Chain Galaxies Research Articles

Was the Milky Way a chain galaxy? Using the IGIMF theory to constrain the thin-disc star formation history and mass

The observed present-day stellar mass function (PDMF) of the solar neighborhood is a mixture of stellar populations born in star-forming events that occurred over the life-time of the thin disk of the Galaxy. Assuming stars form in embedded clusters which have stellar initial mass functions (IMFs) which depend on the metallicity and density of the star-forming gas clumps, the integrated galaxy-wide IMF (IGIMF) can be calculated. The shape of the IGIMF thus depends on the SFR and metallicity. Here, the shape of the PDMF for stars more massive than $1\,M_\odot$ in combination with the mass density in low-mass stars is used to constrain the current star-formation rate (SFR), the star formation history (SFH) and the current stellar plus remnant mass ($M_*$) in the Galactic thin disk. This yields the current SFR, $\dot{M}_*= 4.1^{+3.1}_{-2.8}~M_\odot$yr$^{-1}$, a declining SFH and $M_*=2.1^{+3.0}_{-1.5}\times 10^{11}M_\odot$, respectively, with a V-band stellar mass-to-light ratio of $M_*/L_V=2.79^{+0.48}_{-0.38}$.These values are consistent with independent measurements. We also quantify the surface density of black holes and neutron stars in the Galactic thin disk. The invariant canonical IMF can reproduce the PDMF of the Galaxy as well as the IGIMF, but in the universal IMF framework it is not possible to constrain any of the above Galactic properties. Assuming the IGMF theory is the correct framework and in combination with the vertical velocity dispersion data of stars, it follows that the Milky Way would have appeared as a chain galaxy at high redshift.

Discovery of a Very Large (≈20 kpc) Galaxy at z = 3.72*

Abstract We report the discovery and spectroscopic confirmation of a very large star-forming galaxy, G6025, at . In the rest frame ≈2100 Å, G6025 subtends ≈24 kpc in physical extent when measured from the 1.5σ isophote, in agreement with the parametric size measurements that yield a half-light radius of 4.9 ± 0.5 kpc and a semimajor axis of 12.5 ± 0.1 kpc. It is also very UV-luminous ( ) and young (≈140 ± 60 Myr). Despite its unusual size and luminosity, the stellar population parameters and dust reddening ( and E(B − V) ∼ 0.18 ± 0.05) estimated from the integrated light are similar to those of smaller galaxies at comparable redshifts. The ground-based morphology and spectroscopy show two dominant components, both located off-center, embedded in more diffuse emission. We speculate that G6025 may be a scaled-up version of chain galaxies seen in deep HST imaging or, alternatively, a nearly equal-mass merger involving two super-L* galaxies in its early stage. It lies close to but not within a known massive protocluster at z = 3.78. We find four companions within 6 Mpc from G6025, two of which lie within 1.6 Mpc. While the limited sensitivity of the existing spectroscopy does not allow us to robustly characterize the local environment of G6025, it likely resides in a locally overdense environment. The luminosity, size, and youth of G6025 make it uniquely suited to study the early formation of massive galaxies in the universe.

General Polytropic Self-gravitating Cylinder Free-fall and Accreting Mass String with a Chain of Collapsed Objects

We present a theoretical model framework for general polytropic (GP) hydrodynamic cylinder under self-gravity of infinite length with axial uniformity and axisymmetry. For self-similar dynamic solutions, we derive valuable integrals, analytic asymptotic solutions, sonic critical curves, shock conditions, and global numerical solutions with or without expansion shocks. Among others, we investigate various dynamic solutions featured with central free-fall asymptotic behaviours, corresponding to a collapsed mass string with a sustained dynamic accretion from a surrounding mass reservoir. Depending on the allowed ranges of a scaling index a < -1, such cylindrical dynamic mass accretion rate could be steady, increasing with time and decreasing with time. Physically, such a collapsed mass string or filament would break up into a sequence of sub-clumps and segments as induced by gravitational Jeans instabilities. Depending on the scales involved, such sub-clumps would evolve into collapsed objects or gravitationally bound systems. In diverse astrophysical and cosmological contexts, such a scenario can be adapted on various temporal, spatial and mass scales to form a chain of collapsed clumps and/or compact objects. Examples include the formation of chains of proto-stars, brown dwarfs and gaseous planets along molecular filaments; the formation of luminous massive stars along magnetized spiral arms and circum-nuclear starburst rings in barred spiral galaxies; the formation of chains of compact stellar objects such as white dwarfs, neutron stars, and black holes along a highly condensed mass string. On cosmological scales, one can perceive the formation of chains of galaxies, chains of galaxy clusters or even chains of supermassive and hypermassive black holes in the Universe including the early Universe. All these chains referred to above include possible binaries.

MASSIV: Mass Assembly Survey with SINFONI in VVDS

A key open issue for galaxy evolution and formation models is the understanding of the different mechanisms of galaxy assembly at various cosmic epochs. The aim of this study is to derive the global and spatially-resolved metal content in high-redshift galaxies. Using VLT/SINFONI IFU spectroscopy of a first sample of 50 galaxies at z~1.2 in the MASSIV survey, we are able to measure the Ha and [NII]6584 emission lines. Using the N2 ratio as a proxy for oxygen abundance in the interstellar medium, we measure the metallicity of the sample galaxies. We develop a tool to extract spectra in annular regions of these galaxies, leading to a spatially-resolved estimate of the oxygen abundance in each galaxy. We derive a metallicity gradient for 26 galaxies in our sample and discover a significant fraction of galaxies with a "positive" gradient. Using a simple chemical evolution model, we derive infall rates of pristine gas onto the disks. Seven galaxies display a positive gradient at a high confidence level. Four out of these are interacting and one is a chain galaxy. We suggest that interactions might be responsible for shallowing and even inverting the abundance gradient. We also identify two interesting correlations in our sample: a) galaxies with higher gas velocity dispersion have shallower/positive gradients; and b) metal-poor galaxies tend to show a positive gradient whereas metal-rich ones tend to show a negative one. This last observation can be explained by the infall of metal-poor gas into the center of the disks. We address the question of the origin of this infall under the influence of gas flows triggered by interactions and/or cold gas accretion.

Cores and revived cusps of dark matter haloes in disc galaxy formation through clump clusters

Cusp-core problem is a controversial problem on galactic dark matter haloes. Cosmological N-body simulations has demonstrated that galactic dark matter haloes have a cuspy density profile at the centre. However, baryonic physics may affect the dark matter density profile. For example, it was suggested that adiabatic contraction of baryonic gas makes the dark matter cusp steeper. However, it is still an open question if the gas falls into the galactic centre in smooth adiabatic manner. Recent numerical studies suggested that disc galaxies might experience clumpy phase in their early stage of the disc formation, which could also explain clump clusters and chain galaxies observed in high redshift Universe. In this letter, using numerical simulations with an isolated model, we study how the dark matter halo responds to these clumpy nature of baryon component in disc galaxy formation through the clump cluster phase. Our simulation demonstrates that such clumpy phase leads to a shallower density profile of the dark matter halo in the central region while clumps fall into the centre due to dynamical friction. This mechanism helps to make the central dark matter density profile shallower in the galaxies whose virial mass is as large as 5.0*10^11 solar masses. This phenomenon is caused by reaction to dynamical friction of the stellar clumps against the dark matter halo. The halo draws the clumps into the galactic centre, while kinematically heated by the clumps. We additionally run a dark matter only simulation excluding baryonic component and confirm that the resultant shallower density profile is not due to numerical artifact in the simulation, such as two-body relaxation.

THE SLOAN GREAT WALL. MORPHOLOGY AND GALAXY CONTENT

We present the results of a study of the morphology and galaxy content of the Sloan Great Wall (SGW), the richest galaxy system in the nearby universe. We use the luminosity density field to determine superclusters in the SGW, and the fourth Minkowski functional V3 and the morphological signature (the K1–K2 shapefinder curve) to show the different morphologies of the SGW, from a single filament to a multibranching, clumpy planar system. We show that the richest supercluster in the SGW, SCl 126, and especially its core, resembles a very rich filament, while another rich supercluster in the SGW, SCl 111, resembles a "multispider"—an assembly of high-density regions connected by chains of galaxies. We study the substructure of individual galaxy populations determined by their color in these superclusters using Minkowski functionals and find that in the high-density core of the SGW the clumpiness of red and blue galaxies is similar, but in the outskirts of superclusters the distribution of red galaxies is clumpier than that of blue galaxies. At intermediate densities, the systems of blue galaxies have tunnels through them. We assess the statistical significance of our results using the halo model and smoothed bootstrap. We study the galaxy content and the properties of groups of galaxies in the two richest superclusters of the SGW, paying special attention to bright red galaxies (BRGs) and the first ranked (the most luminous) galaxies in SGW groups. The BRGs are the nearby luminous red galaxies; they are mostly bright and red and typically reside in groups (several groups host five or more BRGs). About one-third of the BRGs are spirals. The scatter of colors of elliptical BRGs is smaller than that of spiral BRGs. About half of the BRGs and of first ranked galaxies in groups have large peculiar velocities. Groups with elliptical BRGs as their first ranked galaxies populate superclusters more uniformly than the groups that have a spiral BRG as their first ranked galaxy. The galaxy and group content of the core of the supercluster SCl 126 shows several differences in comparison with the outskirts of this supercluster and with the supercluster SCl 111. Here, groups with BRGs are richer and have larger velocity dispersions than groups with BRGs in the outskirts of this supercluster and in SCl 111. The fraction of those BRGs that do not belong to any group is the smallest here. In the core of the supercluster SCl 126, the fraction of red galaxies is larger than in the outskirts of this supercluster or in the supercluster SCl 111. Here, the peculiar velocities of the first ranked galaxies are larger than in the outskirts of this supercluster or in the supercluster SCl 111 and the peculiar velocities of elliptical BRGs are larger than those of spiral BRGs, while in the outskirts of this supercluster and in the supercluster SCl 111 the peculiar velocities of spiral BRGs are larger than those of elliptical BRGs. All this suggests that the formation history and evolution of individual neighbor superclusters in the SGW have been different and the SGW is not a genuine physical structure but an assembly of very rich galaxy systems.

Local starbursts seen when the Universe was 2–4 Gyr old

AbstractWe explore the multiwavelength properties of three nearby starburst galaxies: NGC 3079, NGC 7673, and Mrk 08. We established that each of these galaxies has similar rest-frame far-ultraviolet (FUV) morphologies as Lyman-break galaxies (LBGs) at z ~ 1.5 and 4, when the age of the Universe was ~ 4.3 and ~ 1.6 Gyr, respectively. LBGs are at an important stage in galaxy evolution when the Universe had a peak in the star-formation-rate density. Many LBGs are primarily composed of star-forming clumps, i.e., stellar clusters, with a significant lack of older stellar populations. Here, we present the comparison of the spectral-energy distributions (SEDs) of three nearby starburst galaxies with those of typical LBGs. From our nearby sample, each object has been artificially redshifted to observe what the galaxies would look like at z ~ 1 to 4 in the rest-frame FUV. NGC 3079 is an edge-on Seyfert 2 galaxy. It has a bright bulge and is interacting with two other galaxies, with extended Hi only along NGC 3079. The redshifting process changes its appearance, so that at high z it looks like a chain galaxy with multiple knots of star formation and no bulge. NGC 7673 has extended Hi and the star formation is mostly within the inner optical region in the multiple star-forming clumps defining the galaxy morphology. In the FUV, the galaxy looks highly compact with little detail resolved. As it is artificially redshifted, the galaxy continues to look more spherical. Mrk 8 is a merging pair, with the two galaxies observable in the visible spectrum. It is classified as a Wolf–Rayet galaxy, which suggests a very young burst, and is composed of several large star-forming regions. The FUV image does not resolve the separate galaxies, and the appearance remains similar for each redshift. We use the Gini coefficient, M20, and the Sérsic index to quantify the morphologies. The SEDs of the objects have similarities with LBG stellar population models. Because these local galaxies can be studied in more detail, they act as a bridge between nearby observations of starburst galaxies and high-z starburst galaxies such as LBGs.

BULGE AND CLUMP EVOLUTION IN HUBBLE ULTRA DEEP FIELD CLUMP CLUSTERS, CHAINS AND SPIRAL GALAXIES

Clump clusters and chain galaxies in the Hubble Ultra Deep Field are examined for bulges in the NICMOS images. Approximately 50% of the clump clusters and 30% of the chains have relatively red and massive clumps that could be young bulges. Magnitudes and colors are determined for these bulge-like objects and for the bulges in spiral galaxies, and for all of the prominent star-formation clumps in these three galaxy types. The colors are fitted to population evolution models to determine the bulge and clump masses, ages, star-formation rate decay times, and extinctions. The results indicate that bulge-like objects in clump cluster and chain galaxies have similar ages and 2 to 5 times larger masses compared to the star-formation clumps, while the bulges in spirals have ~6 times larger ages and 20 to 30 times larger masses than the clumps. All systems appear to have an underlying red disk population. The masses of star-forming clumps are typically in a range from 10^7 to 10^8 Msun; their ages have a wide range around ~10^2 Myr. Ages and extinctions both decrease with redshift. Star formation is probably the result of gravitational instabilities in the disk gas, in which case the large clump mass in the UDF is the result of a high gas velocity dispersion, 30 km/s or more, combined with a high gas mass column density, ~100 Msun/pc^2. Because clump clusters and chains dominate disk galaxies beyond z~1, the observations suggest that these types represent an early phase in the formation of modern spiral galaxies, when the bulge and inner disk formed.

Observations and modeling of a clumpy galaxy atz = 1.6

We investigate the properties of a clump-cluster galaxy at redshift 1.57. In optical observations, the morphology of this galaxy is dominated by eight star-forming clumps, and its photometric properties are typical of most clump-cluster and chain galaxies. Its complex asymmetrical morphology has led to the suggestion that this system is a group merger of several initially separate proto-galaxies. We performed Hα integral field spectroscopy of this system using SINFONI on VLT UT4. These observations reveal a large-scale velocity gradient throughout the system, but with large local kinematic disturbances. Using a numerical model of gas-rich disk fragmentation, we find that clump interactions and migration can explain the observed disturbed rotation. On the other hand, the global rotation would not be expected for a multiply merging system. We also find that this system follows the relations of stellar mass versus metallicity, star formation rate, and size that are expected for a disk at this redshift. Furthermore, the galaxy exhibits a disk-like radial metallicity gradient. A formation scenario of internal disk fragmentation is therefore the most likely one. A red and metallic central concentration appears to be a bulge in this proto-spiral clumpy galaxy. A chain galaxy at redshift 2.07 in the same field also shows disk-like rotation. Such systems are likely progenitors of present-day bright spiral galaxies, which shape their exponential disks through clump migration and disruption, a process that in turn fuels their bulges. Our results show that disturbed morphologies and kinematics are not necessarily signs of galaxy mergers and interactions, but may instead be produced by the internal evolution of primordial disks.

Rapid Formation of Exponential Disks and Bulges at High Redshift from the Dynamical Evolution of Clump‐Cluster and Chain Galaxies

Many at high redshift have peculiar morphologies dominated by 108-109 M☉ kpc-sized clumps. Using numerical simulations, we show that these clusters can result from fragmentation in gravitationally unstable primordial disks. They appear as galaxies when observed edge-on. In less than 1 Gyr, clump formation, migration, disruption, and interaction with the disk cause these systems to evolve from initially uniform disks into regular spiral with an exponential or double-exponential disk profile and a central bulge. The inner exponential is the initial disk size, and the outer exponential is from material flung out by spiral arms and clump torques. A nuclear black hole may form at the same time as the bulge from smaller black holes that grow inside the dense cores of each clump. The properties and lifetimes of the clumps in our models are consistent with observations of the clumps in high-redshift galaxies, and the stellar motions in our models are consistent with the observed velocity dispersions and lack of organized rotation in chain galaxies. We suggest that violently unstable disks are the first step in spiral galaxy formation. The associated starburst activity gives a short timescale for the initial stellar disk to form.

Galaxy bulges at mid- and high-redshift

AbstractBulges are a major galaxy component in the nearby universe, and are one of the primary features that differentiates and defines galaxies. The origin of bulges can be directly probed in part by examining distant galaxies to search for high redshift bulges, and to study the properties of bulges in formation. We review the evidence for bulges at high redshift in this article, and how by studying bulges through a variety of approaches, including through morphological, colour, and stellar mass selection, we can determine when and how these systems assembled. We argue that the majority of the most massive ‘classical’ bulges are in place byz~ 1.5 − 2, and likely formed very early through major mergers. Other, likely lower mass, bulges form through a secular process along with their disks. Direct observations suggest that these two formation processes are occurring, as spheroids are commonly seen atz&lt; 1, as are disks and spiral galaxies in the form of luminous diffuse objects, clump-clusters, and chain galaxies. However, bulge+disk systems are relatively rare untilz~ 1, suggesting that this structural assembly occurred relatively late.

A Survey of Galaxy Kinematics toz∼1 in the TKRS/GOODS‐N Field. I. Rotation and Dispersion Properties

We present kinematic measurements of a large sample of galaxies from the Team Keck Redshift Survey in the GOODS-N field. We measure line-of-sight velocity dispersions from integrated emission for 1089 galaxies with median redshift 0.637 and spatially resolved kinematics for a subsample of 380 galaxies. This is the largest sample of galaxies to z ~ 1 with kinematics to date and allows us to measure kinematic properties without morphological preselection. Emission-line widths provide a dynamical measurement for the bulk of blue galaxies. To fit the spatially resolved kinematics, we construct models that fit both line-of-sight rotation amplitude and velocity dispersion. Integrated line width correlates well with a combination of the velocity gradient and dispersion and is a robust measure of galaxy kinematics. The spatial extents of emission and continuum are similar, and there is no evidence that line widths are affected by nuclear or clumpy emission. The measured rotation gradient depends strongly on slit position angle alignment with galaxy major axis, but integrated line width does not. Even for galaxies with well-aligned slits, some have kinematics dominated by dispersion (V/σ < 1) rather than rotation. These are probably objects with disordered velocity fields, not dynamically hot stellar systems. About 35% of the spatially resolved sample are dispersion dominated; galaxies that are both dispersion dominated and bright exist at high redshift but appear rare at low redshift. This kinematic morphology may probe galaxies' evolutionary state. It is linked to photometric morphology in HST ACS images: dispersion-dominated galaxies include a higher fraction of irregulars and chain galaxies, while rotation-dominated galaxies are mostly disks and irregulars. Only one-third of chain/hyphen galaxies are dominated by rotation; high-redshift elongated objects cannot be assumed to be inclined disks.

Rings and Bent Chain Galaxies in the GEMS and GOODS Fields

Twenty-four galaxies with rings or partial rings were studied in the GEMS and GOODS fields out to z ~ 1.4. Most resemble local collisional ring galaxies in morphology, size, and clumpy star formation. Clump ages range from 108 to 109 yr, and clump masses go up to several ? 108 M?, based on color evolution models. The clump ages are consistent with the expected lifetimes of ring structures if they are formed by collisions. Fifteen other galaxies that resemble the arcs in partial ring galaxies but have no evident disk emission were also studied. Their clumps have bluer colors at all redshifts compared to the clumps in the ring and partial ring sample, and their clump ages are younger than in rings and partial rings by a factor of ~10. In most respects, they resemble chain galaxies except for their curvature; we refer to them as bent chains. Several rings are symmetric with centered nuclei and no obvious companions. They could be outer Lindblad resonance rings, although some have no obvious bars or spirals to drive them. If these symmetric cases are resonance rings, then they could be the precursors of modern resonance rings, which are only ~30% larger on average. This similarity in radius suggests that the driving pattern speed has not slowed by more by ~30% during the last ~7 Gyr. Those without bars could be examples of dissolved bars.

Optical properties of the NGC 5328 group of galaxies

We present the results of a photometric and spectroscopic study of seven members of the NGC 5328 group of galaxies, a chain of galaxies spanning over 200 kpc (H 0 = 70 km s -1 Mpc -1 ). We analyse the galaxy structure and study the emission-line properties of the group members looking for signatures of star formation and active galactic nucleus activity. We finally attempt to infer, from the modelling of line-strength indices, the stellar population ages of the early-type members. We investigate also the presence of a dwarf galaxy population associated with the bright members. The group is composed of a large fraction of early-type galaxies including NGC 5328 and 5330, two 'bona fide' ellipticals at the centre of the group. In both galaxies no recent star formation episodes are detected by the Hβ versus MgFe line-strength indices of these galaxies. 2MASX J 13524838-2829584 has extremely boxy isophotes which are believed to be connected to a merging event: line-strength indices suggest that this object probably had a recent star formation episode. A warped disc component emerges from the model subtracted image of 2MASX J13530016-2827061, which is interpreted as a signature of an ongoing interaction with the rest of the group. Ongoing star formation and nuclear activity are present in the projected outskirts of the group. The two early-type galaxies 2MASX J13523852-2830444 and 2MASX J13525393-2831421 show spectral signatures of star formation, while a Seyfert 2 type nuclear activity is detected in MCG-5-33-29.

Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations

We investigate the detailed response of gas to the formation of transient and long-lived dynamical structures induced in the early stages of a disk-disk collision, and identify observational signatures of radial gas inflow through a detailed examination of the collision simulation of an equal mass bulge dominated galaxy. Our analysis and discussion mainly focuses around the evolution of the diffuse and dense gas in the early stages of the collision, when the two disks are interacting but have not yet merged. Stars respond to the tidal interaction by forming both transient arms and long lived m = 2 bars, but the gas response is more transient, flowing directly toward the central regions within about 10 years after the initial collision. The rate of inflow declines when more than half of the total gas supply reaches the inner few kpc, where the gas forms a dense nuclear ring inside the stellar bar. The average gas inflow rate to the central 1.8 kpc is ∼ 7M⊙ yr −1 with a peak rate of 17M⊙ yr . Gas with high volume density is found in the inner parts of the post-collision disks at size scales close to the spatial resolution of the simulations, and this may be a direct result of shocks traced by the discontinuity in the gas velocity field. The evolution of gas in a bulgeless progenitor galaxy is also discussed, and a possible link to the “chain galaxy” population observed at high redshifts is inferred. The evolution of the structural parameters such as asymmetry and concentration of both stars and gas are studied in detail. Further, a new structure parameter (the compactness parameter K) that traces the evolution of the size scale of the gas relative to the stellar disk is introduced, and this may be a useful tracer to determine the merger chronology of colliding systems. Non-circular gas kinematics driven by the perturbation of the non-axisymmetric structure can produce distinct emission Department of Astronomy, University of Massachusetts, Amherst, MA 01002 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Department of Astronomy, Case Western Reserve University, Cleveland, OH 44106 Research Corporation Cottrell Scholar and NSF CAREER Fellow

A Constant Bar Fraction out to Redshiftz∼ 1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy

Barlike structures were investigated in a sample of 186 disk galaxies larger than 05 that are in the I-band image of the Tadpole galaxy taken with the Hubble Space Telescope Advanced Camera for Surveys. We found 22 clear cases of barred galaxies, 21 galaxies with small bars that appear primarily as isophotal twists in a contour plot, and 11 cases of peculiar bars in clump-cluster galaxies, which are face-on versions of chain galaxies. The latter bars are probably young, as the galaxies contain only weak interclump emission. Four of the clearly barred galaxies at z ~ 0.8-1.2 have grand-design spirals. The bar fraction was determined as a function of galaxy inclination and compared with the analogous distribution in the local universe. The bar fraction was also determined as a function of galaxy angular size. These distributions suggest that inclination and resolution effects obscure nearly half of the bars in our sample. The bar fraction was also determined as a function of redshift. We found a nearly constant bar fraction of 0.23 ± 0.03 from z ~ 0 to z = 1.1. When corrected for inclination and size effects, this fraction is comparable to the bar fraction in the local universe, ~0.4, which we tabulated for all bar and Hubble types in the Third Reference Catalogue of Bright Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc after correcting for redshift with a Λ-dominated cold dark matter cosmology. The average exponential scale length is ~2 kpc. These are half the sizes of local barred galaxies and not likely to be influenced much by cosmological dimming, because the high-z galaxies are intrinsically brighter. We conclude that galaxy bars were present in normal abundance at least ~8 Gyr ago (z ~ 1); the bars in clump-cluster galaxies may have formed from gaseous disk instabilities and star formation rather than stellar disk instabilities, and bar dissolution cannot be common during a Hubble time unless the bar formation rate is comparable to the dissolution rate. If galaxy interactions trigger bar formation more than bar destruction, then the higher interaction rate in the past suggests relatively few bars actually dissolved in a Hubble time.

Subgalactic Clumps at High Redshift: A Fragmentation Origin?

We investigate the origin of the clumpy structures observed at high redshift, such as chain galaxies. We use a three-dimensional chemodynamical simulation describing the dynamics of stars and a two-phase interstellar medium, as well as feedback processes from the stars. For high efficiency of energy dissipation in the cold cloud medium, the initially gaseous disk fragments and develops several massive clumps of gas and stars. We follow the evolution of the individual clumps and determine their masses, metallicities, and velocities. A few dynamical times after fragmentation of the disk, the clumps merge to build a massive bulge. Calculating Hubble Space Telescope (HST) and UBVRIJHKLM colors, including absorption by interstellar dust, we determine the morphologies and colors of this model in HST images. Several peculiar morphological structures seen in the Hubble Deep Field can be well explained by a fragmented galactic disk model, including chain galaxies and objects consisting of several nearby knots.

Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

Colors and magnitudes were determined for 69 chain galaxies, 58 other linear structures, 32 normal edge-on galaxies, and all of their large star formation clumps in the Hubble Space Telescope Advanced Camera for Surveys field of the Tadpole galaxy. Redshifts of 0.5-2 are inferred from comparisons with published color-evolution models. The linear galaxies have no red nuclear bulges like the normal disk galaxies in our field, but the star formation clumps in each have about the same colors and magnitudes. Light profiles along the linear galaxies tend to be flat, unlike the exponential profiles of normal galaxies. Although the most extreme of the linear objects look like beaded filaments, they are all probably edge-on disks that will evolve to late Hubble type galaxies. The lack of an exponential profile is the result of either a dust scale height that is comparable to the stellar scale height or an intrinsically irregular structure. Examples of galaxies that could be face-on versions of linear galaxies are shown. They have an irregular clumpy structure with no central bulge and with clump colors and magnitudes that are comparable to those in the linear galaxies. Radiative transfer solutions to the magnitudes and surface brightnesses of inclined dusty galaxies suggest that edge-on disks should become more prominent near the detection limit for surface brightness. The surface brightness distribution of the edge-on galaxies in this field confirm this selection effect. The star formation regions are much more massive than in modern galaxies, averaging up to ~109 M☉ for kiloparsec scales.

A Filamentary Structure of Massive Star-forming Galaxies Associated with an X-Ray-absorbed QSO at z =1.8

The genesis of spheroids is central to our understanding of galaxy formation -- they are relatively simple systems, containing about half the stellar mass of the Universe. A major subset of spheroids, massive elliptical galaxies, are preferentially found in clusters where they exhibit old, coeval stellar populations suggesting that they formed synchronously at early epochs. Here we report SCUBA submillimeter imaging of a region around a z=1.8 X-ray selected QSO. The image reveals a remarkable 400 kiloparsec long chain of galaxies each with an obscured star-formation rate sufficiently high to build a massive spheroid in less than 1 Gyr. The large over-density of these galaxies relative to expectations for a random field implies they probably reside in a structure associated with the QSO. We suggest that this star formation is associated with galaxy mergers or encounters within the filament, such as those predicted by the popular hierarchical model of galaxy formation. Our observations suggest that strong absorption in the X-ray spectra of QSOs at high-redshifts may result from a veil of gas thrown up by a merger or merger-induced activity, rather than an orientation-dependent obscuring torus. It is argued that these systems are the precursors of elliptical galaxies found today in the core regions of all rich galaxy clusters.

Discovery of Face-on Counterparts of Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

The background field of the Tadpole galaxy image taken with the Hubble Space Telescope Advanced Camera for Surveys contains 87 clump clusters that are probably low-inclination versions of the 69 chain galaxies that we found in a previous paper. This conclusion follows from the similarity in colors and apparent magnitudes of the brightest clumps in the clump clusters and chain galaxies and from the similarity between the distribution of axial ratios for the combined sample and the distribution for normal disk galaxies. These distributions are consistent with chains and clump clusters being disks viewed at random angles with intrinsic axial ratios of 0.1-0.2 or less. The disks appear to be gas-rich galaxies at z = 1-2 that are forming an early generation of stars in a small number of very large star complexes.