Star Formation in the Hubble Deep Field North

I am currently analyzing the emission line spectra of the approximately 600 galaxies from the sample of Cohen et al. (2000) and Cohen (2001) in the region of the HDF-North with z<1.5. A progress report on this effort of the Caltech Faint Galaxy Redshift Survey is presented.Comment: For the proceedings of the meeting held in Cozumel, March 2002 on Galaxy Evolution: Theory and Observations, to be edited by V. Avila-Reese, C. Firmani, C. Frenk, & C. Allen, RevMexAA SC (2002

Magnesium Isotopes in Halo Stars

We have determined Mg isotope ratios in halo field dwarfs and giants in the globular cluster M71 based on high S/N high spectral resolution (R = 10$^5$) Keck HIRES spectra. Unlike previous claims of an important contribution from intermediate-mass AGB stars to the Galactic halo, we find that our $^{26}$Mg/$^{24}$Mg ratios can be explained by massive stars.Comment: Proceedings, First Stars III, July 16-20, 2007, Santa F

The Chemical Evolution of the Draco Dwarf Spheroidal Galaxy

We present an abundance analysis based on high resolution spectra of 8 stars selected to span the full range in metallicity in the Draco dwarf spheroidal galaxy. We find [Fe/H] for the sample stars ranges from -1.5 to -3.0 dex. Combining our sample with previously published work for a total of 14 luminous Draco giants, we show that the abundance ratios [Na/Fe], [Mg/Fe] and [Si/Fe] for the Draco giants overlap those of Galactic halo giants at the lowest [Fe/H] probed, but are significantly lower for the higher Fe-metallicity Draco stars. For the explosive alpha-elements Ca and Ti, the abundance ratios for Draco giants with [Fe/H] > -2.4 dex are approximately constant and slightly sub-solar, well below values characteristic of Galactic halo stars. The s-process contribution to the production of heavy elements begins at significantly lower Fe-metallicity than in the Galactic halo. Using a toy model we compare the behavior of the abundance ratios within the sample of Draco giants with those from the literature of Galactic globular clusters, and the Carina and Sgr dSph galaxies. The differences appear to be related to the timescale for buildup of the heavy elements, with Draco having the slowest rate. We note the presence of a Draco giant with [Fe/H] < -3.0 dex in our sample, and reaffirm that the inner Galactic halo could have been formed by early accretion of Galactic satellite galaxies and dissolution of young globular clusters, while the outer halo could have formed from those satellite galaxies accreted later.Comment: Submitted to ApJ Dec 22, 2008; accepted June 4, 2009. 75 pages including 22 figures and 9 table

Detailed Abundances of Two Very Metal-Poor Stars in Dwarf Galaxies

The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the nucleosynthetic patterns of one or a few supernovae (SNe). These SNe could have zero metallicity, making metal-poor dSph stars the closest surviving links to Population III stars. Metal-poor dSph stars also help to reveal the formation mechanism of the Milky Way (MW) halo. We present the detailed abundances from Keck/HIRES spectroscopy for two very metal-poor stars in two MW dSphs. One star, in the Sculptor dSph, has [Fe I/H] = -2.40. The other star, in the Ursa Minor dSph, has [Fe I/H] = -3.16. Both stars fall in the previously discovered low-metallicity, high-[α/Fe] plateau. Most abundance ratios of very metal-poor stars in these two dSphs are largely consistent with very metal-poor halo stars. However, the abundances of Na and some r-process elements lie at the lower end of the envelope defined by inner halo stars of similar metallicity. We propose that the metallicity dependence of SN yields is the cause. The earliest SNe in low-mass dSphs have less gas to pollute than the earliest SNe in massive halo progenitors. As a result, dSph stars at –3 < [Fe/H] < –2 sample SNe with [Fe/H] Lt –3, whereas halo stars in the same metallicity range sample SNe with [Fe/H] ~ –3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo

Globular Cluster Abundances and What They Can Tell Us About Galaxy Formation

We review the properties of globular clusters which make them useful for studying the Galactic halo, Galactic chemical evolution, and the early stages of the formation of the Milky Way. We review the evidence that GCs have a chemical inventory similar to those of halo field stars. We discuss the abundance ratios for dSph galaxies and show that it is possible to have formed at least part the Galactic halo field stellar population by dissolving globular clusters and/or accreting dSph galaxies but only if this occurred at an early stage in the formation of the Galaxy. We review the constraints on halo formation timescales deduced from the low Mg isotopic ratios in metal-poor halo field dwarfs which indicate that AGB stars did not have time to contribute significantly, while M71 contains two populations, one without and also one with a substantial AGB contribution. We review the limited evidence for GCs with a second population showing additional contributions from SNII, currently confined to Omega Cen, M54, and M22, all of which may have been the nuclei or central regions of accreted galaxies. We check our own data for additional such GCs, and find preliminary indications that NGC 2419, a massive GC far in the outer Galactic halo, may also belong to this group.Comment: Invited Talk: IAU Symp. 266, Star Clusters - Basic Building Blocks Throughout Time and Space, proceedings to be published by Cambridge University Pres

Nova shells. II - Calibration of the distance scale using novae

Eight new spatially resolved nova shells have been found by imaging with a digital detector through a narrow Hα filter, and two old novae have been recovered. The 11 novae with the best determined maximum luminosities at outburst of the sample of 21 novae with reliable distances are used to derive a M_v(max)-rate of decline relationship. These 11 objects have a mean M_v 15 days past maximum of —5.60 ± 0.45 mag. If I assume that M_v(15) is in fact constant, as is suggested by the theory, then for the full sample of 21 objects I find M_v(max, corr) = —10.70( ±0.30) + 2.41( ±0.23) log (t_2), where t_2 is the time in days to decline 2 mag below maximum light. Having removed all obvious sources of observational error, I deduce that the dispersion in nova outburst luminosity for a fixed rate of decline is 0.45 mag. Previously published surveys of novae in 31 are used to redetermine the distance to that galaxy