On the Dearth of Ultra-faint Extremely Metal-poor Galaxies

Astronomy & Astrophysics, 2004

We have derived element abundances in 310 emission-line galaxies from the Early Data Release of the Sloan Digital Sky Survey (SDSS) for which the [O ] λ4363 emission line was detected, allowing abundance determination by direct methods. We found no extremely metal-deficient galaxy (Z < ∼ Z /12) , probably as a consequence of selection effects in the SDSS sample. The oxygen abundance 12 + log O/H of the SDSS galaxies sample lies in the range from ∼7.6 (Z /12) to ∼8.4 (Z /2). This sample is merged with a sample of ∼100 blue compact dwarf galaxies with high quality spectra containing some very low-metallicity objects to study the abundance patterns of low-metallicity emission-line galaxies. We find that the α elementto-oxygen abundance ratios do not show any significant trends with the oxygen abundance, in agreement with previous studies. The Fe/O abundance ratio is smaller than the solar value, which we interpret as an indication that type Ia supernovae have not yet appeared in these galaxies, implying an age of less than 1−2 Gyr. However, a slight decrease of the Fe/O abundance ratio with increasing metallicity suggests some depletion of iron onto dust in the galaxies with higher metallicities. The N/O abundance ratio ranges from log N/O =-1.6 to −0.8. The fact that no galaxy with log N/O < ∼ −1.6 was discovered implies that local low-metallicity emission-line galaxies are of a different nature than high-redshift damped Lyα systems with log N/O of ∼-2.3 and that their ages are probably larger than 100-300 Myr. Our data indicate the existence of a gradual nitrogen enrichment on a timescale of a few Myr.

Astronomical Journal, 2008

We demonstrate a successful strategy for identifying extremely metal poor galaxies. Our preliminary survey of 24 candidates contains 10 metal poor galaxies of which 4 have 12+log(O/H)<7.65, some of the lowest metallicity blue compact galaxies known to date. Interestingly, our sample of metal poor galaxies have systematically lower metallicity for their luminosity than comparable samples of blue compact galaxies, dIrrs, and normal star-forming galaxies. Our metal poor galaxies share very similar properties, however, with the host galaxies of nearby long-duration gamma-ray bursts (GRBs), including similar metallicity, stellar ages, and star formation rates. We use H\beta to measure the number of OB stars present in our galaxies and estimate a core-collapse supernova rate of ~10^-3 yr^-1. A larger sample of metal poor galaxies may provide new clues into the environment where GRBs form and may provide a list of potential GRB hosts.

Astronomical Journal, 2004

We present follow-up spectra of 39 emission-line galaxies (ELGs) from the KPNO International Spectroscopic Survey (KISS). Many targets were selected as potentially low-metallicity systems based on their absolute B magnitudes and the metallicity-luminosity relation. The spectra, obtained with the Lick 3 m telescope, cover the full optical region from [O ii] kk3726, 3729 to beyond [S ii] kk6717, 6731 and include measurement of [O iii] k4363 in 12 objects. The spectra are presented and tables of the strong line ratios are given. For 12 high signal-to-noise ratio spectra, we determine abundance ratios of oxygen, nitrogen, neon, sulfur, and argon. We find these galaxies to be metal deficient with three systems approaching O/H of 1/25th solar. We compare the abundance results from the temperature-based T e method to the results from the strong-line p 3 method of Pilyguin.

Astronomy and Astrophysics Review, 2000

Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial rôle in the cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields, outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metalpoor galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and the physical processes that are at work in these objects. The question on how they are related and may evolve from one class of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe, we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in the universe. We discuss what QSO absorption lines and known distant galaxies tell us already? We illustrate the importance of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator and discuss the possibility to detect nearly metal-free galaxies at high redshift from Lyα emission.

Mon Notic Roy Astron Soc, 2005

We use a sample of submillimetre-selected galaxies (SMGs) with molecular gas and dynamical mass measurements from the literature to put constraints on the contribution of such galaxies to the total metal budget. Compared to Lyman break galaxies (LBGs), for example, SMGs are rarer (by a factor of 10 or more), but contain much more gas and are more metal rich. We estimate that SMGs brighter than 3 mJy contain only less than 9% of the metals when we combine the observed dynamical masses (few$\times 10^{11}$ \msun), number density ($n\simeq 10^{-4}$ Mpc$^{-3}$), observed gas metallicity (1--2 x solar), and observed gas fractions (~40%) assuming a molecular to neutral hydrogen ratio of 1. Including SMGs fainter than 3 mJy, we estimate that SMGs contain at the most 15% of the metals, where our incompleteness correction is estimated from the dust mass function. Our results are strong upper limits given that high gas fractions and high overall metallicity are mutually exclusive. In summary, SMGs make a significant contribution to the metal budget (< 15%) but not sufficient to solve the `missing metals problem.' A consequence of our results is that SMGs can only add $\approx 3.5$% to $\Omega_{\rm DLA}$, and can not be the source of a significant population of dusty DLAs.

Astronomy and Astrophysics, 2002

We present new results from optical spectroscopy of the brightest Hii region in the dwarf irregular galaxy UKS 1927-177 in Sagittarius (SagDIG). From high signal-to-noise spectra, reddening-corrected line flux ratios have been measured with typical uncertainties of a few percent, from which the oxygen abundance is rediscussed, and new abundance estimates are derived for N and Ne. The O abundance in SagDIG, estimated with the empirical abundance indicator R23 and other methods, is in the range 12 + log(O/H) = 7.26 to 7.50. The fact that SagDIG is ∼ 10 times closer than I Zw 18 makes it an ideal target to test the hypothesis of the existence of young galaxies in the present-day universe. Indeed, stellar photometry suggests that this galaxy may harbor a stellar population older than a few Gyr, and possibly an old stellar component as well. The case of SagDIG therefore supports the view that very low chemical abundances can be maintained throughout the life of a dwarf stellar system, even in the presence of multiple star formation episodes.

Near-infrared colors have been measured for a sample of 31 late-type galaxies in the Pegasus I and Pisces clusters; system luminosities in the sample cover the range — 19 < M H < — 23.5. The color index (J — K) correlates strongly with the absoluteHmagnitude; lower-luminosity systems have bluer colors. These observations are consistent with the assumption that the mean metal abundance of the old disk population decreases systematically with luminosity. The systematic variation oî(B — H) with absolute H magnitude reported recently by Tully et al derives in part from this proposed systematic change of metallicity with luminosity. However, one must still posit a relative increase in the number of newly formed stars and/or a systematic smaller age for lower-luminosity disks in order to fully explain the observed (B — H), H relation.

Astronomy &amp; Astrophysics, 2006

We have re-evaluated empirical expressions for the abundance determination of N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and constructing an appropriate grid of photoionization models with state-of-the art model atmospheres. Using these expressions we have derived heavy element abundances in the ∼310 emission-line galaxies from the Data Release 3 of the Sloan Digital Sky Survey (SDSS) with an observed Hβ flux F(Hβ) > 10 -14 erg s -1 cm -2 and for which the [O iii] λ4363 emission line was detected at least at a 2σ level, allowing abundance determination by direct methods. The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ∼7.1 (Z /30) to ∼8.5 (0.7 Z ). The SDSS sample is merged with a sample of 109 blue compact dwarf (BCD) galaxies with high quality spectra, which contains extremely low-metallicity objects. We use the merged sample to study the abundance patterns of low-metallicity emission-line galaxies. We find that extremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Z /12) are rare in the SDSS sample. The α element-to-oxygen abundance ratios do not show any significant trends with oxygen abundance, in agreement with previous studies, except for a slight increase of Ne/O with increasing metallicity, which we interpret as due to a moderate depletion of O onto grains in the most metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value, by up to 1 dex at the high metallicity end. We also find that Fe/O increases with decreasing Hβ equivalent width EW(Hβ). We interpret this as a sign of strong depletion onto dust grains, and gradual destruction of those grains on a time scale of a few Myr. All the galaxies are found to have log N/O > -1.6, implying that they have a different nature than the subsample of high-redshift damped Lyα systems with log N/O of ∼-2.3 and that their ages are larger than 100-300 Myr. We confirm the apparent increase in N/O with decreasing EW(Hβ), already shown in previous studies, and explain it as the signature of gradual nitrogen ejection by massive stars from the most recent starburst.

We report the discovery of eight new extremely metal-poor galaxies (XMPGs; 12+log(O/H) < 7.65) and the recovery of four previously known or suspected XMPGs (I Zw 18, HS 0822+3542, HS 0837+4717 and A1116+517) using Sloan Digital Sky Survey (SDSS) spectroscopy. These new objects were identified after an analysis of 250,000 galaxy spectra within an area of ∼3000 deg 2 on the sky. Our oxygen abundance determinations have an accuracy of ≤ 0.1 dex and are based on the temperature-sensitive [O iii] λ4363Å line and on the direct calculation of the electron temperature. We briefly discuss a new method of oxygen abundance determinations using the [O ii] λ7319,7330Å lines, which is particularly useful for SDSS emission-line spectra with redshifts ≤ 0.024 since the [O ii] λ3727Å emission line falls outside of the SDSS wavelength range. We detect XMPGs with redshifts ranging from 0.0005 to 0.0443 and M g luminosities from −12. m 4 to −18. m 6. Our eight new XMPGs increase the number of known metaldeficient galaxies by approximately one quarter. The estimated surface density of XMPGs is 0.004 deg −2 for r ≤ 17. m 77.

Astronomy and Astrophysics, 2004

The emission line survey within the Calar Alto Deep Imaging Survey (CADIS) detects galaxies with very low continuum brightness by using an imaging Fabry-Perot interferometer. With spectroscopic follow-up observations of MB > ∼ −19 CADIS galaxies using FORS2 at the VLT and DOLORES at TNG we obtained oxygen abundances of 5 galaxies at z ∼ 0.4 and 10 galaxies at z ∼ 0.64. Combining these measurements with published oxygen abundances of galaxies with MB < ∼ −19 we find evidence that a metallicity-luminosity relation exists at medium redshift, but it is displaced to lower abundances and higher luminosities compared to the metallicityluminosity relation in the local universe. Comparing the observed metallicities and luminosities of galaxies at z < ∼ 3 with Pégase2 chemical evolution models we have found a favoured scenario in which the metallicity of galaxies increases by a factor of ∼ 2 between z ∼ 0.7 and today, and their luminosity decreases by ∼ 0.5 − 0.9 mag.