Dynamics of Lyman Break Galaxies and Their Host Halos (original) (raw)
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Astrophysical Journal, 2010
Lyman break galaxies (LBGs) display a range in structures (from single/compact to clumpy/extended) that is different from typical local star-forming galaxies. Recently, we have introduced a sample of rare, nearby (z<0.3) starbursts that appear to be good analogs of LBGs. These "Lyman Break Analogs" (LBAs) provide an excellent training set for understanding starbursts at different redshifts. We present an application of this by comparing the rest-frame UV/optical morphologies of 30 LBAs with those of sBzK galaxies at z~2, and LBGs at z~3-4 in the HUDF. The UV/optical colors and sizes of LBAs and LBGs are very similar, while the BzK galaxies are somewhat redder and larger. There is significant overlap between the morphologies (G, C, A and M_20) of the local and high-z samples, although the latter are somewhat less concentrated and clumpier. We find that in the majority of LBAs the starbursts appear to be triggered by interactions/mergers. When the images of the LBAs are degraded to the same sensitivity and resolution as the images of LBGs and BzK galaxies, these relatively faint asymmetric features are no longer detectable. This effect is particularly severe in the rest-frame UV. It has been suggested that high-z galaxies experience intense bursts unlike anything seen locally, possibly due to cold flows and instabilities. In part, this is based on the fact that the majority (~70%) of LBGs do not show morphological signatures of mergers. Our results suggest that this evidence is insufficient, since a large fraction of such signatures would likely have been missed in current observations of z>2 galaxies. This leaves open the possibility that clumpy accretion and mergers remain important in driving the evolution of these starbursts, together with rapid gas accretion through other means.
Lyman Break Galaxies as collision-driven starbursts
The 9th astrophysics conference: After the dark ages, when galaxies were young (the Universe at 2<Z<5), 1999
Explaining the nature of the Lyman Break Galaxies (LBGs) recently discovered [1,2] at redshift z ~ 3, is an exciting challenge for the paradigm of hierarchical structure formation. These galaxies are forming stars at a rate comparable to locally rare "starburst" galaxies [3], but are as luminous and numerous as local bright galaxies. In addition, the brightest LBGs have small emission line-widths [4], indicating virial masses of ,~ 1-5 x 101°M®, however LBGs exhibit strong clustering, similar to the properties expected of the most massive (,.~ 1012Mo) dark matter halos [5-8] at this redshift. We explore a possible solution to these apparent paradoxes: that LBGs are a population of collision-driven starburst galaxies which are abundant due to an increased collision rate at high redshift [2,9]. We use high-resolution cosmological Nbody simulations and a hierarchical halo finder to estimate the galaxy collision rate as a function of time in a popular cosmological model (ACDM). We find that appropriate collisions are frequent enough, and the ensuing bursts are plausibly bright enough, to account for most of the LBGs. Although many of the simulated collisions have relatively small masses (~ 101°Mo), they tend to cluster about large-mass halos. They therefore exhibit strong clustering, similar to that observed [1,11,8] and stronger than that of halos. The collision-induced starburst scenario [2,9] thus appears to explain the key observed properties of the high-z galaxies. This picture can be further tested observationally, and distinguished from other scenarios, by more detailed studies of the evolution of the number density of LBGs with redshift and the dependence of their clustering on scale and environment.
The Astrophysical Journal, 2008
used the Galaxy Evolution Explorer (GALEX) UV imaging survey to show that there exists a rare population of nearby compact UV-luminous galaxies (UVLGs) that closely resembles high redshift Lyman break galaxies (LBGs). We present HST images in the UV, optical, and Hα, and resimulate them at the depth and resolution of the GOODS/UDF fields to show that the morphologies of UVLGs are also similar to those of LBGs. Our sample of 8 LBG analogs thus provides detailed insight into the connection between star formation and LBG morphology. Faint tidal features or companions can be seen in all of the rest-frame optical images, suggesting that the starbursts are the result of a merger or interaction. The UV/optical light is dominated by unresolved (∼100-300 pc) super starburst regions (SSBs). A detailed comparison with the galaxies Haro 11 and VV 114 at z = 0.02 indicates that the SSBs themselves consist of diffuse stars and (super) star clusters. The structural features revealed by the new HST images occur on very small physical scales and are thus not detectable in images of high redshift LBGs, except in a few cases where they are magnified by gravitational lensing. We propose, therefore, that LBGs are mergers of gas-rich, relatively low-mass (M * ∼ 10 10 M ⊙ ) systems, and that the mergers trigger the formation of SSBs. If galaxies at high redshifts are dominated by SSBs, then the faint end slope of the luminosity function is predicted to have slope α ∼ 2. Our results are the most direct confirmation to date of models that predict that the main mode of star formation in the early universe was highly collisional.
The Astrophysical Journal, 2009
We report on observations of Lyman-break galaxies (LBGs) selected from the Great Observatories Origins Deep Survey (GOODS) at mean redshifts z ∼ 4, 5 and 6 (B 435 -, V 606 -and i 775 -band dropouts, respectively), obtained with the red-sensitive FORS2 spectrograph at the ESO VLT. This program has yielded spectroscopic identifications for 114 galaxies (∼ 60% of the targeted sample), of which 51 are at z ∼ 4, 31 at z ∼ 5, and 32 at z ∼ 6. We demonstrate that 1 Based on observations made at the European Southern Observatory Very Large Telescope, Paranal, Chile (ESO programme 170.A-0788 The Great Observatories Origins Deep Survey: ESO Public Observations of the SST Legacy / HST Treasury / Chandra Deep Field South). Also based on observations obtained with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under NASA contract NAS 5-26555.
galics- III. Properties of Lyman-break galaxies at a redshift of 3
Monthly Notices of the Royal Astronomical Society, 2004
This paper illustrates how mock observational samples of high-redshift galaxies with sophisticated selection criteria can be extracted from the predictions of galics, a hybrid model of hierarchical galaxy formation that couples the outputs of large cosmological simulations, and semi-analytic recipes to describe dark matter collapse, and the physics of baryons respectively. As an example of this method, we focus on the properties of Lyman Break Galaxies at redshift z ∼ 3 (hereafter LBGs) in a ΛCDM cosmology. With the momaf software package described in a companion paper, we generate a mock observational sample with selection criteria as similar as possible to those implied in the actual observations of z ∼ 3 LBGs by . We need to introduce an additional "maturity" criterion to circonvene subtle effects due to mass resolution in the simulation. We predict a number density of 1.15 arcmin −2 at R ≤ 25.5, in good agreement with the observed number density 1.2 ±0.18 arcmin −2 . Our model allows us to study the efficiency of the selection criterion to capture z ∼ 3 galaxies. We find that the colour contours designed from models of spectrophotometric evolution of stellar populations are able to select more "realistic" galaxies issued from models of hierarchical galaxy formation. We quantify the fraction of interlopers (12 %), and the selection efficiency (85%), and we give estimates of the cosmic variance. We then study the clustering properties of our model LBGs. They are hosted by halos with masses ∼ 1.6 × 10 12 M ⊙ , with a linear bias parameter that decreases with increassing scale from b = 5 to 3. The amplitude and slope of the 2D correlation function is in good agreement with the data. We investigate a series of physical properties: UV extinction (a typical factor 6.2 at 1600Å), stellar masses, metallicities, and Star Formation Rates, and we find them to be in general agreement with observed values. The model also allows us to make predictions at other optical and IR/submm wavelengths, that are easily accessible though queries to a web interfaced relational database. Looking into the future of these LBGs, we predict that 75 % of them end up as massive ellipticals and lenticulars today, even though only 35 % of all our local ellipticals and lenticulars are predicted to have a LBG progenitor. In spite of some shortcomings that come from our simplifying assumptions and the subtle propagation of mass resolution effects, this new 'mock observation' method clearly represents a first step toward a more accurate comparison between hierarchical models of galaxy formation and real observational surveys.
Lyman Break Analogs: Constraints on the Formation of Extreme Starbursts at Low and High Redshift
Proceedings of the International Astronomical Union, 2010
Lyman Break Analogs (LBAs), characterized by high far-UV luminosities and surface brightnesses as detected by GALEX, are intensely star-forming galaxies in the low-redshift universe (z ∼ 0.2), with star formation rates reaching up to 50 times that of the Milky Way. These objects present metallicities, morphologies and other physical properties similar to higher redshift Lyman Break Galaxies (LBGs), motivating the detailed study of LBAs as local laboratories of this high-redshift galaxy population. We present results from our recent integral-field spectroscopy survey of LBAs with Keck/OSIRIS, which shows that these galaxies have the same nebular gas kinematic properties as high-redshift LBGs. We argue that such kinematic studies alone are not an appropriate diagnostic to rule out merger events as the trigger for the observed starburst. Comparison between the kinematic analysis and morphological indices from HST imaging illustrates the difficulties of properly identifying (minor or major) merger events, with no clear correlation between the results using either of the two methods. Artificial redshifting of our data indicates that this problem becomes even worse at high redshift due to surface brightness dimming and resolution loss. Whether mergers could generate the observed kinematic properties is strongly dependent on gas fractions in these galaxies. We present preliminary results of a CARMA survey for LBAs and discuss the implications of the inferred molecular gas masses for formation models.
2005
We report angular correlation function (ACF) of Lyman Break Galaxies (LBGs) with unprecedented statistical quality on the basis of 16,920 LBGs at z = 4 detected in the 1 deg2 sky of the Subaru/XMM-Newton Deep Field. The ACF significantly departs from a power law, and shows an excess on small scale. Particularly, the ACF of LBGs with i ′ < 27.5 have a clear break between the small and large-scale regimes at the angular separation of ≃ 7 ′ ′ whose projected length corresponds to the virial radius of dark halos with a mass of 1011−12M⊙, indicating multiple LBGs residing in a single dark halo. Both on small (2′ ′ < θ < 3 ′′) and large (40′ ′ < θ < 400 ′′) scales, clustering amplitudes monotonically increase with luminosity for the magnitude range of i ′ = 24.5 − 27.5, and the small-scale clustering shows a stronger luminosity dependence than the large-scale clustering. The small-scale bias reaches b ≃ 10 − 50, and the outskirts of small-scale excess extend to a larger ang...
The Astrophysical Journal, 2005
We report angular correlation function (ACF) of Lyman Break Galaxies (LBGs) with unprecedented statistical quality on the basis of 16,920 LBGs at z = 4 detected in the 1 deg 2 sky of the Subaru/XMM-Newton Deep Field. The ACF significantly departs from a power law, and shows an excess on small scale. Particularly, the ACF of LBGs with i ′ < 27.5 have a clear break between the small and large-scale regimes at the angular separation of ≃ 7 ′′ whose projected length corresponds to the virial radius of dark halos with a mass of 10 11−12 M ⊙ , indicating multiple LBGs residing in a single dark halo. Both on small (2 ′′ < θ < 3 ′′) and large (40 ′′ < θ < 400 ′′) scales, clustering amplitudes monotonically increase with luminosity for the magnitude range of i ′ = 24.5 − 27.5, and the small-scale clustering shows a stronger luminosity dependence than the large-scale clustering. The small-scale bias reaches b ≃ 10 − 50, and the outskirts of small-scale excess extend to a larger angular separation for brighter LBGs. The ACF and number density of LBGs can be explained by the cold dark matter model. Subject headings: large-scale structure of universe-galaxies: formation-galaxies: high-redshift 1 Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.
The Astrophysical Journal, 2011
We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z ≃ 7 "z-band dropout" candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular we report on two newly discovered galaxies at redshift ∼ 6.7 in the NTT deep field: both galaxies show a Lyα emission line with rest-frame EWs of the order 15-20Å and luminosities of 2-4×10 42 ergs −1. We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular there is a significant lack of objects with intermediate Lyα EWs (between 20 and 55Å). We conclude that the trend for the fraction of Lyα emission in LBGs that is constantly increasing from z∼3 to z∼6 is most probably reversed from z ∼ 6 to z∼7. Explaining the observed rapid change in the LAE fraction among the drop-out population with reionization requires a fast evolution of the neutral fraction of hydrogen in the Universe. Assuming that the Universe is completely ionized at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we find that our data require a change of the neutral hydrogen fraction of the order ∆χ HI ∼ 0.6 in a time ∆z ∼ 1, provided that the escape fraction does not increase dramatically over the same redshift interval. Subject headings: galaxies: distances and redshifts-galaxies: high-redshift-galaxies: formation * We would like to dedicate this paper in memory of Alan Moorwood, who left us a few days before the paper was submitted. Alan was fundamental to the development of Hawk-I, which enabled this survey and many other important observing programs. He had clear foresight of the instrument's impact on the search for the highest redshift galaxies. More importantly, he always urged us to obtain spectroscopic confirmation of the candidates, and he was eagerly awaiting the results of this effort.