Fifty-seven nearby low surface brightness dwarf galaxies (-10 QQQ ? Mv QQQ ? -16) were searched for globular cluster candidates (GCCs) using Hubble Space Telescope WFPC2 imaging in V and I. The ...sample consists of 18 dwarf spheroidal (dSph), 36 irregular (dIrr), and 3 "transition" type (dIrr/dSph) galaxies with angular sizes less than 3.7 kpc situated at distances 2-6 Mpc in the field and in the nearby groups: M 81, Centaurus A, Sculptor, Canes Venatici I cloud. We find that 650% of dSph, dIrr/dSph, and dIrr galaxies contain GCCs. The fraction of GCCs located near the center of dwarf spheroidal galaxies is QQQ ?2 times higher than for dIrrs. The mean integral color of GCCs in dSphs, (V - I)0 = 1.04 c 0.16 mag, coincides with the corresponding value for Galactic globular clusters and is similar to the blue globular cluster sub-populations in massive early-type galaxies. The color distribution for GCCs in dIrrs shows a clear bimodality with peaks near (V - I)0 = 0.5 and 1.0 mag. Blue GCCs are presumably young with ages t QQQ ? 1 Gyr, while the red GCC population is likely to be older. The detected GCCs have absolute visual magnitudes between Mv = -10 and -5 mag. We find indications for an excess population of faint GCCs with Mv QQQ ? -6.5 mag in both dSph and dIrr galaxies, reminiscent of excess populations of faint globular clusters in nearby Local Group spiral galaxies. The measurement of structural parameters using King-profile fitting reveals that most GCCs have structural parameters similar to extended outer halo globular clusters in the Milky Way and M 31, as well as the recently discovered population of "faint fuzzy" clusters in nearby lenticular galaxies.
Context.
Giant low surface brightness (GLSB) galaxies are an extreme class of objects with very faint and extended gas-rich disks. Malin 1 is the largest GLSB galaxy known to date and one of the ...largest individual spiral galaxies observed so far, but the properties and formation mechanisms of its giant disk are still poorly understood.
Aims.
We used VLT/MUSE IFU spectroscopic observations of Malin 1 to measure the star formation rate (SFR), dust attenuation, and gas metallicity within this intriguing galaxy.
Methods.
We performed a penalized pixel fitting modeling to extract emission line fluxes such as H
α
, H
β
, N
II
6583
and O
III
5007
along the central region as well as from the extended disk of Malin 1.
Results.
Our observations reveal for the first time strong H
α
emission distributed across numerous regions throughout the extended disk of Malin 1. The emission extends to radial distances of ∼100 kpc, which indicates recent star formation activity. We made an estimate of the dust attenuation in the disk of Malin 1 using the Balmer decrement and found that Malin 1 has a mean H
α
attenuation of 0.36 mag. We observe a steep decline in the radial distribution of the SFR surface density (Σ
SFR
) within the inner 20 kpc, followed by a shallow decline in the extended disk. We estimated the gas phase metallicity in Malin 1. We also found for the first time that the metallicity shows a steep gradient from solar metallicity to subsolar values in the inner 20 kpc of the galaxy, followed by a flattening of the metallicity in the extended disk with a relatively high value of ∼0.6
Z
⊙
. We found that the normalized abundance gradient of the inner disk of Malin 1 is similar to the values found in normal galaxies. However, the normalized gradient observed in the outer disk can be considered extreme when compared to other disk galaxies. A comparison of the SFR surface density and gas surface density shows that unlike normal disk galaxies or other low surface brightness galaxies, the outer disk of Malin 1 exhibits a relatively low star formation efficiency based on atomic gas-mass estimates, which may be mildly exacerbated by the vanishing upper molecular gas-mass limits found by recent CO studies.
Conclusions.
With the detection of emission lines in a large part of the extended disk of Malin 1, this work sheds light on the star formation processes in this unique galaxy, highlighting its extended star-forming disk, dust attenuation, almost flat metallicity distribution in the outer disk, and exceptionally low star formation efficiency. Together with previous results, our findings contribute to a more detailed understanding of the formation of the giant disk of Malin 1, and they also constrain possible proposed scenarios of the nature of GLSB galaxies in general.
ABSTRACT We investigate the intrinsic shapes of low-luminosity galaxies in the central 300 kpc of the Virgo Cluster using deep imaging obtained as part of the Next Generation Virgo Cluster Survey ...(NGVS). We build a sample of nearly 300 red-sequence cluster members in the yet-unexplored −14 < Mg < −8 mag range, and we measure their apparent axis ratios, q, through Sérsic fits to their two-dimensional light distribution, which is well described by a constant ellipticity parameter. The resulting distribution of apparent axis ratios is then fit by families of triaxial models with normally distributed intrinsic ellipticities, E = 1 − C/A, and triaxialities, T = (A2 − B2)/(A2 − C2). We develop a Bayesian framework to explore the posterior distribution of the model parameters, which allows us to work directly on discrete data, and to account for individual, surface-brightness-dependent axis ratio uncertainties. For this population we infer a mean intrinsic ellipticity = and a mean triaxiality = . This implies that faint Virgo galaxies are best described as a family of thick, nearly oblate spheroids with mean intrinsic axis ratios 1:0.94:0.57. The core of Virgo lacks highly elongated low-luminosity galaxies, with 95% of the population having q > 0.45. We additionally attempt a study of the intrinsic shapes of Local Group (LG) satellites of similar luminosities. For the LG population we infer a slightly larger mean intrinsic ellipticity = , and the paucity of objects with round apparent shapes translates into more triaxial mean shapes, 1:0.76:0.49. Numerical studies that follow the tidal evolution of satellites within LG-sized halos are in good agreement with the inferred shape distributions, but the mismatch for faint galaxies in Virgo highlights the need for more adequate simulations of this population in the cluster environment. We finally compare the intrinsic shapes of NGVS low-mass galaxies with samples of more massive quiescent systems, and with field, star-forming galaxies of similar luminosities. We find that the intrinsic flattening in this low-luminosity regime is almost independent of the environment in which the galaxy resides, but there is a hint that objects may be slightly rounder in denser environments. The comparable flattening distributions of low-luminosity galaxies that have experienced very different degrees of environmental effects suggest that internal processes are the main drivers of galaxy structure at low masses, with external mechanisms playing a secondary role.
We present measurements of ages, metallicities and α/Fe ratios for 16 globular clusters (GCs) in NGC 147, 185 and 205 and of the central regions of the diffuse galaxy light in NGC 185 and 205. Our ...results are based on spectra obtained with the SCORPIO multislit spectrograph at the 6-m telescope of the Russian Academy of Sciences. We include in our analysis high-quality Hubble Space Telescope/WFPC2 photometry of individual stars in the studied GCs to investigate the influence of their horizontal branch (HB) morphology on the spectroscopic analysis. All our sample GCs appear to be old (T > 8 Gyr) and metal-poor (Z/H≲−1.1), except for the GCs Hubble V in NGC 205 (T= 1.2 ± 0.6 Gyr, Z/H=−0.6 ± 0.2), Hubble VI in NGC 205 (T= 4 ± 2 Gyr, Z/H=−0.8 ± 0.2) and FJJVII in NGC 185 (T= 7 ± 3 Gyr, Z/H=−0.8 ± 0.2). The majority of our GCs sample has solar α/Fe enhancement in contrast to the halo population of GCs in M31 and the Milky Way. The HB morphologies for our sample GCs follow the same behaviour with metallicity as younger halo Galactic GCs. We show that it is unlikely that they bias our spectroscopic age estimates based on Balmer absorption-line indices. Spectroscopic ages and metallicities of the central regions in NGC 205 and 185 coincide with those obtained from colour–magnitude diagrams. The central field stellar populations in these galaxies have approximately the same age as their most central GCs (Hubble V in NGC 205 and FJJIII in NGC 185), but are more metal-rich than the central GCs.
Aims. We present the results of an optical-near-infrared photometric survey of globular cluster systems in early-type galaxies. Our project aims to study correlations between the age structure of the ...globular cluster systems and environmental as well as structural parameters of their host galaxies. Methods. To derive the age structure in globular cluster systems we use combined optical and near-infrared photometry. This combination facilitates the detection of young globular cluster sub- populations if their age differs significantly from the old ( similar to 13 Gyr) population, i.e. if their age is \la7 Gyr. We investigate globular cluster systems in galaxies located in the central regions of galaxy clusters (NGC 1399, M 87, NGC 4478), galaxies which are members of (sub-) groups (NGC 4365, NGC 5846) and isolated galaxies (NGC 3115 and NGC 7192). Further, we look for correlations between globular cluster system properties and structural parameters of the galaxies (velocity dispersion \sigma, color ( ( V- K), ( B-V)), and luminosity M sub( )V. For a quantitative analysis we define the "Methuselah" parameter, {\tt Mth}-based on the fraction of young and old metal-rich globular clusters weighted by age, characterizing the age structure of a globular cluster system. Results. We find that, within our galaxy sample, group galaxies appear to host an intermediate-age globular cluster population, whereas the globular cluster systems in cluster galaxies and in rather isolated galaxies are dominated by an old globular cluster population. A weak correlation of {\tt Mth} with the H_\beta index of the integrated light of the host galaxies is the only trend found with respect to internal galaxy parameters.
We present Lick line-index measurements of extragalactic globular clusters in seven early-type galaxies (NGC 1380, 2434, 3115, 3379, 3585, 5846, and 7192) with different morphological types (E-S0) ...located in field and group/cluster environments. High-quality spectra were taken with the FORS2 instrument at ESO's Very Large Telescope. similar to 50% of our data allows an age resolution Delta t/t approximately 0.3 and a metallicity resolution similar to 0.25-0.4 dex, depending on the absolute metallicity. Globular cluster candidates are selected from deep B, V, R, I, K FORS2/ISAAC photometry with 80-100% success rate inside one effective radius. Using combined optical/near-infrared colour-colour diagrams we present a method to efficiently reduce fore-/background contamination down to 10%. We find clear signs for bi-modality in the globular cluster colour distributions of NGC 1380, 3115, and 3585. The colour distributions of globular clusters in NGC 2434, 3379, 5846, and 7192 are consistent with a broad single-peak distribution. For the analysed globular cluster systems the slopes of projected radial surface density profiles, of the form capital sigma (R) similar to R super(- Gamma ), vary between similar to 0.8 and 2.6. Blue and red globular cluster sub-populations show similar slopes in the clearly bi-modal systems. For galaxies with single-peak globular cluster colour distributions, there is a hint that the blue cluster system seems to have a more extended radial distribution than the red one. Using globular clusters as a tracer population we determine total dynamical masses of host galaxies out to large radii ( similar to 1.6-4.8 R sub(eff)). For the sample we find masses in the range similar to 8.8 x 10 super(10) M. up to similar to 1.2 x 10 super(12) M. The line index data presented here will be used in accompanying papers of this series to derive ages, metallicities and abundance ratios. A compilation of currently available high-quality Lick index measurements for globular clusters in elliptical, lenticular, and late-type galaxies is provided and will serve to augment the current data set.
We present new Hubble Space Telescope (HST) NIC3, near-infrared H-band photometry of globular clusters (GCs) around NGC 4365 and NGC 1399 in combination with archival HST WFPC2 and ACS optical data. ...We find that NGC 4365 has a number of globular clusters with bluer optical colors than expected for their red optical-to-near-infrared colors and an old age. The only known way to explain these colors is with a significant population of intermediate-age (2-8 Gyr) clusters in this elliptical galaxy. On the other hand, our result for NGC 1399 is in agreement with previous spectroscopic work that suggests that its clusters have a large metallicity spread and are nearly all old. In the literature, there are various results from spectroscopic studies of modest samples of NGC 4365 globular clusters. The spectroscopic data allow for either the presence or absence of a significant population of intermediate-age clusters, given the index uncertainties indicated by comparing objects in common between these studies and the few spectroscopic candidates with optical-to-near-IR colors indicative of intermediate ages. Our new near-IR data of the NGC 4365 GC system with a much higher signal-to-noise ratio agree well with earlier published photometry, and both give strong evidence of a significant intermediate-age component. The agreement between the photometric and spectroscopic results for NGC 1399 and other systems lends further confidence to this conclusion and to the effectiveness of the near-IR technique.
We report on the properties of 89 low-mass star clusters located in the vicinity of luminous young clusters ('blue globulars') in the disk of M31. Eighty-two of the clusters are newly detected. We ...have determined their integrated magnitudes and colors, based on a series of Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 exposures in blue and red (HST filters F450W and F814W). The integrated apparent magnitudes range from F450W = 17.5 to 22.5, and the colors indicate a wide range of ages. Stellar color-magnitude diagrams for all clusters were obtained and those with bright enough stars were fit to theoretical isochrones to provide age estimates. The ages range from 12 Myr to >500 Myr. Reddenings, which average E(F450 - F814) = 0.59 with a dispersion of 0.21 mag, were derived from the main-sequence fitting for those clusters. Comparison of these ages and integrated colors with single population theoretical models with solar abundances suggests a color offset of 0.085 mag at the ages tested. Estimated ages for the remaining clusters are based on their measured colors. The age-frequency diagram shows a steep decline of number with age, with a large decrease in number per age interval between the youngest and the oldest clusters detected.
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