China-Chile Proposal

China-Chile Joint Research Fund 2016

Project presentation

• Title: :Star Formation History of Galaxies in Different Environments from MaNGA Spectra

• Abstract (<1000 words):

In the present–day galaxy population, it is well known that there is a strong relation between the properties of galaxies and their stellar masses and environments (Peng et al., 2010, 2012). However, the relative importance of secular evolution (nature) over nurture is not yet clear. Separating the effects of interaction–driven evolution in the observed galaxy properties is not trivial.

- MaNGA spectra and galaxy properties

- SED modelling

- Experience of the team working with MaNGA data

- Experience of the team working together and how this research funding would help in the development of the projects

- Implementation and involvement of Chile researches on MaNGA

• Area of Research: Galaxies

• Names of project PI and Co-Is and their institutions:

Médéric Boquien(1) (PI), Shiyin Shen(2) (PI), Maria Argudo-Fernández(1) (Co-I),

Fangting Yuan(2) (Co-I), Jun Yin(2) (Co-I), Ruixiang Chang(2) (Co-I), Lei Hao(2) (Co-I)

1. - University of Antofagasta, Chile
2. - Shanghai Astronomical Observatory, Chinese Academy of Sciences, China

Scientific Justification

(less than 8 single-spaced pages, including text, graphics, tables and references).

Galaxy evolution in the era of the SDSS–IV and MaNGA surveys

The appearance of galaxies ultimately depends on the properties of their stellar populations: age, mass, chemical composition, etc. As stars form, evolve and die they produce dust and inject vast amounts of metals in the interstellar medium, which have a direct impact on the formation of new stars, shielding and cooling molecular clouds. In case of intense feedback from young, massive stars, star formation can even be quenched altogether. In other words, star formation is the fundamental process governing the formation and evolution of galaxies.

The Sloan Digital Sky Survey (SDSS) has provided us with spectra for a vast number of low redshift galaxies, yielding for each of them information about their star formation, metallicity, and kinematics. However, this information is limited to a single spatial position at the galaxy’s center. This means that this treasure trove of data unfortunately gives us only very limited information on the structure of gradients in galaxies, which would reveal how galaxies have grown over cosmic times. Integral field unit (IFU) observations enable a leap forward: they provide us with an added dimension to the information available for each galaxy. Clues to the nature of the physical processes that drive star formation in galaxies are encoded in the map.

With the advent of the spatially resolved spectroscopy, from integral field spectroscopy (IFS) data and the multi–band images with good spatial coverage from the ultraviolet with GALEX to the infrared with WISE, new observational results are available to explore galaxy assembly models (Pérez et al., 2013; Sánchez-Blázquez et al., 2014; Pan et al., 2015). To elucidate how galaxies grow with time, one needs to recover the SFH, both in space and time, for individual galaxies. In practice, the SFH of a galaxy is determined by finding the most plausible combination of evolved single stellar populations (SSP) that matches its observed spectrum or SED, the so–called “fossil record method”.

MaNGA (Mapping Nearby Galaxies at Apache Point Observatory, Bundy et al. 2016) is an IFS survey designed to investigate the internal kinematic structure and composition of gas and stars in an unprecedented sample of 10,000 nearby galaxies. The survey is one of three core programs in the fourth–generation Sloan Digital Sky Survey (SDSS–IV) that began on 2014 July 1. In previous SDSS programs the observing mode was from a single fiber spectrum. To overcome this limitation and provide spatially resolved spatial maps, MaNGA has adopted a new strategy by bundling fibers together (see Figure 1).

MaNGA will provide us with two–dimensional maps of stellar velocity and velocity dispersion, mean stellar age and star formation rate, stellar metallicity, element abundance ratios, stellar mass surface density, ionized gas velocities, metallicity, and dust extinction for a statistically powerful sample. The galaxies are selected to span a stellar mass interval of nearly 3 orders of magnitude. No cuts are made on color, morphology or environment, so the sample is fully representative of the local galaxy population. In addition to addressing key questions in the formation and evolution of galaxies, the MaNGA survey is designed to follow in the footsteps of SDSS–I/II/III by providing a legacy dataset to the community with enormous discovery potential. Finally, MaNGA will also play a vital role in the coming era of advanced IFU instrumentation, serving as the low–z anchor for interpreting IFU observations of galaxies at higher redshift ranges.

This research program aims at gaining a unique insight into how nearby galaxies formed and evolve by comparing their observed properties with results from state–of–the–art chemical evolution models. Using MaNGA IFUs, we will measure their SFH with unprecedented precision and accuracy combining IFS data from MaNGA with multi–wavelength SED modeling from the ultraviolet to the infrared. We therefore will provide new constraints on the assembly of galaxies across cosmic times.

Coming up next, we will briefly present research on which this projects build upon in Sect. 2. We lay out the research plan for this proposal in Sect. 3.

Figure 1: SDSS three-colour images and MaNGA IFU design for a sample of spiral face-on galaxies.

The involvement on MaNGA projects

Add some figures

MaNGA’s key goals are to understand the “life cycle” of present day galaxies from imprinted clues of their birth and assembly through their ongoing growth via star formation and merging, to their death from quenching at late times.

The MaNGA legacy dataset will address urgent questions in our understanding of galaxy formation, including: 1. the nature of present–day galaxy growth via merging and gas accretion, 2. the processes responsible for terminating star formation in galaxies, and 3. the broad formation history of galaxy subcomponents, including the disk, bulge, and dark matter halo.

Here we briefly present in more detail the building blocks upon which we will carry out this China-Chile collaboration in astronomical research.

Evolution of Spiral Isolated Galaxies with MaNGA

(PI: M. Argudo-Fernández)

From models, it is predicted that disks grow from inner to outer parts of galaxies, the called inside–out model of galaxy formation. This means that first stars form in the center and later in the outer regions. As a consequence, we expect to observe gradients in stellar age and metallicity as a function of galaxy radii. It is very difficult to observe this effect since high spatial and spectral resolution are required. Until recently it was nearly impossible to put precise constraints on the local SFH of distant galaxies. The new IFS data along with multi–wavelength observations provide us with sufficient resolution to study the resolved SFH

along galaxy radii. But separating the effect of the environment on the observed galaxy properties it is also a necessary and challenging question to address. Isolated galaxies and the power of MaNGA data are the key ingredients to reach this goal.

How exactly do galaxies build their disk when they grow in isolation, unaffected by external influences? We aim of characterizing the star formation history of isolated galaxies, under the assumption of the inside-out scenario, and to compare with chemical evolution models using MaNGA IFU data. The sample of isolated galaxies used in this study is selected from the SDSS-based catalogue of Isolated Galaxies (SIG, Argudo-Fernández et al., 2015). Preliminary results are shown in Figure 2. We already studied the effects of the environment in active galactic nuclei (AGN) for these galaxies using one-fiber spectra (Argudo-Fernández et al., 2016). We can also use IFU data to test if AGN is real or it is more related to star formation. We expect, for the very first time, to find some clues on how spiral galaxies form and evolve combining results from the spectral fitting techniques, with the predictions from CEMs, that are being developed in the Shanghai Astronomical Observatory by experts in the field.

Environmental effects on star formation properties in galaxy pairs

(PI: Fangting Yuan)

Merging is a presumably cause to transfer blue and star-forming galaxies to red and dead galaxies. Simulation shows that the interaction of two galaxies in a pair can cause the gas inflow and thus starburst in the center region of the galaxies. Many observations have found that there is enhancement of star formation in galaxy pairs. However, the enhancement may depend on the local environment (see e.g. Tonneson et al., 2014). Also, the spatial extent of the enhanced star formation in pairs is still not clear. Although many studies found the SF

enhancement is in center region, there are also cases showing that the star formation widely spread to bridge and tidal tail regions. A recent work by Moreno et al., 2015 shows that pair interactions can cause the enhancement of star formation in the center, but at the same time suppress the activity at outskirts.

We select the pair sample from the catalogue of groups constructed by Yang et al. (2007) based on SDSS-DR7 data. The spectroscopic redshifts for the companion galaxies are collected not only from SDSS-DR7, but also SDSS-DR10, NED and LAMOST, in order to minimize the fiber collision effect which causes the missing of the spectrum of one member galaxy in a close pair. Especially, the LAMOST data are obtained through the project dedicated to complement the spectroscopic redshifts of SDSS close pairs with LAMOST spectroscopy (Shen et al. 2016). Two galaxies are selected as a pair if they satisfy the following criteria: 1) Both galaxies have spectroscopic redshifts; 2) The projected separation rp is less than 50 kpc; 3) The velocity difference dv is less than 500 km/s. A part of this sample (include ~1000 galaxies) is selected for MaNGA observation, and will obtain the 2D spectra of these pairs in future 4 years.

We focus on exploring how the environment affects the properties in galaxy close pairs. Yang et al. (2007)’s group catalogue already provides us with halo mass of each pairs, which can be used as a parameter to indicate the environment. We also use tidal strength parameters that measure the local galaxy density of each pair (Argudo-Fernández et al., 2015) as another approach to describe the environment. Based on this SDSS pair sample, we can statistically explore the environmental dependence of pairs. On the other hand, MaNGA IFU allows us to study the spatially resolved properties of galaxies, and therefore can further help us clarify in which part the SFR/dust is changed, and whether the spatial variation has some dependence on the environment.

Metallicity evolution in SED modelling for MaNGA spectra

(PI: Shiyin Shen)

To be filled by Shiyin

Médéric’s MaNGA project

(PI: Médéric Boquien)

To be filled by Médéric