1506.01712
Approximately a thousand ultra diffuse galaxies in the Coma cluster
Koda, Yagi, Yamanoi, Komiyama
Report the discovery of 854 ultra diffuse galaxies (UDGs) in the Coma cluster using deep R band images, with partial B, i, and Halpha band coverage, obtained with Subaru. Many of them (332) are MW-sized with very large effective radii of r_e>1.5 kpc. This study was motivated by the recent discovery of 47 UDGs by van Dokkum+(2015); this discovery suggests >1000 UDGs after accounting for the small Subaru fields. The new UDGs show a distribution concentrated around the cluster center, strongly suggesting that the great majority are (likely longtime) cluster members. They are a passively evolving population, lying along the red sequence in the CM diagram with no Halpha signatures. Star formation was, therefore, quenched in the past. They have exponential light profiles, effective radii re~800 pc-5 kpc, effective surface brightnesses mu_e(R)=25-28 mag/arcsec^-2, and stellar masses ~1e7-5e8 Msun. There is also a population of nucleated UDGs. Some MW-sized UDGs appear closer to the cluster center than previously reported; their survival in the strong tidal field, despite their large sizes, possibly indicates a large dark matter fraction protecting the diffuse stellar component. The indicated baryon fraction ~<1% is less than the cosmic average, and thus the gas must have been removed from the possibly massive dark halo. The UDG population appears to be elevated in the Coma cluster compared to the field, indicating that the gas removal mechanism is related primarily to the cluster environment.
1506.01800
Radial alignment of elliptical galaxies by the tidal force of a cluster of galaxies
Rong, et al
Unlike the random radial orientation distribution of field elliptical galaxies, galaxies in a cluster are expected to point preferentially towards the center of the cluster, as a result of the cluster's tidal force on its member galaxies. In this work an analytic model is formulated to simulate this effect. The deformation time scale of a galaxy in a cluster is usually much shorter than the time scale of change of the tidal force; the dynamical process of the tidal interaction within the galaxy can thus be ignored. An equilibrium shape of a galaxy is then assumed to be in the surface of equipotential, which is the sum of the self-gravitational potential of the galaxy and the tidal potential of the cluster at this location. Use a MC method to calculate the radial orientation distribution of these galaxies, by assuming the NFW mass profile of the cluster and the initial ellipticity of field galaxies. The radial angles show a single peak distribution centered at zero. The MC simulations also show that a shift of the reference center from the real cluster center weakens the anisotropy of the radial angle distribution. Therefore, the expected radial alignment cannot be revealed if the distribution of spatial position angle is used instead of that of radial angle. The observed radial orientations of elliptical galaxies in cluster A2744 are consistent with the simulated distribution.
1506.01405
Major mergers going Notts: challenges for modern halo finders
Behroozi, et al
Merging haloes with similar masses (i.e., major mergers) pose significant challenges for halo finders. Compare 5 halo finding algorithms (AHF, HBT, Rockstar, SubFind, and VELOCIraptor) recovery of halo properties for both isolated and cosmo major mergers. Find that halo positions and velocities are often robust, but mass biases exist for every technique. The algorithms also show strong disagreement in the prevalence and duration of major mergers, especially at high redshifts (z>1). This raises significant uncertainties for theoretical models that require major mergers for, e.g., galaxy morphology changes, size changes, or BH growth, as well as for finding Bullet Cluster analogues. All finders not using temporal information also show host halo and sub halo relationship swaps over successive time steps, requiring careful merger tree construction to avoid problematic mass accretion histories. Suggest that future algorithms should combine phase-space and temporal information to avoid the issues presented.
1506.01653
Missing stellar mass in SED fitting: spatially unresolved photometry can underestimate galaxy masses
Sorba, Sawicki
Fit model SED to each pixel in 67 nearby (<z>=0.0057) galaxies using broadband photometry from the SDSS and GALEX. For each galaxy, compare the stellar mass derived by summing the mass of each pixel to that found from fitting the entire galaxy treated as an unresolved point source. Find that, while the pixel-by-pixel and unresolved masses of galaxies with low sSFRs (such as ellipticals and lenticular) are in rough agreement, the unresolved mass estimate for SF galaxies is systematically lower than the measurement from spatially-resolved photometry. The discrepancy is strongly correlated with sSFR, with the highest sSFRs in the sample having masses underestimated by 25% (0.12 dex) when treated as point sources. Found a simple relation to statistically correct mass estimates derived from unresolved broadband SED fitting to the resoled mass estimates: m_resolved = m_unresolved/(-0.0057log(sSFR)+0.34) where sSFR is in units of yr^-1. Study the effect of varying spatial resolution by degrading the image resolution of the largest images and find a sharp decrease in the pixel-by-pixel mass estimate at a physical scale of approximately 3 kpc, which is comparable to spiral arm widths. The effects observed are consistent with the "outshining" idea which posits that the youngest stellar populations mask more massive, older -- and thus fainter -- stellar populations. Although the presence of strong dust lanes can also lead to a drastic difference between resolved and unresolved mass estimates (up to 45% or 0.3 dex) for any individual galaxy, found that resolving dust does not affect mass estimates on average. The strong correlation between mass discrepancy and sSFR is thus most likely due to the outshining systematic bias.
1506.07475
Probing the role of the galactic environment in the formation of stellar clusters; using M83 as a test-bench
Adamo, et al
Present a study of the M83 cluster population, covering the disc of the galaxy between radii of 0.45 and 4.5 kpc. Aim to probe the properties of the cluster population as a function of distance from the galactic center. Observe a net decline in cluster formation efficiency (Gamma, i.e., the amount of star formation happening in bound clusters) from about 19% in the inner region to 7% in the outer part of the galaxy. The recovered Gamma values within different regions of M83 follow the same Gamma versus SFR density relation observed for entire galaxies. Also probe the initial cluster mass function (ICMF) as a function of galactocentric distance. Observe a significant steepening of the ICMF in the outer regions (from -1.90±0.11 to -1.70±0.14) and for the whole galactic cluster population (slope of -2.18±0.07) of M83. Show that this change of slope reflects a more fundamental change of the 'truncation mass' at the high-mass end of the distribution. This can be modeled as a Schechter function slope -2 with an exponential cut-off mass (M_c) that decreases significantly from the inner to the outer regions (from 4.00 to 0.25e5 Msun) while the galactic M_c is ~1.6e5 Msun. The trends in Gamma and ICMF are consistent with the observed radial decrease of the Sigma (H_2), hence in gas pressure. As gas pressure declines, cluster formation becomes less efficient. Conclude that the host galaxy environment appears to regulate 1) the fraction of stars locked in clusters; 2) the upper mass limit of the ICMF, consistently described by a near-universal slope -2 truncated at the high-mass end.
No comments:
Post a Comment