Local Group Galaxies Research Articles

Abundance patterns in early-type galaxies: is there a “knee” in the [Fe/H] vs. [α/Fe] relation?

Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in accordance with their old ages and short formation timescales. In this contribution we aim to resolve the enrichment histories of ETGs. This means we study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe]) separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we show that we can indeed recover the enrichment history per galaxy. We then analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS selected to be ETGs. We separate out those galaxies for which the abundance of iron in stars grows throughout the lifetime of the galaxy, i.e. in which [Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe] parameters are correlated with the mass and velocity dispersion of ETGs. We emphasize that the strongest relation is between [alpha/Fe] and age. This relation falls into two regimes, one with a steep slope for old galaxies and one with a shallow slope for younger ETGs. The vast majority of ETGs in our sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly observed in local group galaxies. This implies that for the vast majority of ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or formed from accreted gas. The properties of the intermediate-age stars in accretion-dominated ETGs indicate that mass growth through late (minor) mergers in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method of reconstructing the stellar enrichment histories of ETGs introduced in this paper promises to constrain the star formation and mass assembly histories of large samples of galaxies in a unique way.

SUSTAINING GALAXY EVOLUTION: THE ROLE OF STELLAR FEEDBACK

We have conducted a near-infrared monitoring campaign at the UK InfraRed Telescope (UKIRT), of the Local Group galaxy M33. The main aim was to identify stars in the very final stage of their evolution, and for which the luminosity is more directly related to the birth mass than the more numerous less-evolved giant stars that continue to increase in luminosity. The pulsating giant stars (AGB and red supergiants) are identified and their distributions are used to derive the star formation rate as a function of age. These stars are also important dust factories; we measure their dust production rates from a combination of our data with Spitzer Space Telescope mid-IR photometry. The mass-loss rates are seen to increase with increasing strength of pulsation and with increasing bolometric luminosity. Low-mass stars lose most of their mass through stellar winds, but even super-AGB stars and red superginats lose~40% of their mass via a dusty stellar wind. We construct a 2-D map of the mass-return rate, showing a radial decline but also local enhancements due to agglomerations of massive stars. By comparing the current star formation rate with total mass input to the ISM, we conclude that the star formation in the central regions of M33 can only be sustained if gas is accreted from further out in the disc or from circum-galactic regions.

MID-INFRARED PERIOD-METALLICITY-LUMINOSITY RELATIONS AND KINEMATICS OF RR LYRAE VARIABLES

We use ALLWISE data release W1- and W2-band epoch photometry collected by the Wide-Field Infrared Survey Explorer (WISE) to determine slopes of the period-luminosity relations for RR Lyrae stars in 15 globular clusters in the corresponding bands. We further combine these results with V- and K-band photometry of Galactic eld RR Lyrae stars to determine the metallicity slopes of the log P F -[Fe/H]-M K , log P F - [Fe/H]-M W1 , and log P F - [Fe/H]- M W2 period-metallicity-luminosity relations. We infer the zero points of these relations and determine the kinematical parameters of thick-disk and halo RR Lyraes via statistical parallax, and estimate the RR Lyrae-based distances to 18 Local-Group galaxies including the center of the Milky Way.

ORIGIN AND EVOLUTION OF STRUCTURE FOR GALAXIES IN THE LOCAL GROUP

The Milky Way did not form in isolation, but is the product of a complex evolution of generations of mergers, collapses, star formation, supernovae and collisional heating, radiative and collisional cooling, and ejected nucleosynthesis. Moreover, all of this occurs in the context of the cosmic expansion, the formation of cosmic laments, dark-matter haloes, spiral density waves, and emerging dark energy. This paper summarizes a review of recent attempts to reconstruct this complex evolution. We compare simulated properties with various observed properties of the Local Group. Among the generic features of simulated systems is the tendency for galactic halos to form within the dark matter laments that dene a supergalactic plane. Gravitational interaction along this structure leads to a streaming ow toward the two dominant galaxies in the cluster. We analyze this alignment and streaming ow and compare with the observed properties of Local-Group galaxies. Our comparison with Local Group properties suggests that some dwarf galaxies in the Local Group are part of a local streaming ow. These simulations also suggest that a signicant fraction of the Galactic halo formed at large distances and arrived later along these streaming ows.

Wolf-Rayet stars in the Small Magellanic Cloud

Wolf-Rayet (WR) stars have a severe impact on their environments owing to their strong ionizing radiation fields and powerful stellar winds. Since these winds are considered to be driven by radiation pressure, it is theoretically expected that the degree of the wind mass-loss depends on the initial metallicity of WR stars. Following our comprehensive studies of WR stars in the Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates for all seven putatively single WN stars known in the SMC. Based on these data, we discuss the impact of a low-metallicity environment on the mass loss and evolution of WR stars. The quantitative analysis of the WN stars is performed with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical properties of our program stars are obtained from fitting synthetic spectra to multi-band observations. In all SMC WN stars, a considerable surface hydrogen abundance is detectable. The majority of these objects have stellar temperatures exceeding 75 kK, while their luminosities range from 10^5.5 to 10^6.1 Lsun. The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M31, and the LMC. By comparing the mass-loss rates derived for WN stars in different Local Group galaxies, we conclude that a clear dependence of the wind mass-loss on the initial metallicity is evident, supporting the current paradigm that WR winds are driven by radiation. A metallicity effect on the evolution of massive stars is obvious from the HRD positions of the SMC WN stars at high temperatures and high luminosities. Standard evolution tracks are not able to reproduce these parameters and the observed surface hydrogen abundances. Homogeneous evolution might provide a better explanation for their evolutionary past.

The Local Group as a time machine: studying the high-redshift Universe with nearby galaxies

We infer the UV luminosities of Local Group galaxies at early cosmic times (z ∼ 2 and z ∼ 7) by combining stellar population synthesis modelling with star formation histories derived from deep colour–magnitude diagrams constructed from Hubble Space Telescope (HST) observations. Our analysis provides a basis for understanding high-z galaxies – including those that may be unobservable even with the James Webb Space Telescope (JWST) – in the context of familiar, well-studied objects in the very low-z Universe. We find that, at the epoch of reionization, all Local Group dwarfs were less luminous than the faintest galaxies detectable in deep HST observations of blank fields. We predict that JWST will observe z ∼ 7 progenitors of galaxies similar to the Large Magellanic Cloud today; however, the HST Frontier Fields initiative may already be observing such galaxies, highlighting the power of gravitational lensing. Consensus reionization models require an extrapolation of the observed blank-field luminosity function (LF) at z ≈ 7 by at least 2 orders of magnitude in order to maintain reionization. This scenario requires the progenitors of the Fornax and Sagittarius dwarf spheroidal galaxies to be contributors to the ionizing background at z ∼ 7. Combined with numerical simulations, our results argue for a break in the UV LF from a faint-end slope of α ∼ −2 at MUV ≲ −13 to α ∼ −1.2 at lower luminosities. Applied to photometric samples at lower redshifts, our analysis suggests that HST observations in lensing fields at z ∼ 2 are capable of probing galaxies with luminosities comparable to the expected progenitor of Fornax.

Deceleration parameter in tilted Friedmann universes

Large-scale peculiar motions are believed to reflect the local inhomogeneity and anisotropy of the universe, triggered by the ongoing process of structure formation. As a result, realistic observers do not follow the smooth Hubble flow but have a peculiar, `tilt', velocity relative to it. Our Local Group of galaxies, in particular, moves with respect to the universal expansion at a speed of roughly 600~km/sec. Relative motion effects are known to interfere with the observations and their interpretation. The strong dipolar anisotropy seen in the Cosmic Microwave Background, for example, is not treated as a sign of real universal anisotropy, but as a mere artifact of our peculiar motion relative to the Hubble flow. With these in mind, we look into the implications of large-scale bulk motions for the kinematics of their associated observers, by adopting a `tilted' Friedmann model. Our aim is to examine whether the deceleration parameter measured in the rest-frame of the bulk flow can differ from that of the actual universe due to relative-motion effects alone. We find that there is a difference, which depends on the speed as well as the scale of the bulk motion. The faster and the smaller the drifting domain, the larger the difference. In principle, this allows relatively slow peculiar velocities to have a disproportionately strong effect on the value of the deceleration parameter measured by observers within bulk flows of, say, few hundred megaparsecs. In fact, under certain circumstances, it is even possible to change the sign of the deceleration parameter. It goes without saying that all these effects vanish identically in the Hubble frame, which makes then an illusion and mere artifact of the observers' relative motion.

Planetary Nebulae and their parent stellar populations. Tracing the mass assembly of M87 and Intracluster light in the Virgo cluster core

AbstractThe diffuse extended outer regions of galaxies are hard to study because they are faint, with typical surface brightness of 1% of the dark night sky. We can tackle this problem by using resolved star tracers which remain visible at large distances from the galaxy centers. This article describes the use of Planetary Nebulae as tracers and the calibration of their properties as indicators of the star formation history, mean age and metallicity of the parent stars in the Milky Way and Local Group galaxies. We then report on the results from a deep, extended, planetary nebulae survey in a 0.5 deg2region centered on the brightest cluster galaxy NGC 4486 (M87) in the Virgo cluster core, carried out with SuprimeCam@Subaru and FLAMES-GIRAFFE@VLT. Two planetary nebulae populations are identified out to 150 kpc distance from the center of M87. One population is associated with the M87 halo and the second one with the intracluster light in the Virgo cluster core. They have different line-of-sight velocity and spatial distributions, as well as different planetary nebulae specific frequencies and luminosity functions. The intracluster planetary nebulae in the surveyed region correspond to a luminosity of four times the luminosity of the Large Magellanic Cloud. The M87 halo planetary nebulae trace an older, more metal-rich, parent stellar population. A substructure detected in the projected phase-space of the line-of-sight velocity vs. major axis distance for the M87 halo planetary nebulae provides evidence for the recent accretion event of a satellite galaxy with luminosity twice that of M33. The satellite stars were tidally stripped about 1 Gyr ago, and reached apocenter at a major axis distance of 60–90 kpc from the center of M87. The M87 halo is still growing significantly at the distances where the substructure is detected.

Dust and molecules in extra-galactic planetary nebulae

AbstractExtra-galactic planetary nebulae (PNe) permit the study of dust and molecules in metallicity environments other than the Galaxy. Their known distances lower the number of free parameters in the observations vs. models comparison, providing strong constraints on the gas-phase and solid-state astrochemistry models. Observations of PNe in the Galaxy and other Local Group galaxies such as the Magellanic Clouds (MC) provide evidence that metallicity affects the production of dust as well as the formation of complex organic molecules and inorganic solid-state compounds in their circumstellar envelopes. In particular, the lower metallicity MC environments seem to be less favorable to dust production and the frequency of carbonaceous dust features and complex fullerene molecules is generally higher with decreasing metallicity. Here, I present an observational review of the dust and molecular content in extra-galactic PNe as compared to their higher metallicity Galactic counterparts. A special attention is given to the level of dust processing and the formation of complex organic molecules (e.g., polycyclic aromatic hydrocarbons, fullerenes, and graphene precursors) depending on metallicity.

PN populations in the local group and distant stellar populations

AbstractOur understanding of galactic structure and evolution is far from complete. Within the past twelve months we have learnt that the Milky Way is about 50% wider than was previously thought. As a consequence, new models are being developed that force us to reassess the kinematic structure of our Galaxy. Similarly, we need to take a fresh look at the halo structure of external galaxies in our Local Group. Studies of stellar populations, star-forming regions, clusters, the interstellar medium, elemental abundances and late stellar evolution are all required in order to understand how galactic assembly has occurred as we see it. PNe play an important role in this investigation by providing a measure of stellar age, mass, abundances, morphology, kinematics and synthesized matter that is returned to the interstellar medium (ISM). Through a method of chemical tagging, halo PNe can reveal evidence of stellar migration and galactic mergers. This is an outline of the advances that have been made towards uncovering the full number of PNe in our Local Group galaxies and beyond. Current numbers are presented and compared to total population estimates based on galactic mass and luminosity. A near complete census of PNe is crucial to understanding the initial-to-final mass relation for stars with mass &gt;1 to &lt;8 times the mass of the sun. It also allows us to extract more evolutionary information from luminosity functions and compare dust-to-gas ratios from PNe in different galactic locations. With new data provided by the Gaia satellite, space-based telescopes and the rise of giant and extra-large telescopes, we are on the verge of observing and understanding objects such as PNe in distant galaxies with the same detail we expected from Galactic observations only a decade ago.

Revisiting the red supergiant progenitors of core-collapse supernovae

AbstractI will present a reanalysis of the properties of the red supergiant (RSG) progenitors of several extragalactic core-collapse, Type II-Plateau supernovae, including SN 2012aw, SN 2013ej, SN 2004et, and SN 2008bk. The observed spectral energy distributions (SEDs) for these stars, from the optical to the infrared, are compared with model SEDs for Galactic RSGs, as well as with the observed SEDs of analogous RSGs in Local Group galaxies. As a result, new estimates of the stellar temperatures and luminosities for the progenitors are obtained, and through comparison with recent massive stellar evolutionary tracks, new inferences are made for the progenitors initial masses. I will place these in the context of the RSG SN progenitor mass spectrum and of the so-called “red supergiant problem.”

Stellar halos around Local Group galaxies

AbstractThe Local Group is now home to 102 known galaxies and candidates, with many new faint galaxies continuing to be discovered. The total stellar mass range spanned by this population covers a factor of close to a billion, from the faintest systems with stellar masses of order a few thousand to the Milky Way and Andromeda, with stellar masses of order 1011M⊙. Here, I discuss the evidence for stellar halos surrounding Local Group galaxies spanning from dwarf scales (with the case of the Andromeda II dwarf spheroidal), though to intermediate mass systems (M33) and finishing with M31. Evidence of extended stellar populations and merging is seen across the luminosity function, indicating that the processes that lead to halo formation are common at all mass scales.

Globular clusters in M31, Local Group, and external galaxies

AbstractThroughout most of the Local Group, globular clusters (GCs) remain recognisable as extended objects in ground-based images taken in good seeing conditions. However, studying the full extent of the GC systems is challenging because of the large sky area that needs to be surveyed and recent years have seen dramatic progress in our knowledge of GC populations in nearby galaxies, thanks to large imaging surveys. At the same time, techniques for deriving detailed abundances from integrated-light spectra of GCs are maturing so that detailed comparisons of the chemical composition for GCs in different galaxies can now be made. Such comparisons may shed important light on the properties of proto-galactic fragments that were accreted onto galaxy halos. Nevertheless, our census of Local Group GCs probably remains far from complete, in particular at low luminosities and for very extended clusters.

AGC 226067: A possible interacting low-mass system

We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source AGC 226067. Originally identified as an ultra-compact high velocity cloud and candidate Local Group galaxy, AGC 226067 is spatially and kinematically coincident with the Virgo cluster, and the identification by multiple groups of an optical counterpart with no resolved stars supports the interpretation that this systems lies at the Virgo distance (D=17 Mpc). The combined observations reveal that the system consists of multiple components: a central HI source associated with the optical counterpart (AGC 226067), a smaller HI-only component (AGC 229490), a second optical component (AGC 229491), and extended low surface brightness HI. Only ~1/4 of the single-dish HI emission is associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x 10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g = 3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC 226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC 229491 are separated by only 0.5'. The current data do not resolve the HI in AGC 229490 and its origin is unclear. We discuss possible scenarios for this system of objects: an interacting system of dwarf galaxies, accretion of material onto AGC 226067, or stripping of material from AGC 226067.

Did globular clusters contribute to the stellar population of the Galactic halo?

AbstractThe origin of Galactic halo stars and the contribution of globular clusters (GC) to this stellar population have long been (and still are) debated. The discovery of multiple stellar populations with peculiar chemical properties in GCs both in the Milky Way and in Local Group galaxies recently brought a renewal on these questions. Indeed most of the scenarios that compete to reproduce the present-day GC characteristics call for fast expulsion of both gas and low-mass stars from these clusters in their early infancy. In this framework, the initial masses of GCs could have been 8 to 25 times higher than their present-day stellar mass, and they could have contributed to 5 to 20 % of the low-mass stars in the Galactic halo. Here we revisit these conclusions, which are in tension with observations of dwarf galaxies and of young massive star clusters in the Local Group. We come back in particular on the paradigm of gas expulsion from massive star clusters, and propose an alternative interpretation of the GC abundance properties. We conclude by proposing a major revision of the current concepts regarding the role massive star clusters play in the assembly of galactic haloes.

The LAMOST survey of background quasars in the vicinity of M31 and M33 – III. results from the 2013 regular survey

In this work, we report new quasars discovered in fields in the vicinity of the Andromeda (M31) and Triangulum (M33) galaxies with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) during the 2013 observational season, the second year of the regular survey. In total, 1330 new quasars are discovered in an area of ∼133 deg2 around M31 and M33. With i magnitudes ranging from 14.79 to 20.0 and redshifts from 0.08 to 4.85, the 1330 new quasars represent a significant increase in the number of identified quasars in fields in the vicinity of M31 and M33. Up to now, there have been a total of 1870 quasars discovered by LAMOST in this area. The much enlarged sample of known quasars in this area can potentially be utilized to construct a precise astrometric reference frame for the measurement of minute proper motions of M31, M33 and their associated substructures, which are vital for understanding the formation and evolution of M31, M33 and the Local Group of galaxies. Moreover, in the sample, there are a total of 45, 98 and 225 quasars with i magnitudes brighter than 17.0, 17.5 and 18.0 respectively. In the aforementioned brightness bins, 15, 35 and 84 quasars are reported here for the first time, and 6, 21 and 81 are reported in our pervious work. In addition, 0, 1 and 6 are from the Sloan Digital Sky Survey and 24, 41 and 54 are from the NED database. These bright quasars provide an invaluable sample to study the kinematics and chemistry of the interstellar/intergalactic medium of the Local Group.

The Local Group Galaxy IC 1613 and its asymptotic giant branch variables

JHKs photometry is presented from a three-year survey of the central regions of the Local Group dwarf irregular galaxy IC1613. The morphologies of the colour-magnitude and colour-colour diagrams are discussed with particular reference to the supergiants and M- and C-type asymptotic giant branch (AGB) stars. Mean JHKs magnitudes, amplitudes and periods are given for five O-rich and nine C-rich Mira variables for which bolometric magnitudes are also estimated. A distance of 750 kpc ($(m-M)_0=24.37\pm 0.08$ mag) is derived for IC1613 by fitting a period-luminosity relation to the C-rich Miras. This is in agreement with values from the literature. The AGB stars exhibit a range of ages. A comparison with theoretical isochrones suggests that four luminous O-rich Miras are as young as $2\times 10^8$ yrs. One of these has a lithium absorption line in its spectrum, demonstrating that it is undergoing hot bottom burning (HBB). This supports the idea that HBB is the cause of the high luminosity of these AGB stars, which puts them above the fundamental period-luminosity (PL) relation. Further studies of similar stars, selected from their positions in the PL diagram, could provide insight into HBB. A much fainter, presumed O-rich, Mira is similar to those found in Galactic globular clusters. The C Miras are of intermediate age. The O-rich variables are not all recognized as O-rich, or even as AGB stars, on the basis of their J-Ks colour. It is important to appreciate this when using near-infrared surveys to classify AGB stars in more distant galaxies.

Evidence for temporal evolution in the M33 disc as traced by its star clusters

We present precision radial velocities and stellar population parameters for 77 star clusters in the Local Group galaxy M33. Our GTC and WHT observations sample both young, massive clusters and known/candidate globular clusters, spanning ages � 10 6 10 10 yr, and metallicities, [M/H] � 1.7 to solar. The cluster system exhibits an age-metallicity relation; the youngest clusters are the most metal-rich. When compared to HI data, clusters with [M/H]� 1.0 and younger than � 4 Gyr are clearly identified as a disc population. The clusters show evidence for strong time evolution in the disc radial metallicity gradient (d[M/H]dt / dR = 0.03 dex/kpc/Gyr). The oldest clusters have stronger, more negative gradients than the youngest clusters in M33. The clusters also show a clear age-velocity dispersion relation. The line of sight velocity dispersions of the clusters increases with age similar to Milky Way open clusters and stars. The general shape of the relation is reproduced by disc heating simulations, and the similarity between the relations in M33 and the Milky Way suggests that heating by substructure, and cooling of the ISM both play a role in shaping this relation. We identify 12 “classical” GCs, six of which are newly identified GC candidates. The GCs are more metal-rich than Milky Way halo clusters, and show weak rotation. The inner (R < 4.5 kpc) GCs exhibit a steep radial metallicity gradient (d[M/H]/dR = 0.29 ± 0.11 dex/kpc) and an exponential-like surface density profile. We argue that these inner GCs are thick disc rather than halo objects.

A UNIVERSAL, TURBULENCE-REGULATED STAR FORMATION LAW: FROM MILKY WAY CLOUDS TO HIGH-REDSHIFT DISK AND STARBURST GALAXIES

While the star formation rate (SFR) of molecular clouds and galaxies is key in understanding galaxy evolution, the physical processes that determine the SFR remain unclear. This uncertainty about the underlying physics has resulted in various different star formation (SF) laws, all having substantial intrinsic scatter. Extending upon previous works that define the column density of star formation () by the gas column density (), we develop a new universal SF law based on the multi-freefall prescription of gas. This new SF law relies predominantly on the probability density function and on the sonic Mach number of the turbulence in the star-forming clouds. By doing so we derive a relation where the SFR correlates with the molecular gas mass per multi-freefall time, whereas previous models had used the average, single-freefall time. We define a new quantity called maximum (multi-freefall) gas consumption rate (MGCR) and show that the actual SFR is only about 0.4% of this maximum possible SFR, confirming the observed low efficiency of SF. We show that placing observations in this new framework ( versus MGCR) yields a significantly improved correlation with 3–4 times reduced scatter compared to previous SF laws and a goodness-of-fit parameter . By inverting our new relationship, we provide sonic Mach number predictions for kiloparsec-scale observations of Local Group galaxies as well as unresolved observations of local and high-redshift disk and starburst galaxies that do not have independent, reliable estimates for the turbulent cloud Mach number.

RED SUPERGIANTS AS COSMIC ABUNDANCE PROBES: THE SCULPTOR GALAXY NGC 300

We present a quantitative spectroscopic study of twenty-seven red supergiants in the Sculptor Galaxy NGC 300. J-band spectra were obtained using KMOS on the VLT and studied with state of the art synthetic spectra including NLTE corrections for the strongest diagnostic lines. We report a central metallicity of [Z]= -0.03 +/- 0.05 with a gradient of -0.083 +/- 0.014 [dex/kpc], in agreement with previous studies of blue supergiants and H II-region auroral line measurements. This result marks the first application of the J-band spectroscopic method to a population of individual red supergiant stars beyond the Local Group of galaxies and reveals the great potential of this technique.