In this paper, we report the discovery of 27 low-surface brightness galaxies, of which 12 are candidates for ultra-diffuse galaxies (UDG) in the Hydra I cluster, based on deep observations taken as ...part of the VST Early-type Galaxy Survey (VEGAS). This first sample of UDG candidates in the Hydra I cluster represents an important step in our project that aims to enlarge the number of confirmed UDGs and, through study of statistically relevant samples, constrain the nature and formation of UDGs. This study presents the main properties of this class of galaxies in the Hydra I cluster. For all UDGs, we analysed the light and colour distribution, and we provide a census of the globular cluster (GC) systems around them. Given the limitations of a reliable GC selection based on two relatively close optical bands only, we find that half of the UDG candidates have a total GC population consistent with zero. Of the other half, two galaxies have a total population larger than zero at 2
σ
level. We estimate the stellar mass, the total number of GCs, and the GC specific frequency (
S
N
). Most of the candidates span a range of stellar masses of 10
7
− 10
8
M
⊙
. Based on the GC population of these newly discovered UDGs, we conclude that most of these galaxies have a standard or low dark matter content, with a halo mass of ≤10
10
M
⊙
.
Abstract
The halo masses Mhalo of low surface brightness (LSB) galaxies are critical measurements for understanding their formation processes. One promising method to estimate a galaxy’s Mhalo is to ...exploit the empirical scaling relation between Mhalo and the number of associated globular clusters (NGC). We use a Bayesian mixture model approach to measure NGC for 175 LSB 23 ≤ 〈μe,r〉 (mag arcsec−2) ≤ 28 galaxies in the Fornax cluster using the Fornax Deep Survey data; this is the largest sample of low-mass galaxies so-far analysed for this kind of study. The proximity of the Fornax cluster means that we can measure galaxies with much smaller physical sizes 0.3 ≤ re,r (kpc) ≤ 9.5 compared to previous studies of the GC systems of LSB galaxies, probing stellar masses down to M* ∼ 105 M⊙. The sample also includes 12 ultra-diffuse galaxies (UDGs), with projected r-band half-light radii greater than 1.5 kpc. Our results are consistent with an extrapolation of the M*−Mhalo relation predicted from abundance matching. In particular, our UDG measurements are consistent with dwarf-sized haloes, having typical masses between 1010 and 1011 M⊙. Overall, our UDG sample is statistically indistinguishable from smaller LSB galaxies in the same magnitude range. We do not find any candidates likely to be as rich as some of those found in the Coma cluster. We suggest that environment might play a role in producing GC-rich LSB galaxies.
In the core of the Fornax cluster, on the west side of NGC 1399, we have detected a previously unknown region of intracluster light (ICL). It is made up by several faint ( mag arcsec−2) patches of ...diffuse light. The bulk of the ICL is located in between the three bright galaxies in the core, NGC 1387, NGC 1379, and NGC 1381, at arcmin (∼58-230 kpc) from the central galaxy NGC 1399. We show that the ICL is the counterpart in the diffuse light of the known over-density in the population of blue globular clusters (GCs). The total g-band luminosity of the ICL is L , which is ∼5% of the total luminosity of NGC 1399. This is consistent with the fraction of the blue GCs in the same region of the cluster. The ICL has mag, which is similar to the colors in the halo of the bright galaxies in the cluster core. The new findings were compared with theoretical predictions for the ICL formation and they support a scenario in which the intracluster population detected in the core of the Fornax cluster is build up by the tidal stripping of material (stars and GCs) from galaxy outskirts in a close passage with the central brightest galaxy (cD). Moreover, the diffuse form of the ICL and its location close to the core of the cluster is expected in a dynamically evolved cluster like Fornax.
Context. The dynamical mass-to-light (M/L) ratios of massive ultra-compact dwarf galaxies (UCDs) are about 50% higher than predicted by stellar population models. Aims. Here we investigate the ...possibility that these apparently elevated M/L ratios of UCDs are caused by a central black hole (BH) that heats up the internal motion of stars. We focus on a sample of ~50 extragalactic UCDs from the literature for which velocity dispersions and structural parameters have been measured. Methods. To be self-consistent in our BH mass estimates, we first redetermine the dynamical masses and M/L ratios of our sample UCDs, using up-to-date distance moduli and a consistent treatment of aperture and seeing effects. On average, the homogeneously redetermined dynamical mass and M/L ratios agree to within 5% with previous literature results. We calculate the ratio Ψ = (M/L)dyn/(M/L)pop between the dynamical and the stellar population M/L for an assumed age of 13 Gyr. Ψ > 1 indicates an elevated dynamical M/L ratio, suggesting dark mass on top of a canonical stellar population of old age. For all UCDs with Ψ > 1 we estimate the mass of a hypothetical central black hole needed to reproduce the observed integrated velocity dispersion Results. Massive UCDs (M > 107 M⊙) have an average Ψ = 1.7 ± 0.2, implying notable amounts of dark mass in them. We find that, on average, central BH masses of 10–15% of the UCD mass can explain these elevated dynamical M/L ratios. The implied BH masses in UCDs range from several 105 M⊙ to several 107 M⊙. In the MBH-luminosity plane, UCDs are offset by about two orders of magnitude in luminosity from the relation derived for galaxies. Our findings can be interpreted such that massive UCDs originate from progenitor galaxies with masses around ~109 M⊙, and that those progenitors have SMBH occupation fractions of ~60–100%. The suggested UCD progenitor masses agree with predictions from the tidal stripping scenario. Also, the typical BH mass fractions of nuclear clusters in such ~109 M⊙ galaxy bulges agree with the 10–15% BH fraction estimated for UCDs. Lower-mass UCDs (M < 107 M⊙) exhibit a bimodal distribution in Ψ, suggestive of a coexistence of massive globular clusters and tidally stripped galaxies in this mass regime. Conclusions. Central BHs as relict tracers of tidally stripped progenitor galaxies are a plausible explanation for the elevated dynamical M/L ratios of UCDs. Direct observational tests of this scenario are suggested.
Aims. We analyse the properties of the early-type dwarf galaxy population ($M_V > -17$ mag) in the Hydra I cluster. We investigate the galaxy luminosity function (LF), the colour-magnitude relation ...(CMR), and the magnitude-surface brightness relation down to MV ~ -10 mag. Another goal of this study is to find candidates for ultra-compact dwarf galaxies (UCDs) in Hydra I. Methods. Two spectroscopic surveys performed with Magellan I/LDSS2 at Las Campanas Observatory and VLT/VIMOS, as well as deep VLT/FORS1 images in V and I bands, covering the central parts of the cluster, were examined. We identify cluster members by radial velocity measurements and select other cluster galaxy candidates by their morphology and low surface brightness. The candidates' total magnitudes and central surface brightnesses were derived from the analysis of their surface brightness profiles. To determine the faint-end slope of the LF, the galaxy number counts are completeness corrected. Results. We obtain radial velocities for 126 objects and identify 32 cluster members, of which 5 are previously uncatalogued dwarf galaxies. One possible UCD candidate with MV = -13.26 mag is found. Our sample of $\simeq$100 morphologically selected dwarf galaxies with $M_V>-17$ mag defines a CMR that extends the CMR of the giant cluster galaxies to the magnitude limit of our survey (MV ~ -10 mag). It matches the relations found for the Local Group (LG) and the Fornax cluster dwarf galaxies almost perfectly. The Hydra I dwarf galaxies also follow a magnitude-surface brightness relation that is very similar to that of the LG dwarf galaxies. Moreover, we observe a continuous relation for dwarf galaxies and giant early-type galaxies when plotting the central surface brightness $\mu_0$ of a Sérsic model vs. the galaxy magnitude. The effective radius is found to be largely independent of the luminosity for MV > -18 mag. It is consistent with a constant value of $R_{\mathrm{e}}$ ~ 0.8 kpc. We present the photometric parameters of the galaxies as the Hydra I Cluster Catalogue (HCC). By fitting a Schechter function to the luminosity distribution, we derive a very flat faint-end slope of the LF (α = -1.13 ± 0.04), whereas fitting a power law for MV > -14 mag gives α = -1.40 ± 0.18. Conclusions. Our findings of a continuous CMR and $\mu_0$ - MV relation for dwarf and giant early-type galaxies suggests that they are the same class of objects. The similarity of those relations to other environments like the LG implies that internal processes could be more important for their global photometric properties than external influences.
Context. One formation channel discussed for ultra-compact dwarf galaxies (UCDs) is that of massive star clusters, and the other main scenario is that of tidally transformed dwarf galaxies. Aims. We ...aim at quantifying the specific frequency of UCDs in a range of environments and at relating this to the frequency of star clusters and potential progenitor dwarf galaxies. Are the frequencies of UCDs consistent with being the bright tail of the globular cluster luminosity function (GCLF)? Methods. We propose a definition for the specific frequency of UCDs, SN,UCD = NUCD100.4(MV,host − MV,0)cw. The parameter MV,0 is the zero point of the definition, chosen such that the specific frequency of UCDs is the same as those of globular clusters, SN,GC, if UCDs follow a simple extrapolation of the GCLF. Considering UCDs as compact stellar systems with MV < −10.25 mag (mass above ~2 × 106 M⊙), it is MV,0 = −20 mag. The parameter cw is a correction term to take the dependence of the GCLF width σ on the host galaxy luminosity into account. We apply our definition of SN,UCD to results of spectroscopic UCD searches in the Fornax, Hydra, and Centaurus galaxy clusters, two Hickson compact groups, and the Local Group. This includes a large database of 180 confirmed UCDs in Fornax. Results. We find that the specific frequencies derived for UCDs match those of GCs very well, to within 10–50%. The ratio \hbox{$\frac{S_{\rm N,UCD}}{S_{\rm N,GC}}$}SN,UCDSN,GC is 1.00 ± 0.44 for the four environments Fornax, Hydra, Centaurus, and Local Group, which have SN,GC values. This good match also holds for individual giant galaxies in Fornax and in the Fornax intracluster-space. The error ranges of the derived UCD specific frequencies in the various environments then imply that not more than ~50% of UCDs were formed from dwarf galaxies. We show that such a scenario would require ≳90% of primordial dwarfs in galaxy cluster centers (<100 kpc) to have been stripped of their stars. Conclusions. We conclude that the number counts of UCDs are fully consistent with them being the bright tail of the GC population. From a statistical point of view there is no need to invoke an additional formation channel.
As part of the Fornax Deep Survey with the ESO VLT Survey Telescope, we present new g- and r-band mosaics of the SW group of the Fornax Cluster. It covers an area of 3 × 2 square degrees around the ...central galaxy NGC 1316. The deep photometry, the high spatial resolution of OmegaCam, and the large covered area allow us to study the galaxy structure, trace stellar halo formation, and look at the galaxy environment. We map the surface brightness profile out to 33′ (∼200 kpc ∼ 15Re) from the galaxy center, down to arcsec−2 and arcsec−2. This allow us to estimate the scales of the main components dominating the light distribution, which are the central spheroid, inside 5 5 (∼33 kpc), and the outer stellar envelope. Data analysis suggests that we are catching in the act the second phase of the mass assembly in this galaxy, since the accretion of smaller satellites is going on in both components. The outer envelope of NGC 1316 still hosts the remnants of the accreted satellite galaxies that are forming the stellar halo. We discuss the possible formation scenarios for NGC 1316, by comparing the observed properties (morphology, colors, gas content, kinematics, and dynamics) with predictions from cosmological simulations of galaxy formation. We find that (i) the central spheroid could result from at least one merging event (it could be a preexisting early-type disk galaxy with a lower-mass companion) and (ii) the stellar envelope comes from the gradual accretion of small satellites.
Ultra-compact dwarf galaxies (UCDs) are stellar systems with masses of around 107 to 108M⊙ and half-mass radii of 10–100 pc. They have some properties in common with massive globular clusters, ...however dynamical mass estimates have shown that UCDs have mass-to-light ratios which are on average about twice as large than those of globular clusters at comparable metallicity, and tend to be larger than what one would expect for old stellar systems composed out of stars with standard mass functions. One possible explanation for elevated high mass-to-light ratios in UCDs is the existence of a substantial amount of dark matter, which could have ended up in UCDs if they are the remnant nuclei of tidally stripped dwarf galaxies, and dark matter was dragged into these nuclei prior to tidal stripping through, for example, adiabatic gas infall. Tidal stripping of dwarf galaxies has also been suggested as the origin of several massive globular clusters like Omega Cen, in which case one should expect that globular clusters also form with substantial amounts of dark matter in them. In this paper, we present collisional N-body simulations which study the co-evolution of a system composed out of stars and dark matter. We find that the dark matter gets removed from the central regions of such systems due to dynamical friction and mass segregation of stars. The friction time-scale is significantly shorter than a Hubble time for typical globular clusters, while most UCDs have friction times much longer than a Hubble time. Therefore, a significant dark matter fraction remains within the half-mass radius of present-day UCDs, making dark matter a viable explanation for the elevated M/L ratios of UCDs. If at least some globular clusters formed in a way similar to UCDs, we predict a substantial amount of dark matter in their outer parts.
Aims.
We aim to measure the Sloan
r
′, Sloan
i
′,
J
, and
Ks
magnitudes of Starlink’s STARLINK-1130 (Darksat) and STARLINK-1113 low Earth orbit (LEO) communication satellites and determine the ...effectiveness of the Darksat darkening treatment from the optical to the near-infrared (NIR).
Methods.
Four observations of Starlink’s LEO communication satellites, Darksat and STARLINK-1113, were conducted on two nights with two telescopes. The Chakana 0.6 m telescope at the Ckoirama observatory (Chile) observed both satellites on 5 Mar. 2020 (UTC) and 7 Mar. 2020 (UTC) using a Sloan
r
′ and Sloan
i
′ filter, respectively. The ESO VISTA 4.1 m telescope with the VIRCAM instrument observed both satellites on 5 Mar. 2020 (UTC) and 7 Mar. 2020 (UTC) in the NIR
J
-band and
Ks
-band, respectively.
Results.
The calibration, image processing, and analysis of the Darksat images give
r
≈ 5.6 mag,
i
≈ 5.0 mag,
J
≈ 4.2 mag, and
Ks
≈ 4.0 mag when scaled to a range of 550 km (airmass = 1) and corrected for the solar incidence and observer phase angles. In comparison, the STARLINK-1113 images give
r
≈ 4.9 mag,
i
≈ 4.4 mag,
J
≈ 3.8 mag, and
Ks
≈ 3.6 mag when corrected for range, solar incidence, and observer phase angles. The data and results presented in this work show that the special darkening coating used by Starlink for Darksat has darkened the Sloan
r
′ magnitude by 50%, Sloan
i
′ magnitude by 42%, NIR
J
magnitude by 32%, and NIR
Ks
magnitude by 28%.
Conclusions.
The results show that both satellites increase in reflective brightness with increasing wavelength and that the effectiveness of the darkening treatment is reduced at longer wavelengths. This shows that the mitigation strategies being developed by Starlink and other LEO satellite operators need to take into account other wavelengths, not just the optical. This work highlights the continued importance of obtaining multi-wavelength observations of many different LEO satellites in order to characterise their reflective properties and to aid the community in developing impact simulations and developing mitigation tools.
Context. The internal dynamics of ultra-compact dwarf galaxies (UCDs) has attracted increasing attention, with most of the UCDs studied to date located in the Virgo cluster. Aims. Our aim is to ...perform a comprehensive census of the internal dynamics of UCDs in the Fornax cluster, and to shed light on the nature of the interface between star clusters and galaxies. Methods. We obtained high-resolution spectra of 23 Fornax UCDs with –$10.4>M_V>-13.5$ mag ($10^6<{ M/M_{\sun}<10^8}$), using FLAMES/Giraffe at the VLT. This is the largest homogeneous data set of UCD internal dynamics assembled to date. We derive dynamical $M/L$ ratios for 15 UCDs covered by HST imaging. Results. In the MV–σ plane, UCDs with $M_V<-12$ mag are consistent with the extrapolated Faber-Jackson relation for luminous elliptical galaxies, while most of the fainter UCDs are closer to the extrapolated globular cluster (GC) relation. At a given metallicity, Fornax UCDs have, on average, $M/L$ ratios lower by 30–40% than Virgo UCDs, suggesting possible differences in age or dark matter content between Fornax and Virgo UCDs. For our sample of Fornax UCDs we find no significant correlation between $M/L$ ratio and mass. We combine our data with available $M/L$ ratio measurements of compact stellar systems with $10^4<{ M/M_{\sun}}<10^8$ M, and normalise all $M/L$ estimates to solar metallicity. We find that UCDs ($M \gtrsim 2 \times 10^6~M_{\sun}$) have $M/L$ ratios twice as large as GCs ($M\lesssim 2 \times 10^6~M_{\sun}$). We argue that dynamical evolution has probably had only a small effect on the current $M/L$ ratios of objects in the combined sample, implying that stellar population models tend to under-predict dynamical $M/L$ ratios of UCDs and over-predict those of GCs. Considering the scaling relations of stellar spheroids, we find that UCDs align well along the “Fundamental Manifold”. UCDs can be considered the small-scale end of the galaxy sequence in this context. The alignment for UCDs is especially clear for $r_{\rm e} \gtrsim 7$ pc, which corresponds to dynamical relaxation times that exceed a Hubble time. In contrast, globular clusters exhibit a broader scatter and do not appear to align along the manifold. Conclusions. We argue that UCDs are the smallest dynamically un-relaxed stellar systems, with $M\gtrsim 2 \times 10^6~M_{\sun}$ and $7\lesssim{r_{\rm e}/{\rm pc}} \lesssim 100$. Future studies should aim at explaining the elevated $M/L$ ratios of UCDs and the environmental dependence of their properties.
PDF
