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Register a new userThe impact of the gas distribution on the determination of dynamical masses of galaxies using unresolved observations
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The dynamical mass (M_dyn) is a key property of any galaxy, yet a determination of M_dyn is not straight-forward if spatially resolved measurements are not available. This situation occurs in single-dish HI observations of the local universe, but also frequently in high-redshift observations. M_dyn-measurements in high-redshift galaxies are commonly obtained through observations of the CO line, the most abundant tracer of the molecular medium. Even though the CO linewidth can in most cases be determined with reasonable accuracy, a measurement of the size of the emitting region is typically challenging given current facilities. We show how the integrated spectra (`global profiles) of a variety of galaxy models depend on the spatial distribution of the tracer gas as well as its velocity dispersion. We demonstrate that the choice of tracer emission line significantly affects the shape of the global profiles. In particular, in the case of high (~50 kms-1) velocity dispersions, compact tracers (such as CO) result in Gaussian-like (non-double-horned) profiles, as is indeed frequently seen in high-redshift observations. We determine at which radii the rotation curve reaches the rotation velocity corresponding to the velocity width, and find that for each tracer this happens at a well-defined radius: HI velocity widths typically originate at ~5 optical scale lengths, while CO velocity widths trace the rotation velocity at ~2 scale lengths. We additionally explore other distributions to take into account that CO distributions at high redshift likely differ from those at low redshift. Our models, while not trying to reproduce individual galaxies, define characteristic radii that can be used in conjunction with the measured velocity widths in order to define dynamical masses consistent with the assumed gas distribution.
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Supermassive black holes reside in the nuclei of most galaxies. Accurately determining their mass is key to understand how the population evolves over time and how the black holes relate to their host galaxies. Beyond the local universe, the mass is commonly estimated assuming virialized motion of gas in the close vicinity to the active black holes, traced through broad emission lines. However, this procedure has uncertainties associated with the unknown distribution of the gas clouds. Here we show that the comparison of black hole masses derived from the properties of the central accretion disc with the virial mass estimate provides a correcting factor, for the virial mass estimations, that is inversely proportional to the observed width of the broad emission lines. Our results suggest that line-of-sight inclination of gas in a planar distribution can account for this effect. However, radiation pressure effects on the distribution of gas can also reproduce our findings. Regardless of the physical origin, our findings contribute to mitigate the uncertainties in current black hole mass estimations and, in turn, will help to further understand the evolution of distant supermassive black holes and their host galaxies.
We study the stellar populations of the brightest group galaxies (BGGs) in groups with different dynamical states, using GAMA survey data. We use two independent, luminosity dependent indicators to probe the relaxedness of their groups; the magnitude gap between the two most luminous galaxies ($Delta M_{12}$), and offset between BGG and the luminosity center ($D_{offset}$) of the group. Combined, these two indicators were previously found useful for identifying relaxed and unrelaxed groups. We find that the BGGs of unrelaxed groups have significantly bluer NUV-r colours than in relaxed groups. This is also true at the fixed sersic index. We find the bluer colours cannot be explained away by differing dust fraction, suggesting there are real differences in their stellar populations. SFRs derived from SED-fitting tend to be higher in unrelaxed systems. This is in part because of a greater fraction of BGGs with non-elliptical morphology, but also because unrelaxed systems have larger numbers of mergers, some of which may bring fuel for star formation. The SED-fitted stellar metallicities of BGGs in unrelaxed systems also tend to be higher by around 0.05 dex, perhaps because their building blocks were more massive. We find that the $Delta M_{12}$ parameter is the most important parameter behind the observed differences in the relaxed/unrelaxed groups, in contrast with the previous study of Trevisan et al. (2017). We also find that groups selected to be unrelaxed using our criteria tend to have higher velocity offsets between the BGG and their group.
In this paper we analyse the methodology to derive the bar pattern speed from dynamical simulations. The results are robust to the changes in the vertical-scale height and in the mass-to-light (M/L) ratios. There is a small range of parameters for which the kinematics can be fitted. We have also taken into account the use of different type of dynamical modelling and the effect of using 2-D vs 1-D models in deriving the pattern speeds. We conclude that the derivation of the bar streaming motions and strength and position of shocks is not greatly affected by the fluid dynamical model used. We show new results on the derivation of the pattern speed for NGC 1530. The best fit pattern speed is around 10 km/s/kpc, which corresponds to a R_cor/R_bar = 1.4, implying a slower bar than previously derived from more indirect assumptions. With this pattern speed, the global and most local kinematic features are beautifully reproduced. However, the simulations fail to reproduce the velocity gradients close to some bright HII regions in the bar. We have shown from the study of the Halpha equivalent widths that the HII regions that are located further away from the bar dust-lane in its leading side, downstream from the main bar dust-lane, are older than the rest by 1.5-2.5 Myr. In addition, a clear spatial correlation was found between the location of HII regions, dust spurs on the trailing side of the bar dust-lane, and the loci of maximum velocity gradients parallel to the bar major axis.
We derive the stellar-to-halo mass relation (SHMR), namely $f_starpropto M_star/M_{rm h}$ versus $M_star$ and $M_{rm h}$, for early-type galaxies from their near-IR luminosities (for $M_star$) and the position-velocity distributions of their globular cluster systems (for $M_{rm h}$). Our individual estimates of $M_{rm h}$ are based on fitting a dynamical model with a distribution function expressed in terms of action-angle variables and imposing a prior on $M_{rm h}$ from the concentration-mass relation in the standard $Lambda$CDM cosmology. We find that the SHMR for early-type galaxies declines with mass beyond a peak at $M_starsim 5times 10^{10}M_odot$ and $M_{rm h}sim 10^{12}M_odot$ (near the mass of the Milky Way). This result is consistent with the standard SHMR derived by abundance matching for the general population of galaxies, and with previous, less robust derivations of the SHMR for early types. However, it contrasts sharply with the monotonically rising SHMR for late types derived from extended HI rotation curves and the same $Lambda$CDM prior on $M_{rm h}$ as we adopt for early types. The SHMR for massive galaxies varies more or less continuously, from rising to falling, with decreasing disc fraction and decreasing Hubble type. We also show that the different SHMRs for late and early types are consistent with the similar scaling relations between their stellar velocities and masses (Tully-Fisher and Faber-Jackson relations). Differences in the relations between the stellar and halo virial velocities account for the similarity of the scaling relations. We argue that all these empirical findings are natural consequences of a picture in which galactic discs are built mainly by smooth and gradual inflow, regulated by feedback from young stars, while galactic spheroids are built by a cooperation between merging, black-hole fuelling, and feedback from AGNs.
The 2020s are poised to continue the past two decades of significant advances based on observations of dwarf galaxies in the nearby universe. Upcoming wide-field photometric surveys will probe substantially deeper than previous data sets, pushing the discovery frontier for new dwarf galaxies to fainter magnitudes, lower surface brightnesses, and larger distances. These dwarfs will be compelling targets for testing models of galaxy formation and cosmology, including the properties of dark matter and possible modifications to gravity. However, most of the science that can be extracted from nearby dwarf galaxies relies on spectroscopy with large telescopes. We suggest that maximizing the scientific impact of near-future imaging surveys will require both major spectroscopic surveys on 6-10m telescopes and multiplexed spectroscopy with even larger apertures.
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