arXiv subjects

Christopher J. Conselice

Publications and source records attributed to Christopher J. Conselice.

At least 289 records · Page 16Linked to original sources

arXiv2009

A Comparison of Galaxy Merger History Observations and Predictions from Semi-Analytic Models

We present a detailed analysis of predicted galaxy-galaxy merger fractions and rates in the Millennium simulation and compare these with the most up to date observations of the same quantities up to z~3. We carry out our analysis by considering the predicted merger history in the Millennium simulation within a given time interval, as a function of stellar mass. This method, as opposed to pair fraction counts, considers mergers that have already taken place, and allows a more direct comparison with the observed rates and fractions measured with the concentration-asymmetry-clumpiness (CAS) method. We examine the evolution of the predicted merger fraction and rate in the Millennium simulation for galaxies with stellar masses M_* ~ 10^9 - 10^12 M_sun. We find that the predicted merger rates and fractions match the observations well for galaxies with M_* > 10^11 M_sun at z<2, while significant discrepancies occur at lower stellar masses, and at z>2 for M_* > 10^11 M_sun systems. At z>2 the simulations underpredict the observed merger fractions by a factor of 4-10. The shape of the predicted merger fraction and rate evolutions are similar to the observations up to z~2, and peak at 1 10^11 M_sun. We discuss possible reasons for these discrepancies, and compare different realisations of the Millennium simulation to understand the effect of varying the physical implementation of feedback. We conclude that the comparison is potentially affected by a number of issues, including uncertainties in interpreting the observations and simulations in terms of the assumed merger mass ratios and merger time-scales. (abridged)

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arXiv2009

The Structures of Distant Galaxies - IV: A New Empirical Measurement of the Time-Scale for Galaxy Mergers - Implications for the Merger History

Understanding the role of mergers in galaxy formation is one of the most outstanding problems in extragalactic astronomy. While we now have an idea for how the merger fraction evolves at redshifts z < 3, converting this merger fraction into merger rates, and therefore how many mergers an average galaxy undergoes during its history, is still uncertain. The main reason for this is that the inferred number of mergers depends highly upon the time-scale observational methods are sensitive for finding ongoing or past mergers. While there are several theoretical and model-based estimates of merger times, there is currently no empirical measure of this time-scale. We present the first observationally based measurement of merger times utilising the observed decline in the galaxy major merger fraction at z < 1.2 based on > 20,000 galaxies in the Extended Groth Strip and COSMOS surveys. Using a new methodology described in this paper, we determine how long a galaxy remains identifiable as a merging system within the CAS system. We find a maximum CAS major merger time-scale of 1.1+/-0.3 Gyr at z < 1.2, and a most likely CAS merger time-scale of 0.6+/-0.3 Gyr, in good agreement with results from N-body simulations. Utilizing this time-scale we measure the number of major mergers galaxies with masses M_{*} > 10^{10} M_0 undergo at z < 1.2, with a total number N_m = 0.90_{-0.23}^{+0.44}. We further show that this time-scale is inconsistent with a star formation origin for ultra-high asymmetries, thereby providing further evidence that structural methods are able to locate mostly merging galaxies.

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arXiv2009

The Structures of Distant Galaxies - II: Diverse Galaxy Structures and Local Environments at z = 4-6; Implications for Early Galaxy Assembly

We present an analysis of the structures, sizes, star formation rates, and local environmental properties of galaxies at z~4-6, utilising deep Hubble Space Telescope imaging of the Hubble Ultra Deep Field. The galaxies we study are selected with the Lyman-break drop-out technique, using galaxies which are B-,V-, and i-drops, which effectively selects UV bright starbursting galaxies between z=4 and z=6. Our primary observational finding is that starbursting galaxies at z>4 have a diversity in structure, with roughly 30% appearing distorted and asymmetric, while the majority are smooth and apparently undisturbed systems. We utilize several methods to compute the inferred assembly rates for these distorted early galaxies including utilising the CAS system and pair counts. Overall, we find a similar fraction of galaxies which are in pairs as the fraction which have a distorted structure. Using the CAS methodology, and our best-estimate for merger time-scales, we find that the total number of inferred effective mergers for M_{*} > 10^9-10 M_sol galaxies at z<6 is N_m = 4.2^{+4.1}_{-1.4}. The more common symmetrical systems display a remarkable scaling relation between the concentration of light and their half-light radii, revealing the earliest known galaxy scaling relationship, and demonstrating that some galaxies at z>4 are likely in a relaxed state. Systems which are asymmetric do not display a correlation between size and half-light radii, and are generally larger than the symmetric smooth systems. The time-scale for the formation of these smooth systems is 0.5-1 Gyr, suggesting that most of these galaxies are formed through coordinated very rapid gas collapses and star formation over a size of 1-2 kpc, or from merger events at z > 10. (Abridged)

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arXiv2009

History of Galaxy Interactions and their Impact on Star Formation over the Last 7 Gyr from GEMS

We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z~0.24--0.80 (lookback time T_b~3--7 Gyr) using 3698 (M*>=1e9 Msun) galaxies with GEMS HST, COMBO-17, and Spitzer data. Our results are: (1) Among 790 high mass (M*>=2.5e10 Msun) galaxies, the visually-based merger fraction over z~0.24--0.80, ranges from 9%+-5% to 8%+-2%. Lower limits on the major and minor merger fractions over this interval range from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first approximate empirical estimate of the frequency of minor mergers at z<1. For a visibility timescale of ~0.5 Gyr, it follows that over T_b~3--7 Gyr, ~68% of high mass systems have undergone a merger of mass ratio >1/10, with ~16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous `major or minor' mergers. The mean merger rate is a few x 1e-4 Gyr-1 Mpc-3. (2) We compare the empirical merger fraction and rate for high mass galaxies to a suite of Lambda CDM-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of five dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ~3698 M*>=1e9 Msun galaxies, we find that the mean SFR of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z~0.24--0.80. Visibly merging systems only account for less than 30% of the cosmic SFR density over T_b~3--7 Gyr. This suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

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arXiv2009

Constraining Star Formation and AGN in z~2 Massive Galaxies using High Resolution MERLIN Radio Observations

We present high spatial resolution MERLIN 1.4GHz radio observations of two high redshift (z~2) sources, RGJ123623 (HDF147) and RGJ123617 (HDF130), selected as the brightest radio sources from a sample of submillimetre-faint radio galaxies. They have starburst classifications from their rest-frame UV spectra. However, their radio morphologies are remarkably compact (<80mas and <65mas respectively), demanding that the radio luminosity be dominated by Active Galactic Nuclei (AGN) rather than starbursts. Near-IR imaging (HST NICMOS F160W) shows large scale sizes (R_(1/2)~0.75", diameters ~12kpc) and SED fitting to photometric points (optical through the mid-IR) reveals massive (~5x10^(11) M_sun), old (a few Gyr) stellar populations. Both sources have low flux densities at observed 24um and are undetected in observed 70um and 850um, suggesting a low mass of interstellar dust. They are also formally undetected in the ultra-deep 2Ms Chandra data, suggesting that any AGN activity is likely intrinsically weak. We suggest both galaxies have evolved stellar populations, low star formation rates, and low accretion rates onto massive black holes (10^(8.6) M_sun) whose radio luminosity is weakly beamed (by factors of a few). A cluster-like environment has been identified near HDF130 by an over-density of galaxies at z=1.99, reinforcing the claim that clusters lead to more rapid evolution in galaxy populations. These observations suggest that high-resolution radio (MERLIN) can be a superb diagnostic tool of AGN in the diverse galaxy populations at z~2.

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arXiv2008

The Structures of Distant Galaxies - III: The Merger History of over 20,000 Massive Galaxies at z < 1.2

Utilizing deep Hubble Space Telescope imaging from the two largest field galaxy surveys, the Extended Groth Strip (EGS) and the COSMOS survey, we examine the structural properties, and derive the merger history for 21,902 galaxies with M_*>10^{10} M_0 at z<1.2. We examine the structural CAS parameters of these galaxies, deriving merger fractions, at 0.2 10^{10} M_0, the merger fraction can be parameterised by f_m = f_0*(1+z)^m with the power-law slope m=2.3+/-0.4. By using the best available z = 0 prior the slope increases to m=3.8+/-0.2, showing how critical the measurement of local merger properties are for deriving the evolution of the merger fraction. We furthermore show that the merger fraction derived through structure is roughly a factor of 3-6 higher than pair fractions. Based on the latest cosmological simulations of mergers we show that this ratio is predicted, and that both methods are likely tracing the merger fraction and rate properly. We calculate, utilising merger time scales from simulations, and previously published merger fractions that the merger rate of galaxies with M_*>10^{10} M_0 increases linearly between z = 0.7 and z = 3, and that a typical M_*>10^{10} M_0 galaxy undergoes between 1-2 major mergers at z<1.2.

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arXiv2008

A Surprisingly High Pair Fraction for Extremely Massive Galaxies at z ~ 3 in the GOODS NICMOS Survey

We calculate the major pair fraction and derive the major merger fraction and rate for 82 massive ($M_{*}>10^{11}M_{\odot}$) galaxies at $1.7 < z < 3.0$ utilising deep HST NICMOS data taken in the GOODS North and South fields. For the first time, our NICMOS data provides imaging with sufficient angular resolution and depth to collate a sufficiently large sample of massive galaxies at z $>$ 1.5 to reliably measure their pair fraction history. We find strong evidence that the pair fraction of massive galaxies evolves with redshift. We calculate a pair fraction of $f_{m}$ = 0.29 +/- 0.06 for our whole sample at $1.7 < z < 3.0$. Specifically, we fit a power law function of the form $f_{m}=f_{0}(1+z)^{m}$ to a combined sample of low redshift data from Conselice et al. (2007) and recently acquired high redshift data from the GOODS NICMOS Survey. We find a best fit to the free parameters of $f_{0}$ = 0.008 +/- 0.003 and $m$ = 3.0 +/- 0.4. We go on to fit a theoretically motivated Press-Schechter curve to this data. This Press-Schechter fit, and the data, show no sign of levelling off or turning over, implying that the merger fraction of massive galaxies continues to rise with redshift out to z $\sim$ 3. Since previous work has established that the merger fraction for lower mass galaxies turns over at z $\sim$ 1.5 - 2.0, this is evidence that higher mass galaxies experience more mergers earlier than their lower mass counterparts, i.e. a galaxy assembly downsizing. Finally, we calculate a merger rate at z = 2.6 of $\Re$ $<$ 5 $\times$ 10$^{5}$ Gpc$^{-3}$ Gyr$^{-1}$, which experiences no significant change to $\Re$ $<$ 1.2 $\times$ 10$^{5}$ Gpc$^{-3}$ Gyr$^{-1}$ at z = 0.5. This corresponds to an average $M_{*}>10^{11}M_{\odot}$ galaxy experiencing 1.7 +/- 0.5 mergers between z = 3 and z = 0.

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arXiv2008

Extremely Compact Massive Galaxies at 1.7<z<3

We measure and analyse the sizes of 82 massive (M >= 10^11 M_Sun) galaxies at 1.7 2. We split our sample into disk-like (Sersic index n<=2) and spheroid-like (Sersic index n>2) galaxies, and find that at a given stellar mass, disk-like galaxies at z~2.3 are a factor of 2.6+/-0.3 smaller than present day equal mass systems, and spheroid-like galaxies at the same redshift are 4.3+/-0.7 times smaller than comparatively massive elliptical galaxies today. We furthermore show that the stellar mass densities of very massive galaxies at z~2.5 are similar to present-day globular clusters with values ~2x10^10 M_Sun kpc^-3

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arXiv2008

Ubiquitous outflows in DEEP2 spectra of star-forming galaxies at z=1.4

Galactic winds are a prime suspect for the metal enrichment of the intergalactic medium and may have a strong influence on the chemical evolution of galaxies and the nature of QSO absorption line systems. We use a sample of 1406 galaxy spectra at z~1.4 from the DEEP2 redshift survey to show that blueshifted Mg II 2796, 2803 A absorption is ubiquitous in starforming galaxies at this epoch. This is the first detection of frequent outflowing galactic winds at z~1. The presence and depth of absorption are independent of AGN spectral signatures or galaxy morphology; major mergers are not a prerequisite for driving a galactic wind from massive galaxies. Outflows are found in coadded spectra of galaxies spanning a range of 30x in stellar mass and 10x in star formation rate (SFR), calibrated from K-band and from MIPS IR fluxes. The outflows have column densities of order N_H ~ 10^20 cm^-2 and characteristic velocities of ~ 300-500 km/sec, with absorption seen out to 1000 km/sec in the most massive, highest SFR galaxies. The velocities suggest that the outflowing gas can escape into the IGM and that massive galaxies can produce cosmologically and chemically significant outflows. Both the Mg II equivalent width and the outflow velocity are larger for galaxies of higher stellar mass and SFR, with V_wind ~ SFR^0.3, similar to the scaling in low redshift IR-luminous galaxies. The high frequency of outflows in the star-forming galaxy population at z~1 indicates that galactic winds occur in the progenitors of massive spirals as well as those of ellipticals. The increase of outflow velocity with mass and SFR constrains theoretical models of galaxy evolution that include feedback from galactic winds, and may favor momentum-driven models for the wind physics.

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arXiv2008

Size evolution of the most massive galaxies at 1.7<z<3 from GOODS NICMOS survey imaging

We measure the sizes of 82 massive (M>10^11 M_sun) galaxies at 1.7 2, confirming the extreme compactness of these galaxies. We split our sample into disk-like (n<2) and spheroid-like (n>2) galaxies based on their Sersic indices, and find that at a given stellar mass disk-like galaxies at z~2.3 are a factor of 2.6+/-0.3 smaller than present day equal mass systems, and spheroid-like galaxies at the same redshifts are 4.3+/-0.7 smaller than comparatively massive elliptical galaxies today. At z>2 our results are compatible with both a leveling off, or a mild evolution in size. Furthermore, the high density (~2x10^10 M_sun kpc^-3) of massive galaxies at these redshifts, which are similar to present day globular clusters, possibly makes any further evolution in sizes beyond z=3 unlikely.

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arXiv2008

Exploring the evolutionary paths of the most massive galaxies since z~2

We use Spitzer MIPS data from the FIDEL Legacy Project in the Extended Groth Strip to analyze the stellar mass assembly of massive (M>10^11 M_sun) galaxies at z<2 as a function of structural parameters. We find 24 micron emission for more than 85% of the massive galaxies morphologically classified as disks, and for more than 57% of the massive systems morphologically classified as spheroids at any redshift, with about 8% of sources harboring a bright X-ray and/or infrared emitting AGN. More noticeably, 60% of all compact massive galaxies at z=1-2 are detected at 24 micron, even when rest-frame optical colors reveal that they are dead and evolving passively. For spheroid-like galaxies at a given stellar mass, the sizes of MIPS non-detections are smaller by a factor of 1.2 in comparison with IR-bright sources. We find that disk-like massive galaxies present specific SFRs ranging from 0.04 to 0.2 Gyr^-1 at z<1 (SFRs ranging from 1 to 10 M_sun/yr), typically a factor of 3-6 higher than massive spheroid-like objects in the same redshift range. At z>1, and more pronouncedly at z>1.3, the median specific SFRs of the disks and spheroids detected by MIPS are very similar, ranging from 0.1 to 1 Gyr^-1 (SFR=10-200 M_sun/yr). We estimate that massive spheroid-like galaxies may have doubled (at the most) their stellar mass from star-forming events at z<2: less than 20% mass increase at 1.7<z<2.0, up to 40% more at 1.1<z<1.7, and less than 20% additional increase at z<1. Disk-like galaxies may have tripled (at the most) their stellar mass at z<2 from star formation alone: up to 40% mass increase at 1.7<z<2.0, and less than 180% additional increase below z=1.7 occurred at a steady rate.

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arXiv2008

Star Formation and Merging in Massive Galaxies at z < 2

Observing massive galaxies at various redshifts is one of the most straightforward and direct approaches towards understanding galaxy formation. There is now largely a consensus that the massive galaxy (M_* > 10^11 M_0) population is fully formed by z~1, based on mass and luminosity functions. However, we argue that the latest data can only rule out number and mass density evolution of a factor of > 2-3 at z < 1.5. We furthermore show that the star formation history of M_* > 10^11 M_0 galaxies reveals that 40+/-5% of galaxies with M_* > 10^11 M_0 at z~1 are undergoing star formation that effectively doubles their stellar mass between z = 0.4 - 1.4. These massive galaxies also undergo 0.9^+0.7_-0.5 major mergers during this same time period.

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arXiv2008

The Redshift Evolution of Wet, Dry, and Mixed Galaxy Mergers from Close Galaxy Pairs in the DEEP2 Galaxy Redshift Survey

We study the redshift evolution of galaxy pair fractions and merger rates for different types of galaxies using kinematic pairs selected from the DEEP2 Redshift Survey. By parameterizing the evolution of the pair fraction as (1+z)^{m}, we find that the companion rate increases mildly with redshift with m = 0.41+-0.20 for all galaxies with -21 < M_B^{e} < -19. Blue galaxies show slightly faster evolution in the blue companion rate with m = 1.27+-0.35 while red galaxies have had fewer red companions in the past as evidenced by the negative slope m = -0.92+-0.59. We find that at low redshift the pair fraction within the red sequence exceeds that of the blue cloud, indicating a higher merger probability among red galaxies compared to that among the blue galaxies. With further assumptions on the merger timescale and the fraction of pairs that will merge, the galaxy major merger rates for 0.1 < z <1.2 are estimated to be ~10^{-3}h^{3}Mpc^{-3}Gyr^{-1} with a factor of 2 uncertainty. At z ~ 1.1, 68% of mergers are wet, 8% of mergers are dry, and 24% of mergers are mixed, compared to 31% wet mergers, 25% dry mergers, and 44% mixed mergers at z ~ 0.1. The growth of dry merger rates with decreasing redshift is mainly due to the increase in the co-moving number density of red galaxies over time. About 22% to 54% of present-day L^{*} galaxies have experienced major mergers since z ~ 1.2, depending on the definition of major mergers. Moreover, 24% of the red galaxies at the present epoch have had dry mergers with luminosity ratios between 1:4 and 4:1 since z ~ 1. Our results also suggest that the wet mergers and/or mixed mergers may be partially responsible for producing red galaxies with intermediate masses while a significant portion of massive red galaxies is assembled through dry mergers at later times.

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arXiv2007

Hierarchical Disk Galaxy Assembly as the Origin of Scatter in the z~1 Stellar Mass Tully-Fisher Relation

Recent observations of distant disk galaxies show that there is little to no evolution in the relation between maximum rotation speed and stellar mass at z < 1.2. There is however a significant scatter between these two quantities whose origin is uncertain. We show in this paper that this scatter is at least partially the result of galaxy merging, revealing that disk galaxy growth at z < 1 is fundamentally hierarchical. We carry this out by calculating CAS (concentration, asymmetry, clumpiness) structural parameters using archival Hubble Space Telescope imaging of 91 high-redshift disk galaxies at 0.4 < z < 1.0 with robustly measured stellar masses and rotational maximum velocities taken from Conselice et al. (2005). We separate our sample into two redshift bins divided at z = 0.7, and investigate deviations from the stellar-mass Tully-Fisher relation in both the M_* and V_max directions, and how these correlate with structural asymmetries. We find a significant (> 3 sigma) correlation between the residuals from the stellar-mass Tully-Fisher relation in both the M_* and V_max directions, and high asymmetries. This result holds after we remove contributions from star formation and edge-on galaxies which can produce higher asymmetries unrelated to merging. While there are a few cases in which our disk galaxies have very large asymmetries, and are potentially involved in major mergers, in general these asymmetries are smaller than the major merger limit. It is therefore likely that these galaxies are forming hierarchically through minor galaxy mergers, which is also suggested by the constant slope and zero point of the stellar mass Tully-Fisher relation during the same epoch.

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arXiv2007

The Structures of Distant Galaxies I: Galaxy Structures and the Merger Rate to z~3 in the Hubble Ultra-Deep Field

This paper begins a series in which we examine the structures of distant galaxies to directly determine the history of their formation modes. We start this series by examining the structures of z_F850LP < 27 galaxies in the Hubble Ultra-Deep field, the deepest high-resolution optical image taken to date. We investigate a few basic features of galaxy structure using this image. These include: (1) The agreement of visual eye-ball classifications and non-parametric quantitative (CAS, Gini/M_20) methods; (2) How distant galaxy quantitative structures can vary as a function of rest-frame wavelength; and (3) The evolution of distant galaxy structures up to z~3. One of our major conclusions is that the majority of galaxies with z_850 < 27 are peculiar in appearance, and that galaxy assembly is rapidly occurring at these magnitudes, even up to the present time. We find a general agreement between galaxy classification by eye and through quantitative methods, as well as a general agreement between the CAS and the Gini/M_20 parameters. We find that the Gini/M_20 method appears to find a larger number of galaxy mergers than the CAS system, but contains a larger contamination from non-mergers. We furthermore calculate the merger rate of galaxies in the UDF up to z~3, finding an increase with redshift as well as stellar mass, confirming previous work in the Hubble Deep Field. We find that massive galaxies with M_{*} > 10^10 M_0 undergo 4.3_+0.8^-0.8 major galaxy mergers at z < 3, with all of this merging occurring at z > 1.

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arXiv2007

AEGIS: New Evidence Linking Active Galactic Nuclei to the Quenching of Star Formation

Utilizing Chandra X-ray observations in the All-wavelength Extended Groth Strip International Survey (AEGIS) we identify 241 X-ray selected Active Galactic Nuclei (AGNs, L > 10^{42} ergs/s) and study the properties of their host galaxies in the range 0.4 < z < 1.4. By making use of infrared photometry from Palomar Observatory and BRI imaging from the Canada-France-Hawaii Telescope, we estimate AGN host galaxy stellar masses and show that both stellar mass and photometric redshift estimates (where necessary) are robust to the possible contamination from AGNs in our X-ray selected sample. Accounting for the photometric and X-ray sensitivity limits of the survey, we construct the stellar mass function of X-ray selected AGN host galaxies and find that their abundance decreases by a factor of ~2 since z~1, but remains roughly flat as a function of stellar mass. We compare the abundance of AGN hosts to the rate of star formation quenching observed in the total galaxy population. If the timescale for X-ray detectable AGN activity is roughly 0.5-1 Gyr--as suggested by black hole demographics and recent simulations--then we deduce that the inferred AGN "trigger" rate matches the star formation quenching rate, suggesting a link between these phenomena. However, given the large range of nuclear accretion rates we infer for the most massive and red hosts, X-ray selected AGNs may not be directly responsible for quenching star formation.

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arXiv2007

Keck spectroscopy of the faint dwarf elliptical galaxy population in the Perseus Cluster core: mixed stellar populations and a flat luminosity function

We present the result of a photometric and Keck-LRIS spectroscopic study of dwarf galaxies in the core of the Perseus Cluster, down to a magnitude of M_B = -12.5. Spectra were obtained for twenty-three dwarf-galaxy candidates, from which we measure radial velocities and stellar population characteristics from absorption line indices. From radial velocities obtained using these spectra we confirm twelve systems as cluster members, with the remaining eleven as non-members. Using these newly confirmed cluster members, we are able to extend the confirmed colour-magnitude relation for the Perseus Cluster down to M_B = -12.5. We confirm an increase in the scatter about the colour magnitude relationship below M_B = -15.5, but reject the hypothesis that very red dwarfs are cluster members. We measure the faint-end slope of the luminosity function between M_B = -18 and M_B = -12.5, finding alpha = -1.26 \pm 0.06, which is similar to that of the field. This implies that an overabundance of dwarf galaxies does not exist in the core of the Perseus Cluster. By comparing metal and Balmer absorption line indices with alpha-enhanced single stellar population models, we derive ages and metallicities for these newly confirmed cluster members. We find two distinct dwarf elliptical populations: an old, metal poor population with ages ~ 8 Gyr and metallicities [Fe/H] < -0.33, and a young, metal rich population with ages < 5 Gyr and metallicities [Fe/H] > -0.33. Dwarf galaxies in the Perseus Cluster are therefore not a simple homogeneous population, but rather exhibit a range in age and metallicity.

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arXiv2007

Galaxy Bulges at Mid and High-Redshift

Bulges 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 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 by z ~ 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 at z > 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 until z ~ 1, suggesting that this structural assembly occurred relatively late.

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