arXiv subjects

Christopher J. Conselice

Publications and source records attributed to Christopher J. Conselice.

At least 217 records · Page 12Linked to original sources

arXiv2015

Evolution of the brightest cluster galaxies: the influence of morphology, stellar mass and environment

Using a sample of 425 nearby Brightest Cluster Galaxies (BCGs) from von der Linden et al. (2007), we study the relationship between their internal properties (stellar masses, structural parameters and morphologies) and their environment. More massive BCGs tend to inhabit denser regions and more massive clusters than lower mass BCGs. Furthermore, cDs, which are BCGs with particularly extended envelopes, seem to prefer marginally denser regions and tend to be hosted by more massive halos than elliptical BCGs. cD and elliptical BCGs show parallel positive correlations between their stellar masses and environmental densities. However, at a fixed environmental density, cDs are, on average, ~40% more massive. Our results, together with the findings of previous studies, suggest an evolutionary link between elliptical and cD BCGs. We suggest that most present-day cDs started their life as ellipticals, which subsequently grew in stellar mass and size due to mergers. In this process, the cD envelope developed. The large scatter in the stellar masses and sizes of the cDs reflects their different merger histories. The growth of the BCGs in mass and size seems to be linked to the hierarchical growth of the structures they inhabit: as the groups and clusters became denser and more massive, the BCGs at their centres also grew. This process is nearing completion since the majority (~60%) of the BCGs in the local Universe have cD morphology. However, the presence of galaxies with intermediate morphological classes (between ellipticals and cDs) suggests that the growth and morphological transformation of some BCGs is still ongoing.

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arXiv2015

A Critical Assessment of Stellar Mass Measurement Methods

In this paper we perform a comprehensive study of the main sources of random and systematic errors in stellar mass measurement for galaxies using their Spectral Energy Distributions (SEDs). We use mock galaxy catalogs with simulated multi-waveband photometry (from U-band to mid-infrared) and known redshift, stellar mass, age and extinction for individual galaxies. Given different parameters affecting stellar mass measurement (photometric S/N ratios, SED fitting errors, systematic effects, the inherent degeneracies and correlated errors), we formulated different simulated galaxy catalogs to quantify these effects individually. We studied the sensitivity of stellar mass estimates to the codes/methods used, population synthesis models, star formation histories, nebular emission line contributions, photometric uncertainties, extinction and age. For each simulated galaxy, the difference between the input stellar masses and those estimated using different simulation catalogs, $Δ\log(M)$, was calculated and used to identify the most fundamental parameters affecting stellar masses. We measured different components of the error budget, with the results listed as follows: (1). no significant bias was found among different codes/methods, with all having comparable scatter; (2). A source of error is found to be due to photometric uncertainties and low resolution in age and extinction grids; (3). The median of stellar masses among different methods provides a stable measure of the mass associated with any given galaxy; (4). The deviations in stellar mass strongly correlate with those in age, with a weaker correlation with extinction; (5). the scatter in the stellar masses due to free parameters are quantified, with the sensitivity of the stellar mass to both the population synthesis codes and inclusion of nebular emission lines studied.

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arXiv2015

Tracing galaxy populations through cosmic time: A critical test of methods for connecting the same galaxies between different redshifts at $z < 3$

Connecting galaxies with their descendants (or progenitors) at different redshifts can yield strong constraints on galaxy evolution. Observational studies have historically selected samples of galaxies using a physical quantity, such as stellar mass, either above a constant limit or at a constant cumulative number density. Investigation into the efficacy of these selection methods has not been fully explored. Using a set of four semi-analytical models based on the output of the Millennium Simulation, we find that selecting galaxies at a constant number density (in the range $-4.3 < \log\ n\ [\mathrm{Mpc}^{-3}\ h^{3}] < -3.0$) is superior to a constant stellar mass selected sample, although it still has significant limitations. Recovery of the average stellar mass, stellar mass density and average star-formation rate is highly dependent on the choice of number density but can all be recovered to within $<50\%$ at the commonly employed choice of $\log\ n\ [\mathrm{Mpc}^{-3}\ h^{3}] = -4.0$, corresponding to $\log M_\odot / h \sim 11.2$ at $z=0$, but this increases at lower mass limits. We show that there is a large scatter between the location of a given galaxy in a rank ordering based on stellar mass between different redshifts. We find that the inferred velocity dispersion may be a better tracer of galaxy properties, although further investigation is warranted into simulating this property. Finally, we find that over large redshift ranges selection at a constant number density is more effective in tracing the progenitors of modern galaxies than vice-versa.

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arXiv2015

A Deep Hubble and Keck Search for Definitive Identification of Lyman Continuum Emitters at z~3.1

Narrow-band imaging of the rest-frame Lyman continuum (LyC) of galaxies at z~3.1 has produced a large number of candidate LyC-emitting galaxies. These samples are contaminated by galaxies at lower redshift. To better understand LyC escape, we need an uncontaminated sample of galaxies that emit strongly in the LyC. Here we present deep Hubble imaging of five bright galaxies at z~3.1 that had previously been identified as candidate LyC-emitters with ground-based images. The WFC3 F336W images probe the LyC of galaxies at z>3.06 and provide an order-of-magnitude increase in spatial resolution over ground-based imaging. The non-ionizing UV images often show multiple galaxies (or components) within ~1'' of the candidate LyC emission seen from the ground. In each case, only one of the components is emitting light in the F336W filter, which would indicate LyC escape if that component is at z>3.06. We use Keck/NIRSPEC near-IR spectroscopy to measure redshifts of these components to distinguish LyC-emitters from foreground contamination. We find that two candidates are low redshift contaminants, one candidate had a previously misidentified redshift, and the other two cannot be confirmed as LyC-emitters. The level of contamination is consistent with previous estimates. For the galaxies with z>3.06, we derive strong 1 sigma limits on the relative escape fraction between 0.07 and 0.09. We still do not have a sample of definitive LyC-emitters, and a much larger study of low luminosity galaxies is required. The combination of high resolution imaging and deep spectroscopy is critical for distinguishing LyC-emitters from foreground contaminants.

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arXiv2015

Clumpy Galaxies in CANDELS. I. The Definition of UV Clumps and the Fraction of Clumpy Galaxies at 0.5<z<3

Although giant clumps of stars are crucial to galaxy formation and evolution, the most basic demographics of clumps are still uncertain, mainly because the definition of clumps has not been thoroughly discussed. In this paper, we study the basic demographics of clumps in star-forming galaxies (SFGs) at 0.5 0.5. The redshift evolution of Fclumpy changes with the stellar mass (M*) of the galaxies. Low-mass (log(M*/Msun)<9.8) galaxies keep an almost constant Fclumpy of about 60% from z~3.0 to z~0.5. Intermediate-mass and massive galaxies drop their Fclumpy from 55% at z~3.0 to 40% and 15%, respectively, at z~0.5. We find that (1) the trend of disk stabilization predicted by violent disk instability matches the Fclumpy trend of massive galaxies; (2) minor mergers are a viable explanation of the Fclumpy trend of intermediate-mass galaxies at z<1.5, given a realistic observability timescale; and (3) major mergers are unlikely responsible for the Fclumpy trend in all masses at z<1.5. The clump contribution to the rest-frame UV light of SFGs shows a broad peak around galaxies with log(M*/Msun)~10.5 at all redshifts, possibly linked to the molecular gas fraction of the galaxies. (Abridged)

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arXiv2015

The Link Between Morphology and Structure of Brightest Cluster Galaxies: Automatic Identification of cDs

We study a large sample of 625 low-redshift brightest cluster galaxies (BCGs) and link their morphologies to their structural properties. We derive visual morphologies and find that ~57% of the BCGs are cD galaxies, ~13% are ellipticals, and ~21% belong to the intermediate classes mostly between E and cD. There is a continuous distribution in the properties of the BCG's envelopes, ranging from undetected (E class) to clearly detected (cD class), with intermediate classes (E/cD and cD/E) showing the increasing degrees of the envelope presence. A minority (~7%) of BCGs have disk morphologies, with spirals and S0s in similar proportions, and the rest (~2%) are mergers. After carefully fitting the galaxies light distributions by using one-component (Sersic) and two-component (Sersic+Exponential) models, we find a clear link between the BCGs morphologies and their structures and conclude that a combination of the best-fit parameters derived from the fits can be used to separate cD galaxies from non-cD BCGs. In particular, cDs and non-cDs show very different distributions in the $R_e$--$RFF$ plane, where $R_e$ is the effective radius and $RFF$ (the residual flux fraction) measures the proportion of the galaxy flux present in the residual images after subtracting the models. In general, cDs have larger $R_e$ and $RFF$ values than ellipticals. Therefore we find, in a statistically robust way, a boundary separating cD and non-cD BCGs in this parameter space. BCGs with cD morphology can be selected with reasonably high completeness (~75%) and low contamination (~20%). This automatic and objective technique can be applied to any current or future BCG sample with good quality images.

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arXiv2014

Galaxy And Mass Assembly (GAMA): mass-size relations of z$<$0.1 galaxies subdivided by Sérsic index, colour and morphology

We use data from the Galaxy And Mass Assembly (GAMA) survey in the redshift range 0.01$<$z$<$0.1 (8399 galaxies in $g$ to $K_s$ bands) to derive the stellar mass $-$ half-light radius relations for various divisions of 'early' and 'late'-type samples. We find the choice of division between early and late (i.e., colour, shape, morphology) is not particularly critical, however, the adopted mass limits and sample selections (i.e., the careful rejection of outliers and use of robust fitting methods) are important. In particular we note that for samples extending to low stellar mass limits ($<10^{10}\mathcal{M_{\odot}}$) the Sérsic index bimodality, evident for high mass systems, becomes less distinct and no-longer acts as a reliable separator of early- and late-type systems. The final set of stellar mass $-$ half-light radius relations are reported for a variety of galaxy population subsets in 10 bands ($ugrizZYJHK_s$) and are intended to provide a comprehensive low-z benchmark for the many ongoing high-z studies. Exploring the variation of the stellar mass $-$ half-light radius relations with wavelength we confirm earlier findings that galaxies appear more compact at longer wavelengths albeit at a smaller level than previously noted: at $10^{10}\mathcal{M_{\odot}}$ both spiral systems and ellipticals show a decrease in size of 13% from $g$ to $K_s$ (which is near linear in log wavelength). Finally we note that the sizes used in this work are derived from 2D Sérsic light profile fitting (using GALFIT3), i.e., elliptical semi-major half light radii, improving on earlier low-z benchmarks based on circular apertures.

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arXiv2014

On the intermediate-redshift central stellar mass-halo mass relation, and implications for the evolution of the most massive galaxies since z~1

The stellar mass-halo mass relation is a key constraint in all semi-analytic, numerical, and semi-empirical models of galaxy formation and evolution. However, its exact shape and redshift dependence remain debated. Several recent works support a relation in the local Universe steeper than previously thought. Based on the comparisons with a variety of data on massive central galaxies, we show that this steepening holds up to z~1, for stellar masses Mstar>2e11 Msun. Specifically, we find significant evidence for a high-mass end slope of β>0.35-0.70, instead of the usual β~0.20-0.30 reported by a number of previous results. When including the independent constraints from the recent BOSS clustering measurements, the data, independent of any systematic errors in stellar masses, tend to favor a model with a very small scatter (< 0.15 dex) in stellar mass at fixed halo mass, in the redshift range z < 0.8 and for Mstar>3e11 Msun, suggesting a close connection between massive galaxies and host halos even at relatively recent epochs. We discuss the implications of our results with respect to the evolution of the most massive galaxies since z~1.

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arXiv2014

Scaling Relations and X-ray Properties of Moderate-Luminosity Galaxy Clusters from 0.3 < z < 0.6 with XMM-Newton

We present new X-ray temperatures and improved X-ray luminosity estimates for 15 new and archival XMM-Newton observations of galaxy clusters at intermediate redshift with mass and luminosities near the galaxy group/cluster division (M2500 < $2.4\times 10^{14} h_{70}^{-1} M_\odot$, L < $2\times 10^{44}$ erg $s^{-1}$, 0.3< z < 0.6). These clusters have weak-lensing mass measurements based on Hubble Space Telescope observations of clusters representative of an X-ray selected sample (the ROSAT 160SD survey). The angular resolution of XMM-Newton allows us to disentangle the emission of these galaxy clusters from nearby point sources, which significantly contaminated previous X-ray luminosity estimates for six of the fifteen clusters. We extend cluster scaling relations between X-ray luminosity, temperature, and weak-lensing mass for low-mass, X-ray-selected clusters out to redshift $\approx$0.45. These relations are important for cosmology and the astrophysics of feedback in galaxy groups and clusters. Our joint analysis with a sample of 50 clusters in a similar redshift range but with larger masses (M500 < $21.9 \times 10^{14} M_\odot$, $0.15 \leq z \leq 0.55$) from the Canadian Cluster Comparison Project finds that within r2500, $M \propto L^{0.44 +/- 0.05}$, $T \propto L^{0.23 +/- 0.02}$, and $M \propto T^{1.9 +/- 0.2}$. The estimated intrinsic scatter in the M-L relation for the combined sample is reduced to $σ_{log(M|L)}=0.10$, from $σ_{log(M|L)}=0.26$ with the original ROSAT measurements. We also find an intrinsic scatter for the T-L relation, $σ_{log(T|L)}=0.07 +/- 0.01$.

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arXiv2014

Galaxy And Mass Assembly (GAMA): Refining the Local Galaxy Merger Rate using Morphological Information

We use the Galaxy And Mass Assembly (GAMA) survey to measure the local Universe mass dependent merger fraction and merger rate using galaxy pairs and the CAS structural method, which identifies highly asymmetric merger candidate galaxies. Our goals are to determine which types of mergers produce highly asymmetrical galaxies, and to provide a new measurement of the local galaxy major merger rate. We examine galaxy pairs at stellar mass limits down to $M_{*} = 10^{8}M_{\odot}$ with mass ratios of $<$100:1 and line of sight velocity differences of $ΔV<500$ km s$^{-1}$. We find a significant increase in mean asymmetries for projected separations less than the sum of the individual galaxy's Petrosian 90 radii. For systems in major merger pairs with mass ratios of $<$4:1 both galaxies in the pair show a strong increase in asymmetry, while in minor merger systems (with mass ratios of $>$4:1) the lower mass companion becomes highly asymmetric, while the larger galaxy is much less affected. The fraction of highly asymmetric paired galaxies which have a major merger companion is highest for the most massive galaxies and drops progressively with decreasing mass. We calculate that the mass dependent major merger fraction is fairly constant at $\sim1.3-2\%$ between $10^{9.5}<M_{*}<10^{11.5} M_{\odot}$, and increases to $\sim4\%$ at lower masses. When the observability time scales are taken into consideration, the major merger rate is found to approximately triple over the mass range we consider. The total co-moving volume major merger rate over the range $10^{8.0}<M_{*}<10^{11.5} M_{\odot}$ is $(1.2 \pm 0.5) \times 10^{-3}$ $h^{3}_{70}$ Mpc$^{-3}$ Gyr$^{-1}$.

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arXiv2014

Minor vs Major Mergers: The Stellar Mass Growth of Massive Galaxies from z=3 using Number Density Selection Techniques

We present a study on the stellar mass growth of the progenitors of local massive galaxies with a variety of number density selections with $n\le1\times10^{-4}\,\rm{Mpc^{-3}}$ (corresponding to $M_*=10^{11.24}\rm{M_{\odot}}$ at z=0.3) in the redshift range $0.3<z<3.0$. We select the progenitors of massive galaxies using a constant number density selection, and one which is adjusted to account for major mergers. We find that the progenitors of massive galaxies grow by a factor of four in total stellar mass over this redshift range. On average the stellar mass added via the processes of star formation, major and minor mergers account for $24\pm8\%$, $17\pm15\%$ and $34\pm14\%$, respectively, of the total galaxy stellar mass at $z=0.3$. Therefore $51\pm20\%$ of the total stellar mass in massive galaxies at $z=0.3$ is created externally to their z=3 progenitors. We explore the implication of these results on the cold gas accretion rate and size evolution of the progenitors of most massive galaxies over the same redshift range. We find an average gas accretion rate of $\sim66\pm32\,\rm{M_{\odot}yr^{-1}}$ over the redshift range of $1.5<z<3.0$. We find that the size evolution of a galaxy sample selected this way is on average lower than the findings of other investigations.

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arXiv2014

Galaxy Zoo: CANDELS Barred Disks and Bar Fractions

The formation of bars in disk galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in disks decreases from the local Universe to z ~ 1, and by z > 1 simulations predict that bar features in dynamically mature disks should be extremely rare. Here we report the discovery of strong barred structures in massive disk galaxies at z ~ 1.5 in deep rest-frame optical images from CANDELS. From within a sample of 876 disk galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a sub-sample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5< z < 2 (f_bar = 10.7 +6.3 -3.5% after correcting for incompleteness) does not significantly evolve. We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disk galaxies have evolved over the last 11 billion years.

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arXiv2014

Galaxy And Mass Assembly: Deconstructing Bimodality - I. Red ones and blue ones

We measure the mass functions for generically red and blue galaxies, using a z < 0.12 sample of log M* > 8.7 field galaxies from the Galaxy And Mass Assembly (GAMA) survey. Our motivation is that, as we show, the dominant uncertainty in existing measurements stems from how 'red' and 'blue' galaxies have been selected/defined. Accordingly, we model our data as two naturally overlapping populations, each with their own mass function and colour-mass relation, which enables us characterise the two populations without having to specify a priori which galaxies are 'red' and 'blue'. Our results then provide the means to derive objective operational definitions for the terms 'red' and 'blue', which are based on the phenomenology of the colour-mass diagrams. Informed by this descriptive modelling, we show that: 1.) after accounting for dust, the stellar colours of 'blue' galaxies do not depend strongly on mass; 2.) the tight, flat 'dead sequence' does not extend much below log M* ~ 10.5; instead, 3.) the stellar colours of 'red' galaxies vary rather strongly with mass, such that lower mass 'red' galaxies have bluer stellar populations; 4.) below log M* ~ 9.3, the 'red' population dissolves into obscurity, and it becomes problematic to talk about two distinct populations; as a consequence, 5.) it is hard to meaningfully constrain the shape, including the possibility of an upturn, of the 'red' galaxy mass function below log M* ~ 9. Points 1-4 provide meaningful targets for models of galaxy formation and evolution to aim for.

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arXiv2014

The mass evolution of the first galaxies: stellar mass functions and star formation rates at $4 < z < 7$ in the CANDELS GOODS-South field

We measure new estimates for the galaxy stellar mass function and star formation rates for samples of galaxies at $z \sim 4,~5,~6~\&~7$ using data in the CANDELS GOODS South field. The deep near-infrared observations allow us to construct the stellar mass function at $z \geq 6$ directly for the first time. We estimate stellar masses for our sample by fitting the observed spectral energy distributions with synthetic stellar populations, including nebular line and continuum emission. The observed UV luminosity functions for the samples are consistent with previous observations, however we find that the observed $M_{UV}$ - M$_{*}$ relation has a shallow slope more consistent with a constant mass to light ratio and a normalisation which evolves with redshift. Our stellar mass functions have steep low-mass slopes ($α\approx -1.9$), steeper than previously observed at these redshifts and closer to that of the UV luminosity function. Integrating our new mass functions, we find the observed stellar mass density evolves from $\log_{10} ρ_{*} = 6.64^{+0.58}_{-0.89}$ at $z \sim 7$ to $7.36\pm0.06$ $\text{M}_{\odot} \text{Mpc}^{-3}$ at $z \sim 4$. Finally, combining the measured UV continuum slopes ($β$) with their rest-frame UV luminosities, we calculate dust corrected star-formation rates (SFR) for our sample. We find the specific star-formation rate for a fixed stellar mass increases with redshift whilst the global SFR density falls rapidly over this period. Our new SFR density estimates are higher than previously observed at this redshift.

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arXiv2014

Galaxy And Mass Assembly (GAMA): Stellar mass functions by Hubble type

We present an estimate of the galaxy stellar mass function and its division by morphological type in the local (0.025 < z < 0.06) Universe. Adopting robust morphological classifications as previously presented (Kelvin et al.) for a sample of 3,727 galaxies taken from the Galaxy And Mass Assembly survey, we define a local volume and stellar mass limited sub-sample of 2,711 galaxies to a lower stellar mass limit of M = 10^9.0 M_sun. We confirm that the galaxy stellar mass function is well described by a double Schechter function given by M* = 10^10.64 M_sun, α1 = -0.43, ϕ*1 = 4.18 dex^-1 Mpc^-3, α2 = -1.50 and ϕ*2 = 0.74 dex^-1 Mpc^-3. The constituent morphological-type stellar mass functions are well sampled above our lower stellar mass limit, excepting the faint little blue spheroid population of galaxies. We find approximately 71+3-4% of the stellar mass in the local Universe is found within spheroid dominated galaxies; ellipticals and S0-Sas. The remaining 29+4-3% falls predominantly within late type disk dominated systems, Sab-Scds and Sd-Irrs. Adopting reasonable bulge-to-total ratios implies that approximately half the stellar mass today resides in spheroidal structures, and half in disk structures. Within this local sample, we find approximate stellar mass proportions for E : S0-Sa : Sab-Scd : Sd-Irr of 34 : 37 : 24 : 5.

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arXiv2014

Galaxy Formation as a Cosmological Tool. I: The Galaxy Merger History as a Measure of Cosmological Parameters

As galaxy formation and evolution over long cosmic time-scales depends to a large degree on the structure of the universe, the assembly history of galaxies is potentially a powerful approach for learning about the universe itself. In this paper we examine the merger history of dark matter halos based on the Extended Press-Schechter formalism as a function of cosmological parameters, redshift and halo mass. We calculate how major halo mergers are influenced by changes in the cosmological values of $Ω_{\rm m}$, $Ω_Λ$, $σ_{8}$, the dark matter particle temperature (warm vs. cold dark matter), and the value of a constant and evolving equation of state parameter $w(z)$. We find that the merger fraction at a given halo mass varies by up to a factor of three for halos forming under the assumption of Cold Dark Matter, within different underling cosmological parameters. We find that the current measurements of the merger history, as measured through observed galaxy pairs as well as through structure, are in agreement with the concordance cosmology with the current best fit giving $1 - Ω_{\rm m} = Ω_{\rm Λ} = 0.84^{+0.16}_{-0.17}$. To obtain a more accurate constraint competitive with recently measured cosmological parameters from Planck and WMAP requires a measured merger accuracy of $δf_{\rm m} \sim 0.01$, implying surveys with an accurately measured merger history over 2 - 20 deg$^{2}$, which will be feasible with the next generation of imaging and spectroscopic surveys such as Euclid and LSST.

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arXiv2014

Galactic conformity and central / satellite quenching, from the satellite profiles of M$^{\ast}$ galaxies at $0.4<z<1.9$ in the UKIDSS UDS

We explore the redshift evolution of a curious correlation between the star-formation properties of central galaxies and their satellites (`galactic conformity') at intermediate to high redshift ($0.4 9.7$, around central galaxies at the characteristic Schechter function mass, ${\rm M} \sim {\rm M}^{\ast}$. We fit the radial profiles of satellite number densities with simple power laws, finding slopes in the range -1.1 to -1.4 for mass-selected satellites, and -1.3 to -1.6 for passive satellites. We confirm the tendency for passive satellites to be preferentially located around passive central galaxies at $3σ$ significance and show that it exists to at least $z\sim2$. Meanwhile, the quenched fraction of satellites around star-forming galaxies is consistent with field galaxies of equal stellar masses. We find no convincing evidence for a redshift-dependent evolution of these trends. One simple interpretation of these results is that only passive central galaxies occupy an environment that is capable of independently shutting off star-formation in satellite galaxies. By examining the satellites of higher stellar mass star-forming galaxies (${\rm log} ({\rm M}_*/{\rm M}_{\odot}) > 11$), we conclude that the origin of galactic conformity is unlikely to be exclusively due to the host dark-matter halo mass. A halo-mass-independent correlation could be established by either formation bias or a more physical connection between central and satellite star-formation histories. For the latter, we argue that a star-formation (or AGN) related outburst event from the central galaxy could establish a hot halo environment which is then capable of quenching both central and satellite galaxies.

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arXiv2014

The Evolution of Galaxy Structure over Cosmic Time

I present a comprehensive review of the evolution of galaxy structure in the universe from the first galaxies we can currently observe at z~6 down to galaxies we see in the local universe. I further address how these changes reveal galaxy formation processes that galaxy structural analyses can provide. This review is pedagogical and begins with a detailed discussion of the major methods in which galaxies are studied morphologically and structurally. This includes the well-established visual method; Sersic fitting to measure galaxy sizes and surface brightness profile shapes; non-parametric structural methods including the concentration (C), asymmetry (A), clumpiness (S) (CAS) method, as well as newer structural indices. Included is a discussion of how these structural indices measure fundamental properties of galaxies such as their scale, star formation rate, and ongoing merger activity. Extensive observational results are shown demonstrating how broad galaxy morphologies and structures change with time up to z~3, from small, compact and peculiar systems in the distant universe to the formation of the Hubble sequence we find today. This review further addresses how structural methods measure accurately the merger history out to z~3. The properties and evolution of bulges, disks, bars, and at z>1 large star forming clumps are also described, along with how morphological galaxy quenching occurs. Furthermore, the role of environment in producing structure in galaxies over cosmic time is treated. Alongside the evolution of general structure, I also delineate how galaxy sizes change with time, with measured sizes up to a factor of 2-5 smaller at high redshift at a given stellar mass. This review concludes with a discussion of how galaxy structure reveals the formation mechanisms behind galaxies, providing a new and unique way to test theories of galaxy formation.

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