C++ threads and class methods. Problem with memory leakage

I have a problem with threading in C++, I want to parallelly call same method on different class objects. However, program encounters some problem with that. From time to time it crashes and sometimes runs through code part, but always return a ridiculously large number as an ID, always in the place of 0. The class body:

class Gas{
public:
    Gas(const int id, const int imax, const int jmax){
        id_ = id;
        NX_ = imax;
        NR_ = jmax;
    }
private:
    int id_;                            //id of specimen
    int NX_;                            //quantity of elements in the X direction
    int NR_;                            //quantity of elements in the R direction

    std::vector<double> fr_;            //molar fraction
    std::vector<double> ms_;            //mass source
    std::vector<double> ms_old_;        //mass source value from previous iteration - for source relaxation
};

The class method:

double Gas::initialize(){
        int i, j, k;
        fr_.resize(NX_ * NR_);
        ms_.resize(NX_ * NR_);
        ms_old_.resize(NX_ * NR_);

        std::cout << "ID: " << id_ << '\n';


        k = 0;
        for(i = 0; i < NX_; i++){
            for(j = 0; j < NR_; j++){
                fr_[k] = 0.01;
                ms_[k] = 0.;
                k++;
            }
        }
    }

Here is the threading implementation in the code:

std::vector<Gas> gases;
std::vector<Gas*> ptr_gas(6);
for(i = 0; i < 6; i++){                                     //creating vector holding objects representing specific gases
        gases.push_back(Gas(i, 10, 5));
        ptr_gas[i] = &gases[i];
    }

std::vector<std::thread*> th_gas;
int i = 0;
for(auto &X : ptr_gas){
    std::thread *thr = new std::thread(&Gas::initialize, ptr_gas[i]);
    th_gas.push_back(thr);
    i++;
}

for(auto &X : th_gas){
    X->join();
    delete X;
}
th_gas.clear();     

The output of std::cout I put inside the method goes like this:

ID: ID: 1
4999616
ID: 5
ID: 4
ID: 2
ID: 3

It looks like there is some problem with joining of the first of created threads. Any suggestions on how to avoid that problem? I have already checked ptr_gas addresses and they are correct.

std::vector<Gas> gases;
std::vector<Gas*> ptr_gas(6);
for(i = 0; i < 6; i++){                                     //creating vector holding objects representing specific gases
        gases.push_back(Gas(i, grid));
        ptr_gas[i] = &gases[i];
    }

Your pointers in ptr_gas get "invalid" because "gases" will reallocate when you push_back.