Leo I Group Research Articles

A possible dwarf galaxy satellite-of-satellite problem in ΛCDM

Dark matter clusters on all scales, and it is therefore expected that even substructure should host its own substructure. Using the Extragalactic Distance Database, we searched for dwarf-galaxy satellites of dwarf galaxies, that is, satellite-of-satellite galaxies, corresponding to these substructures of substructure. From investigation of Hubble Space Telescope data for 117 dwarf galaxies, we report the discovery of a previously unknown dwarf galaxy around the ultra-diffuse M96 companion M96-DF6 at 10.2 Mpc in the Leo-I group. We confirm its dwarf-galaxy nature as a stellar overdensity. Modeling its structural parameters with a growth-curve analysis, we find that it is an ultrafaint dwarf galaxy with a luminosity of 1.5 × 105 L⊙, which is 135 times fainter than its host. Based on its close projection to M96-DF6, it is unlikely that their association occurs simply by chance. We compare the luminosity ratio of this and three other known satellite-of-satellite systems with results from two different cosmological sets of ΛCDM simulations. For the observed stellar mass range of the central dwarf galaxies, the simulated dwarfs have a higher luminosity ratio between the central dwarf and its first satellite (≈10 000) than observed (≈100), excluding the Large and Small Magellanic Cloud (LMC/SMC) system. No simulated dwarf analog at these observed stellar masses has the observed luminosity ratio. This cannot be due to missing resolution, because it is the brightest subhalos that are missing. This may indicate that there is a satellite-of-satellite (SoS) problem for ΛCDM in the stellar-mass range between 106 and 108 M⊙, the regime of the classical dwarf galaxies. However, simulated dwarf models at both a lower (< 106 M⊙) and higher (> 108 M⊙) stellar mass have comparable luminosity ratios. For the higher-stellar-mass systems, the LMC/SMC system is reproduced by simulations; for the lower stellar masses, no observed satellite-of-satellite system has been observed to date. More observations and simulations of satellite-of-satellite systems are needed to assess whether the luminosity ratio is at odds with ΛCDM.

The pride of lions around Messier 105

ABSTRACTWe undertook a search for new dwarf galaxies in the Leo-I group using the data from the DECaLS digital sky survey. Five new presumed members of this group have been found in a wide vicinity of ${\rm M}\, 105 ({\rm NGC}\, 3379$). Currently, the group has a population of 83 galaxies, 33 of which have measured radial velocities. More than half of the group members belong to early types with no signs of ongoing star formation. About a quarter of the galaxies are outside the group’s virial radius, R= 385 kpc. The presence of multiple systems with a size of about 15 kpc is evident in the group, but there are no noticeable global flat or filamentary substructures. The luminosity function of the group looks to be deficient in galaxies with absolute magnitudes in the interval MB = [−18, −15] mag. The ${\rm M}\, 105$ group is characterized by a radial velocity dispersion of 136 km s−1, orbital mass estimate (5.76 ± 1.32) × 1012 M⊙, and the total mass-to-K-band-luminosity ratio (17.8 ± 4.1)M⊙/L⊙. The neighbouring group of galaxies around ${\rm M}\, 66 ({\rm NGC}\, 3627$) has a similar virial radius, 390 kpc, velocity dispersion, 135 km s−1, and total mass-to-luminosity ratio, (15.6 ± 3.9)M⊙/L⊙. Both groups in the Leo constellation are approaching the Local Group with a velocity of about 100 km s−1. In the background of the ${\rm M}\, 105$ group, we noted a group of 6 galaxies with an unusually low virial mass-to-luminosity ratio, MT/LK = (4.1 ± 2.2) M⊙/L⊙.

The Leo-I group: new dwarf galaxy and ultra diffuse galaxy candidates

Context. The study of dwarf galaxies and their environments provides crucial test beds for predictions of cosmological models and insights into the structure formation on small cosmological scales. In recent years, many problems on the scale of groups of galaxies has challenged the current standard model of cosmology. Aims. Our aim is to increase the sample of known galaxies in the Leo-I group, which contains the M 96 subgroup and the Leo Triplet. This galaxy aggregate is located at the edge of the Local Volume at a mean distance of 10.7 Mpc. Methods. We employed image enhancing techniques to search for low surface brightness objects in publicly available gr images taken by the Sloan Digital Sky Survey within 500 square degrees around the Leo-I group. Once detected, we performed surface photometry and compared their structural parameters to other known dwarf galaxies in the nearby universe. Results. We found 36 new dwarf galaxy candidates within the search area. Their morphology and structural parameters resemble known dwarfs in other groups. Among the candidates five or six galaxies are considered as ultra diffuse galaxy candidates. If confirmed, they would be some of the closest examples of this galaxy type. We assessed the luminosity function of the Leo-I group and find it to be considerably rich in dwarf galaxies, with twice the number of galaxies as the Local Group at a limiting magnitude of MV = −10 and a steeper faint-end slope.

New dwarf galaxy candidates in the Leo-I Group

We have carried out a search for low-surface-brightness dwarf galaxies in the region of the Leo-I Group (M96) in images of the second Palomar Sky Survey. We found a total of 36 likely dwarf members of the group with typical magnitudes B t ∼18m–19m in an area of sky covering 120 square degrees. Half of these galaxies are absent from known catalogs and lists of galaxies. The radial-velocity dispersion calculated for 19 galaxies is 130 km/s. The Leo-I Group has a mean distance from the Sun of 10.4 Mpc, a mean projected radius of 352 kpc, an integrated luminosity of 6.7 × 1010 L ⊙, a virial mass-to-luminosity ratio of 107 M ⊙/L ⊙, and a mean crossing time of 2.7 Gyr. The group shows evidence for a radial segregation of the galaxies according to morphological type and luminosity, suggesting that the group is in a state of dynamical relaxation. The subsystem of bright galaxies in the Leo-I Group is smaller in size (250 kpc) and has a lower velocity dispersion (92 km/s), resulting in a lower virial mass-to-luminosity ratio (34 M ⊙/L ⊙), as is typical of the Local Group and other nearby groups of galaxies.

Detection of hot plasma around M96 in the Leo-I group

AbstractThe nearby (D = 11 Mpc) sparse group of galaxies, Leo-I, is in many respects unique. It is the nearest group containing both bright spirals (M96 and M95) and a bright elliptical (M105). A giant (diameter ca. 200 kpc) intergalactic Hɪ ring orbits the central M105/NGC3384 galaxy pair and appears to interact with M96. If M96 is really in the group core, the Leo-I group provides an unusually “clean” route to determining the Hubble constant. In our 22 ksec ASCA SIS exposure of M96 we have detected diffuse X-ray emission extending more than 10 arcminutes North of M96, in the direction of the Hɪ ring. The morphology and spectral characteristics of the diffuse emission shows that M96 has recently interacted with the Hɪ ring, indicating that M96, the Hɪ ring and the central galaxy M105 are at the same distance within a few percent.

The Cepheid distance to M96 and the Hubble constant

ABSTRA C T HST WFPC2 observations of Cepheids in M96 (NGC 3368) are used to find a distance to that galaxy of 11:2 ^ 1:0 Mpc. This estimate is based on a calibration of the Cepheid period‐luminosity relation in the Large Magellanic Cloud, and includes a correction for the difference in metallicity between the two systems. There are good reasons for believing M96 is at the same distance as four E/S0 galaxies in the Leo-I group, and hence we calibrate secondary distance indicators based on the early-type galaxies, namely the fundamental plane and surface-brightness-fluctuation method. Also the Type Ia supernova 1998bu occurred in M96 itself and is used to calibrate the SN Ia distance scale. These methods reach to recession velocities of greater than 5000 km s 21 and can therefore allow us to evaluate the Hubble constant without reference to the peculiar velocity of M96 itself. In fact, these indicators agree well between themselves and hence we find H0a 67 ^ 7k m s 21 Mpc 21

The Distance to the Coma Cluster from Surface Brightness Fluctuations

We report on the first determination of the distance to the Coma Cluster from surface brightness fluctuation (SBF) measurements obtained from Hubble Space Telescope WFPC2 observations of the bright E0 galaxy NGC 4881 in the Coma Cluster and ground-based observations of the standard E1 galaxy NGC 3379 in the Leo-I group. Relative distances based on the I-band fluctuation magnitude, ${u{m}{7016}}$ -->I, are strongly dependent on the metallicity and age of the stellar population. However, the radial changes in the stellar populations of the two giant ellipticals NGC 3379 and NGC 4881 are well described by published Mg2 gradients, and the ground-based measurements of ${u{m}{7016}}$ -->I at several radial points in NGC 3379 are used to calibrate ${u{m}{7016}}$ -->I in terms of the Mg2 index. The distance to NGC 3379, assumed to be identical to the average SBF distance of the Leo-I group, is combined with the new SBF measurements of NGC 4881 to obtain a Coma Cluster distance of 102 ± 14 Mpc. Combining this distance with the cosmic recession velocity of the Coma Cluster (7186 ± 428 km s-1), we find the Hubble constant to be H0 = 71 ± 11 km s-1 Mpc-1.

Calibration of the Fundamental Plane Zero Point in the Leo I Group and an Estimate of the Hubble Constant

We derive new effective radii and total magnitudes for 5 E and S0 galaxies in the Leo-I group from wide-field CCD images. These are used in conjunction with recent literature velocity data to construct the fundamental plane (FP) of the Leo-I group. The rms scatter that we find is only 6 % in distance. The zero point of this relation provides a calibration of the FP as a distance indicator and directly determines the angular diameter distance ratio between the Leo-I group and more distant clusters. In the language of Jerjen and Tammann (1993) we determine a cosmic velocity of the Leo-I group of 757+-68 km/s relative to the Coma cluster, or 796+-57 km/s relative to a frame of 9 clusters. Combining this velocity with the Cepheid distance to M96, a member of Leo-I, we find the Hubble constant to be H_0=67+-8 km/s/Mpc or H_0=70+-7 km/s/Mpc for each case. The distance we obtain for the Coma cluster itself (108+-12 Mpc) is in good agreement with a number of other recent estimates.

Determination of the Hubble constant from observations of Cepheid variables in the galaxy M96

New Hubble Space Telescope observations of Cepheid variable stars in the nearby galaxy M96 give a distance to the host galaxy group, Leo-I, of 11.6+/-0.8 Mpc. This value, used in conjunction with several reliable secondary indicators of relative distance, constrains the distances to more remote galaxy clusters, and yields a value of the Hubble constant (Ho=69+/-8 km/s/Mpc) that is independent of the velocity of the Leo-I group itself.