What type of objects are being detected as $z\sim 3$ "Lyman break galaxies"?
Are they predominantly the most massive galaxies at that epoch, or are many of
them smaller galaxies undergoing a short-lived burst of merger-induced star
formation? We attempt to address this question using high-resolution
cosmological hydrodynamic simulations including star formation and feedback.
Our $\Lambda$CDM simulation, together with Bruzual-Charlot population synthesis
models, reproduces the observed number density and luminosity function of Lyman
break galaxies when dust is incorporated. The inclusion of dust is crucial for
this agreement. In our simulation, these galaxies are predominantly the most
massive objects at this epoch, and have a significant population of older
stars. Nevertheless, it is possible that our simulations lack the resolution
and requisite physics to produce starbursts, despite having a physical
resolution of $\la 700$ pc at z=3. Thus we cannot rule out merger-induced
starburst galaxies also contributing to the observed population of
high-redshift objects.Comment: 5 pages, contribution to the Proceedings of Rencontres
  Internationales de l'IGRAP, Clustering at High Redshift, Marseille 199

Davé, Romeel

Gardner, Jeffrey P.

Hernquist, Lars

Katz, Neal

Weinberg, David H.

English

arXiv

What type of objects are being detected as $z\sim 3$  Lyman break galaxies ? Are they predominantly the most massive galaxies at that epoch, or are many of them smaller galaxies undergoing a short-lived burst of merger-induced star formation? We attempt to address this question using high-resolution cosmological hydrodynamic simulations including star formation and feedback. Our $\Lambda$CDM simulation, together with Bruzual-Charlot population synthesis models, reproduces the observed number density and luminosity function of Lyman break galaxies when dust is incorporated. The inclusion of dust is crucial for this agreement. In our simulation, these galaxies are predominantly the most massive objects at this epoch, and have a significant population of older stars. Nevertheless, it is possible that our simulations lack the resolution and requisite physics to produce starbursts, despite having a physical resolution of $\la 700$ pc at z=3. Thus we cannot rule out merger-induced starburst galaxies also contributing to the observed population of high-redshift objects

Dave, R

Gardner, J

Hernquist, L

Katz, N

Weinberg, D

ScholarWorks@UMass Amherst

University of Massachusetts AmherstScholarWorks@UMass AmherstAstronomy Department Faculty Publication Series Astronomy1999The Nature of Lyman Break Galaxies inCosmological Hydrodynamic SimulationsR DaveJ GardnerL HernquistN KatzUniversity of Massachusetts - AmherstD WeinbergFollow this and additional works at: https://scholarworks.umass.edu/astro_faculty_pubsPart of the Astrophysics and Astronomy CommonsThis Article is brought to you for free and open access by the Astronomy at ScholarWorks@UMass Amherst. It has been accepted for inclusion inAstronomy Department Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, pleasecontact scholarworks@library.umass.edu.Recommended CitationDave, R; Gardner, J; Hernquist, L; Katz, N; and Weinberg, D, "The Nature of Lyman Break Galaxies in Cosmological HydrodynamicSimulations" (1999). Clustering at High Redshift. 979.Retrieved from https://scholarworks.umass.edu/astro_faculty_pubs/979arXiv:astro-ph/9910220v1  12 Oct 1999The Evolution of Galaxies on Cosmological TimescalesASP Conference Series, Vol. 3× 108, 1999J. E. Beckman, and T. J. Mahoney, eds.The Nature of Lyman Break Galaxies in CosmologicalHydrodynamic SimulationsRomeel Dave´Princeton University Observatory, Princeton, NJ 08544Jeffrey P. GardnerDept. of Astronomy, University of Washington, Seattle, WA 98195Lars HernquistHarvard-Smithsonian Center for Astrophysics, Cambridge, MA, 02138Neal KatzDept. of Astronomy, University of Massachusetts, Amherst, MA, 01003David H. WeinbergDept. of Astronomy, Ohio State University, Columbus, OH 43210Abstract. What type of objects are being detected as z ∼ 3 “Lymanbreak galaxies”? Are they predominantly the most massive galaxies atthat epoch, or are many of them smaller galaxies undergoing a short-lived burst of merger-induced star formation? We attempt to addressthis question using high-resolution cosmological hydrodynamic simula-tions including star formation and feedback. Our ΛCDM simulation,together with Bruzual-Charlot population synthesis models, reproducesthe observed number density and luminosity function of Lyman breakgalaxies when dust is incorporated. The inclusion of dust is crucial forthis agreement. In our simulation, these galaxies are predominantly themost massive objects at this epoch, and have a significant populationof older stars. Nevertheless, it is possible that our simulations lack theresolution and requisite physics to produce starbursts, despite having aphysical resolution of∼< 700 pc at z = 3. Thus we cannot rule out merger-induced starburst galaxies also contributing to the observed populationof high-redshift objects.1. IntroductionThe detection of large numbers of high-redshift galaxies using the Lyman breaktechnique has greatly furthered our understanding of early galaxy formation.A variety of arguments, from clustering[1] to semi-analytic modeling[2] to N-body simulations[3], suggest that these Lyman break galaxies (LBGs) form in12 R. Dave´ et al.highly biased, rare density peaks in the early universe. However, the natureof these galaxies remains controversial. Are they the most massive galaxy con-tained in these peaks, having quiescently formed stars for some time[4]? Orare they smaller galaxies residing in large potential wells that are undergoingan short-lived merger-induced starburst[5]? The key to answering this questionis to determine the mass of the underlying galaxy. This may be done observa-tionally[6] or by modeling processes of galaxy formation[7]. So far, only N-bodyand semi-analytic techniques have been applied, and the results vary, dependingprimarily on what is assumed for merger-induced starbursts. In principle, hy-drodynamic simulations of galaxy formation including star formation, togetherwith population synthesis models, can directly address these questions within agiven cosmology. That is what we investigate in these proceedings.2. Simulation and AnalysisWe simulate a random 11.111h−1Mpc cube in a ΛCDM universe, with Ωm = 0.4,ΩΛ = 0.6, H0 = 65, n = 0.95, and Ωb = 0.02h−2. We use Parallel TreeSPHto advance 1283 gas and 1283 dark matter particles from z = 49 to z = 3.Our spatial resolution is 1.7h−1 comoving kpc (equivalent Plummer softening),implying that at z = 3 our physical resolution is ∼ 640pc. Our mass resolutionis mSPH = 1.3× 107M⊙ and mdark = 1× 108M⊙. Using a 60-particle criterionfor our simulated galaxy completeness limit[8] implies that we are resolving mostgalaxies with Mbaryonic ∼> 8× 108M⊙.We include star formation and thermal feedback[9]. At z = 3, we identifygalaxies using Spline Kernel Interpolative DENMAX (SKID), and compile a listof star formation events in each galaxy. Since gas is gradually converted intostars in each SPH particle, a given particle can have up to 20 star formationevents. We treat each event as an instantaneous single-burst population usingBruzual & Charlot’s GISSEL98[10], assuming a Scalo IMF with Z = 0.4Z⊙1.We sum the spectra for all events in a galaxy to produce its rest-frame spectrumat z = 3. We apply a correction for dust absorption using a galactic extinctionlaw[11] with AV = 1.0. We then redshift the spectra to z = 0 and apply UnGRfilter functions[12] to obtain the observed broad-band colors for our simulatedgalaxy population. Note that no K-correction is necessary since we redshift thespectrum prior to applying the filters.3. The Simulated Lyman Break Galaxy PopulationOur simulation produces 1238 galaxies at z = 3. Figure 1 shows the luminosityfunctions Φ in R (solid histogram), G (dotted line) and Un (dashed line) ofthese galaxies. Note that Φ(Un) is shown without any attenuation due to HIalong the line of sight. The left and right panels show Φ without and withdust, respectively. The turnover above R∼> 28 is likely due to resolution effects,while the lack of galaxies with R∼< 24 is due to our small volume. Between1Using a Salpeter or Miller-Scalo IMF results in more LBGs. However, dust plays a larger rolein determining galaxy properties.The Nature of Lyman Break Galaxies 3these values, our luminosity function (with dust) is in rough agreement withthe observed R-band luminosity function [13], shown as the solid curve down toR = 27 (the current observational limit), although somewhat steeper. In reality,there is probably a range of dust extinctions, and this will tend to flatten Φ.Figure 1: Luminosity function of high-redshift galaxies in Un, G and R, without (leftpanel) and with (right panel) dust.The number of Lyman break galaxies expected for this cosmology and vol-ume is ∼ 7 [1,13], though this number could be higher due to source confu-sion [15]. With dust included, we produce 7 galaxies with R < 25.5, of which6 satisfy the LBG color selection, in reasonable agreement with observations.Without dust, there are 38. Not surprisingly, the number density of simulatedLBGs is highly sensitive to the amount of dust included, and undoubtedly tothe type and distribution of dust as well.Figure 2: (Un + 2)−G vs. G − R of simulated galaxies, without (left panel) and with(right panel) dust. Triangles have R < 25.5, dots have R > 25.5.Color selection is at the heart of the Lyman break technique. In Figure 2we show Un−G vs. G−R plots of our simulated galaxies, with an arbitrary twomagnitudes of extinction added to Un to crudely mimic intervening HI absorp-tion. Triangles represent galaxies with R < 25.5, and dots are the remaininggalaxies. The Lyman break color selection is up and to the left of the dashedboundary. Left and right panels show without and with dust, respectively. Dustmoves galaxies to higher G − R, and somewhat higher Un − G. Most galaxiesat z = 3 fall within the color selection, but significantly more dust would movethe bright galaxies outside the G−R < 1.2 criterion.4 R. Dave´ et al.Figure 3: Stellar mass vs. R, without dust (left panel) and with dust (right panel).We now investigate the mass of simulated LBGs. Figure 3 shows the stellarmass vs. R-band magnitude. The horizontal line demarcates R = 25.5, themagnitude limit of the observed LBG sample. While there is some scatter, theclear trend is that the brightest objects are also the most massive ones. Thescatter increases to smaller masses, and is slightly larger in G and Un, but oursimulations indicate that LBGs are the most massive galaxies at z = 3.Figure 4: Star formation rate per unit stellar mass as a function of time (age of theuniverse), in three different R-band magnitude ranges.Figure 4 shows the evolution of the star formation rate per solar mass ofstars for three galaxy samples: R < 25.5 (left panel), 25.5 < R < 27.5 (middlepanel), and 27.5 < R < 29.5 (right panel). The brightest galaxies have beenforming stars the longest, typically for over a Gyr by z = 3. Thus they contain asignificant older stellar population. Fainter (and smaller) galaxies have formedthe bulk of their stars more recently.4. ConclusionsOur simulation roughly reproduces the number density and luminosity functionof LBGs for a reasonable value of dust extinction. It suggests that LBGs are themost massive objects at z ∼ 3, and that they contain a significant older stellarpopulation.While this simulation puts forth a consistent picture for the nature ofLBGs, we cannot rule out the aforementioned alternative scenario that LBGsare smaller starbursting galaxies. The reason is that starburst regions are typi-cally a few hundred parsecs across, and therefore below our resolution. The starformation rate in our simulations is tied primarily to the local density (using aThe Nature of Lyman Break Galaxies 5Schmidt Law) which is limited by resolution. Thus we do not effectively mimic“starbursts” as would occur in a much higher resolution merger simulation[14].Conversely, some semi-analytic models insert such starburst behavior explicitly,so it is not surprising that they obtain different results.At present, it is not feasible to run simulations of sufficient resolution toresolve starbursts while still modeling a random cosmological volume. Further-more, starbursts are likely to be governed by many other physical processesthat we are only crudely modeling at present, such as feedback and ionization.Thus we cannot rule out the possibility that smaller starbursting systems alsocontribute to the observed Lyman break galaxy population. Nevertheless, oursimulation with reasonable physical parameters is able to reproduce the basicobserved properties of this population without including such objects.Acknowledgments. Thanks to Kurt Adelberger for the UnGR filter re-sponse functions. Thanks to Stephane Charlot for GISSEL98. Thanks to RachelSomerville and Tsafrir Kolatt for helpful comments. The simulation was run onthe NCSA Origin 2000.References[1] Adelberger, K. L., Steidel, C. C., Giavalisco, M., Dickinson, M., Pettini, M.,& Kellogg, M. 1998, ApJ, 505, 18[2] Baugh, C. M., Cole, S., Frenk, C. S., & Lacey, C. G. 1998, ApJ, 498, 504[3] Wechsler, R. H., Gross, M., Primack, J. R., Blumenthal, G. R., & Dekel, A.1998, ApJ, 506, 19[4] Steidel, C. C., Adelberger, K. L., Dickinson, M., Giavalisco, M., Pettini, M.,& Kellogg, M. 1998, ApJ, 492, 428[5] Lowenthal, J. D., Koo, D., Gu´zman, R., Gallego, J., Phillips, A. C., Faber,S. M., Vogt, N., Illingworth, G. D., & Gronwall, C. 1997, ApJ, 481, 673[6] Pettini, M., Kellogg, M., Steidel, C. C., Dickinson, M., Adelberger, K. L., &Giavalisco, M. 1998, ApJ, 508, 539[7] Kolatt, T. S., Bullock, J. S., Somerville, R. S., Sigad, Y., Jonsson, P.,Kravtsov, A. V., Klypin, A. A., Primack, J. R., Faber, S. M., Dekel,A. 1999, ApJ, 523, L109[8] Weinberg, D. H., Dave´, R., Gardner, J. P., Hernquist, L., & Katz, N. 1999 in”Photometric Redshifts and High Redshift Galaxies”, eds. R. Weymann,L. Storrie-Lombardi, M. Sawicki & R. Brunner, (SF: ASP Conf Series)[9] Katz, N., Weinberg D.H., & Hernquist, L. 1996, ApJS, 105, 19[10] Bruzual, G. A., & Charlot, S. 1993, ApJ, 405, 538[11] Cardelli, J. A., Clayton, G. C., & Mathis, J. 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The Nature of Lyman Break Galaxies in Cosmological Hydrodynamic Simulations

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