Molecular gas has still only been detected beyond the R25 radius in a few galaxies. Is this due to the low H2 content or to the difficulty of using Carbon Monoxide (CO) to trace H2? Similarly, star formation (SF) decreases sharply in the outer disks of spirals although HI is often plentiful; is the decrease in SF because there is little H2 or because the SF is very inefficient in the outer disk environment?

Existing observations suggest that while outer disk clouds tend to be smaller (steeper mass function), their CO brightness temperature is only slightly lower than in the inner disk, at least when observed with sufficiently high angular resolution. In near-solar metallicity galaxies (Z ⩾ 0.5Zsol ), the CO does not become intrinsically difficult to detect when H2 is present, even in the outer disk. While more observations of CO or other means of tracing H2 in the outer disks are necessary, current data tend to show that the SF rate per unit H2 remains approximately constant with galactocentric distance, indicating that the star formation proceeds normally but the transformation of HI into H2 is very slow in the outer disk.

Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations, they are molecular with H2 the dominant species and CO the best available. When the abundances of carbon and oxygen are low compared to hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies CO forms slowly and is easily destroyed, so it cannot accumulate inside dense clouds. Then we lose our ability to trace the gas in regions of star formation and we lose critical information on the temperatures, densities, and velocities of the material that collapses. I will report on high resolution observations with ALMA of CO clouds in the local group dwarf irregular galaxy WLM, which has a metallicity that is 13% of the solar value and 50% lower than the previous CO detection threshold and the properties derived of very small dense CO clouds mapped..

Disk galaxies in cosmological numerical simulations grow by accreting gas from the cosmic web. This gas reaches the external disk, and then spirals in dragged along by tidal forces and/or disk instabilities. The importance of gas infall is as clear from numerical simulations as it is obscure to observations. Extremely metal poor (XMP) galaxies seem to be the best example we have of the gas accretion process at work. They have large off-center starbursts which show significant metallicity drop compared with the host galaxy. This observation is naturally explained as a gas accretion event caught in the act. We present preliminary results of the kinematical properties of the metal poor starbursts in XMPs, which suggest that the starbursts are kinematically decoupled entities within the host galaxy.

The HI in galaxies often extends past their conventionally defined optical extent. I report results from our team which has been probing low intensity star formation in outer disks using imaging in Hα and ultraviolet. Using a sample of hundreds of HI selected galaxies, we confirm that outer disk HII regions and extended UV disks are common. Hence outer disks are not dormant but are dimly forming stars. Although the ultraviolet light in galaxies is more centrally concentrated than the HI, the UV/HI ratio (the Star Formation Efficiency) is nearly constant, with a slight dependency on surface brightness. This result is well accounted for in a model where disks maintain a constant stability parameter Q. This model also accounts for how the ISM and star formation are distributed in the bright parts of galaxies, and how HI appears to trace the distribution of dark matter in galaxy outskirts.

The stellar kinematics of the discs of S0 galaxies (as obtained using planetary nebulae, PNe, and integrated stellar light data) is comparable to that of spiral galaxies, with similar flat rotation curves and falling velocity dispersion profiles, but they present a larger amount of random motions. The only other tracer available to probe the kinematics of individual early-type galaxies are globular clusters (GCs). GCs’ formation is intimately connected to a galaxy major star forming event(s) and GCs are, therefore, good proofs of galaxy formation histories. We directly compare a sample of PNe, GCs, and stellar velocities out to 4 effective radii, in the S0 galaxies NGC 2768 and NGC 1023. In particular, we test a new method for studying GC properties and we find that these two lenticular galaxies are consistent with being formed through different formation mechanisms.

We revisit the well known H i size-mass relation of galaxies with an unprecedented large sample of over 500 galaxies (Wang et al. 2016). We find that the relation and the scatter about the relation do not depend on the galaxy type, optical luminosity or H i richness. It indicates a fundamental unrecognised mechanism that drives the distribution of H i in different galaxies.

We focused on the stellar age-velocity dispersion relation (AVR). We performed the N-body/SPH simulations to investigate the origin of AVR. As a results, we found that AVR is not consistent with simple stellar heating.

In this short write-up, I will concentrate on a few topics of interest. In the 1970s I found very extended HI disks in galaxies such as NGC 5055 and NGC 2841, out to 2 - 2.5 times the Holmberg radius. Since these galaxies are warped, a “tilted ring model” allows rotation curves to be derived, and evidence for dark matter to be found. The evaluation of the amount of dark matter is hampered by a disk-halo degeneracy, which can possibly be broken by observations of velocity dispersions in both the MgI region and the CaII region.

Of the three kinds of neutral gas found outside the stellar disks of Local Group galaxies, only the products of interaction, like the Magellanic Stream, have a clearly understandable origin. Both the high-velocity clouds and the faint H I between M31 and M33 remain a mystery. New observations of the region between M31 and M33 with the Green Bank Telescope show that the H I there resides in clouds with a size and mass similar to that of dwarf galaxies, but without stars. These clouds might be products of an interaction, or condensations in the hot circumgalactic medium of M31, but both these models have difficulties. The prevalence of clouds like this in the Local Group remains to be determined.

The current IAU Symposium is closely connected to the EU-funded network DAGAL (Detailed Anatomy of Galaxies), with the final annual network meeting of DAGAL being at the core of this international symposium. In this short paper, we give an overview of DAGAL, its training activities, and some of the scientific advances that have been made under its umbrella.

Detection of optical surface brightness structures with magnitudes fainter than 30 mag/arcsec2 has remained elusive in current photometric deep surveys. We are conducting a new imaging strategy to cross that frontier and address the study of outskirts of galaxies and theories of galaxy formation.

In this work we study the effect of satellite accretion on the building-up of the radial stellar age distribution in the discs of spiral galaxies. In addition, we analyse its effect on other chemical and dynamical properties of these systems up to their outskirts.

Local analog galaxies play an important role in understanding the properties of high-redshift galaxies. We present a method to select a type of local analog that closely resembles the ionized interstellar medium conditions in high-redshift galaxies. These galaxies are selected based on their locations in the [O III]/Hβ versus [N II]/Hα nebular emission-line diagnostic diagram. The ionization parameters and electron densities in these analogs are comparable to those in z ≃ 2 − 3 galaxies, but higher than those in normal SDSS galaxies by ≃ 0.6 dex and ≃ 0.9 dex, respectively. We find that the high sSFR and SFR surface density can enhance the electron densities and the ionization parameters, but still cannot fully explain the difference in ISM condition between nearby galaxies and the local analogs/high-redshift galaxies.

Recent chemical abundance measurements of damped Lyman-alpha absorbers (DLAs) revealed a large intrinsic scatter in their metallicities. We discuss a semi-analytic model that was specifically designed to study this scatter by tracing the chemical evolution of the interstellar matter in small regions of the Universe with different mean density, from over- to underdense regions. It is shown that different histories of structure formation in these regions are reflected in the chemical properties of the proto-galaxies. We also address deuterium abundance measurements, which constitute a complementary probe of the star formation and infall histories.

We investigate the structure and dynamics of the Milky Way (MW) disk stars based on the analysis of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) data, to infer the past evolution histories of the MW disk component(s) possibly affected by radial migration and/or satellite accretions. APOGEE is the first near-infrared spectroscopic survey for a large number of the MW disk stars, providing their radial velocities and chemical abundances without significant dust extinction effects. We here adopt red-clump (RC) stars (Bovy et al. 2014), for which the distances from the Sun are determined precisely, and analyze their radial velocities and chemical abundances in the MW disk regions covering from the Galactocentric distance, R, of 5 kpc to 14 kpc. We investigate their dynamical properties, such as mean rotational velocities, 〈V φ〉 and velocity dispersions, as a function of R, based on the MCMC Bayesian method. We find that at all radii, the dynamics of alpha-poor stars, which are candidates of young disk stars, is much different from that of alpha-rich stars, which are candidates of old disk stars. We find that our Jeans analysis for our sample stars reveals characteristic spatial and dynamical properties of the MW disk, which are generally in agreement with the recent independent work by Bovy et al. (2015) but with a different method from ours.

This project is aimed at understanding the dependence of star formation on the environment by analysing young stellar populations in two very different positions in disk galaxies: circumnuclear and outer disk giant regions. Integral field spectroscopy (IFS) provide an ideal means to achieve these goals providing simultaneous spatial and spectral resolution. Here we present the characterization of the work sample, composed by 671 outer regions and 725 inner regions from 263 isolated spirals galaxies observed by the CALIFA survey. The wide number of regions in both samples allows us to obtain statistically relevant results about the influence of metallicity, density and environment on star formation, and how it disseminates over the galaxy, to obtain evolutionary stories for the star-forming regions and to compare our results with models of massive star formation and galactic chemical evolution.

The mid-infrared provides a unique view of galaxy stellar populations, sensitive to both the integrated light of old, low-mass stars and to individual dusty mass-losing stars. We present results from an extended Spitzer/IRAC survey of M31 with total lengths of 6.6 and 4.4 degrees along the major and minor axes, respectively. The integrated surface brightness profile proves to be surprisingly difficult to trace in the outskirts of the galaxy, but we can also investigate the disk/halo transition via a star count profile, with careful correction for foreground and background contamination. Our point-source catalog allows us to report on mid-infrared properties of individual objects in the outskirts of M31, via cross-correlation with PAndAS, WISE, and other catalogs.

As part of the Z-PAndAS Keck II DEIMOS survey of resolved stars in our neighboring galaxy, Andromeda (M31), we have built up a unique data set of measured velocities and chemistries for thousands of stars in the Andromeda stellar halo, particularly probing its rich and complex substructure. In this contribution, we will discuss the structural, dynamical and chemical properties of Andromeda's dwarf spheroidal galaxies, and how there is no observational evidence for a difference in the evolutionary histories of those found on and off M31's vast plane of satellites. We will also discuss a possible extension to the most significant merger event in M31 - the Giant Southern Stream - and how we can use this feature to refine our understanding of M31's mass profile, and its complex evolution.

Initial efforts to identify extended UV disk (XUV-disk) galaxies were confined to nearby targets using image products from early in the GALEX mission. We developed a beta Zooniverse-based citizen science project to address this issue, specifically (1) allowing a dramatically larger galaxy sample by crowd-sourcing blink comparison UV-optical image inspection to volunteers, and (2) incorporating all archived GALEX data for each target considered. We aim to widely deploy this project to the public within the upcoming year.

We have discovered that warm gas flows along galaxy major and minor axes detected out to 200 kpc. Our results are derived from a sample of HST-imaged isolated galaxies with nearby background quasars used to probe their 105K CGM detected in HST/COS UV spectra (traced by Ovi absorption). We constrain the geometry of the gas to reside between 20-40 degrees of the projected major axis and within 60 degrees of the projected minor axis, with little-to-no gas found in between. Furthermore, strong absorption systems tend to be found along the minor axes of star-forming galaxies. All of our results are consistent with the current view of the CGM originating from major axis-fed inflows/recycled gas and from minor axis-driven outflows.