arXiv subjects

Eva K. Grebel

Publications and source records attributed to Eva K. Grebel.

At least 163 records · Page 9Linked to original sources

arXiv2007

Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. IV. The color-magnitude relation

We present an analysis of the optical colors of 413 Virgo cluster early-type dwarf galaxies (dEs), based on Sloan Digital Sky Survey imaging data. Our study comprises (1) a comparison of the color-magnitude relation (CMR) of the different dE subclasses that we identified in Paper III of this series, (2) a comparison of the shape of the CMR in low and high-density regions, (3) an analysis of the scatter of the CMR, and (4) an interpretation of the observed colors with ages and metallicities from population synthesis models. We find that the CMRs of nucleated (dE(N)) and non-nucleated dEs (dE(nN)) are significantly different from each other, with similar colors at fainter magnitudes (r > 17 mag), but increasingly redder colors of the dE(N)s at brighter magnitudes. We interpret this with older ages and/or higher metallicities of the brighter dE(N)s. The dEs with disk features have similar colors as the dE(N)s and seem to be only slightly younger and/or less metal-rich on average. Furthermore, we find a small but significant dependence of the CMR on local projected galaxy number density, consistently seen in all of u-r, g-r, and g-i, and weakly i-z. We deduce that a significant intrinsic color scatter of the CMR is present, even when allowing for a distance spread of our galaxies. No increase of the CMR scatter at fainter magnitudes is observed down to r = 17 mag (Mr = -14 mag). The color residuals, i.e., the offsets of the data points from the linear fit to the CMR, are clearly correlated with each other in all colors for the dE(N)s and for the full dE sample. We conclude that there must be at least two different formation channels for early-type dwarfs in order to explain the heterogeneity of this class of galaxy. (Abridged)

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arXiv2007

A Wide-Field View of Leo II -- A Structural Analysis Using the SDSS

Using SDSS I data, we have analysed the stellar distribution of the Leo II dwarf spheroidal galaxy (distance of 233 kpc) to search for evidence of tidal deformation. The existing SDSS photometric catalogue contains gaps in regions of high stellar crowding, hence we filled the area at the centre of Leo II using the DAOPHOT algorithm applied to the SDSS images. The combined DAOPHOT-SDSS dataset contains three-filter photometry over a 4x4 square degree region centred on Leo II. By defining a mask in three-filter colour-magnitude space, we removed the majority of foreground field stars. We have measured the following Leo II structural parameters: a core radius of r_c = 2.64 +/- 0.19 arcmin (178 +/- 13 pc), a tidal radius of r_t = 9.33 +/- 0.47 arcmin (632 +/- 32 pc) and a total V-band luminosity of L_V = (7.4 +/- 2.0) times 10^5 L_sun (M_V = -9.9 +/- 0.3). Our comprehensive analysis of the Leo II structure did not reveal any significant signs of tidal distortion. The internal structure of this object contains only mild isophotal twisting. A small overdensity was discovered appoximately 4.5 tidal radii from the Leo II centre, however we conclude it is unlikely to be material tidally stripped from Leo II based on its stellar population, and is most likely a foreground overdensity of stars. Our results indicate that the influence of the Galactic graviational field on the structure of Leo II has been relatively mild. We rederived the mass-to-light ratio of this system using existing kinematic data combined with our improved structural measurements, and favour the scenario in which Leo II is strongly dominated by dark matter with (M/L)_V ~ 100 in solar units.

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arXiv2007

The Tully-Fisher Relation and Its Residuals for a Broadly Selected Sample of Galaxies

We measure the relation between galaxy luminosity and disk circular velocity (the Tully-Fisher [TF] relation), in the g, r, i, and z-bands, for a broadly selected sample of galaxies from the Sloan Digital Sky Survey, with the goal of providing well defined observational constraints for theoretical models of galaxy formation. The input sample of 234 galaxies has a roughly flat distribution of absolute magnitudes in the range -18.5 > Mr > -22, and our only morphological selection is an axis-ratio cut b/a < 0.6 to allow accurate inclination corrections. Long-slit spectroscopy yields usable H-alpha rotation curves for 162 galaxies. Observational errors, including distance errors due to peculiar velocities, are small compared to the intrinsic scatter of the TF relation. The slope of the forward TF relation steepens from -5.5 +/- 0.2 mag/log(km/s) in the g-band to -6.6 +/- 0.2 mag/log(km/s) in the z-band. The intrinsic scatter is approximately 0.4 mag in all bands. The scatter is not dominated by rare outliers or by any particular class of galaxies, though it drops slightly, to 0.36 mag, if we restrict the sample to nearly bulgeless systems. Correlations of TF residuals with other galaxy properties are weak: bluer galaxies are significantly brighter than average in the g-band but only marginally brighter in the i-band; more concentrated galaxies are slightly fainter than average; and the TF residual is virtually independent of half-light radius, contrary to the trend expected for gravitationally dominant disks. The observed residual correlations do not account for most of the intrinsic scatter, implying that this scatter is instead driven largely by variations in the ratio of dark to luminous matter within the disk galaxy population.

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arXiv2007

Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. III. Subpopulations: distributions, shapes, origins

From a quantitative analysis of 413 Virgo cluster early-type dwarf galaxies (dEs) with Sloan Digital Sky Survey imaging data, we find that the dE class can be divided into multiple subpopulations that differ significantly in their morphology and clustering properties. Three dE subclasses are shaped like thick disks and show no central clustering: (1) dEs with disk features like spiral arms or bars, (2) dEs with central star formation, and (3) ordinary, bright dEs that have no or only a weak nucleus. These populations probably formed from infalling progenitor galaxies. In contrast, ordinary nucleated dEs follow the picture of classical dwarf elliptical galaxies in that they are spheroidal objects and are centrally clustered like E and S0 galaxies, indicating that they have resided in the cluster since a long time, or were formed along with it. These results define a morphology-density relation within the dE class. We find that the difference in the clustering properties of nucleated dEs and dEs with no or only a weak nucleus is not caused by selection biases, as opposed to previously reported suggestions. The correlation between surface brightness and observed axial ratio favors oblate shapes for all subclasses, but our derivation of intrinsic axial ratios indicates the presence of at least some triaxiality. We discuss possible interrelations and formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) of these dE subpopulations.

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arXiv2006

Empirical Color Transformations Between SDSS Photometry and Other Photometric Systems

We present empirical color transformations between the Sloan Digital Sky Survey (SDSS) ugriz photometry and Johnson-Cousins UBVRI system and Becker's RGU system, respectively. Owing to the magnitude of data that is becoming available in the SDSS photometric system it is particularly important to be able to convert between this new system and traditional photometric systems. Unlike earlier published transformations we based our calculations on stars actually measured by the SDSS with the SDSS 2.5-m telescope. The photometric database of the SDSS provides in a sense a single-epoch set of 'tertiary standards' covering more than one quarter of the sky. Our transformations should facilitate their use to easily and reliably derive the corresponding approximate Johnson-Cousins or RGU magnitudes. The SDSS survey covers a number of areas that were previously established as standard fields in the Johnson-Cousins system, in particular, fields established by Landolt and by Stetson. We used these overlapping fields to create well-photometered star samples on which our calculated transformations are based. For the RGU photometry we used fields observed in the framework of the new Basel high-latitude field star survey. We calculated empirical color transformations between SDSS photometry and Johnson-Cousins UBVRI and Becker's RGU system. For all transformations we found linear relations to be sufficient. Furthermore we showed that the transformations between the Johnson-Cousins and the SDSS system have a slight dependence on metallicity.

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arXiv2006

Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. II. Early-type dwarfs with central star formation

Despite the common picture of an early-type dwarf (dE) as a quiescent galaxy with no star formation and little gas, we identify 23 dEs that have blue central colors caused by recent or ongoing star formation in our sample of 476 Virgo cluster dEs. In addition, 14 objects that were mostly classified as (candidate) BCDs have similar properties. Among the certain cluster members, the dEs with blue centers reach a fraction of more than 15% of the dE population at brighter (B<=16) magnitudes. A spectral analysis of the centers of 16 galaxies reveals in all cases an underlying old population that dominates the mass, with M(old)>=90% for all but one object. Therefore the majority of these galaxies will appear like ordinary dEs within ~one Gigayear or less after the last episode of star formation. Their overall gas content is less than that of dwarf irregular galaxies, but higher than that of ordinary dEs. Their flattening distribution suggests the shape of a thick disk, similar to what has been found for dEs with disk features in Paper I of this series. Their projected spatial distribution shows no central clustering, and their distribution with projected local density follows that of irregular galaxies, indicative of an unrelaxed population. This is corroborated by their velocity distribution, which displays two side peaks characteristic of recent infall. We discuss possible formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) that might be able to explain both the disk shape and the central star formation of the dEs with blue centers.

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arXiv2006

Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. I. On the possible disk nature of bright early-type dwarfs

We present a systematic search for disks in 476 Virgo cluster early-type dwarf (dE) galaxies. Disk features (spiral arms, edge-on disks, or bars) were identified by applying unsharp masks to a combined image from three bands (g, r, i), as well as by subtracting the axisymmetric light distribution of each galaxy from that image. 14 objects are unambiguous identifications of disks, 10 objects show 'probable disk' features, and 17 objects show 'possible disk' features. The number fraction of these galaxies, for which we introduce the term dEdi, reaches more than 50% at the bright end of the dE population, and decreases to less than 5% for magnitudes B>16. The luminosity function of our full dE sample can be explained by a superposition of dEdis and ordinary dEs, strongly suggesting that dEdis are a distinct type of galaxy. This is supported by the projected spatial distribution: dEdis show basically no clustering and roughly follow the spatial distribution of spirals and irregulars, whereas ordinary dEs are distributed similarly to the strongly clustered E/S0 galaxies. While the flattening distribution of ordinary dEs is typical for spheroidal objects, the distribution of dEdis is significantly different and agrees with their being flat oblate objects. We therefore conclude that the dEdis are not spheroidal galaxies that just have an embedded disk component, but are instead a population of genuine disk galaxies. Several dEdis display well-defined spiral arms with grand design features that clearly differ from the flocculent, open arms typical for late-type spirals that have frequently been proposed as progenitors of early-type dwarfs. This raises the question of what process is able to create such spiral arms - with pitch angles like those of Sab/Sb galaxies - in bulgeless dwarf galaxies. (Abridged)

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arXiv2006

Local Group(s)

The properties of the galaxies of the Local Group are reviewed, followed by a brief discussion of nearby groups. The galaxy groups in our vicinity - the M81 group, the Cen A group, and the IC 342/Maffei group - are in many respects Local Group analogs: Their luminosity functions, galaxy content, fractional galaxy type distribution, crossing times, masses, and zero-velocity surface radii are similar to those of the Local Group. Also, the nearby groups usually consist of two subgroups, some of which approach each other and may ultimately merge to form a fossil group. These poor groups contrast with the less evolved, loose and extended galaxy ``clouds'' such as the Scl group and the CVn I cloud. These are characterized by long crossing times, are dominated by gas-rich, late-type galaxies, and lack gas-deficient, low luminosity early-type dwarfs. These clouds may be groups still in formation. The local Hubble flow derived from the clouds and groups is very cold.

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arXiv2006

Andromeda X, A New Dwarf Spheroidal Satellite of M31: Photometry

We report the discovery of Andromeda X, a new dwarf spheroidal satellite of M31, based on stellar photometry from the Sloan Digital Sky Survey (SDSS). Using follow-up imaging data we have estimated its distance and other physical properties. We find that Andromeda X has a dereddened central surface brightness of mu_V,0 ~ 26.7 mag arcsec^-2 and a total apparent magnitude of V_tot ~ 16.1, which at the derived distance modulus, (m - M)_0 ~ 24.12 - 24.34, yields an absolute magnitude of M_V ~ -8.1 +/- 0.5; these values are quite comparable to those of Andromeda IX, a previously-discovered low luminosity M31 satellite. The discoveries of Andromeda IX and Andromeda X suggest that such extremely faint satellites may be plentiful in the Local Group.

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arXiv2005

Dark Matter and Stellar Mass in the Luminous Regions of Disk Galaxies

We investigate the correlations among stellar mass (M_*), disk scale length (R_d), and rotation velocity at 2.2 disk scale lengths (V_2.2) for a sample of 81 disk-dominated galaxies (disk/total >= 0.9) selected from the SDSS. We measure V_2.2 from long-slit H-alpha rotation curves and infer M_* from galaxy i-band luminosities (L_i) and g-r colors. We find logarithmic slopes of 2.60+/-0.13 and 3.05+/-0.12 for the L_i-V_2.2 and M_*-V_2.2 relations, somewhat shallower than most previous studies, with intrinsic scatter of 0.13 dex and 0.16 dex. Our direct estimates of the total-to-stellar mass ratio within 2.2R_d, assuming a Kroupa IMF, yield a median ratio of 2.4 for M_*>10^10 Msun and 4.4 for M_*=10^9-10^10 Msun, with large scatter at a given M_* and R_d. The typical ratio of the rotation speed predicted for the stellar disk alone to the observed rotation speed at 2.2R_d is ~0.65. The distribution of R_d at fixed M_* is broad, but we find no correlation between disk size and the residual from the M_*-V_2.2 relation, implying that this relation is an approximately edge-on view of the disk galaxy fundamental plane. Independent of the assumed IMF, this result implies that stellar disks do not, on average, dominate the mass within 2.2R_d. We discuss our results in the context of infall models of disk formation in cold dark matter halos. A model with a disk-to-halo mass ratio m_d=0.05 provides a reasonable match to the R_d-M_* distribution for spin parameters λranging from ~0.04-0.08, and it yields a reasonable match to the mean M_*-V_2.2 relation. A model with m_d=0.1 predicts overly strong correlations between disk size and M_*-V_2.2 residual. Explaining the wide range of halo-to-disk mass ratios within 2.2R_d requires significant scatter in m_d values, with systematically lower m_d for galaxies with lower $M_*$.

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arXiv2005

A Search for PNe in Nearby Galaxies with SDSS Imaging Data

We present the latest results from our project to search for new planetary nebulae in nearby galaxies using Sloan Digital Sky Survey (SDSS) imaging data. Our method is based on photometric criteria and can be applied to galaxies where PNe appear as point sources. We applied these criteria to the whole area of M31 as scanned by SDSS, detecting 130 new PN candidates and 30 known PNe. All selected PNe candidates are located in the outer regions of M31. For 85 candidates follow-up spectroscopy was obtained with the 2.2m telescope at Calar Alto Observatory. The observations show that our method has a detection efficiency of about 82%. We discuss the 2D velocity field of the outer part of M31 based on our observed PN data. The PNe suggest an exponential disk scale length of 13 kpc along the minor axis. We discovered two PNe along the line of sight to Andromeda NE, a very low surface brightness giant stellar structure in the outer halo of M31. These two PNe are located at projected distances of ~48 kpc and ~41 kpc from the center of M31 and are the most distant PNe in M31 found up to now. Our data support the idea that Andromeda NE is located at the distance of M31. No PNe associated with other M31 satellites observed by the SDSS were found. Applying our method to other SDSS regions we checked data for the Local Group galaxies Sextans, Draco, Leo I, Pegasus, Sextans B and Leo A and recovered a known PN in Leo A. We re-measured its O/H and for the first time determined abundances of N/H, S/H, He/H as well as the electron number density Ne. We argue that the PN progenitor was formed ~1.5 Gyr ago during the strongest episode of star formation in Leo A.

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arXiv2005

Near-Field Cosmology with Local Group Dwarf Spheroidals

The Local Group offers an excellent laboratory for near-field cosmology by permitting us to use the resolved stellar content of its constituent galaxies as probes of galaxy formation and evolution, which in turn is an important means for testing cosmological models of hierarchical structure formation. In this review, we discuss the the least massive, yet most numerous type of galaxy in the Local Group, the dwarf spheroidal galaxies, and compare their properties to cosmological predictions. In particular, we point out problems found with a simple building block scenario and with effects expected from reionization. We show that the star formation histories of dSphs are inconsistent with the predicted cessation of star formation after reionization; instead, extended star formation episodes are observed. The Galactic dSphs contain in part prominent intermediate-age populations, whereas the Galactic halo does not. Conversely, the M31 dSphs are almost entirely old, while the M31 halo contains a substantial intermediate-age population. These differences in the population structure as well as the differences in the modes of star formation inferred from [alpha/Fe] ratios make dSphs unlikely major contributors to the build-up of the Galactic and M31 halo unless most of the accretion occurred at early epochs. On the other hand, there is clear evidence for ongoing harassment and accretion of a number of dSphs.

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arXiv2005

Stellar Populations in the Local Group of Galaxies

The characteristics and properties of the stellar populations and evolutionary histories of Local Group galaxies are summarized and compared to predictions of cosmological models. No clear signature of the re-ionization epoch is observed; in particular, there is no cessation of star formation activity in low-mass dwarf galaxies at the end of re-ionization. Arguments against the morphological transformation of dwarf irregular into dwarf spheroidal galaxies are derived from their pronounced evolutionary differences at early epochs as evidenced by the offset in the metallicity-luminosity relation between gas-rich and gas-poor dwarfs. While there is increasing evidence for past and ongoing accretion events the overall importance of dwarf galaxies as building blocks remains unclear considering their differences in modes of star formation and detailed chemistry.

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arXiv2005

The Arches Cluster - Evidence for a Truncated Mass Function?

We have analyzed high-resolution, adaptive optics (AO) HK observations of the Arches cluster obtained with NAOS/CONICA. With a spatial resolution of 84 mas, the cluster center is uniquely resolved. From these data the present-day mass function (MF) of Arches is derived down to about 4 Msun. The integrated MF as well as the core and 2nd annulus MFs are consistent with a turn-over at 6-7 Msun. This turn-over indicates severe depletion of intermediate and low-mass stars in the Arches cluster, possibly caused by its evolution in the Galactic Center environment. The Arches MF represents the first resolved observation of a starburst cluster exhibiting a low-mass truncated MF. This finding has severe implications for stellar population synthesis modelling of extragalactic starbursts, the derivation of integrated properties such as the total mass of star clusters in dense environments, the survival of low-mass remnants from starburst populations, and chemical enrichment during starburst phases.

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arXiv2005

Spectrophotometry of Sextans A and B: Chemical Abundances of HII Regions and Planetary Nebulae

We present the results of high quality long-slit spectroscopy of planetary nebulae (PNe) and HII regions in the two dwarf irregular galaxies Sextans A and B, which belong to a small group of galaxies just outside the Local Group. In Sextans A we obtained the element abundances in its only known PN and in three HII regions with the classical Te method. The oxygen abundances in these three HII regions of Sextans A are all consistent within the individual rms uncertainties, with the average 12+log(O/H) = 7.54+/-0.06. The oxygen abundance of the PN in Sextans A is, however, significantly higher: 12+$\log$(O/H) = 8.02+/-0.05. This PN is even more enriched in nitrogen and helium, suggesting a classification as a PN of Type I. In Sextans B we obtained spectra of one PN and six HII regions. Element abundances with the Te method could be derived for the PN and three of the HII regions. For two of these HII regions, which have a separation of only ~70 pc in projection, the oxygen abundances do not differ within the rms uncertainties, with a mean of 12+log(O/H) = 7.53+/-0.05. The third HII region, which is about 0.6 kpc northeast from the former two, is twice as metal-rich, with 12+log(O/H) = 7.84+/-0.05. This suggests considerable inhomogeneity in the present-day metallicity distribution in Sextans B. (Abridged)

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arXiv2005

The colours of Virgo dEs as seen by SDSS

A colour analysis of a carefully selected sample of over 200 Virgo cluster dEs and 8 giant ellipticals (Es) in the SDSS yields the following results: a) the nucleated dEs (dENs) follow a tight colour-magnitude relation (CMR) which does not broaden significantly at fainter (Mr > -15) magnitudes, b) the CMR of dENs smoothly transitions to the CMR of Es, but the latter changes its slope at about Mr = -20, c) the brighter dENs are consistent with ages of t > 5Gyr and a decrease of metallicity towards fainter magnitudes, but a possible additional luminosity-age relation is not ruled out, d) it is crucial to treat dS0s separately from dEs since their different colour properties would otherwise bias the comparison of nucleated and non-nucleated dEs (dEnoNs), e) the dEnoNs show a weak trend towards bluer colours than dENs.

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arXiv2005

On the origin of the radial mass density profile of the Galactic halo Globular Cluster System

We investigate what may be the origin of the presently observed spatial distribution of the mass of the Galactic Old Halo globular cluster system. We propose its radial mass density profile to be a relic of the distribution of the cold baryonic material in the protoGalaxy. Assuming that this one arises from the profile of the whole protoGalaxy minus the contribution of the dark matter (and a small contribution of the hot gas by which the protoglobular clouds were bound), we show that the mass distributions around the Galactic centre of this cold gas and of the Old Halo agree satisfactorily. In order to demonstrate our hypothesis even more conclusively, we simulate the evolution with time, up to an age of 15 Gyr, of a putative globular cluster system whose initial mass distribution in the Galactic halo follows the profile of the cold protogalactic gas. We show that beyond a galactocentric distance of order 2 to 3 kpc, the initial shape of such a mass density profile is preserved in spite of the complete destruction of some globular clusters and the partial evaporation of some others. This result is almost independent of the choice of the initial mass function for the globular clusters, which is still ill-determined. The shape of these evolved cluster system mass density profiles also agree with the presently observed profile of the Old Halo globular cluster system, thus strengthening our hypothesis. Our result might suggest that the flattening shown by the Old Halo mass density profile at short distance from the Galactic centre is, at least partly, of primordial origin.

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arXiv2004

Andromeda IX: A New Dwarf Spheroidal Satellite of M31

We report the discovery of a new dwarf spheroidal satellite of M31, Andromeda IX, based on resolved stellar photometry from the Sloan Digital Sky Survey (SDSS). Using both SDSS and public archival data we have estimated its distance and other physical properties, and compared these to the properties of a previously known dwarf spheroidal companion, Andromeda V, also observed by SDSS. Andromeda IX is the lowest surface brightness galaxy found to date (mu_{V} ~ 26.8 mag arcsec^-2), and at the distance we estimate from the position of the tip of Andromeda IX's red giant branch, (m - M)_{0} ~ 24.5 (805 kpc), Andromeda IX would also be the faintest galaxy known (M_{V} ~ -8.3).

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