Nearby Galaxies

X-ray observations of the hot circum-galactic medium.

X-ray survey of the hot coronae around nearby disk galaxies.

This project includes Chandra survey of the X-ray emitting hot gas corona around a sample of 53 nearby highly inclined disk (spiral or S0) galaxies (Li & Wang 2013a,b; Li et al. 2014; Wang et al. 2016), a sample of 5 nearby isolated S0 galaxies (Li et al. 2011), as well as a few case studies (Li et al. 2008,2009; including Suzaku observations of NGC5866, Li 2015a). Out basic conclusion is that the X-ray emission within a few tens of kpc from the galactic center is mostly produced by stellar feedback (starburst or Type Ia supernovae), and the detected soft X-ray emission only accounts for ~1% of the supernova energy injection rate. Please see the listed references here for details of these works.

Circum-Galactic Medium of MASsive Spirals (CGM-MASS).

The above X-ray survey of nearby galaxies is mostly composed of L* or sub-L* galaxies (typically less massive than the Milky Way or Andromeda galaxy). In massive galaxies, due to their much steeper gravitational potential, gravitational heating in the form of accretion shock and compression may be important in producing X-ray emitting hot gas. We have conducted an XMM-Newton survey of 5 extremely massive (at least 3 times as massive as the Milky Way Galaxy) isolated spiral galaxies in the local universe, which is based on an XMM-Newton AO-13 large program. The key scientific goal is to search for the differences in the properties of the hot gaseous halo of super-L* and L*/sub-L* galaxies and to search for the missing baryons in the form of the extended hot gas halo. Introduction of the sample galaxies and an initial case study can be found in Li et al. (2016c), and follow-up papers with more scientific results will be submitted soon.

XMM-Newton survey of M31 halo.

Existing observations indicate that most of the X-ray emission from the hot circum-galactic medium or the corona of a galaxy can be attributed to stellar feedback, while the accretion induced hot halo gas, as predicted by galaxy formation models, is not detected. This is a large (510 ks) XMM-Newton survey of the M31 halo approved in AO-16 (2017-2018), in order to search for the accreted hot halo. We will study the spatial distribution of hot gas density, temperature, and metallicity. We will also estimate the strength of collisional ionization to better model the UV absorption lines from the HST observed background AGN. These analyses will help us to explore if the accreted circum-galactic medium is previously not detected because they have low density and distribute at large radii, or they have too low temperature to emit X-ray, or they have too low metallicity so X-ray emissivity.

Non-thermal hard X-ray emission from the nuclear superbubble of NGC3079.

The cosmic ray dominated Fermi bubbles in the Milky Way are produced either by a starburst or an AGN. We can gain further insight into the physics of superbubble by studying similar bubbles around other galaxies. Perhaps the best analog is in NGC 3079, which hosts a smaller and probably younger bubble, so offers an enormous opportunity for advancement. We propose Chandra observation to study the non-thermal hard X-ray emission. With the new data, we can discriminate if cosmic rays are accelerated by individual supernova remnants or by the bubble as a whole. Also, we can determine the primary energy loss channel for cosmic rays — inverse Compton or synchrotron emission. Finally, we will measure the pressures of the thermal and non-thermal components, which dictate the expansion of the system. This is an approved 100ks Chandra Cycle 18 (2017-2018) program.

Other X-ray projects on the hot circum-galactic medium.

Our group have also obtained a lot of new data on the hot circum-galactic medium in the last Chandra and XMM-Newton cycle (􏰂~1 Ms approved in 2016). In addition to my PI program on NGC 3079 and M31 halo, I also serve as co-I on a 224ks XMM-Newton observation to study the temperature and metallicity distribution of the hot circum-galactic medium of NGC3079 (PI: Hodges-Kluck), a 156ks XMM-Newton observation to study the hot circum-galactic medium of the ultra-diffuse galaxy Dragonfly 44 (PI: Hodges-Kluck), a 50ks Chandra observation to search for X-ray emission from the intra-group medium around a galaxy with an X-ray bright background AGN which can be used for absorption line studies (PI: Bregman). See here for details.

I am also a member of the Lynx Baryon Cycles working group. Lynx, previously called the X-ray Surveyor, is a large mission concept identified in the 2013 NASA Astrophysics Roadmap. With its high angular resolution, sensitivity, and energy resolution, Lynx will greatly improve our understanding of the extended hot circum-galactic medium through both broad and narrow band imaging observations and absorption line studies of background AGN.

Radio continuum halo.

I have been collaborating with the CHANG-ES group (PI: Judith Irwin) on the study of the radio continuum halo of nearby galaxies, which are mostly produced by CR electrons in galactic magnetic field (Irwin et al. 2012a,b). In addition to contributing in X-ray observations and scientific discussions (Damas-Segovia et al. 2016; Irwin et al. 2017), I also led on statistical analysis of the sample (Li et al. 2016a, using data from Wiegert et al. 2015), with major results include: (1) the radio-Infrared (IR) relation is slightly superlinear at low frequency; (2) ∼ 5% of SN energy is converted to CRs whose leptonic component produces the radio emission; (3) galaxies with higher star formation rate (SFR) convert a larger fraction of their SN energy into CRs than into hot gas.

Optical and IR emission lines from the halo.

I expanded my research to UV, optical, and IR astronomy after coming to University of Michigan at the end of 2014. Emission lines in these bands trace the cooler (than X-ray emitting) phase of the circum-galactic medium.

The projects I am working on (as PI) now include:

Hα imaging of the warm ionized gas around the CGM-MASS galaxies

Data of NGC 5908 has been taken in May 2016 with the 2.4m telescope at MDM observatory. Below is the abstract of the proposal.

The extended diffuse ionized gas (eDIG) around galaxies traces the galactic disk-halo interaction. However, the escape fraction of ionizing photons and the major ionization mechanism are still poorly constrained, and direct evidence of cool/hot gas interaction is still rare. We herein propose deep Hα observations of NGC 5908 and its bright companion NGC 5905, as a pilot multi-wavelength case study for our XMM-Newton sample of the most massive spiral galaxies in the local Universe. Combining the deep Hα and XMM-Newton X-ray/UV observations, we will (1) determine the escape fraction of the ionizing photon based on the Hα and XMM-Newton/OM UV images; (2) estimate the relative contribution of photo and collisionally ionization by constraining the thermal pressure of cool gas from the X-ray data and an assumption of pressure balance between cool/hot gases; (3) search for large scale Hα structures and signatures of cool-hot gas interaction.

SOFIA observation of far-IR emission lines from the nuclear superbubble of NGC 3079

This is a scheduled (on late Feb. and early Mar., 2017) 3 hour SOFIA observation, with the abstract presented below.

Context: Superbubbles produced by nuclear starbursts or the AGN in the center of galaxies play a key role in exchanging energy and mass between galaxies and their environments. Far-IR band contains principle atomic and ionized ISM cooling lines which are critical in understanding the superbubble structure and the physical processes involved in producing them.

Aims: Our major goal is to study the spatial distribution and physical conditions of the neutral and ionized gas in and around the ∼1.3 kpc-diameter superbubble of the nearby edge-on galaxy NGC 3079. This superbubble represents one of the most energetic galactic outflow in the local Universe.

Methods: We propose SOFIA/FIFI-LS spatially resolved spectroscopy studies of the nuclear superbubble of NGC 3079. We will search for IR emission lines such as [O III] λλ52, 88 μm, [N II] λ122 μm, [C II] λ158 μm, and compare their spatial distributions to the multi-wavelength archival data tracing different ISM phases. In particular, in the local Universe, compared to Herschel/PACS or SOFIA/GREAT, FIFI-LS is unique in covering the key diagnostic [O III] λ52 μm line which traces high density and high temperature ionized gas. We will model the lines and the IR continuum in order to derive or constrain some physical parameters of the neutral and ionized gas. We will also study the energetics of different gas phases in the superbubble.

Anticipated Results: Comparing the spatial distributions of IR line flux and kinematics to those of optical emission lines will help us to determine if the neutral and ionized cool gas are coupled to the warmer gas or not. Constraining the density and temperature of the gas by modeling the IR emission lines and continuum further help us to determine the origin of the lines, i.e., from the diffuse gas or belong to photodissociation regions at the surface of molecular cloudlets entrained in the outflow. Using the outflow velocities estimated from the line shift and the mass of cool atomic/ionized gas derived from the modeled gas properties, we will also estimate the kinematic energy contained in the cool gas and compare the energy budget among different gas phases of the superbubble.

Molecular gas in and out of the galactic disk.

Molecular gas flowing in and out of the galaxies are directly linked to the triggering and quenching of the star formation activity. I started working on mm-wave data after coming to Michigan. The major projects I am working on now include:

IRAM 30m mm-wave observations of the molecular gas in the CGM-MASS galaxies

This project is based on 23.9 hour IRAM 30m observations of the CO lines in 7 massive isolated spiral galaxies largely overlapping with the CGM-MASS sample. The abstract is presented below:

Molecular gas in isolated massive spiral galaxies plays a key role in star formation and gas circulation between galaxies and the circum-galactic medium (CGM). We propose IRAM 30m CO observations of the seven CGM-MASS galaxies in the northern sky, in order to measure the content and distribution of the molecular gas in these most massive spiral galaxies in the local Universe. We will compare the molecular gas budget of CGM-MASS galaxies to other samples, in order to distinguish if an internal or external origin of the molecular gas better describe the relation between the observed hot halo cooling rate, the SFR, and the molecular gas content. We will also compare the star formation efficiency of our sample to lower mass galaxies, in order to understand their low star formation activity.

ALMA observations of the molecular outflow in NGC 5775

I am also collaborating with the CHANG-ES group on the ALMA observations of the molecular outflow in NGC 5775 (PI: Kepley).

UV emission and absorption lines from the CGM.

I am working on some proposed projects on UV absorption line studies of the cool circum-galactic medium.

I am also a member of the CAFE (Census of WHIM Accretion Feedback Explorer) group. CAFE is a proposed joint space mission of the Chinese Academy of Science and the European Space Agency, aiming at mapping the circum-galactic and inter-galactic medium of nearby galaxies through narrow band imaging of Lyα and O VI emission lines. Initial funding has been awarded to some Chinese institutes for preliminary studies. I have been involved in selecting the sample and determining the specific scientific goals. CAFE will be the only UV mission covering the O VI line (best tracer of the ~105 K CGM) in the local universe in the foreseeable future.

Supernova Remnants (SNRs)

SNRs are the major Galactic factories producing CRs. My previous and ongoing research is focused on understanding the broad band emission of SNRs, which is mostly composed of thermal X-ray emission from hot plasma and CR emission from radio to γ-ray.

Spatially resolved X-ray spectroscopy of the historical SNR SN1006.

I developed tools for spatially resolved spectroscopy analysis of extended X-ray sources and applied them to the XMM-Newton large program of SN1006 (PI: Decourchelle). We produced images of physical parameters and performed statistical analysis to study the geometry and hydrodynamical state of this famous Galactic CR accelerator (Li et al. 2015b, 2016b).

NuSTAR characterization of the synchrotron emission of SN1006.

This work is based on the largest (~400ks) guest observer program of NuSTAR Cycle 1 (PI: Li), which is aiming at detecting and resolving the synchrotron emission from TeV CR electrons beyond the energy coverage of Chandra and XMM-Newton (to ~20 keV). We have finished data analysis and an initial draft paper, which is under discussion with co-authors and will be submitted soon.

Broad band spectral energy distribution (SED) of SNR-molecular cloud (MC) interacting systems.

We are now conducting a multi-wavelength survey of a sample of 22 SNRs with evidences of interacting with MCs, which are promising sites of hadronic CR emissions. Molecular line data from IRAM 30m or Delingha 13.7m telescopes are or will be available for most of the SNRs in the sample, including the recently completed 36.5 hr IRAM 30m observation of Cas A (PI: Li). We have also submitted a Fermi proposal to detect GeV emission from the SNRs with the latest Fermi data. TeV data from ground-based Cherenkov telescopes (HESS, MILAGRO, MAGIC, CANGAROO, or VERITAS) of some of the SNRs will be provided by collaborators in CEA, Saclay. We will then model the broad band SED from radio to TeV, aiming at studying the relation between SNR shock/cloud interaction and the production of hadronic or leptonic CR emissions (e.g., Miceli et al. 2016).

Other projects

High-redshift AGN

Wang et al. (2017)

Star forming regions in the face-on galaxy M101

Sun et al. (2012)

References can be found here