Research main | My Group Research

STARBURSTS AND GALAXY OUTFLOWS

Some galaxies produce stars at a frenzied pace, going through the available reservoir of gas in a very short time. What is at the root of the starburst phenomenon? What drives the short timescales and high efficiencies? What controls the length of the star formation episode: gas depletion, or feedback that prevents further star formation?

We are studying these problems in a number of nearby galaxies, particularly the prototypical starbursts M 82 and NGC 253. We have targeted the nuclear starburst NGC 253 with a number of ALMA observations. We were the first to detect and characterize its molecular wind, and we continue to study the nuclear starburst region. We have also studied outflows in the nearest quasar, located in the core of the ultra luminous galaxy F11119+3257.

This is our image of molecular gas in NGC 253, one of the nearest starbursts (Bolatto et al. 2013). It was featured in Nature as one of the 2013 images of the year.

REPRESENTATIVE MULTI-WAVELENGTH GALAXY SURVEYS

The supply of molecular gas and the efficiency with which it is converted into stars are among the principal determinants of galaxy evolution. Nonetheless, extragalactic surveys of molecular gas conspicuously lag behind optical surveys in terms of unbiased sampling of the galaxy population. How does the efficiency of star formation changes within and among galaxies? Why are red galaxies red? And, how do galaxies grow? These questions can only be answered with the data produced by such surveys.

The EDGE survey studies the distribution of molecular gas in a statistically significant sample of over one hundred galaxies. The survey targets the J = 1-0 transition of CO and its 13CO isotopologue, which trace the bulk of the cold, star-forming gas. Paired with matched IFU mapping of the entire optical spectrum from CALIFA, this survey enables studies of resolved star formation efficiencies and histories, gas and stellar kinematics, nebular extinctions, and ionized gas properties across the Hubble sequence (Bolatto et al. 2017, Levy et al. 2018, 2019). We are currently pursuing ancillary observations, and proposing for a companion survey using ALMA.

THE HIGH-REDSHIFT UNIVERSE

Large surveys of molecular gas in normal galaxies at z~1-2, such as PHIBSS, reveal the properties of main sequence galaxies during the peak of star formation history of the cosmos (Tacconi et al. 2013, 2017). What are the conditions in these objects? Why are these galaxies forming stars at such a high rate? Does star formation proceed in similar conditions, or are these galaxies so gas-rich that global disk instabilities play a major role? On the larger context, what does the molecular gas luminosity function look like throughout the history of the universe? (Lenkic et al. 2020)

The figure show the field surrounding one of the PHIBSS targets, which is in the center. This object has two other nearby galaxies that appear to be more molecule-rich than the target source, and are not particularly apparent in the HST optical images. These chance discoveries allow us to study the clustering properties of gas-rich main sequence galaxies at z~2, and ultimately constrain the CO luminosity function at this redshift (Lenkic et al., in prep.).

PROBES OF THE INTERSTELLAR MEDIUM IN GALAXIES

Some of the brightest far-infrared and submillimeter-wave lines in galaxies are fine-structure transitions from the interstellar medium. They carry a wealth of information and can be studied from the ground with instruments such as ALMA for sources at medium and high redshifts. For local galaxies, many of these lines can be studied from space with Herschel or using a stratospheric observatory like SOFIA. These transitions allow us to diagnose the star formation activity of distant galaxies (Herrera-Camus et al. 2015, 2016), and physical conditions such as density, radiation field, and pressure.

The figure shows the results of our study on the thermal pressure in the disks of nearby galaxies (Herrera-Camus et al. 2017). This is the first time data of this type has been obtained for other galaxies like the Milky-Way. The plot shows the distribution of pressures in lines-of-sight dominated by atomic gas, derived under different assumptions (different fractions of "dark" molecular gas and cold neutral medium). The distribution is very similar to values obtained in the disk of the Milky Way using two other methods by different authors.

HIGH SPATIAL-RESOLUTION MOLECULAR SURVEYS OF NEARBY GALAXIES

Nearby galaxies offer some of the best laboratories to study the formation and evolution of Giant Molecular Clouds, the sites of the formation of new stars. With modern interferometers and single-dish instruments we can study the gas at tens of parsec resolution, and determine its properties by analyzing combinations of molecular transitions. High-dipole molecules such as HCN require high densities to be excited, while CO 1-0 is excited in relatively low density molecular gas due to a combination of its low critical density and its high optical depth, which helps pumping it radiatively. Isolopologues of CO, such as 13CO, lack this second advantage and are thus excited at intermediate densities. Using a combination of these and other lines is thus a very powerful probe of the distribution of gas densities in the ISM. Other lines accessible at mm wavelengths, such as HNCO or SiO, are probes of ice or dust destruction in shocks, and ratios higher-J transitions of different molecules (or sometimes ratios of metastable transitions of ammonia or methanol) are sensitive to temperature. Other physical information is provided by the local velocity dispersion. Pulling all this information together in a spatially-resolved manner is challenging, but linking it to galaxy properties and activitiy promises to be the next frontier at understanding the mechanisms of galaxy evolution.

The image shows a CO 1-0 mosaic of peak temperature of the galaxy IC 342 obtained by the Argus heterodyne array at the GBT (Li et al, in prep). The beam is illustrated in the bottom left corner. The central bar and the arms are clearly visible, as are inter-arm clouds. Large single-dishes (GBT, LMT, IRAM) and interferometers (ALMA, PdBI) are the only instruments that have the sensitivity to do this type of work.

MAGELLANIC CLOUD SCIENCE AND THE LOW METALLICITY UNIVERSE

In a bottom-up Lambda-CDM cosmology, the building blocks of galaxies are smaller systems similar to today's dwarf galaxies. Like those early-time systems, dwarf galaxies in the present universe are also poor in heavy elements, adding another dimension to the study of the physics of the interstellar gas. The lack of heavy elements affects the abundance of dust (which is made mostly of silicon, carbon, oxygen, and iron) relative to gas, which has widespread implications for a number of physical processes. This makes dwarf galaxies key laboratories to understand how the universe works. Two of the nearest examples of massive gas-rich dwarf galaxies are the Large and Small Magellanic Clouds (the LMC is massive enough to be classified as a small galaxy rather than a dwarf galaxy). Because of their nearby distances the spatial scales we can explore with observations are very small, making "Galactic scale" science (such as the study of individual stars) possible on these objects.

I lead and are part of a number of efforts to understand better the environment of the Clouds and other dwarf galaxies in the local universe. Some of these work has used Spitzer, Herschel, and SOFIA as well as ALMA, and in the future JWST (e.g., Leroy et al. 2007, Lopez et al. 2011, Sandstrom et al. 2012, Jameson et al. 2018).

EXCITING NEW FACILITIES

The lifeblood of astronomy is cutting-edge facilities that provide the remote-sensing data that we use to make sense of the universe that surrounds us. The images depict concepts for new facilities. The one on the left shows a ground based radio interferometer, the ngVLA, a NRAO instrument based in the Plains of Saint Augustine in New Mexico and extending with 300-km baselines, operating between 1 GHz and 116 GHz and with an order-of-magnitude improvement in collecting area over the VLA and ALMA. The one on the center shows the conceptual science that can be tackled with the Origins Space Telescope, a 5-8 m diameter cooled far-infrared NASA space observatory that can peer into the dust enshrouded cores of galaxies and detect protogalaxies at the edge of the universe. The right hand figure is the patch for EXCLAIM, a NASA-GSFC balloon experiment that will carry out intensity mapping at 500 GHz, as well as mapping of [CI] and CO in our own Milky Way and nearby galaxies.

I am the co-chair of the Science Advisory Council for the ngVLA, and participate in the science groups of the SPICA infrared telescope. I was also a member and chair of the ALMA Science Advisory Committee and led its conceptual instrumentation development plan recommendation, and part of the Advisory Board for the Green Bank Observatory.