We have obtained deep Hubble Space Telescope (HST) imaging of 19 dwarf galaxy candidates in the vicinity of M101. Advanced Camera for Surveys HST photometry for two of these objects showed resolved ...stellar populations and tip of the red giant branch derived distances (D ∼ 7 Mpc) consistent with M101 group membership. The remaining 17 were found to have no resolved stellar populations, meaning they are either part of the background NGC 5485 group or are distant low surface brightness (LSB) galaxies. It is noteworthy that many LSB objects that had previously been assumed to be M101 group members based on projection have been shown to be background objects, indicating the need for future diffuse dwarf surveys to be very careful in drawing conclusions about group membership without robust distance estimates. In this work we update the satellite luminosity function of M101 based on the presence of these new objects down to MV = −8.2. M101 is a sparsely populated system with only nine satellites down to MV −8, as compared with 26 for M31 and 24.5 7.7 for the median host in the Local Volume. This makes M101 by far the sparsest group probed to this depth, although M94 is even sparser to the depth at which it has been examined (MV = −9.1). M101 and M94 share several properties that mark them as unusual compared with the other Local Volume galaxies examined: they have a very sparse satellite population but also have high star-forming fractions among these satellites; such properties are also found in the galaxies examined as part of the Satellites around Galactic Analogs survey. We suggest that these properties appear to be tied to the wider galactic environment, with more isolated galaxies showing sparse satellite populations that are more likely to have had recent star formation, while those in dense environments have more satellites that tend to have no ongoing star formation. Overall, our results show a level of halo-to-halo scatter between galaxies of similar mass that is larger than is predicted in the lambda cold dark matter model.
We have conducted a search of a 9 deg2 region of the Canada-France-Hawaii-Telescope Legacy Survey around the Milky Way analog M101 (D ∼ 7 Mpc), in order to look for previously unknown ...low-surface-brightness galaxies. This search has uncovered 38 new low-surface-brightness dwarf candidates, and confirmed 11 previously reported galaxies, all with central surface brightness (g, 0) > 23 mag arcsec−2, potentially extending the satellite luminosity function for the M101 group by ∼1.2 mag. The search was conducted using an algorithm that nearly automates the detection of diffuse dwarf galaxies. The candidates' small sizes and low surface brightnesses mean that the faintest of these objects would likely be missed by traditional visual or computer detection techniques. The dwarf galaxy candidates span a range of −7.1 ≥ Mg ≥ −10.2 and half-light radii of 118-540 pc at the distance of M101, and they are well fit by simple Sérsic surface brightness profiles. These properties are consistent with dwarfs in the Local Group, and to match the Local Group luminosity function, ∼10-20 of these candidates should be satellites of M101. Association with a massive host is supported by the lack of detected star formation and the overdensity of candidates around M101 compared to the field. The spatial distribution of the dwarf candidates is highly asymmetric, and concentrated to the northeast of M101, therefore distance measurements will be required to determine if these are genuine members of the M101 group.
We report the discovery of two ultra-diffuse galaxies (UDGs) that show clear evidence of an association with tidal material and an interaction with a larger galaxy halo, found during a search of the ...Wide portion of the Canada-France-Hawaii Telescope Legacy Survey. The two new UDGs, NGC 2708-Dw1 and NGC 5631-Dw1, are faint (Mg = −13.7 and −11.8 mag), extended (rh = 2.60 and 2.15 kpc), and have low central surface brightness ( (g, 0) = 24.9 and 27.3 mag arcsec−2), while the stellar stream associated with each has a surface brightness (g) 28.2 mag arcsec−2. These observations provide evidence that the origin of some UDGs may connect to galaxy interactions, either by transforming normal dwarf galaxies by expanding them, or because UDGs can collapse out of tidal material (i.e., they are tidal dwarf galaxies). Further work is needed to understand the fraction of the UDG population "formed" through galaxy interactions, and wide field searches for diffuse dwarf galaxies will provide further clues to the origin of these enigmatic stellar systems.
We present the widest-field resolved stellar map to date of the closest (D~ 3.8 Mpc) massive elliptical galaxy NGC 5128 (Centaurus A; Cen A), extending out to a projected galactocentric radius of ...~150 kpc. The data set is part of our ongoing Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) utilizing the Magellan/Megacam imager. We resolve a population of old red giant branch (RGB) stars down to ~1.5 mag below the tip of the RGB, reaching surface brightness limits as low as muv,0~ 32 mag arcsec super(-2). The resulting spatial stellar density map highlights a plethora of previously unknown streams, shells, and satellites, including the first tidally disrupting dwarf around Cen A (CenA-MM-Dw3), which underline its active accretion history. We report 13 previously unknown dwarf satellite candidates, of which 9 are confirmed to be at the distance of Cen A (the remaining 4 are not resolved into stars), with magnitudes in the range Mv=-7.2 to -13.0, central surface brightness values of muv,0= 25.4-26.9 mag arcsec super(-2), and half-light radii of rh= 0.22-2.92 kpc. These values are in line with Local Group dwarfs but also lie at the faint/diffuse end of their distribution; interestingly, CenA-MM-Dw3 has similar properties to the recently discovered ultradiffuse galaxies in Virgo and Coma. Most of the new dwarfs are fainter than the previously known Cen A satellites. The newly discovered dwarfs and halo substructures are discussed in light of their stellar populations, and they are compared to those discovered by the PAndAS survey of M31.
We have obtained deep Hubble Space Telescope (HST) imaging of four faint and ultra-faint dwarf galaxy candidates in the vicinity of M101-Dw21, Dw22, Dw23 and Dw35, originally discovered by Bennet et ...al. Previous distance estimates using the surface brightness fluctuation technique have suggested that these four dwarf candidates are the only remaining viable M101 satellites identified in ground-based imaging out to the virial radius of M101 (D 250 kpc). Advanced Camera for Surveys imaging of all four dwarf candidates shows no associated resolved stellar populations, indicating that they are thus background galaxies. We confirm this by generating simulated HST color-magnitude diagrams of similar brightness dwarfs at the distance of M101. Our targets would have displayed clear, resolved red giant branches with dozens of stars if they had been associated with M101. With this information, we construct a satellite luminosity function for M101, which is 90% complete to MV = −7.7 mag and 50% complete to MV = −7.4 mag, that extends into the ultra-faint dwarf galaxy regime. The M101 system is remarkably poor in satellites in comparison to the Milky Way and M31, with only eight satellites down to an absolute magnitude of MV = −7.7 mag, compared to the 14 and 26 seen in the Milky Way and M31, respectively. Further observations of Milky Way analogs are needed to understand the halo-to-halo scatter in their faint satellite systems, and connect them with expectations from cosmological simulations.
Context.
A new, self-consistent, scale-free theory of stellar convection was recently developed (SFCT) in which velocities, dimensions, and energy fluxes carried by the convective elements are ...defined in a rest frame co-moving with the convective element itself. As the dynamics of the problem is formulated in a different framework with respect to the mixing length theory (MLT), the SFCT equations are sufficient to determine all the properties of stellar convection in accordance with the physics of the environment alone, with no need for the mixing length parameter (MLP). Subsequently, the SFCT was improved by introducing suitable boundary conditions at the surface of the external convective zones of the stars, and the first stellar models and evolutionary tracks on the Hertzsprung–Russell diagram were calculated.
Aims.
The SFCT received alternatively positive and negative attention that spurred us to reconsider the whole problem. In this work, we aim to re-examine the physical foundations and results of the SFCT, elucidate some misconceptions on its physical foundations, reply to reported criticisms, and present some recent improvements to the SFCT.
Methods.
The analysis was done using the same formalism of the previous studies, but novel arguments and demonstrations are added to better justify the controversial points, in particular the relaxation of instantaneous hydrostatic equilibrium between a convective element and the surrounding medium.
Results.
The main results include (i) a novel detailed discussion of the boundary conditions to ensure that the temperature gradients in the outermost regions of a star are adequate for analyses of stability or instability in asteroseismology; (ii) a quantitative comparison with the MLT; and, finally, (iii) the recovery of the MLT as a particular case of the SFCT, but also in this case with no need for the MLP.
Conclusions.
In conclusion, the SFCT is a step forward with respect to the classical MLT.
We present deep imaging of the most distant dwarf discovered by the Dark Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry reaches ~3 mag deeper than previous work and ...allows us to confirm the presence of a stellar cluster whose position is consistent with Eri II's center. This makes Eri II, at Mv= -7.1, the least luminous galaxy known to host a (possibly central) cluster. The cluster is partially resolved, and at Mv= -3.5 it accounts for ~4% of Eri II's luminosity. We derive updated structural parameters for Eri II, which has a half-light radius of ~280 pc and is elongated ( epsilon ~ 0.48) at a measured distance of D~ 370 kpc. The color-magnitude diagram displays a blue, extended horizontal branch, as well as a less populated red horizontal branch. A central concentration of stars brighter than the old main-sequence turnoff hints at a possible intermediate-age (~3 Gyr) population; alternatively, these sources could be blue straggler stars. A deep Green Bank Telescope observation of Eri II reveals no associated atomic gas.
Context.
The extended stellar halos of galaxies contain important clues for investigating their assembly history and evolution.
Aims.
We investigate the resolved stellar content and the extended halo ...of NGC 5128 as a function of galactocentric distance, and trace the halo outward to its currently detectable limits.
Methods.
We used
Hubble
Space Telescope images obtained with the WFPC2, ACS, and WFC3 cameras equipped with
F
606
W
and
F
814
W
filters to resolve individual red giant branch (RGB) stars in 28 independent pointings across the halo of NGC 5128. The stellar halo analysis for 14 of these pointings is presented here for the first time. Star counts from deep
V
I
color-magnitude diagrams reaching at least 1.5 mag below the tip of the RGB are used to derive the surface density distribution of the halo. The contamination by Milky Way stars is assessed with a new control field, with models, and by combining optical and near-IR photometry.
Results.
We present a new calibration of the WFC3
F
606
W
+
F
814
W
photometry to the ground-based
V
I
photometric system. The photometry shows that the stellar halo of NGC 5128 is dominated by old RGB stars that are present in all fields. The
V
-band surface brightness of fields changes from 23 to 32 mag arcsec
−2
between the innermost field only 8.3 kpc from the galaxy center to our outermost halo fields, which are located 140 kpc away from the center along the major axis and 92 kpc along the minor axis. Within the inner ∼30 kpc, we also find evidence for a 2 − 3 Gyr old population traced by asymptotic giant branch stars that are brighter than the tip of the RGB. This population contributes only up to 10% in total stellar mass if it is 2 Gyr old, but a larger fraction of 30 − 40% is required if its age is 3 Gyr. The stellar surface density profile is well fit by a classic
r
1/4
curve or a simple power-law form ∼
r
−3.1
over the full radial range, with no obvious break in the slope, but with large field-to-field scatter. The ellipticity measured from integrated-light photometry in the inner parts,
e
= (
b
/
a
) = 0.77, flattens to
e
= 0.54 ± 0.02 beyond 30 kpc. Considering the flattening of the outer halo, the projection of the elliptical isophote on the semimajor axis for our most distant field reaches nearly 30 effective radii.
We analyze the optical counterpart to the ultra-compact high velocity cloud AGC 226067, utilizing imaging taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The ...color-magnitude diagram of the main body of AGC 226067 reveals an exclusively young stellar population, with an age of ∼7-50 Myr, and is consistent with a metallicity of Fe/H ∼ −0.3 as previous work has measured via H ii region spectroscopy. Additionally, the color-magnitude diagram is consistent with a distance of D 17 Mpc, suggesting an association with the Virgo cluster. A secondary stellar system located ∼1 6 (∼8 kpc) away in projection has a similar stellar population. The lack of an old red giant branch ( 5 Gyr) is contrasted with a serendipitously discovered Virgo dwarf in the ACS field of view (Dw J122147+132853), and the total diffuse light from AGC 226067 is consistent with the luminosity function of the resolved ∼7-50 Myr stellar population. The main body of AGC 226067 has a MV = −11.3 0.3, or Mstars = 5.4 1.3 × 104 M given the stellar population. We searched 20 deg2 of imaging data adjacent to AGC 226067 in the Virgo Cluster, and found two similar stellar systems dominated by a blue stellar population, far from any massive galaxy counterpart-if this population has star-formation properties that are similar to those of AGC 226067, it implies ∼0.1 M yr−1 in Virgo intracluster star formation. Given its unusual stellar population, AGC 226067 is likely a stripped remnant and is plausibly the result of compressed gas from the ram pressure stripped M86 subgroup (∼350 kpc away in projection) as it falls into the Virgo Cluster.
A census of the satellite population around dwarf galaxy primary hosts in environments outside the Local Group is essential to understanding Λ cold dark matter galaxy formation and evolution on the ...smallest scales. We present deep optical Hubble Space Telescope imaging of the gas-rich, faint dwarf galaxy Antlia B (MV = −9.4)-a likely satellite of NGC 3109 (D = 1.3 Mpc)-discovered as part of our ongoing survey of primary host galaxies similar to the Magellanic Clouds. We derive a new tip of the red giant branch distance of D = 1.35 0.06 Mpc (m − M = 25.65 0.10), consistent with membership in the nearby NGC 3109 dwarf association. The color-magnitude diagram (CMD) shows both a prominent old, metal-poor stellar component and confirms a small population of young, blue stars with ages 1 Gyr. We use the CMD fitting algorithm MATCH to derive the star formation history (SFH) and find that it is consistent with the typical dwarf irregular or transitional dwarf galaxy (dTrans) in the Local Group. Antlia B shows relatively constant stellar mass growth for the first ∼10-11 Gyr and almost no growth in the last ∼2-3 Gyr. Despite being gas-rich, Antlia B shows no evidence of active star formation (i.e., no H emission) and should therefore be classified as a dTrans dwarf. Both Antlia B and the Antlia dwarf (dTrans) are likely satellites of NGC 3109, suggesting that the cessation of ongoing star formation in these galaxies may be environmentally driven. Future work studying the gas kinematics and distribution in Antlia B will explore this scenario in greater detail. Our work highlights the fact that detailed studies of nearby dwarf galaxies in a variety of environments may continue to shed light on the processes that drive the SFH and evolution of dwarf galaxies more generally.
PDF
