Starburst Intensity Limit of Galaxies at z~5-6

The peak star formation intensity in starburst galaxies does not vary significantly from the local universe to redshift z~6. We arrive at this conclusion through new surface brightness measurements of 47 starburst galaxies at z~5-6, doubling the redshift range for such observations. These galaxies are spectroscopically confirmed in the Hubble Ultra Deep Field (HUDF) through the GRism ACS program for Extragalactic Science (GRAPES) project. The starburst intensity limit for galaxies at z~5-6 agree with those at z~3-4 and z~0 to within a factor of a few, after correcting for cosmological surface brightness dimming and for dust. The most natural interpretation of this constancy over cosmic time is that the same physical mechanisms limit starburst intensity at all redshifts up to z~6 (be they galactic winds, gravitational instability, or something else). We do see two trends with redshift: First, the UV spectral slope of galaxies at z~5-6 is bluer than that of z~3 galaxies, suggesting an increase in dust content over time. Second, the galaxy sizes from z~3 to z~6 scale approximately as the Hubble parameter 1/H(z). Thus, galaxies at z~6 are high redshift starbursts, much like their local analogs except for slightly bluer colors, smaller physical sizes, and correspondingly lower overall luminosities. If we now assume a constant maximum star formation intensity, the differences in observed surface brightness between z~0 and z~6 are consistent with standard expanding cosmology and strongly inconsistent with tired light model.Comment: Accepted for publication in ApJ (23 pages, 5 figures). Minor changes to tex

Dynamics of ``Small Galaxies'' in the Hubble Deep Field

We have previously found in the Hubble Deep Field a significant angular correlation of faint, high color-redshift objects on scales below one arcsecond, or several kiloparsecs in metric size. We examine the correlation and nearest neighbor statistics to conclude that 38% of these objects in the HDF have a companion within one arcsecond, three times the number expected in a random distribution with the same number of objects. We examine three dynamical scenarios for these object multiplets: 1) the objects are star-forming regions within normal galaxies, whose disks have been relatively dimmed by K-correction and surface brightness dimming; 2) they are fragments merging into large galaxies; 3) they are satellites accreting onto normal L_* galaxies. We find that hypothesis 1 is most tenable. First, large galaxies in the process of a merger formation would have accumulated too much mass in their centers (5e12 M_sun inside 2 kpc) to correspond to present day objects. Second, accretion by dynamical friction occurs with a predictable density vs. radius slope, not seen among the faint HDF objects. Since the dynamical friction time is roughly (1 Gyr), a steady-state should have been reached by redshift z < 5. Star-forming regions within galaxies clearly present no dynamical problems. Since large spirals would still appear as such in the HDF, we favor a scenario in which the faint compact sources in the HDF are giant starforming regions within small normal galaxies, such as Magellanic irregulars. Finally we checked that reduction in mass-to-light from induced star-formation cannot alone explain the luminosity overdensity.Comment: AASTeX 4.0 (preprint), 4 PostScript figure

Clustering of Lyman alpha emitters at z ~ 4.5

We present the clustering properties of 151 Lyman alpha emitting galaxies at z ~ 4.5 selected from the Large Area Lyman Alpha (LALA) survey. Our catalog covers an area of 36' x 36' observed with five narrowband filters. We assume that the angular correlation function w(theta) is well represented by a power law A_w = Theta^(-beta) with slope beta = 0.8, and we find A_w = 6.73 +/- 1.80. We then calculate the correlation length r_0 of the real-space two-point correlation function xi(r) = (r/r_0)^(-1.8) from A_w through the Limber transformation, assuming a flat, Lambda-dominated universe. Neglecting contamination, we find r_0 = 3.20 +/- 0.42 Mpc/h. Taking into account a possible 28% contamination by randomly distributed sources, we find r_0 = 4.61 +/- 0.6 Mpc/h. We compare these results with the expectations for the clustering of dark matter halos at this redshift in a Cold Dark Matter model, and find that the measured clustering strength can be reproduced if these objects reside in halos with a minimum mass of 1-2 times 10^11 Solar masses/h. Our estimated correlation length implies a bias of b ~ 3.7, similar to that of Lyman-break galaxies (LBG) at z ~ 3.8-4.9. However, Lyman alpha emitters are a factor of ~ 2-16 rarer than LBGs with a similar bias value and implied host halo mass. Therefore, one plausible scenario seems to be that Lyman alpha emitters occupy host halos of roughly the same mass as LBGs, but shine with a relatively low duty cycle of 6-50%.Comment: 23 pages in preprint format, 4 figures, ApJ accepte

Detecting Primordial Stars

We discuss the expected properties of the first stellar generations in the Universe. We find that it is possible to discern truly primordial populations from the next generation of stars by measuring the metallicity of high-z star forming objects. The very low background of the future James Webb Space Telescope (JWST) will enable it to image and study first-light sources at very high redshifts, whereas its relatively small collecting area limits its capability in obtaining spectra of z~10-15 first-light sources to either the bright end of their luminosity function or to strongly lensed sources. With a suitable investment of observing time JWST will be able to detect individual Population III supernovae, thus identifying the very first stars that formed in the Universe.Comment: [8 pages, 5 figures] Invited Talk, to appear in IMF@50: The Stellar Initial Mass Function Fifty Years Later, eds E. Corbelli, F. Palla, and H. Zinnecker (Dordrecht: Kluwer

The Dynamical Masses, Densities, and Star Formation Scaling Relations of Lyman Alpha Galaxies

We present the first dynamical mass measurements for Lyman alpha galaxies at high redshift, based on velocity dispersion measurements from rest-frame optical emission lines and size measurements from HST imaging, for a sample of nine galaxies drawn from four surveys. These measurements enable us to study the nature of Lyman alpha galaxies in the context of galaxy scaling relations. The resulting dynamical masses range from 1e9 to 1e10 solar masses. We also fit stellar population models to our sample, and use them to plot the Lyman alpha sample on a stellar mass vs. line width relation. Overall, the Lyman alpha galaxies follow well the scaling relation established by observing star forming galaxies at lower redshift (and without regard for Lyman alpha emission), though in 1/3 of the Lyman alpha galaxies, lower-mass fits are also acceptable. In all cases, the dynamical masses agree with established stellarmass-linewidth relation. Using the dynamical masses as an upper limit on gas mass, we show that Lyman alpha galaxies resemble starbursts (rather than "normal" galaxies) in the relation between gas mass surface density and star formation activity, in spite of relatively modest star formation rates. Finally, we examine the mass densities of these galaxies, and show that their future evolution likely requires dissipational ("wet") merging. In short, we find that Lyman alpha galaxies are low mass cousins of larger starbursts.Comment: Submitted to The Astrophysical Journal. 23 pp including three figures and four table

Variable polarization in the optical afterglow of GRB 021004

We present polarimetric observations of the afterglow of gamma-ray burst (GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among the observations shows a 45 degree change in the position angle from 9 hours after the burst to 16 hours after the burst, and comparison with published data from later epochs even shows a 90 degree change between 9 and 89 hours after the burst. The degree of linear polarization shows a marginal change, but is also consistent with being constant in time. In the context of currently available models for changes in the polarization of GRBs, a homogeneous jet with an early break time of t_b ~ 1 day provides a good explanation of our data. The break time is a factor 2 to 6 earlier than has been found from the analysis of the optical light curve. The change in the position angle of the polarization rules out a structured jet model for the GRB.Comment: 5 pages, 2 figures. Published in A&A letter

First Results from the Large Area Lyman Alpha Survey

We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching known high redshift Lyman alpha galaxies (i.e., observed equivalent width above 80 Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in an 80 Angstrom filter) have an observed surface density corresponding to 11000 +- 700 per square degree per unit redshift at z=4.5. Spatial variation in this surface density is apparent on comparison between counts in 6561 and 6730 Angstrom filters. Early spectroscopic followup results from the Keck telescope included three sources meeting our criteria for good Lyman alpha candidates. Of these, one is confirmed as a z=4.52 source, while another remains consistent with either z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies per square degree per unit redshift.Comment: 10 pages, 2 figures (3 .ps files), uses AASTeX 4. Submitted to The Astrophysical Journal Letter

How to Tell a Jet from a Balloon: A Proposed Test for Beaming in Gamma Ray Bursts

If gamma ray bursts are highly collimated, the energy requirements of each event may be reduced by several (~ 4-6) orders of magnitude, and the event rate increased correspondingly. Extreme conditions in gamma ray bursters lead to highly relativistic motions (bulk Lorentz factors Gamma > 100). This results in strong forward beaming of the emitted radiation in the observer's rest frame. Thus, all information on gamma ray bursts comes from those ejecta emitted in a narrow cone (opening angle 1/Gamma) pointing towards the observer. We are at present ignorant of whether there are ejecta outside that cone or not. The recent detection of longer wavelength transients following gamma ray bursts allows an empirical test of whether gamma ray bursts are collimated jets or spherical fireballs. The bulk Lorentz factor of the burst ejecta will decrease with time after the event, as the ejecta sweep up the surrounding medium. Thus, radiation from the ejecta is beamed into an ever increasing solid angle as the burst remnant evolves. It follows that if gamma ray bursts are highly collimated, many more optical and radio transients should be observed without associated gamma rays than with them. Published supernova searches may contain enough data to test the most extreme models of gamma ray beaming. We close with a brief discussion of other possible consequences of beaming, including its effect on the evolution of burst remnants.Comment: Original replaced with accepted refereed manuscript. 11 pages, uses AASTeX 4.0 LaTeX macros. To be published in The Astrophysical Journal Letters, vol. 487, p. L1 (20 September 1997

Probing the Reionization History of the Universe using the Cosmic Microwave Background Polarization

The recent discovery of a Gunn--Peterson (GP) trough in the spectrum of the redshift 6.28 SDSS quasar has raised the tantalizing possibility that we have detected the reionization of the universe. However, a neutral fraction (of hydrogen) as small as 0.1% is sufficient to cause the GP trough, hence its detection alone cannot rule out reionization at a much earlier epoch. The Cosmic Microwave Background (CMB) polarization anisotropy offers an alternative way to explore the dark age of the universe. We show that for most models constrained by the current CMB data and by the discovery of a GP trough (showing that reionization occurred at z > 6.3), MAP can detect the reionization signature in the polarization power spectrum. The expected 1-sigma error on the measurement of the electron optical depth is around 0.03 with a weak dependence on the value of that optical depth. Such a constraint on the optical depth will allow MAP to achieve a 1-sigma error on the amplitude of the primordial power spectrum of 6%. MAP with two years (Planck with one year) of observation can distinguish a model with 50% (6%) partial ionization between redshifts of 6.3 and 20 from a model in which hydrogen was completely neutral at redshifts greater than 6.3. Planck will be able to distinguish between different reionization histories even when they imply the same optical depth to electron scattering for the CMB photons.Comment: ApJ version. Added Figure 2 and reference

Spectroscopic Confirmation of Three Redshift 5.7 Lyman-alpha Emitters from the Large Area Lyman Alpha Survey

Narrow-band searches for Lyman alpha emission are an efficient way of identifying star-forming galaxies at high redshifts. We present Keck telescope spectra confirming redshifts z = 5.7 for three objects discovered in the Large Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory. All three spectra show strong, narrow emission lines with the asymmetric profile that is characteristically produced in high redshift Lyman alpha emitters by preferential HI absorption in the blue wing of the line. These objects are undetected in deep Bw, V, R, and 6600A narrow-band images from the NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and Lyman alpha forest absorption at redshift z = 5.7. All three objects show large equivalent widths (>= 150A in the rest-frame), suggesting at least one of the following: a top-heavy initial mass function, very low stellar metallicity, or the presence of an active nucleus. We consider the case for an active nucleus to be weak in all three objects due to the limited width of the Lyman alpha emission line (< 500 km/s) and the absence of any other indicator of quasar activity. The three confirmed high redshift objects were among four spectroscopically observed targets drawn from the sample of 18 candidates presented by Rhoads and Malhotra (2001). Thus, these spectra support the Lyman alpha emitter population statistics from our earlier photometric study, which imply little evolution in number density from z=5.7 to z=4.5 and provide strong evidence that the reionization redshift is greater than 5.7.Comment: Submitted to AJ, June 2002. 15 pages, AASTe