Introduction to the special issue : From RIDGE to Ridge 2000

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 12–17, doi:10.5670/oceanog.2012.01.Articles in this special issue of Oceanography represent a compendium of research that spans the disciplinary and thematic breadth of the National Science Foundation's Ridge 2000 Program, as well as its geographic focal points. The mid-ocean ridge (MOR) crest is where much of Earth's volcanism is focused and where most submarine volcanic activity occurs. If we could look down from space at our planet with the ocean drained, the MOR's topography and shape, along with its intervening fracture zones, would resemble the seams on a baseball, with the ocean basins dominating our planetary panorama. The volcanic seafloor is hidden beneath the green-blue waters of the world's ocean, yet therein lie fundamental clues to how our planet works and has evolved over billions of years, something that was not clearly understood 65 years ago—witness the following quote from H.H. Hess (1962) in his essay on "geopoetry" and commentary on J.H.F. Umbgrove's (1947) comprehensive summary of Earth and ocean history: The birth of the oceans is a matter of conjecture, the subsequent history is obscure, and the present structure is just beginning to be understood. Fascinating speculation on these subjects has been plentiful, but not much of it predating the last decade [the 1950s] holds water.This special issue was funded by a supplement to the Ridge 2000 Office grant at the Woods Hole Oceanographic Institution (NSF-OCE-0838923)

Remarks on simple interpolation between Jordanian twists

In this paper, we propose a simple generalization of the locally r-symmetric Jordanian twist, resulting in the one-parameter family of Jordanian twists. All the proposed twists differ by the coboundary twists and produce the same Jordanian deformation of the corresponding Lie algebra. They all provide the $\kappa$-Minkowski spacetime commutation relations. Constructions from noncommutative coordinates to the star product and coproduct, and from the star product to the coproduct and the twist are presented. The corresponding twist in the Hopf algebroid approach is given. Our results are presented symbolically by a diagram relating all of the possible constructions.Comment: 12 page

The ACS LCID project. X. The Star Formation History of IC 1613: Revisiting the Over-Cooling Problem

We present an analysis of the star formation history (SFH) of a field near the half light radius in the Local Group dwarf irregular galaxy IC 1613 based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our observations reach the oldest main sequence turn-off, allowing a time resolution at the oldest ages of ~1 Gyr. Our analysis shows that the SFH of the observed field in IC 1613 is consistent with being constant over the entire lifetime of the galaxy. These observations rule out an early dominant episode of star formation in IC 1613. We compare the SFH of IC 1613 with expectations from cosmological models. Since most of the mass is in place at early times for low mass halos, a naive expectation is that most of the star formation should have taken place at early times. Models in which star formation follows mass accretion result in too many stars formed early and gas mass fractions which are too low today (the "over-cooling problem"). The depth of the present photometry of IC 1613 shows that, at a resolution of ~1 Gyr, the star formation rate is consistent with being constant, at even the earliest times, which is difficult to achieve in models where star formation follows mass assembly.Comment: 13 pages, 12 figures, accepted for publication in the Ap

The Progenitors of Dwarf Spheroidal Galaxies

Dwarf spheroidal (dSph) galaxies present an evolutionary puzzle that we explore in 40 early- and late-type dwarfs in the Local Group and nearby field. Although dSphs formed stars over extended periods, today all but one are free of detectable interstellar matter (ISM), even in the Fornax dSph, where stars still formed 100 Myr ago. Combining metallicities for red giants with HI data from the literature, we show that the well-known offset in luminosity-metallicity (L-Z) relations for dSphs and dwarf irregular (dIrr) galaxies exists also when comparing only their old stellar populations: dSphs have higher mean stellar metallicities for a fixed luminosity. Evidently younger dSphs experienced more efficient enrichment than young dIrrs. Dwarf galaxies, whose locus in the L-Z diagram is consistent with that of dSphs even for baryonic luminosities, are the ``transition-type dwarfs'' Phoenix, DDO210, LGS3, Antlia, and KKR25. They have mixed dIrr/dSph morphologies, low stellar masses, low angular momentum, and HI contents of less than a few 10^6 solar masses. Unlike dIrrs, many transition-type dwarfs would closely resemble dSphs if their gas were removed; they are likely dSph progenitors. As gas removal is key, we consider the empirical evidence for various gas removal processes. We suggest that internal gas removal mechanisms are inadequate and favor ram pressure stripping to make dSphs. A combination of initial conditions and environment seems to support the formation of dSphs, which appear to form from small galaxies with active early star formation, whose evolution halts due to externally induced gas loss. Transition-type dwarfs then are dSphs that kept their ISM, and therefore should replace dSphs in isolated locations where stripping is ineffective. (Abridged)Comment: 25 pages in AASTeX two-column preprint style, 1 table, 3 figures. Accepted for publication in the Astronomical Journal (April 2003 issue

Chemical abundance ratios of galactic globular clusters from modelling integrated light spectroscopy

We use our new, flux-calibrated stellar population model of absorption-line indices to derive ages, metallicities, and various element abundance ratios from integrated light spectroscopy of galactic globular clusters. The ages agree well with the literature and are all consistent with the age of the universe. There is a considerable scatter, though, and we obtain systematically larger ages than CMD determinations mostly for metal-rich globular clusters. The metallicities agree well with literature values on the Zinn & West scale, if we adopt iron abundance [Fe/H] for those clusters whose ages agree with the CMD ages. It turns out that the derivation of individual element abundance ratios is not reliable at [Fe/H]<-1 dex, while the [alpha/Fe] ratio is robust at all metallicities. We find general enhancement of light and alpha elements, as expected, with significant variations for some elements. The elements O and Mg follow the same general enhancement with almost identical distributions of [O/Fe] and [Mg/Fe]. We obtain slightly lower [C/Fe] and very high [N/Fe] ratios, instead. This chemical anomaly, commonly attributed to self-enrichment, is well known in globular clusters from individual stellar spectroscopy. It is the first time that this pattern is obtained also from the integrated light. The alpha elements follow a pattern such that the heavier elements Ca and Ti are less enhanced. More specifically, the [Ca/Fe] and [Ti/Fe] ratios are lower than [O/Fe] and [Mg/Fe] by about 0.2 dex. This trend is also seen in recent determinations of element abundances in globular cluster and field stars of the Milky Way. This suggests that Type Ia supernovae contribute significantly to the enrichment of the heavier alpha elements as predicted by nucleosynthesis calculations and galactic chemical evolution models.Comment: MNRAS, re-submitted including referee's comments (minor revision), update on CMD globular cluster age

Leo A: A Late-Blooming Survivor of the Epoch of Reionization in the Local Group

As part of a major program to use isolated Local Group dwarf galaxies as near-field probes of cosmology, we have obtained deep images of the dwarf irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble Space Telescope. From these images we have constructed a color-magnitude diagram (CMD) reaching apparent [absolute] magnitudes of (M475, M814) > (29.0 [+4.4], 27.9 [+3.4]), the deepest ever achieved for any irregular galaxy beyond the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of all the star formation that ever occurred in Leo A happened more recently than 8 Gyr ago. The CMD shows only a very small amount of star formation in the first few billion years after the Big Bang; a possible burst at the oldest ages cannot be claimed with high confidence. The peak SFR occurred ~1.5-4 Gyr ago, at a level 5-10 times the current value. Our modelling indicates that Leo A has experienced very little metallicity evolution; the mean inferred metallicity is consistent with measurements of the present-day gas-phase oxygen abundance. We cannot exclude a scenario in which ALL of the ancient star formation occurred prior to the end of the era of reionization, but it seems unlikely that the lack of star formation prior to ~8 Gyr ago was due to early loss or exhaustion of the in situ gas reservoir.Comment: Accepted for publication in the Astrophysical Journal Letters; 5 pages, 3 figures (2 in color); uses emulateapj.st

The ACS LCID project. V. The Star Formation History of the Dwarf Galaxy \objectname[]{LGS-3}: Clues for Cosmic Reionization and Feedback

We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy \objectname[]{LGS-3} (Pisces) based on deep photometry obtained with the {\it Advanced Camera for Surveys} onboard the {\it Hubble Space Telescope}. Our analysis shows that the SFH of \objectname[]{LGS-3} is dominated by a main episode $\sim 11.7$ Gyr ago with a duration of $\sim$ 1.4 Gyr which formed $\sim 90$ of the stars. Subsequently, \objectname[]{LGS-3} continued forming stars until the present, although at a much lower rate. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of \objectname[]{LGS-3} with expectations from cosmological models. Most or all the star formation was produced in \objectname[]{LGS-3} after the reionization epoch, assumed to be completed at $z\sim6$ or $\sim 12.7$ Gyr ago. The total mass of the galaxy is estimated to be between 2 and $4\times 10^8$ M$_\odot$, corresponding to circular velocities between 28 km\ s$^{-1}$ to 36 km\ s$^{-1}$. These values are close to but somewhat above the limit of 30 km\ s$^{-1}$ below which the UV background is expected to prevent any star formation after reionization. Feedback from SNe associated with the initial episode of star formation (mechanical luminosity from SNe $L_w=5.3\times 10^{38}$ erg s$^{-1}$) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of \objectname[]{LGS-3}; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.Comment: accepted to be published at Ap

On the Origin of the Supergiant HI Shell and Putative Companion in NGC 6822

We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large HI hole. The resulting deep color magnitude diagrams show that NGC 6822 has formed >50% of its stars in the last ~5 Gyr. The star formation histories of all six positions are similar over the most recent 500 Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming that the lifetime of the structure is longer than 500 Myr; such long-lived structures have now been observed in multiple systems and may be the norm in galaxies with solid-body rotation. The old stellar populations (red giants and red clump stars) of the putative companion are consistent with those of the extended halo of NGC 6822; this argues against the interpretation of this structure as a bona fide interacting companion galaxy and against its being linked to the formation of the HI hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended HI structure in NGC 6822 is a warped disk inclined to the line of sight.Comment: The Astrophysical Journal, in press. Full-resolution version available on request from the first autho

The ACS Nearby Galaxy Survey Treasury I. The Star Formation History of the M81 Outer Disk

The ACS Nearby Galaxy Survey Treasury (ANGST) is a large Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) treasury program to obtain resolved stellar photometry for a volume-limited sample of galaxies out to 4 Mpc. As part of this program, we have obtained deep ACS imaging of a field in the outer disk of the large spiral galaxy M81. The field contains the outskirts of a spiral arm as well as an area containing no current star formation. Our imaging results in a color-magnitude diagram (CMD) reaching to F814W = 28.8 and F606W = 29.5, one magnitude fainter than the red clump. Through detailed modeling of the full CMD, we quantify the age and metallicity distribution of the stellar populations contained in the field. The mean metallicity in the field is -1<[M/H]<0 and only a small fraction of stars have ages <~1 Gyr. The results show that most of the stars in this outer disk field were formed by z~1 and that the arm structure at this radius has a lifetime of >~100 Myr. We discuss the measured evolution of the M81 disk in the context of surveys of high-redshift disk galaxies and deep stellar photometry of other nearby galaxies. All of these indicate that massive spiral disks are mostly formed by z~1 and that they have experienced rapid metal enrichment.Comment: 26 pages, 2 tables, 18 figures, accepted for publication in A

Complexity on Small Scales III: Iron and alpha Element Abundances in the Carina Dwarf Spheroidal Galaxy

We have obtained high-resolution spectroscopy of ten red giants in the Carina dwarf spheroidal (dSph) with UVES at the ESO/VLT. Here we present the abundances of O,Na,Mg,Si,Ca,Ti and Fe. By comparing the iron abundances [Fe/H] with calcium triplet (CaT) metallicities we show that the empirical CaT technique yields good agreement with the high-resolution data for [Fe/H]>-2 dex, but tends to deviate at lower metallicities. We identify two metal poor stars with iron abundances of -2.72 and -2.50 dex. These stars are found to have enhanced [alpha/Fe] ratios similar to those of stars in the Milky Way halo. However, the bulk of the Carina red giants are depleted in the [alpha/Fe] abundance ratios with respect to the Galactic halo at a given metallicity. One of our targets, with a [Fe/H] of -1.5 dex, is considerably depleted in almost all of the alpha-elements by ~0.5 dex compared to the solar values. Such a low [alpha/Fe] can be produced by stochastical fluctuations in terms of an incomplete mixing of single Type Ia and II SNe events into the ISM. Our derived element ratios are consistent with the episodic and extended SF in Carina known from its color-magnitude diagram. We find a considerable star-to-star scatter in the abundance ratios. This suggests that Carina's SF history varies with position within the galaxy, with incomplete mixing. Alternatively, the SF rate is so low that the high-mass stellar IMF is sparsely populated, as statistically expected in low-mass star clusters, leading to real scatter in the resultant mass-integrated yields. Both ideas are consistent with slow stochastic SF in dissolving associations, so that one may not speak of a single SF history at a detailed level (Abridged).Comment: 23 pages, 8 figures, accepted for publication in the A