Searcharxiv⌕ Search

arXiv subjects

Eva K. Grebel

Publications and source records attributed to Eva K. Grebel.

At least 181 records · Page 10Linked to original sources
arXiv2004

A New Giant Stellar Structure in the Outer Halo of M31

The Sloan Digital Sky Survey has revealed an overdensity of luminous red giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we have called Andromeda NE. The line-of-sight distance to Andromeda NE is within approximately 50 kpc of M31; Andromeda NE is not a physically unrelated projection. Andromeda NE has a g-band absolute magnitude of ~ -11.6 and central surface brightness of ~ 29 mag/sq.arcsec, making it nearly two orders of magnitude more diffuse than any known Local Group dwarf galaxy at that luminosity. Based on its distance and morphology, Andromeda NE is likely undergoing tidal disruption. Andromeda NE's red giant branch color is unlike that of M31's present-day outer disk or the stellar stream reported by Ibata et al. (2001), arguing against a direct link between Andromeda NE and these structures. However, Andromeda NE has a red giant branch color similar to that of the G1 clump; it is possible that these structures are both material torn off of M31's disk in the distant past, or that these are both part of one ancient stellar stream.

astro-ph↗
arXiv2004

Spectroscopy of PNe in Sextans A, Sextans B, NGC 3109 and Fornax

Planetary nebulae (PNe) and HII regions provide a probe of the chemical enrichment and star formation history of a galaxy from intermediate ages to the present day. We present the first results of NTT spectroscopy of HII regions and/or PNe in four nearby dwarf galaxies: Sextans A, Sextans B, NGC 3109, and Fornax. For all PNe and some of the HII regions in these galaxies we have obtained elemental abundances via the classic Te-method based on the detection of the [OIII] 4363 line. The oxygen abundances in three HII regions of Sextans A are all consistent within the individual rms uncertainties. The oxygen abundance in the PN of Sextans A is however significantly higher. This PN is even more enriched in nitrogen and helium, implying its classification as a PN of Type I. The presumably unaffected PN abundances of S and Ar are well below those in the HII regions, indicating a lower metallicity at the epoch of the PN progenitor formation. For two HII regions in Sextans B, the oxygen abundances do not differ within the rms uncertainties. The third one is, however, twice as metal-rich, providing evidence for the inhomogeneity of the current metallicity distribution in Sextans B. For the PN in Sextans B we measured an O/H that is consistent with that of the low-metallicity HII regions. For NGC 3109 our preliminary results indicate that the oxygen abundances of PNe and HII regions are all within a small range of $\pm$0.15 dex. For the PN in Fornax, Ne, Ar and S abundances suggest that the ISM metallicity was $\sim$0.3 dex lower at the epoch of the PN progenitor's formation, compared to the O/H value derived for the PN (abridged).

astro-ph↗
arXiv2004

Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies

Cold dark matter models for galaxy formation predict that low-mass systems will be the first sites of star formation. As these objects have shallow gravitational potential wells, the subsequent growth of their stellar populations may be halted by heating and gas loss due to reionization. This effect has been suggested to have profoundly influenced properties of present-day dwarf galaxies, including their stellar populations and even survival as visible galaxies. In this Letter we draw on results from quantitative studies of Local Group dwarf galaxy star formation histories, especially for Milky Way satellites, to show that no clear signature exists for a widespread evolutionary impact from reionization. All nearby dwarf galaxies studied in sufficient detail contain ancient populations indistinguishable in age from the oldest Galactic globular clusters. Ancient star formation activity proceeded over several Gyr, and some dwarf spheroidal galaxies even experienced fairly continuous star formation until just a few Gyr ago. Despite their uniformly low masses, their star formation histories differ considerably. The evolutionary histories of nearby dwarf galaxies appear to reflect influences from a variety of local processes rather than a dominant effect from reionization.

astro-ph↗
arXiv2004

The Magellanic Clouds Photometric Survey: The Large Magellanic Cloud Stellar Catalog and Extinction Map

We present our catalog of U, B, V, and I stellar photometry of the central 64 sq. deg. area of the Large Magellanic Cloud. Internal and external astrometric and photometric tests using existing optical photometry (U, B, and V from Massey's bright star catalog and I from the near-infrared sky survey DENIS) are used to confirm our observational uncertainty estimates. We fit stellar atmosphere models to the optical data to check the consistency of the photometry for individual stars across the passbands and to estimate the line-of-sight extinction. Finally, we use the estimated line-of-sight extinctions to produce an extinction map across the Large Magellanic Cloud, confirm the variation of extinction as a function of stellar population, and produce a simple geometrical model for the extinction as a function of stellar population.

astro-ph↗
arXiv2004

Strong Emission Line HII Galaxies in the Sloan Digital Sky Survey. I. Catalog of DR1 Objects with Oxygen Abundances from Te easurements

We present the first edition of the {S}DSS {H}II-galaxies with {O}xygen abundances {C}atalog (SHOC), which is a listing of strong emission-line galaxies (ELGs) from the Sloan Digital Sky Survey (SDSS). Oxygen abundances have been obtained with the classic Te-method. We describe the method exploiting the SDSS database to construct this sample. The selection procedures are described and discussed in detail, as well as some problems encountered in the process of deriving reliable emission line parameters. The method was applied to the SDSS Data Release 1 (DR1). We present 612 SDSS emission-line galaxies (624 separate SDSS targets in total), for which the oxygen abundances 12+log(O/H) have r.m.s. uncertainties <0.20 dex. The subsample of 263 ELGs have an uncertainty <0.10 dex, while 459 ELGs have an uncertainty <0.15 dex. The catalog includes the main parameters of all selected ELGs, the intensities and equivalent widths of hydrogen and oxygen emission lines, as well as oxygen abundances with their uncertainties. The information on the presence of Wolf-Rayet blue and/or red bumps in 109 galaxies is also included. With the use of combined g,r,i SDSS images we performed visual morphological classification of all SHOC galaxies. 461 galaxies (~75%) are classified as confident or probable blue compact galaxies (BCG/BCG?), 78 as irregular ones, 20 as low surface brightness galaxies (LSBG), 10 as obviously interacting and 43 as spiral galaxies. In creating the catalog, 30 narrow line AGN and 69 LINERs were also identified; these are also presented apart of the main catalog. We outline briefly the content of the catalog, and the prospects of its use for statistical studies of the star formation and chemical evolution issues. Some of these studies will be presented in the forthcoming paper (abridged).

astro-ph↗
arXiv2004

The Evolutionary History of Local Group Irregular Galaxies

Irregular (Irr) galaxies are gas-rich objects with recent or ongoing star formation. In absence of spiral density waves, star formation occurs largely stochastically. The scattered star-forming regions tend to be long-lived and migrate slowly. Older populations have a spatially more extended and regular distribution. In fast-rotating Irrs high star formation rates with stronger concentration toward the galaxies' center are observed, and cluster formation is facilitated. In slowly or nonrotating dwarf Irrs (dIrrs) star formation occurs more quiescently, and the formation of OB associations is common. Irrs and dIrrs tend to experience continuous star formation with amplitude variations and to continue to form stars for another Hubble time. Irrs exhibit lower effective yields than spirals, and [alpha/Fe] ratios below the solar value. This may be indicative of fewer Type II SNe and lower astration rates in their past. Alternatively, many metals may be lost from the shallow galaxy potential wells due to selective winds. The differences in the metallicity-luminosity relation between dIrrs and dwarf spheroidals (which, despite their lower masses, tend to have too high a metallicity for their luminosity as compared to dIrrs) further supports the idea of slow astration and slow enrichment in dIrrs. The current data on age-metallicity relations are still too sparse to distinguish between infall, leaky-box, and closed-box models. The preferred location of dIrrs in the outer parts of galaxy groups and clusters and in the field as well as the positive correlation between gas content and distance from massive galaxies indicate that these dIrrs probably have not yet experienced significant interactions or galaxy harassment. (Abridged)

astro-ph↗
arXiv2004

WIYN Survey for Carbon Stars in the M31 and Cetus Dwarf Spheroidal Galaxies: Evolutionary Implications

We report results of a photometric survey with the WIYN telescope for carbon stars in the M 31 dwarf spheroidal (dSph) companions And III, And V, And VI, and And VII, as well as in the relatively isolated Local Group dSph Cetus. We find three carbon-star candidates in And VII and one carbon star in each And VI and Cetus. Comparing the carbon star content with other Local Group dwarf galaxies, we argue against the presence of substantial intermediate-age stellar populations in the all of the galaxies surveyed with the exception of And VII. We discuss these results in the context of the origin of the Andromeda dSphs and conclude that these are ancient galaxies, most of which ceased star formation long before the main merger events in M31. The M31 dSphs therefore show less diversity in star formation histories than the Galactic dSph companions, or the M31 dE companions, as illustrated by NGC147 which was surveyed as a calibration object. All of our dSph targets except And V have candidate carbon stars below the tip of the RGB, which resemble CH stars found in globular clusters. We estimate that 0.3% of stars in the dSphs are CH stars, presumably as a result of C pollution from a binary companion. Comparisons with CH star frequencies in globular clusters could constrain the impact of dense environments on the frequency of this form of binary star evolution.

astro-ph↗
arXiv2003

A Low Latitude Halo Stream around the Milky Way

We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l = 180 deg to l = 227 deg with turnoff magnitude $g \sim 19.5$; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R = 18 +/- 2 kpc from the Galactic Center above the Galactic plane, and has R = 20 +/- 2 kpc in the region sampled below the Galactic plane. The evidence includes five hundred SDSS spectroscopic radial velocities of stars within 30 degrees of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km/s at (l,b) = (198, -27), 22 km/s at (l,b) = (225, 28), 30 km/s at (l,b) = (188, 24), and 30 km/s at (l,b) = (182, 27) degrees. The structure rotates in the same prograde direction as the Galactic disk stars, but with a circular velocity of 110+/-25 km/s. The narrow measured velocity dispersion is inconsistent with power law spheroid or thick disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km/s at (l, b) = (165, -55) deg. We estimate a preliminary metallicity from the Ca II (K) line and color of the turnoff stars of [Fe/H] = -1.6 with a dispersion of 0.3 dex and note that the turnoff color is consistent with that of the spheroid population. We interpret our measurements as evidence for a tidally disrupted satellite of $2 \times 10^7$ to $5 \times 10^8 \rm M_\odot$ which rings the Galaxy.

astro-ph↗
arXiv2003

On the C and N Abundances of 47 Tucanae Main Sequence Stars

We report the results of an analysis of CN and CH band strengths among a large sample of 47 Tucanae main-sequence and turn-off stars presented earlier by Harbeck et al. The resulting C and N abundances derived from synthetic spectra demonstrate: 1) A strongly anti-correlated relationship between [C/Fe] and [N/Fe] with the CN-strong stars exhibiting depleted carbon and enhanced nitrogen. 2) The abundances of both elements agree remarkably well with those found among the evolved red giants of the cluster implying little change in surface abundances from at least Mv ~ +6.5 to the tip of the red giant branch. 3) The pattern of C-depletions and N-enhancements are quite similar to those seen among the turn-off stars of M71, a cluster of almost identical metallicity but lower central concentration and escape velocity. At the same time, similar if not smaller N-enhancements and larger C-depletions are evident among like stars in the more metal-poor cluster M5. We interpret these results, as did Harbeck et al., as evidence of the operation of some pollution/accretion event early in the cluster history - the most likely source being AGB ejecta. However, the present results rule out simple surface pollution and suggest that a substantial fraction of the present stars' masses must be involved.

astro-ph↗
arXiv2003

Low-Surface-Brightness Galaxies in the Sloan Digital Sky Survey. I. Search Method and Test Sample

In this paper we present results of a pilot study to use imaging data from the Sloan Digital Sky Survey (SDSS) to search for low-surface-brightness (LSB) galaxies. For our pilot study we use a test sample of 92 galaxies from the catalog of Impey et al. (1996) distributed over 93 SDSS fields of the Early Data Release (EDR). Many galaxies from the test sample are either LSB or dwarf galaxies. To deal with the SDSS data most effectively a new photometry software was created, which is described in this paper. We present the results of the selection algorithms applied to these 93 EDR fields. Two galaxies from the Impey et al. test sample are very likely artifacts, as confirmed by follow-up imaging. With our algorithms, we were able to recover 87 of the 90 remaining test sample galaxies, implying a detection rate of $\sim$96.5%. The three missed galaxies fall too close to very bright stars or galaxies. In addition, 42 new galaxies with parameters similar to the test sample objects were found in these EDR fields (i.e., $\sim$47% additional galaxies). We present the main photometric parameters of all identified galaxies and carry out first statistical comparisons. We tested the quality of our photometry by comparing the magnitudes for our test sample galaxies and other bright galaxies with values from the literature. All these tests yielded consistent results. We briefly discuss a few unusual galaxies found in our pilot study, including an LSB galaxy with a two-component disk and ten new giant LSB galaxies.

astro-ph↗
arXiv2003

Sagittarius Tidal Debris 90 kpc from the Galactic Center

A new overdensity of A-colored stars in distant parts of the Milky Way's stellar halo, at a dereddened SDSS magnitude of g_0 = 20.3, is presented. Identification of associated variable RR Lyrae candidates supports the claim that these are blue horizontal branch stars. The inferred distance of these stars from the Galactic center is 90 kpc, assuming the absolute magnitude of these stars is M_g_0 = 0.7 and that the Sun is 8.5 kpc from the Galactic center. The new tidal debris is within 10 kpc of same plane as other confirmed tidal debris from the disruption of the Sagittarius dwarf galaxy, and could be associated with the trailing tidal arm. Distances to the Sagittarius stream estimated from M stars are about 13% smaller than our inferred distances. The tidal debris has a width of at least 10 degrees, and is traced for more than 20 degrees across the sky. The globular cluster NGC 2419 is located within the detected tidal debris, and may also have once been associated with the Sagittarius dwarf galaxy.

astro-ph↗
arXiv2003

The near-IR properties and continuum shapes of high redshift quasars from the Sloan Digital Sky Survey

We present J-H-K' photometry for a sample of 45 high redshift quasars found by the Sloan Digital Sky Survey. The sample was originally selected on the basis of optical colors and spans a redshift range from 3.6 to 5.03. Our photometry reflects the rest-frame SED longward of Ly alpha for all redshifts. The results show that the near-IR colors of high redshift quasars are quite uniform. We have modelled the continuum shape of the quasars (from just beyond Ly alpha to ~4000 A) with a power law of the form f_nu \propto nu^alpha, and find =-0.57 with a scatter of 0.33. This value is similar to what is found for lower redshift quasars over the same restframe wavelength range, and we conclude that there is hardly any evolution in the continuum properties of optically selected quasars up to redshift 5. The spectral indices found by combining near-IR with optical photometry are in general consistent but slightly flatter than what is found for the same quasars using the optical spectra and photometry alone, showing that the continuum region used to determine the spectral indices can somewhat influence the results.

astro-ph↗
arXiv2003

CN Variations in NGC 7006

Rotationally induced mixing with subsequent dredge-up of nucleosynthesized material is discussed as a second parameter of the horizontal branch morphology in globular clusters. CNO abundances have been proposed as tracers of the dredge up of processed material. \ngc is a prominent example of a second parameter GC: Its HB morphology is too red for its metallicity. We present spectroscopic measurements of CN molecular band strengths S(3839) and CH band CH(4300) strengths for 12 giants in \ngc to test rotationally-driven mixing as a second parameter in this cluster. Our observations reveal (i) a scatter in star-to-star CN absorption strengths with the same amplitude as seen in other GCs of the same metallicity, but different HB morphologies; (ii) a possible continuous distribution of CN absorption strength with a preference for CN-enriched stars, and (iii) a possible weak radial gradient in the number ratio of CN-strong and CN-weak stars. We argue against the hypothesis that CN-variations are directly correlated with the second parameter effect of the HB morphology. However, the small sample of stars measured in \ngc prevents us from drawing firm conclusions. Finally, we identify one star of our sample as a foreground dwarf carbon star.

astro-ph↗
arXiv2003

Discovery of Eight New Extremely Metal--Poor Galaxies in the Sloan Digital Sky Survey

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 (IZw18, HS0822+3542, HS0837+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 $\le$ 0.1 dex and are based on the temperature-sensitive [O {\sc iii}] $λ$4363 Åline and on the direct calculation of the electron temperature. We briefly discuss a new method of oxygen abundance determinations using the [O {\sc ii}] $λ$7319,7330 Å lines, which is particularly useful for SDSS emission-line spectra with redshifts $\le$~0.024 since the [O {\sc ii}] $λ$3727 Åemission 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\fm4 to $-$18\fm6. Our eight new XMPGs increase the number of known metal-deficient galaxies by approximately one quarter. The estimated surface density of XMPGs is 0.004 deg$^{-2}$ for $r$ $\le$ 17\fm77.

astro-ph↗
arXiv2003

Nebular Abundances of Nearby Southern Dwarf Galaxies

The results of optical spectroscopy of H II regions in a sample of southern dwarf irregulars consisting of five dwarf galaxies in the Centaurus A group, four dwarfs in the Sculptor group, and eight additional dwarf galaxies are presented. Oxygen abundances are derived using the direct method where [O III]4363 is detected; otherwise, abundances are derived with the bright-line method using the McGaugh and the Pilyugin calibrations. ESO358-G060 has the lowest oxygen abundance (12+log(O/H) = 7.32), which is comparable to the value for the second most metal-poor galaxy known (SBS0335-052). In all, new oxygen abundances are reported for nine dwarf galaxies; updated values are presented for the remaining galaxies. Derived oxygen abundances are in the range from 3% to 26% of the solar value. Oxygen abundances for dwarfs in the southern sample are consistent with the metallicity-luminosity relationship defined by a control sample of dwarf irregulars with [O III]4363 abundances and well-measured distances. However, NGC5264 appears to have an (upper branch) oxygen abundance approximately two to three times higher than other dwarfs at similar luminosities. Nitrogen-to-oxygen and neon-to-oxygen abundance ratios are also reported; in particular, IC1613 and IC5152 show elevated nitrogen-to-oxygen ratios for their oxygen abundances.

astro-ph↗
arXiv2003

Observing the dark matter density profile of isolated galaxies

Using the Sloan Digital Sky Survey (SDSS), we probe the halo mass distribution by studying the velocities of satellites orbiting isolated galaxies. In a subsample that covers 2500 sq. degrees on the sky, we detect about 3000 satellites with absolute blue magnitudes going down to M_B = -14; most of the satellites have M_B=-16 to -18, comparable to the magnitudes of M32 and the Magellanic Clouds. After a careful, model-independent removal of interlopers, we find that the line-of-sight velocity dispersion of satellites declines with distance to the primary. For an L* galaxy the r.m.s. line-of-sight velocity changes from ~120 km/s at 20 kpc to ~60 km/s at 350 kpc. This decline agrees remarkably well with theoretical expectations, as all modern cosmological models predict that the density of dark matter in the peripheral parts of galaxies declines as rho_DM propto r^{-3}. Thus, for the first time we find direct observational evidence of the density decline predicted by cosmological models; we also note that this result contradicts alternative theories of gravity such as MOND. We also find that the velocity dispersion of satellites within 100 kpc scales with the absolute magnitude of the central galaxy as sigma propto L^{0.3}; this is very close to the Tully-Fisher relation for normal spiral galaxies.

astro-ph↗
arXiv2003

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)

astro-ph↗