arXiv subjects

Enrico V. Held

Publications and source records attributed to Enrico V. Held.

At least 19 recordsLinked to original sources

arXiv2016

Chemistry and Kinematics of the Late-Forming Dwarf Irregular Galaxies Leo A, Aquarius, and Sagittarius DIG

We present Keck/DEIMOS spectroscopy of individual stars in the relatively isolated Local Group dwarf galaxies Leo A, Aquarius, and the Sagittarius dwarf irregular galaxy. The three galaxies--but especially Leo A and Aquarius--share in common delayed star formation histories relative to many other isolated dwarf galaxies. The stars in all three galaxies are supported by dispersion. We found no evidence of stellar velocity structure, even for Aquarius, which has rotating HI gas. The velocity dispersions indicate that all three galaxies are dark matter-dominated, with dark-to-baryonic mass ratios ranging from $4.4^{+1.1}_{-0.8}$ (SagDIG) to $9.6^{+2.5}_{-1.8}$ (Aquarius). Leo A and SagDIG have lower stellar metallicities than Aquarius, and they also have higher gas fractions, both of which would be expected if Aquarius were farther along in its chemical evolution. The metallicity distribution of Leo A is inconsistent with a Closed or Leaky Box model of chemical evolution, suggesting that the galaxy was pre-enriched or acquired external gas during star formation. The metallicities of stars increased steadily for all three galaxies, but possibly at different rates. The [$α$/Fe] ratios at a given [Fe/H] are lower than that of the Sculptor dwarf spheroidal galaxy, which indicates more extended star formation histories than Sculptor, consistent with photometrically derived star formation histories. Overall, the bulk kinematic and chemical properties for the late-forming dwarf galaxies do not diverge significantly from those of less delayed dwarf galaxies, including dwarf spheroidal galaxies.

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arXiv2014

The mass-metallicity relation of galaxies up to redshift 0.35

Our research on the age-metallicity and mass-metallicity relations of galaxies is presented and compared to the most recent investigations in the field. We have been able to measure oxygen abundances using the direct method for objects spanning four orders of magnitude in mass, and probing the last 4 Gyr of galaxy evolution. We have found preliminary evidence that the metallicity evolution is consistent with expectations based on age-metallicity relations obtained with low resolution stellar spectra of resolved Local Group galaxies.

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arXiv2012

Homogeneous Metallicities and Radial Velocities for Galactic Globular Clusters

Well determined radial velocities and abundances are essential for analyzing the properties of the Globular Cluster system of the Milky Way. However more than 50% of these clusters have no spectroscopic measure of their metallicity. In this context, this work provides new radial velocities and abundances for twenty Milky Way globular clusters which lack or have poorly known values for these quantities. The radial velocities and abundances are derived from spectra obtained at the Ca II triplet using the FORS2 imager and spectrograph at the VLT, calibrated with spectra of red giants in a number of clusters with well determined abundances. For about half of the clusters in our sample we present significant revisions of the existing velocities or abundances, or both. We also confirm the existence of a sizable abundance spread in the globular cluster M54, which lies at the center of the Sagittarius dwarf galaxy. In addition evidence is provided for the existence of a small intrinsic internal abundance spread (sigma [Fe/H](int) ~ 0.11-0.14 dex, similar to that of M54) in the luminous distant globular cluster NGC 5824. This cluster thus joins the small number of Galactic globular clusters known to possess internal metallicity ([Fe/H]) spreads.

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arXiv2010

The Star Formation History of Isolated Dwarf UGC4879

Recent observations of UGC4879 with the Advanced Camera for Surveys on the Hubble Space Telescope confirm that it is a nearby isolated dwarf irregular galaxy. We measure a distance of 1.36+/-0.03 Mpc using the Tip of the Red Giant Branch method. This distance puts UGC4879 beyond the radius of first turnaround of the Local Group and ~700 kpc from its nearest neighbor Leo A. This isolation makes this galaxy an ideal laboratory for studying pristine star formation uncomplicated by interactions with other galaxies. We present the star formation history of UGC4879 derived from simulated color-magnitude diagrams.

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arXiv2010

Hubble Space Telescope survey of the Perseus Cluster -III: The effect of local environment on dwarf galaxies

We present the results of a Hubble Space Telescope (HST) study of dwarf galaxies in the outer regions of the nearby rich Perseus Cluster, down to M_V = -12, and compare these with the dwarf population in the cluster core from our previous HST imaging. In this paper we examine how properties such as the colour magnitude relation, structure and morphology are affected by environment for the lowest mass galaxies. Dwarf galaxies are excellent tracers of the effects of environment due to their low masses, allowing us to derive their environmentally based evolution, which is more subtle in more massive galaxies. We identify 11 dwarf elliptical (dE) and dwarf spheroidal (dSph) galaxies in the outer regions of Perseus, all of which are previously unstudied. We measure the (V-I)_0 colours of our newly discovered dEs, and find that these dwarfs lie on the same red sequence as those in the cluster core. The morphologies of these dwarfs are examined by quantifying their light distributions using CAS parameters, and we find that dEs in the cluster outskirts are on average more disturbed than those in the core, with =0.13\pm0.09 and =0.18\pm0.08, compared to =0.02\pm0.04, =0.01\pm0.07 for those in the core. Based on these results, we infer that these objects are ``transition dwarfs'', likely in the process of transforming from late-type to early type galaxies as they infall into the cluster, with their colours transforming before their structures. When we compare the number counts for both the core and outer regions of the cluster, we find that below M_V = -12, the counts in the outer regions of the cluster exceed those in the core. This is evidence that in the very dense region of the cluster, dwarfs are unable to survive unless they are sufficiently massive to prevent their disruption by the cluster potential and interactions with other galaxies.

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arXiv2009

Smooth, undisturbed dwarf spheroidal galaxies in the Perseus Cluster core: Implications for dark matter content

Using deep HST/ACS observations of the core of the Perseus Cluster, we identify a large population of dwarf elliptical galaxies down to M_V = -12. All these dwarfs are remarkably smooth in appearance, showing no evidence for internal features that could be the result of tidal processes or star formation induced by the cluster potential. Based on these observations and the relatively large sizes of these dwarfs, we argue that at least some must have a large dark matter component to prevent their disruption by the cluster potential. We further derive a new method to quantify the dark matter content of cluster dSphs without the use of kinematics, which are impossible to obtain at these distances. We find that mass-to-light ratios for dwarfs in the core of the Perseus Cluster are comparable to those found for Local Group dSphs, ranging between M/L ~ 1 and 120. This is evidence that dwarf spheroidals reside in dark matter subhalos that protect them from tidal processes in the cores of dense clusters.

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arXiv2009

Variable Stars in the Fornax dSph Galaxy. III. The Globular Cluster Fornax 5

We present a new study of the variable star population in globular cluster 5 of the Fornax dwarf spheroidal galaxy, based on B and V time series photometry obtained with the MagIC camera of the 6.5 m Magellan Clay telescope and complementary HST archive data. Light curves and accurate periodicities were obtained for 30 RR Lyrae stars and 1 SX Phoenicis variable. The RR Lyrae sample includes 15 fundamental-mode pulsators, 13 first-overtone pulsators, 1 candidate double-mode pulsator and one RR Lyrae star with uncertain type classification. The average and minimum periods of the ab-type RR Lyrae stars, =0.590 days, P(ab,min)=0.53297 days, and the position in the horizontal branch type--metallicity plane, indicate that the cluster has Oosterhoff-intermediate properties, basically confirming previous indications by Mackey & Gilmore (2003b), although with some differences both in the period and type classification of individual variables. The average apparent magnitude of the Fornax 5 RR Lyrae stars is =21.35 +/- 0.02 mag (sigma=0.07 mag, average on 14 stars more likely belonging to the cluster, and having well sampled light curves). This value leads to a true distance modulus of mu0=20.76 +/- 0.07 (d=141.9 (+4.6;-4.5) kpc) if we adopt for the cluster the metal abundance by Buonanno et al. (1998; [Fe/H]=-2.20 +/- 0.20), or mu0=20.66 +/- 0.07 (d=135.5 (+4.4;-4.3) kpc), if we adopt Strader et al.'s (2003) metal abundance ([Fe/H]=-1.73 +/- 0.13).

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arXiv2008

A New Hubble Space Telescope Distance to NGC 1569: Starburst Properties and IC 342 Group Membership

We present deep HST ACS/WFC photometry of the dwarf irregular galaxy NGC 1569, one of the closest and strongest nearby starburst galaxies. These data allow us, for the first time, to unequivocally detect the tip of the red giant branch and thereby determine the distance to NGC 1569. We find that this galaxy is 3.36 +/- 0.20 Mpc away, considerably farther away than the typically assumed distance of 2.2 +/- 0.6 Mpc. Previously thought to be an isolated galaxy due to its shorter distance, our new distance firmly establishes NGC 1569 as a member of the IC 342 group of galaxies. The higher density environment may help explain the starburst nature of NGC 1569, since starbursts are often triggered by galaxy interactions. On the other hand, the longer distance implies that NGC 1569 is an even more extreme starburst galaxy than previously believed. Previous estimates of the rate of star formation for stars younger than ~ 1 Gyr become stronger by more than a factor of 2. Stars older than this were not constrained by previous studies. The dynamical masses of NGC 1569's three super star clusters, which are already known as some of the most massive ever discovered, increase by ~53% to 6-7x10^5 solar masses.

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arXiv2008

Luminosity-Metallicity Relation for dIrr Galaxies in the Near-Infrared

(abridged) The present work is a first step to collect homogeneous abundances and near-infrared (NIR) luminosities for a sample of dwarf irregular (dIrr) galaxies, located in nearby groups. The use of NIR luminosities is intended to provide a better proxy to mass than the blue luminosities commonly used in the literature; in addition, selecting group members reduces the impact of uncertain distances. Accurate abundances are derived to assess the galaxy metallicity. Optical spectra are collected for Hii regions in the dIrrs, allowing the determination of oxygen abundances by means of the temperature-sensitive method. For each dIrr galaxy H-band imaging is performed and the total magnitudes are measured via surface photometry. This high-quality database allows us to build a well-defined luminosity-metallicity relation in the range -13 >= M(H) >= -20. The scatter around its linear fit is reduced to 0.11 dex, the lowest of all relations currently available. There might exist a difference between the relation for dIrrs and the relation for giant galaxies, although a firm conclusion should await direct abundance determinations for a significant sample of massive galaxies. This new dataset provides an improved luminosity-metallicity relation, based on a standard NIR band, for dwarf star-forming galaxies. The relation can now be compared with some confidence to the predictions of models of galaxy evolution. Exciting follow-ups of this work are (a) exploring groups with higher densities, (b) exploring nearby galaxy clusters to probe environmental effects on the luminosity-metallicity relation, and (c) deriving direct oxygen abundances in the central regions of star-forming giant galaxies, to settle the question of a possible dichotomy between the chemical evolution of dwarfs and that of massive galaxies.

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arXiv2008

Variable stars in the Fornax dSph Galaxy. II. Pulsating stars below the horizontal branch

We have carried out an intensive survey of the northern region of the Fornax dwarf spheroidal galaxy with the aim of detecting the galaxy's short--period pulsating stars (P<0.25 days). Observations collected over three consecutive nights with the Wide Field Imager of the 2.2m MPI telescope at ESO allowed us to detect 85 high-amplitude (0.20-1.00 mag in B-light) variable stars with periods in the range from 0.046 to 0.126 days, similar to SX Phoenicis stars in Galactic metal-poor stellar populations. The plots of the observed periods vs. the B and V magnitudes show a dispersion largely exceeding the observational errors. To disentangle the matter, we separated the first-overtone from the fundamental-mode pulsators and tentatively identified a group of subluminous variables, about 0.35 mag fainter than the others. Their nature as either metal-poor intermediate-age stars or stars formed by the merging of close binary systems is discussed. The rich sample of the Fornax variables also led us to reconstruct the Period-Luminosity relation for short-period pulsating stars. An excellent linear fit, M(V)=-1.83(+/-0.08)-3.65(+/-0.07) log P(fund), was obtained using 153 Delta Scuti and SX Phoenicis stars in a number of different stellar systems.

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arXiv2005

The stellar populations of local dwarfs

Recent progress in our knowledge of stellar populations in local dwarf spheroidal galaxies is briefly discussed. A few results are summarized including wide field observations of stellar populations and their spatial variations, studies of AGB and variable stars, extension to near-infrared wavelengths, and the interpretation effort based on synthetic color-magnitude diagrams and chemical evolution models.

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arXiv2005

The near-IR luminosity-metallicity relation of dwarf irregular galaxies

We report on the recent developments of our long-term investigation of the near-IR luminosity-metallicity relation for dwarf irregular galaxies in nearby groups. A very well-defined relation is emerging from our observational database, and a preliminary discussion of its implications is given.

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arXiv2002

VLT spectroscopy of globular clusters in the Sombrero galaxy

We have obtained intermediate-resolution VLT spectroscopy of 75 globular cluster candidates around the Sa galaxy M104 (NGC4594). Fifty-seven candidates out to ~ 40 kpc in the halo of the galaxy were confirmed to be bona-fide globular clusters, 27 of which are new. A first analysis of the velocities provides only marginal evidence for rotation of the cluster system. From Hbeta line strengths, almost all of the clusters in our sample have ages that are consistent, within the errors, with Milky Way globular clusters. Only a few clusters may be 1-2 Gyr old, and bulge and halo clusters appear coeval. The absorption line indices follow the correlations established for the Milky Way clusters. Metallicities are derived based upon new empirical calibrations with Galactic globular clusters taking into account the non-linear behavior of some indices (e.g., Mg2). Our sample of globular clusters in NGC4594 spans a metallicity range of -2.13 < [Fe/H] < +0.26 dex, and the median metallicity of the system is [Fe/H] = -0.85. Thus, our data provide evidence that some of the clusters have super-solar metallicity. Overall, the abundance distribution of the cluster system is consistent with a bimodal distribution with peaks at [Fe/H] ~- 1.7 and -0.7. However, the radial change in the metallicity distribution of clusters may not be straightforwardly explained by a varying mixture of two sub-populations of red and blue clusters.

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arXiv2002

Wide field photometry of the M104 globular cluster system

We present preliminary results of a wide field study of the globular cluster system of NGC4594, the Sombrero galaxy. The galaxy was observed in B, V, and R using the Wide Field Imager on the ESO 2.2m telescope. Using color and shape criteria to select a sample of highly probable globular cluster candidates, we measured the radial density profile of clusters out to 40' (100 Kpc) in the galaxy halo. The colors are consistent with the bimodal color distribution observed in previous studies. The red cluster candidates show a clear central concentration relative to the blue clusters. The population of red clusters does not appear significantly flattened, thus indicating that they are associated to the galaxy bulge rather than to the disk.

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arXiv2001

Dwarf galaxies in the Local Group: the VLT perspective

Recent results on the evolution of Local Group dwarf galaxies obtained from VLT imaging and spectroscopy are briefly reviewed, and prospects for dwarf galaxy research at the VLT are discussed in the light of the current and forthcoming instrumentation. Some aspects of future instrument developments, such as deep wide-field imaging at both optical and near-infrared wavelengths that may be of advantage for research on the evolution of dwarf galaxies, are briefly discussed.

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arXiv2001

Population gradients in nearby dwarf galaxies

Recent results on the detection of substructure in dwarf spheroidal galaxies are discussed. In most cases they show that, when a galaxy experiences multiple SF episodes, the intermediate age population is more centrally concentrated than the old population, and that the recent SF is even more concentrated towards the central regions. Moreover, it appears that the spatial distribution of stars becomes more and more irregular as younger and younger subpopulations are considered. We illustrate how wide-area observations, allowing a substantial coverage of dwarf spheroidals (in particular the satellites of the Milky Way), has provided evidence for cosmologically old populations in every dSph galaxy. All these results are placed in the broader scenario of population gradients in nearby dwarf galaxies of different morphological types.

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arXiv1999

The elusive old population of the dwarf spheroidal galaxy Leo I

We report the discovery of a significant old population in the dwarf spheroidal (dSph) galaxy Leo I as a result of a wide-area search with the ESO New Technology Telescope. Studies of the stellar content of Local Group dwarf galaxies have shown the presence of an old stellar population in almost all of the dwarf spheroidals. The only exception was Leo I, which alone appeared to have delayed its initial star formation episode until just a few Gyr ago. The color-magnitude diagram of Leo I now reveals an extended horizontal branch, unambiguously indicating the presence of an old, metal-poor population in the outer regions of this galaxy. Yet we find little evidence for a stellar population gradient, at least outside R > 2' (0.16 kpc), since the old horizontal branch stars of Leo I are radially distributed as their more numerous intermediate-age helium-burning counterparts. The discovery of a definitely old population in the predominantly young dwarf spheroidal galaxy Leo I points to a sharply defined first epoch of star formation common to all of the Local Group dSph's as well as to the halo of the Milky Way.

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arXiv1999

The stellar populations of the Fornax dwarf spheroidal galaxy

(abridged) We present B,V,I CCD photometry of about 40000 stars in four regions of the Fornax dSph. Using the resultant CMDs, many already known age-tracers are investigated, and new CMD features are also detected: we show that blue HB stars may be present in the outer regions, and measure the luminosity of the AGB bump. We measure a corrected distance modulus (m-M)o = 20.70+/-0.12, based on the RGB tip luminosity, which is in good agreement with that obtained from the mean magnitude of old HB stars [(m-M)o=20.76+/-0.04] and with previous results. The (B-I) distribution of the RG stars can be approximately described as the superposition of two populations. The dominant component, comprising ~70% of the red giant stars, consists of relatively metal-enriched intermediate-age stars, whose mean metallicity is [Fe/H]=-1.39+/-0.15 (the age-corrected metallicity would be [Fe/H] ~ -1.0+/-0.15). The dominant intermediate-age component has an intrinsic color dispersion σ(B-I)=0.06+/-0.01mag, corresponding to a relatively low abundance dispersion, σ[Fe/H]=0.12+/-0.02dex. In addition, there is a small population of giants on the blue side of the RGB, whose spatial distribution is consistent with that of old horizontal branch stars, and completely different from that of the younger population. This unambiguously qualifies them as old and metal-poor. The exceptional color width of the Fornax RGB is therefore due to the presence of two main populations, yielding a large abundance range (-2.0<[Fe/H]<-0.7). This evidence suggests a scenario in which this galaxy started forming a stellar halo and its surrounding clusters together about 10-13 Gyr ago, then a major SF episode occurred after several Gyr.

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