolution of the L–T relation, particularly at low temperatures. The model is energetically plausible, but seems to exceed the observed heating rates of intermediate-temperature clusters. Introducing the heating process into the model requires changes to a number of model parameters in order to retain a good match to the observed galaxy properties. With the revised parameters, the best-fitting luminosity function is comparable to that presented in Bower et al. The new model makes a fundamental step forward, providing a unified model of galaxy and cluster ICM formation. However, the detailed comparison with the data is not completely satisfactory, and we highlight key areas for improvement.

ZEN2: a narrowJ-band search forz∼ 9 Lyα emitting galaxies directed towards three lensing clusters

We present the results of a continuing survey to detect Ly alpha emitting galaxies at redshifts z similar to 9: the z equals nine' (ZEN) survey. We have obtained deep VLT Infrared Spectrometer and Array Camera observations in the narrow J-band filter NB119 directed towards three massive lensing clusters: Abell clusters 1689, 1835 and 114. The foreground clusters provide a magnified view of the distant Universe and permit a sensitive test for the presence of very high redshift galaxies. We search for z similar to 9 Ly alpha emitting galaxies displaying a significant narrow-band excess relative to accompanying J-band observations that remain undetected in Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) optical images of each field. No sources consistent with this criterion are detected above the unlensed 90 per cent point-source flux limit of the narrow-band image, F-NB = 3.7 x 10(-18) erg s(-1) cm(-2). To date, the total coverage of the ZEN survey has sampled a volume at z similar to 9 of approximately 1700 comoving Mpc(3) to a Lya emission luminosity of 10(43) erg s(-1). We conclude by considering the prospects for detecting z similar to 9 Ly alpha emitting galaxies in light of both observed galaxy properties at z 7.

Stellar Feedback in Dwarf Galaxy Formation

Dwarf galaxies pose substantial challenges for cosmological models. In particular, current models predict a dark-matter density that is divergent at the center, which is in sharp contrast with observations that indicate a core of roughly constant density. Energy feedback, from supernova explosions and stellar winds, has been proposed as a major factor shaping the evolution of dwarf galaxies. We present detailed cosmological simulations with sufficient resolution both to model the relevant physical processes and to directly assess the impact of stellar feedback on observable properties of dwarf galaxies. We show that feedback drives large-scale, bulk motions of the interstellar gas, resulting in substantial gravitational potential fluctuations and a consequent reduction in the central matter density, bringing the theoretical predictions in agreement with observations.

The Recent Star Formation Histories of Nearby Galaxies

Recent (≤ 0.5 Gyr) star formation histories have a large impact on the observable properties of galaxies. Using HST/ACS observations, we have used the blue helium burning (BHeB) stars to construct spatially resolved star formation histories of M81 group dwarf galaxies with a time resolution of roughly 30 Myr over the last 500 Myr. We have designed a sample of ten galaxies spanning ranges of 6 magnitudes in luminosity, 1000 in current star formation rate, and 0.5 dex in metallicity. The ACS observations allow us to directly observe the strength and spatial relationships of all of the recent star formation in these galaxies. These observations are complemented by high-quality ancillary data (e.g., Spitzer, UV/optical/H-alpha/NIR, VLA HI). Our resolved star formation maps will be compared with star formation rates inferred from H-alpha, UV, and IR observations – allowing an independent calibration of these techniques. Given the ranges in metallicity, these observations will provide calibrations of stellar evolution tracks for young, low metallicity stars. These observations will also enable us to construct prescriptions of how star formation and feedback depend on metallicity, size, gas content, and current star formation rates in galaxies. Finally, I note that the new observations becoming available as a part of the ANGST (ACS Nearby Galaxies Survey Treasury Program) will allow a large number of dwarf galaxies to be analyzed in this way.

Strong Emission-Line Galaxies at Low Redshifts in the Field around the Quasar SDSSp J104433.04−012502.2

Abstract We discuss the observational properties of strong emission-line galaxies at low redshifts found by our deep imaging survey for high-redshift Ly$\alpha$ emitters. In our survey, we used a narrow-band filter, NB816 ($\lambda_{\mathrm{center}} = 8150 \,{Å}$ with $\mbox{FWHM} = 120 \,\mbox{Å}$ FWHM$= 120 \,{Å}$), and an intermediate-band filter, IA827 ($\lambda_{\mathrm{center}} = 8270 \,{Å}$ with $\mbox{FWHM} = 340 \,\mbox{Å}$ FWHM$= 340 \,{Å}$). In this survey, 62 NB816-excess ($\gt 0.9 \,\mathrm{mag} \gt 0.9 \,\mathrm{mag}$) and 21 IA827-excess ($\gt 0.8 \,\mathrm{mag} \gt 0.8 \,\mathrm{mag}$) objects were found. Among them, we found 20 NB816-excess and 4 IA827-excess Ly$\alpha$ emitter candidates. Therefore, it turns out that 42 NB816-excess and 17 IA827-excess objects are strong emission-line objects at lower redshifts. Since 4 objects in the two low-$z$ samples are common, the total number of strong low-$z$ emitters is 55. Applying our photometric redshift technique, we identified 7 $\mathrm{H}\alpha$ emitters at $z \approx 0.24$, 20 $\mathrm{H}\beta$–[O III] ones at $z \approx 0.65$, and 11 [O II] ones at $z \approx 1.19$. However, we could not determine reliable photometric redshifts of the remaining 17 emitters. The distributions of their rest frame equivalent widths are consistently understood based on recent studies of galaxy evolution from $z \sim 1$ to $z \sim 0$.

The Millennium Galaxy Catalogue: on the natural subdivision of galaxies

The distribution of global photometric, spectroscopic, structural and morphological parame- ters for a well-defined sample of 350 nearby galaxies has been examined. The usual trends were recovered demonstrating that E/S0 galaxies are redder, more quiescent, more centrally concentrated and possess larger Sersic indices than late-type galaxies. Multivariate statistical analyses were performed to examine the distribution of all parameters simultaneously. The main result of these analyses was the existence of only two classes of galaxies, corresponding closely to early and late types. Linear discriminant analysis (LDA) was able to reproduce the classifications of early- and late-type galaxies with high success, but further refinement of galaxy types was not reproduced in the distribution of observed galaxy properties. A principal components analysis (PCA) showed that the major variance of the parameter set corresponded to a distinction between early and late types, highlighting the importance of the distinction. A hierarchical clustering analysis (HCA) revealed only two clear natural classes within the parameter set, closely corresponding to early and late types. Early and late types are clearly distinct and the distinction is of fundamental importance. In contrast, late types from Sa to Irr are smoothly distributed throughout the parameter space. A population of galaxies classified by eye as elliptical/lenticular, and exhibiting concentration indices similar to early types were found to have a significant star formation activity. These galaxies are preferentially faint, suggesting they are low-mass systems. Ke yw ords: galaxies: evolution - galaxies: fundamental parameters.

Orbital structure of collisionless merger remnants: on the origin of photometric and kinematic properties of elliptical and S0 galaxies

We present a detailed investigation of the relation between the orbital content of merger remnants and observable properties of elliptical and S0 galaxies. Our analysis is based on the statistical sample of collisionless mergers of disc galaxies with different mass ratios and orbital parameters, published by Naab & Burkert. We use the spectral method by Carpintero & Aguilar to determine the orbital content of every remnant and correlate it with its intrinsic shape, and its projected kinematic and photometric properties. We discuss the influence of the bulge component and varying pericentre distances. The classified orbit families are box orbits, minor-axis tubes, inner and outer major-axis tubes and boxlets. In general, box orbits dominate the inner parts of the remnant. Major and minor-axis tubes become dominant at intermediate radii and boxlets at large radii. The two most abundant orbit classes are the minor-axis tubes and the box orbits. Their ratio seems to determine the basic properties of a remnant. On average, the fraction of minor-axis tubes increases by a factor of 2 from a merger mass ratio of 1:1 to 4:1, whereas the fraction of box orbits decreases by 10 per cent. At a given mass the central velocity dispersion of a remnant scales with the ratio of minor-axis tubes to box orbits. Interestingly, the division line between rotational supported systems and pressure supported objects, (υmaj/σ0)*= 0.7, turns out to coincide with a box to minor-axis tube ratio of unity. The observed h3-υ/σ anticorrelation for ellipticals cannot be reproduced by collisionless merger remnants. We propose that this can only be reconciled by an additional physical process that significantly reduces the box orbit content. Remnants that are dominated by minor-axis tube orbits have predominantly discy projections. Boxy remnants have always a box to minor-axis tube ratio larger than one. This study will enable us to identify observed ellipticals that could have formed, in the collisionless limit, by gas-poor disc mergers. In addition, it demonstrates how observable properties of spheroidal stellar systems are connected with their intrinsic orbital structure.

Direct evidence of the receding “torus” around central nuclei of powerful radio sources

The presence of obscuring material (or a dusty `torus') in active galactic nuclei (AGN) is central to the unification model for AGN. Two models, the multi-population model for radio sources and the receding torus model, are capable of describing observational properties of powerful radio galaxies and radio quasars. Here, I study the changes of the opening angle of the obscuring torus with radio luminosity at 151 MHz, [O III] emission-line luminosity and cosmic epoch aiming to discriminate between two working models. An analytical expression relating the half opening angle of the torus to the mean projected linear sizes of double radio galaxies and quasars is derived. The sizes of powerful double radio sources taken from the combined sample of 3CRR, 6CE and 7CR complete samples are used to estimate the torus opening angle. I found a statistically significant correlation between the half opening angle of the torus and [O III] emission-line luminosity. The opening angle increases from 20 to 60 degrees with increasing [O III] emission-line luminosity. This correlation is interpreted as direct evidence of the receding torus around central engines of powerful double radio sources.

Cosmic Gamma-Ray Bursts: The Big Picture

SummaryA “typical” GRB occurs in a star-forming region of a galaxy at a redshift z~1. In currently popular models, it is caused by the collapse of a massive star which has exhausted its nuclear fuel supply. The star collapses to a black hole threaded by a strong magnetic field, and possibly fed by an accretion torus. Through a variety of processes, electrons are accelerated and gamma-rays, X-rays, optical light, and radio emission ensue, with durations from seconds to years. In this talk, I will review the general observational properties of bursts, their afterglows and host galaxies, and some of the open questions about them.

Element segregation in giant galaxies and X-ray clusters

We examine the process of element segregation by gravity in giant elliptical galaxies and X-ray clusters. We solve the full set of flow equations developed by Burgers for a multi-component fluid, under the assumption that magnetic fields slow the diffusion by a constant factor F_B. Compared to the previous calculations that neglected the residual heat flow terms, we find that diffusion is faster by ~20%. In clusters we find that the diffusion changes the local abundance typically by factors of 1+0.3(T/10^8 K)^1.5/F_B and 1+0.15(T/10^8 K)^1.5/F_B for helium and heavy elements, respectively, where T is the gas temperature. In elliptical galaxies, the corresponding factors are 1+0.2(T/10^7 K)^1.5/F_B and 1+0.1(T/10^7 K)^1.5/F_B, respectively. If the suppression factor F_B is modest, diffusion could significantly affect observational properties of hot X-ray clusters and cD galaxies. In particular, diffusion steepens the baryon distribution, increases the total X-ray luminosity, and changes the spectrum and evolution of stars that form out of the helium-rich gas. Detection of these diffusion signatures would allow to gauge the significance of the magnetic fields, which also inhibit thermal heat conduction as a mechanism for eliminating cooling flows in the same environments.

Chemical Enrichment in Damped Lyα Systems from Hierarchical Galaxy Formation Models

We investigate chemical enrichment in damped Ly? (DLA) systems in the hierarchical structure formation scenario using a semianalytic model of galaxy formation. The model developed by Nagashima, Totani, Gouda & Yoshii takes into account various selection effects on high-redshift galaxies and can reproduce fundamental observational properties of galaxies, such as luminosity functions and number-magnitude/redshift relations. DLA systems offer the possibility of measuring metal abundance more accurately than faint galaxies. For example, recent measurements of the zinc abundance can help in understanding processes of metal pollution and star formation in DLA systems because zinc is virtually unaffected by dust depletion. Here we focus on this advantage for observations exploring the metallicity evolution in DLA systems at high redshifts. We can consistently show the metallicity evolution for reasonable models. The models also reproduce fundamental properties of the local galaxy population. This result suggests that the chemical evolution of DLA systems can be consistently reconciled with the observational features of typical galaxies. We also investigate other properties of DLA systems (column density distribution and mass density of cold gas) and find that star formation in massive galaxies should be more active than that in low-mass galaxies, which is consistent with the results of Nagashima et al. and Cole et al. in which the star formation timescale is set by the cold gas mass fraction in local spiral galaxies. Finally, we discuss host galaxies associated with DLA systems. We conclude from the observations that they primarily consist of sub-L* and/or dwarf galaxies.

How does clustering depend on environment?

According to the current paradigm, galaxies form and reside in extended cold dark matter (CDM) halos and in turn are key tracers of cosmological structure. Understanding how different types of galaxies occupy halos of different masses is one of the major challenges facing extragalactic astrophysics. The observed galaxy properties depend on the environment surrounding the galaxy. Within the framework of most galaxy formation models the environmental dependence of the galaxy population is mainly due to the change of the halo mass function with large-scale environment. Such models make precise predictions for how galaxy clustering should depend on environment. We will illustrate this by presenting analytical models of dark matter and galaxy clustering along with results obtained from numerical simulations. With these results we can hope to obtain a better understanding of the link between galaxies and dark matter and thereby constrain galaxy formation models. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Starburst-AGN Connection: A Lesson from High-zPowerful Radio Galaxies

Powerful radio galaxies at high redshift are highly useful in studies of early evolution of AGN-hosting galaxies because their observed optical and near infrared light are dominated by their stellar population rather than the nonthermal continuum emitted by the central engine of AGNs. In addition, the presence of AGN activity in them implies that a supermassive black hole has been already made in their nuclei. These properties allow us to investigate a possible starburst-AGN connection in early universe and then provide some crucial hints for the formation mechanism of supermassive black holes. Taking observational properties of high-z powerful radio galaxies into account, we discuss a possible formation mechanism of supermassive black holes in their nuclei.

Detailed comparison of the structures and kinematics of simulated and observed barred galaxies

We examine the observable properties of simulated barred galaxies including radial mass profiles, edge-on structure and kinematics, bar lengths and pattern speed evolution for detailed comparison to real systems. We have run several simulations in which bars are created through inherent instabilities in self-consistent simulations of a realistic disc+halo galaxy model with a disc-dominated, flat rotation curve. These simulations were run at high (N=20M particles) and low (N=500K) resolution to test numerical convergence. We determine the pattern speeds in simulations directly from the phase angle of the bar versus time and the Tremaine-Weinberg method. Fundamental dynamics do not change between the high and low resolution, suggesting that convergence has been reached in this case. We find the higher resolution is needed to simulate structural and kinematic properties accurately. The edge-on view of the higher-resolution system clearly shows the bending instability and formation of a peanut-shaped bulge. We determined bar lengths by different means to determine the simulated bar is fast, with a corotation to bar length ratio under 1.5. Simulated bars in these models form with pattern speeds slower than those observed and slow down during their evolution. Dynamical friction between the bar and dark halo is responsible for this deceleration, as revealed by the transfer of angular momentum between the disc and the halo. However, even though the pattern speed is reduced at later times, the instantaneous scale length of the disc has grown sufficiently for the bar motion to agree with many observations. By using a different model and simulation technique than other authors, we are able to compare the robustness of these methods.

Analysis of the Interaction Effects in the Southern Galaxy Pair Tol 1238−364 and ESO 381‐G009

In the context of the connection among galaxy-galaxy interaction, starbursts, and nuclear activity, we present and discuss a quantitative morphological analysis based on BVR images and a detailed spectroscopic investigation of two interacting galaxies, the Seyfert 2 Tol 1238-364 (IC 3639) and its companion ESO 381-G009, forming a triple system with ESO 381-G006. Broadband optical photometry is complemented by Hα imaging, which provides information about the distribution of star-forming regions across the galaxies. Long-slit spectroscopic data obtained at different position angles of the slit are employed to determine the physical conditions of circumnuclear and extranuclear regions. A mixture of thermal and nonthermal ionizing radiation is found in the surroundings of the nucleus of Tol 1238-364, and the energy budget supports the presence of a circumnuclear starburst. Several regions in both the galaxies show anomalous line ratios: additional ionization by shock heating and low ionization of some extranuclear H II regions are suggested as possible explanations. An analysis of the emission-line profiles reveals the presence of a broad Hα component in the nuclear region of Tol 1238-364. Independent estimates of the star formation rates (SFRs) were obtained through flux-calibrated Hα images and far-infrared (FIR) emission in the four IRAS bands. Overall, SFR densities have been compared with the SFR densities derived from Hα emission in the individual regions of the galaxies sampled by long-slit spectra. In both the galaxies an enhancement of the star formation activity with respect to isolated galaxies is revealed. The prevalence of starburst or nuclear activity has been examined through FIR color indices. The interaction scenario is discussed on the basis of the observed galaxy properties.

Formation of polar ring galaxies

Polar ring galaxies are peculiar systems in which a gas rich, nearly polar ring surrounds an early-type or elliptical host galaxy. Two formation scenarios for these objects have been proposed: they are thought to form either in major galaxy mergers or by tidal accretion of the polar material from a gas rich donor galaxy. Both scenarios are studied through N-body simulations including gas dynamics and star formation. Constraints on physical parameters are drawn out, in order to determine which scenario is the most likely to occur. Polar ring galaxies from each scenario are compared with observations and we discuss whether the accretion scenario and the merging scenario account for observational properties of polar ring galaxies. The conclusion of this study is that the accretion scenario is both the most likely and the most supported by observations. Even if the merging scenario is rather robust, most polar ring galaxies are shown to be the result of tidal gas accretion events.