Bernardo Cervantes Sodi

Assistant Professor @ Instituto de Radioastronomía y Astrofísica,

                                    Universidad Nacional Autónoma de México

  About Me  --  Research  --  Studies  --  Academic Appointments  --  Publications  --  Group Members  --  Contact

Photo: Bukhansan National Park by B. Cervantes Sodi

About Me Top

I'm an astrophysicist working at the Instituto de Radioastronomía y Astrofísica of the National Autonomous University of Mexico (UNAM). I was born in Mexico City where I spent my student years. I got my bachelor's degree in Physics Engineering at Universidad Iberoamericana and then my Masters and PhD in Astrophysics at UNAM. I then moved to Korea to work as a postdoc at KASI, then to Shanghai to work with prof. Cheng Li at the Shanghai Astronomical Observatory and finally to Seoul to work with prof. Changbom Park at the Korea Institute for Advance Study.

My research field is Extragalactic Astrophysics. I am interested in how galaxies acquire their present day structural parameters, specially how angular momentum is acquired, the influence of the environment on the evolution of galaxies and more recently I'm trying to understand why some galaxies develop stellar bars and why some others don't, although they share similar overall physical parameters.

Research Top

About 30% of galaxies in the local Unvierse present elongated features in their centers, such as the one shown in the next figure showing the famous galaxy NGC 1300. These elongated features are commonly know as bars and are conformed by stars, gas and dust. If you take a careful look of the image you will also notice many background galaxies, farther away than NGC 1300, many of them also barred galaxies.

Numerical simulations have shown that rotating disks are highly unstable and form spontaneously bars. From the earliest simulations, it was noted that the inclusion of a gravitationally dominating dark matter halo helps to stabilize the disk and prevent and/or delay the formation of the bar. Later on, several theoretical studies have addressed this issue but until recently, no observational effort was directed to prove or disprove such claim. In a couple of papers we studied if the fraction of barred galaxies depends on the stellar-to-halo mass ratio, in order to see if the amount of dark matter has any relevance on the presence of bars in spiral galaxies.

Using a large sample of galaxies (about ~10,000 galaxies) form the Sloan Digital Sky Survey (SDSS), we first looked at the dependence of the bar fraction (fbar) on the stellar-to-halo mass ratio. The next figure on the left clearly shows that as the stellar-to-halo mass ratio increases,
fbar increases, specially for the case of strong bars. It can be argued that this effect is due entirely by the dependence the bar fraction shows with the stellar mass. To see if fbar  presents a variation with varing halo mass at fixed stellar mass, we looked at  in a 2D plane shown in the next figure, right panels. The color contours denote the bar fraction for the full sample, including strong and weak bars, but also differentiating between the two types. It is clear, at least for the case of strong bars, that at fixed stellar mass, there is a systematic increase of fbar with decreasing halo mass. This results suggest that dark matter halos have a stabilizing effect against bar formation. For more details on this work check our paper.


The dark matter halo masses for the previous results were based on an indirect method by looking at the spatial distribution of galaxies in the SDSS and assigning masses by ranking galaxies according to their luminosities. To double-check that our results were insensitive to this method of assigning masses, I studied the same problem but estimating halo masses by looking at the kinematic information of the HI gas of those galaxies. In the next figure I present the bar fraction as a function of stellar mass splitting the sample in two, a high halo mass sub-sample and a low halo-mass subsample. The two columns correspond to two different halo mass estimators. Again, for the case of strong bars, the bar fraction of galaxies having heavy dark matter halos is statistically lower than that of the sub-sample with low halo masses, supporting our previous results.


Having HI mass information already at hand, we also explored if the gas content of galaxies has any influence on the presence of bars. We found, as previous studies had already reported, that the bar fraction decreases with increasing gas-to-stellar mass ratio. This result can be consequence of two different mechanisms:  (i) strong bars promote the consumption of atomic gas and, (ii) gas prevents the formation/growth of bars. Our results show that barred galaxies in our sample are not consuming their gas in a more efficient way than their unbarred counterparts (look at the gas consumption timescale in the figure), hence favoring the second explanation; increasing the gas content in disk galaxies prevents the formation of bars, they grow more slowly or they are destroyed directly or in- directly by the presence of gas.


More details here.

More recently, in collaboration with my student Osbaldo Sánchez, we have been studying Low Surface Brightness galaxies (LSBs). These galaxies are interesting because they are very extended systems with low density stellar disks and by consequence, strongly dark matter dominated. Here some examples, taken again form the SDSS:


Given that LSBs are gravitationally dominated by dark matter at all radii, is logical to expect that the fraction of LSBs hosting stellar bars would be low. This is exactly the case as we show in the next figure, where we contrast the bar fraction of LSBs in comparison with the same fraction for High Surface Brigthness galaxies (HSBs).


Studies Top

Academic Appointments Top


* Assistant Professor

     Instituto de Radioastronomía y Astrofísica,
     Universidad Nacional Autónoma de México, UNAM.
     Morelia, Michoacan, México.
     February 2015 – to date

* Research FellowKIAS_logo

     School of Physics,
     Korea Institute for Advanced Study
     Under the supervision of Prof. Changbom Park
     October 2013 – December 2014

 * Postdoctoral Researchershao_logo

     Max-Planck-Institute Partner Group at
     Shanghai Astronomical Observatory
     Under the supervision of Prof. Cheng Li
     September 2011 – August 2013

 * Postdoctoral Researcherkasi_logo

     Korea Astronomy and Space Science Institute
     June 2009 - July 2011


Publications Top


Updated list of publications in the ADS

Group Members Top

Osbaldo Sánchez García.
Got his bachelor's degree in Physics at Universidad Autónoma de Zacatecas, under the supervision of José Luis Saucedo and Bernardo Cervantes Sodi. He is now pursuing his Master's degree at IRyA studying the likelihood of low surface brightness galaxies hosting stellar bars.

Luis Enrique Pérez Montaño.
Got his bachelor's degree in Physics at Universidad Autónoma de Puebla. He started working with me during a summer program for undergraduate research at IRyA in 2016 and after joining our Master's program he continued his research focused on determining the galactic spin of low surface brightness galaxies to see if their spin is the physical parameter responsible for their characteristic features.

Contact Top

Instituto de Radioastronomía y Astrofísicaarcoiris

UNAM Campus Morelia

Antigua Carretera a Pátzcuaro # 8701
Col. Ex Hacienda San José de la Huerta
Morelia, Michoacán, México
C.P. 58089

b.cervantes at
From Morelia: 322 27 77 ext. 42654
From Mexico City: 56 23 27 77 ext. 42654
Office no. 117


Picture at Iztaccihuatl, Mexico