This is a guest post by María Argudo-Fernández (University of Antofagasta).
Esta entrada de blog está escrita por nuestra invitada María Argudo-Fernández (Universidad de Antofagasta).

It is well known that the environment where a galaxy resides plays an important role in its formation and evolution. Galaxies in the MaNGA sample have many different morphologies. There are elliptical, lenticular, and spiral galaxies, but there are also peculiar galaxies such as dwarfs and mergers or interacting galaxies. There are also very bright and large galaxies, with many many stars, but MaNGA is also observing smaller and fainter galaxies. All of these are properties that we know have some relation with the galaxy environment. For example, there are more elliptical galaxies in denser environments than you can find in the field, which is mainly populated by spiral galaxies. This relation is known as the morphology-density relation.

Es bien conocido que el entorno donde viven las galaxias juega un papel importante en su formación y evolución. Las galaxias de la muestra de MaNGA muestran muchas formas diferentes. Hay galaxias que son elípticas, lenticulares y espirales, pero también hay galaxias peculiares como galaxias enanas y fusiones de galaxias o galaxias en interacción. También hay galaxias de gran tamaño, galaxias con muchísimas estrellas y galaxias muy brillantes, aunque MaNGA también ha observado galaxias más pequeñas y galaxias más débiles. Todas estas son propiedades que sabemos que tienen una relación con el entorno. Por ejemplo, se pueden encontrar más galaxias de tipo elíptico en entornos más densos que en el campo, el cual está poblado mayormente por galaxias de tipo espiral. Esta relación se conoce como la relación morfología-densidad.

MaNGA is opening a new window for observing nearby galaxies. For instance, MaNGA is allowing us to study how galaxies are forming new stars or how galaxies are quenching their star formation. We can also explore how and how fast stars and gas are moving within galaxies, or how old the stars are in different regions of the galaxies (for example bulge and disk). Now it’s our time to investigate what is the role of the environment on these properties, and for that we need to characterise the neighbourhood of the MaNGA galaxies.

MaNGA está abriendo una nueva forma de observar galaxias cercanas. Por ejemplo, MaNGA nos está permitiendo estudiar cómo las galaxias están formando nuevas estrellas o cómo está cesando su formación estelar. También podemos explorar cómo y cuán rápido se mueven las estrellas y el gas dentro de las galaxias, o qué edad tienen las estrellas en diferentes zonas de las galaxias (por ejemplo en el bulbo o en el disco). Ahora es nuestra oportunidad para investigar el papel que juega el entorno en estas propiedades, y para ello necesitamos caracterizar el vecindario alrededor de las galaxias de MaNGA.

How can we do this? We first need to understand how galaxies are distributed. Galaxies are not homogeneously distributed in the Universe. Through gravitational influence galaxies tend to cluster in larger structures as clusters, filaments, and walls, leaving large voids between them.

To better understand this let’s think about people. People are not homogeneously distributed on the planet, and their numbers grow somehow affected by the conditions of the region they live in. The highest populations are concentrated in the largest cities, in the order of tens of millions, and surrounded by suburbs. Moving away from the city we find smaller cities, villages, and in a very extreme situation we could find tribes and hermits in the most isolated regions.

¿Cómo podemos hacerlo? Tenemos que parametrizar el vecindario alrededor de cada galaxia de MaNGA. Primero necesitamos entender cómo se distribuyen las galaxias. La distribución de las galaxias en el universo no es homogénea. Las galaxias tienden a agruparse por influencia gravitatoria en estructuras cada vez más densas, como cúmulos, filamentos y paredes, dejando grandes vacíos entremedio.

Para entender esto mejor pensemos acerca de las personas. Las personas no están distribuidas homogéneamente en el planeta, y de alguna forma crecen influenciadas por las condiciones de la región en la que viven. Las mayores concentraciones de personas se encuentran en las ciudades más grandes, del orden de decenas de millones, que están además rodeadas de suburbios. Conforme nos alejamos de las grandes ciudades encontramos ciudades más pequeñas, pueblos, y en situaciones más extremas, incluso tribus y personas ermitañas en las regiones más aisladas.

We have some methods to identify the neighbourhood around galaxies. We first define a perimeter (what we name a physical volume) around each MaNGA galaxy. This perimeter can contain a few houses around each galaxy (what we refer as the local environment), a district (what we refer as the intermediate or group environment), or a full city (what we refer as the large-scale environment). We then calculate different parameters in these volumes. For example the local density parameter tells us how many neighbour galaxies are living in that volume. Another parameter, the tidal strength, estimate the gravitational influence that each neighbour galaxy exerts on our favourite MaNGA galaxy. We also use more sophisticated methods to relate MaNGA galaxies with the biggest structures in the Universe (clusters, filaments, sheets, and voids).

Nosotros tenemos algunos métodos para identificar el vecindario alrededor de las galaxias. Para ello primero definimos un perímetro alrededor de cada galaxia de MaNGA (lo que llamamos un volumen físico). Éste perímetro puede contener desde unas pocas casas alrededor (a lo que nos referimos como entorno local), un barrio (a lo que nos referimos como entorno intermedio o grupal), o una ciudad completa (a lo que nos referimos como entorno a gran escala). Una vez definidos estos volúmenes podemos calcular diferentes parámetros. La densidad local, por ejemplo, nos dice cuántas galaxias vecinas viven en ese volumen. Con otro parámetro, el parámetro de marea, estimar la influencia gravitatoria que ejercen todas las galaxias vecinas en mi galaxia MaNGA favorita. También usamos otros parámetros más sofisticados para relacionar las galaxias de MaNGA con las mayores estructuras del universo (los cúmulos, los filamentos, las paredes y los vacíos).

In the Galaxy Environment for MaNGA Galaxies (GEMA) value added catalogue we are providing the quantification of the environment for all MaNGA galaxies observed in the Fiftheenth Data Release of the Sloan Digital Sky Survey (DR15). We have compiled these and other environment parameters, and some of them have been already used to explore the influence on the environment on MaNGA galaxies. For example, using the tidal strength, we have found that the galaxies with counter-rotating stars and gas tend to be more isolated than galaxies where the gas is rotating the same direction than their stars (Chen et al. 2017​, Jin et al. 2017). On the other hand, it seems that the age and metallicity gradients in galaxies (from the center of the galaxies to the outskirts) are not affected by the local and the large-scale environments (Zheng et al. 2017, Goddard et al. 2017).

The GEMA catalogue is publicly available in DR15 here! You can play with it to explore where you favourite MaNGA galaxy lives.

En el catálogo de valor añadido GEMA (Galaxy Environment for MaNGA Galaxies, por sus siglas en inglés), proveemos la cuantificación del entorno para las galaxias de MaNGA observadas en el SDSS-DR15. Hemos calculado éstos y otros parámetros de entorno, donde ya hemos usado algunos de ellos para explorar la influencia del entorno en galaxias de MaNGA. Por ejemplo, hemos encontrado que las galaxias donde sus estrellas y el gas están contra-rotando tienden a estar más aisladas que galaxias similares pero donde sus estrellas y el gas rotan en el mismo sentido (Chen et al. 2017​, Jin et al. 2017). Por otra parte, parece que los gradientes de la edad y metalicidad en las galaxias (desde el centro hacia las partes externas) no están afectados ni por el entorno local ni por el entorno a gran escala (Zheng et al. 2017, Goddard et al. 2017).

El catálogo GEMA está disponible al público en el DR15! Te invitamos a jugar con él para explorar dónde vive tu galaxia MaNGA favorita.

This is a guest post by David V. Stark (Kavli Institute for the Physics and Mathematics of Universe, University of Tokyo).

The SDSS-IV MaNGA survey is providing the most comprehensive census of the stellar and ionized gas content of local galaxies to date, but there is another major component of galaxies the SDSS telescope does not see: the cold gas. Cold gas plays the key role of fueling the formation of new stars. Galaxies with ongoing star formation tend to have lots of cold gas, while those with no ongoing star formation have very little cold gas. Figuring out how and why galaxies acquire, consume, and/or lose their gas over time is fundamentally important to our understanding of galaxy evolution as a whole.

Typically, the largest component of cold gas within galaxies takes the form of neutral hydrogen atoms floating around at very low densities. In the astronomical community, this component is referred to as HI (which in this case is not an enthusiastic greeting, but is rather pronounced “H one”). Our ability to see HI is thanks to a very small transition within hydrogen atoms where the proton and electron go from spinning in the same direction to spinning in opposite directions. This “spin-flip” transition releases a tiny amount of energy in the form a electromagnetic radiation with a wavelength of 21 centimeters. Such a long wavelength lies in the radio regime of the electromagnetic spectrum, so is invisible to optical telescopes like that used for the MaNGA survey. Thankfully there are radio telescopes specifically designed to detect this radiation.

The HI-MaNGA survey led by Professor Karen Masters and myself is an ongoing observing program to measure the HI content of MaNGA galaxies using the 100m Green Bank Telescope (GBT). Located within the Radio Quiet Zone of West Virginia, USA, the GBT is one of the world’s premier radio telescopes, and its large collecting area and “quiet” surroundings makes it an excellent tool to measure the faint 21cm emission from MaNGA galaxies that lie as much as hundreds of megaparsecs away.

The GBT cannot provide pictures of MaNGA galaxies in the same way as optical telescopes, but rather acts like a spectrometer with a single spatial pixel that measures all the emission from an area on the sky that is about 270 times larger than a single fiber in the MaNGA IFUs. So while we do not map the HI within galaxies, we do measure the integrated radio spectrum emitted by each galaxy. From this spectrum we can measure two fundamental properties: (1) The total amount of light emitted at 21cm, which is directly proportional to the amount of HI gas, and (2) the spread of the 21cm emission line, which reflects a galaxy’s rotation speed and can be used to place crucial constraints on the total enclosed mass (stars, gas, and dark matter).

Data for the first 331 galaxies from HI-MaNGA has been released as a Value Added Catalog in SDSS Data Release 15, with both the processed radio spectra and derived properties made available. This first release is just a taste of what is to come; additional data has been collected for over 2000 additional MaNGA galaxies, and observations are continuing as we speak. This work would not be possible without the amazing team of undergradute and graduate students who have helped with, and continue to help with, observations and data reduction: Zach Pace, Frederika Phipps, Alaina Bonilla, Nile Samanso, Catherine Witherspoon, Catherine Fielder, Emily Harrington, Shoaib Shamsi, Daniel Finnegan, and Lucy Newnham.

Stay tuned for a lot more data and a ton of interesting science!